Textbook of Zoology· Vertebrates

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Abercrombie, M., A DICTIONARY OF BIOLOGY Penguin
Hickman, C. J. and
Johnson, M. L.
Andrewartha, H. G. INTRODUCTION TO THE STUDY OF Chapman & Hall
ANIMAL POPULATIONS
Bailey, N. T. J. STATISTICAL METHODS IN BIOLOGY English Universities
Press
Barrington, E. J. W. INVERTEBRATE STRUCTURE Nelson
Chapman, R. F. THE INSECTS: STRUCTURE AND English Uni~·ersities
FUNCTION Press
Clark, L. R., Geier, P. W., THE ECOLOGY OF INSECT POPULA- Chapman & Hall
Hughes, R. D. and Morris, TIONS IN THEORY AND PRACTICE
R. F.
Hayes, W. THE GENETICS OF BACTERIA AND Blackwell Scientific
THEIR VIRUSES
Elton, C. S. THE ECOLOGY OF INVASIONS BY Methuen
ANIMALS AND PLANTS
Hughes, G. M. COMPARATIVE PHYSIOLOGY OF Heinemann Educational
VERTEBRATE RESPIRATION
Imms, A. D. A GENERAL TEXTBOOK OF ENTO- Methuen
MOLOGY
Lockwood, A. P.M. ANIMAL BODY FLUIDS AND THEIR Heinemann Educational
REGULATION
Manning, A. AN INTRODUCTION TO ANIMAL Arnold
BEHAVIOUR
Marshall, A. J. and A TEXTBOOK OF INVERTEBRATE Macmillan
Williams, W. D. ZOOLOGY
Paul, J. CELL BIOLOGY Heinemann Educational
Soulsby, E. J. C. MONNIG'S HELMINTHS, ARTHRO- Bailliere Tindall
PODS AND PROTOZOA OF DOMESTI-
CATED ANIMALS
Stephenson, G. MATHEMATICAL METHODS FOR Longman
SCIENCE STUDENTS
Weatherburn, C. E. A FIRST COURSE IN MATHE- Cambridge University
MA TICAL STATISTICS Press
Wigglesworth, V. B. THE PRINCIPLES OF INSECT Methuen
PHYSIOLOGY
Young, J. Z. THE LIFE OF VERTEBRATES Oxford University Press
Textbook of
Zoology
Vertebrates

Revised and largely rewritten by the late A. J. MARSHALL, D.Phil., D.Sc.

sometime Professor of Zoology and Comparative Physiology in Monash University

This is the Seventh Edition of A Textbook of Zoology, Vol. II

by the late T. JEFFERY PARKER, D.Sc., F.R.S.
sometime Professor of Biology in the University of Otago, Dunedin

and the late WILLIAM A. HASWELL, M.A., D.Sc., F.R.S.

sometime Professor of Biology in the University of Sydney

English Language Book Society and Macmillan Education

© Macmillan & Co Ltd 1962
All rights reserved. No part of this publication may be
reproduced or transmitted, in any form or by any means,
without permission.

First Edition 1898

Second Edition 1910
Third Edition 1921
Fourth Edition 1928
Fifth Edition 1930
Sixth Edition 1940
Reprinted 1943, 1947, 1949, 1951
Seventh Edition 1962
Reprinted 1963
Reprinted with corrections 1964
Reprinted 1966, 1967, 1972

ELBS edition first published 1972

Published by
THE MACMILLAN PRESS LTD
London and Basingstoke
Associated companies in New York
Toronto Dublin Melbourne
Johannesburg and Madras

SBN 333 05391 5 (hard cover)

333 02829 5 (paper cover)

ELBS edition: SBN 333 14085 0 (paper cover)

ISBN 978-1-349-00200-9 ISBN 978-1-349-00198-9 (eBook)
DOI 10.1007/978-1-349-00198-9

The paperback edition of this book is sold subject to the

condition that it shall not, by way of trade or otherwise, be
lent, resold, hired out, or otherwise circulated without
the publisher's prior consent, in any form of binding or
cover other than that in which it is published and without
a similar condition including this condition being
imposed on the subsequent purchaser.
 • • •
In conclusion, it must be pointed out that in order adequately
to solve the problems of Zoology they must be approached from
all sides. From the time of Cuvier to that of Owen comparative
anatomy was the dominant branch of the science, and there was
a tendency to depreciate the work of the 'mere' systematist and
outdoor naturalist. For the last five and twenty years em-
bryology has been in the ascendant, and the 'mere' anatomist
has been somewhat overshadowed. To-day, hopeful signs of a
renewed interest in ethology-the study of living animals under
natural conditions-are accompanied by a tendency to look
upon all laboratory work as necrology rather than biology-the
study of corpses rather thim of living things. But nothing is
more certain than that if the new 'natural history' is to be
superior to the old-more scientific, more concerned with the
solution of general problems-it can only be by utilising to the
full all that has been learnt in the laboratory in the departments
of anatomy, physiology and embryology. (Concluding paragraph,
Parker and Haswell, rst ed., r898.)

A2
 PREFACE TO THE SEVENTH EDITION

I T is now more than sixty years since the initial publication of' Parker and
Haswell'. Many changes were made in Volume II by Forster-Cooper in
his Sixth Edition of 1940, but much of the original nineteenth-century
text has nevertheless remained relatively unaltered. Consequently no apology
is offered for the almost complete revision that follows.
When the publishers invited me to undertake the Seventh Edition they
were kind enough to give me a free hand. For a while I thought of altering
the whole scheme of the volume to one of my own devising. However, I
decided (like my immediate predecessor) against the disruption of the 'general
plan of a text-book that has served well so many generations of students'.
In general I have attempted, within the limits of available space, to retain
basic morphology and yet at the same time emphasise functional aspects and,
where possible, present animals as living creatures rather than as laboratory
specimens. In this I have been helped by my wife, Jane Marshall, whose
otherwise unacknowledged pen-drawings are scattered through the text:
a collaboration, incidentally, that was responsible once or twice for minor
domestic dissension owing to the conflicting claims of art and accuracy.
An innovation in the present volume is the inclusion of the elements of a
classification under each illustration of a whole, or almost whole, animal.
This may help the student more or less painlessly to familiarise himself with
the animal's systematic position. Some trouble too, has been taken to provide
a lavish cross-reference system so that comparative information can be found
quickly and without tediously repetitive consultation of the Index or Contents.
It was at first hoped to include an appendix on the principles (and pitfalls)
of systematics but there was no room for it. In any case, students can nowa-
days easily consult Mayr, Linsley and Usinger's Methods and Principles of
Systematic Zoology (1953) or Simpson's The Principles of Classification and a
Classification of Mammals (1945). (The last-named is additionally noteworthy
in its lack of the solemnity that so many scientists believe to be indispensable
to good scholarship.) Today, with such books readily available, only
egotism or perversity, rather than ignorance, can be responsible for an extension
of the taxonomic morass into which so many groups have been thrust: an
unhealthy swamp that will need the labour of a century to remove.
The question of references has been one of some difficulty. Clearly it is
VII
 viii PREFACE
impossible to cite every source of information, most of which has been taken
from original papers. A compromise has been made. In the first place, key
references, leading to many others, have been included in the bibliography.
Where an authority has been cited personally in the text, the publication date
has been deliberately omitted. It is believed that it will provide a mild exer-
cise in ingenuity and bibliographic skill for the honours man to discover the
source from which he requires further information.
Another debatable point has been the appropriate amount of palreontology
that should be given in a text-book of zoology. Some of my colleagues are of
the opinion that there was already included rather too much information about
fossil forms. I believe, however, that it is impossible to gain a good general
understanding of living animals-either in form or function-without an
adequate appreciation of what is known of their ancestry. Therefore the
fossil sections have been expanded.
The elimination of N eoceratodus as the lung-fish 'type', and the substitution
of Protopterus, needs brief explanation. The former is so rare, and rigidly
protected, that not even an Australian student can confidently hope ever to
dissect one. Living Protopterus, on the other hand, can be bought in unlimited
numbers in fish-markets over wide areas of Africa. Thus they are available
(for demonstration purposes at least) to university departments anywhere.
It may seem surprising, in this view, that Salmo is retained as the teleost
'type'. However, there is employed for the purpose today (in the many
countries where this book is used) such a wide diversity of bony fishes that any
new choice would have only limited and local value.
Had I not had my first systematic instruction in zoology and physiology
from students of Haswell, and made my first supervised dissections in the school
that he founded, I would never have essayed the present revision. Yet, once
begun, the task became surprisingly agreeable. It has been lightened by the
rigorous but kindly criticism of friends who read sections of the manuscript
dealing with subjects of their special interests. These are as follows:
E. C. Amoroso A.]. E. Cave E. W. Knight-Jones D. L. Serventy
F. ]. Aumonier G. E. H. Foxon K. A. Kermack V. B. Sheffer
E. W. Baxter F. C. Fraser Dennis Lacy J. R. Simons
A. d'A. Bellairs P. H. Greenwood D. A. McDonald R. Strahan
Ruth Bellairs Frank Goldby ]. D. Macdonald E. I. White
R. J. H. Brown I. Griffiths N. B. Marshall H. P. Whiting
G. H. 0. Burgess R. W. Hayman ]. A. C. Nichol
P.M. Butler William Holmes H. K. Pusey
D. B. Carlisle C. Burdon-Jones W. D. L. Ride
If this edition is found to be deserving of any commendation, much of it is
properly due to them. Some rewrote special pieces, and a few, in addition,
 PREFACE IX

provided illustrations (below which acknowledgement is made). Almost all

drew my attention to errors or half-truths, including, I have to confess, a few
that I succeeded in introducing myself. On occasions, my benefactors dis-
agreed in a most stimulating way among themselves-principally on minor
matters of taxonomy and terminology-and so it is more than ever necessary
to add on their behalf the customary disclaimer concerning responsibility.
Nobody other than myself has seen more than a comparatively small part of
the manuscript.
It is a final pleasure to acknowledge the loan by the British Museum (Natural
History) of specimens for illustration, and the constant assistance given me
by the Library staffs of University College and, particularly, the Zoological
Society of London.
A. J. M.
Victoria
Australia I96o

NOTE ON THE 1964 REPRINT

The 1964 reprint with corrections has been improved by the further criticism
of some of the above-named, and additionally by valuable comments from
Drs. D. F. Dorward, A. K. Lee, and J. W. Warren.
A.]. M.
Victoria
Australia I964
 CONTENTS
PHYLUM CHORDATA r

SUB-PHYLUM ADELOCHORDATA (BEMICHORDATA) 5

CLASS ENTEROPNEUSTA . 5
External characters and crelom (p. 5); alimentary canal and associated structures
(p. 8); stomochord (proboscis or buccal diverticulum) (p. 10); blood-vascular
system (p. 1o); nervous system (p. II); locomotion (p. II}; reproductive organs
(p. II); development (p. II); tornaria (p. 13).
CLASS PTEROBRANCBIA . 14
Blood-vascular system (p. 17); reproduction (p. 17); affinities (p. 20).

SUB-PHYLUM TUNICATA (UROCHORDATA) 20

CLASS ASCIDIACEA . 21
Example of the mass. (The Ascidian or Sea-squirt (Ascidia) (p. 21); body-wall and
atrial cavity (p. 22); pharynx (p. 23); alimentary canal (p. 25); blood-vascular
system (p. 25); excretion (p. 27); nervous system (p. 27); neural gland (p. 27);
reproductive system (p. 27); development and metamorphosis (p. 28).
General Organisation of Ascidians: General features (p. 34); composite ascidians
(p. 36.)
CLASS TBALIACEA 36
Orders Doliolida (Cyclomyaria) (p. 36) ; Pyrosomida (p. 39); Salpida (Desmomyaria)
(p. 41).
CLASS LARVACEA (APPENDICULARIA) 44
General features, affinities (p. 46).

SUB-PHYLUM ACRANIA (CEPHALOCHORDATA) 47

Example of the Sub-phylum.-The Lancelet (Amphioxus) (Branchiostoma) (p. 47);
body-wall (p. 49); skeleton (p. so); alimentary canal and associated structures (p. 51);
atrium (p. 53) ; ingestion, digestion, and absorption (p. 53); crelom (p. 54); blood-
vascular system (p. 54); excretory organs (p. 56) ; nervous system (p. 57); organs
of special sense (p. 59); reproductive system (p. 59); development (p. 6o); affinities
(p. 67).

GENERAL INTRODUCTION TO CRANIATE (VERTEBRATE} ·FORMS 69

Craniate Tissues: epithelial tissue (p. 70); connective tissue (p. 70); adipose
tissue (p. 70); blood (p. 70); lymph (p. 71); cartilage (p. 71); bone (p. 71); muscle
(p. 71); neural tissue (p. 71); ectodermal derivatives (p. 71); mesodermal deriva-
tives (p. 71); endodermal derivatives (p. 71).
The Craniate Body: External characters (p. 78); skeleton (p. 88); endoskeleton
(p. 89); alimentary canal and associated structures (p. 102); organs of respiration
(p. 105); blood-vascular system (p. 108); lymphatics (p. IIS); nervous system
(p. II6); somatic motor nerves (p. 130); branchial arch nerves (p. 130); sensory
organs (p. 135); olfactory organ (p. 137); eye (p. 139); development of the eye
(p. 142); apparatus of audition and equilibration (p. 143); endocrine organs
(p. 147); urinogenital organs (p. 154); development (p. r6o); metamerism (p. r6r).

SUB-PHYLUM CRANIATA (VERTEBRATA) 163

xi
Xll CONTENTS

SUPER-CLASS AGNATHA .
CLASS EUPBANERIDA 167
CLASS HETEROSTRACI (PTERASPIDA) 168
CLASS ANASPIDA r7o
CLASS OSTEOSTRACI (CEPBALASPIDA) 171
C<ELOLEPIDA. 174
CLASS PETB.OliYZONTIA 175
Example of the Class.-The Lamprey (Petromyzon) 176
External Characters (p. 177); endoskeleton (p. r8o); muscles (p. r84); alimentary
canal and associated structures (p. r84); respiratory organs (p. 185); circulatory
system (p. r86); nervous system (p. 187); organs of special sense (p. 190); urino.
genital system (p. 193); development (p. 194).
CLASS JIYXINOIDEA . 197
Comparison of the Myxinoidea with the Lamprey r98

SUPER-CLASS GNATBOSTOMATA . 203

CLASS PLACODERMI (APBETOBYOIDEA) 205
Sub-class Acanthodii . 207
Sub-class Arthrodira 210
Sub-class Petalichthyida 212
Sub-class Antiarchi (Pterichthyomorphi) 214
Sub-class Rhenanida . 216
Sub-class Paheospondylia 217
CLASS ELASMOBRANCIDI (CHONDRICHTHYES) (SELACHD) 219
Orders Cladoselachi (Pleuropterygii) (p. 222); Pleuracanthodii (lchthyotoini) (p. 224);
Protoselachii (p. 224); Euselachii (p. 226).
Example of the Class.-A Dogfish (Scyliorhinus ( = Scyllium), Squalus, Brachcelurus,
etc.) (p. 229); general external features (p. 229); skeleton (p. 231); alimentary
canal and associated structures (p. 237); respiration (p. 239); blood-vascular
system (p. 240); nervous system (p. 245); organs of special sense (p. 249); endo-
crine organs (p. 251); excretion and osmoregulation (p. 252); reproduction (p. 254).
Sub-class Bradyodonti 256
Order Holocephali 256
External characters (p. 258); endoskeleton (p. 259); alimentary canal (p. 261);
respiratory organs, heart, brain (p. 262); urinogenital organs and reproduction
(p. 263).
General Organiaation of Elasmobranchs: External characters (p. 265); in-
tegument (p. 267); endoskeleton (p. 267); skull (p. 268); electric organs (p. 271);
alimentary canal and associated structures (p. 272); respiratory organs (p. 273);
blood-vascular system (p. 273); brain (p. 274); organs of special sense (p. 274);
audio-equilibration (p. 275); endocrine organs (p. 276); urinogenital organs and
reproduction (p. 276); placentation (p. 278); development (p. 279).
CLASS OSTEICHTHYES 282
Sub-class Actinopterygii 282
Super-order Chondrostei 285
Orders Palseoniscoidea (p. 285); Acipenseroidei (p. 288); Polypterini (Cladistia)
(p. 290).
Super-order Holostei . 29 r
Orders Semionotoidea (p. 292); Amioidea (p. 292).
Super-order Teleostei . 293
Soft-rayed orders (p. 296); Intermediate orders (p. 296); Spiny-rayed and allied
orders (p. 297).
Example of the Sub-class.-The Brook or Brown Trout (Salmo jario) 298
External characters (p. 298); skin and exoskeleton (p. 300); endoskeleton (p. 300);
muscles (p. 309); crelom (p. 309); alimentary canal and associated structures
 CONTENTS xiii
(p. 309); nervous system (p. 312); autonomic nervous system (p. 314); organs of
special sense (p. 315); osmoregulation and excretion (p. 317); endocrine organs
(seep. 346); reproduction (p. 318); development (p. 319).
General Organisation of Actinopterygian&: External form (p. 320); exoskeleton
(p. 329); endoskeleton (p. 330); electric organs (p. 334); alimentary canal and
associated structures (p. 335); respiratory organs (p. 337); swim.bladder (p. 339);
blood.vascular system (p. 342); brain (p. 344); exteroception (p. 344); endocrine
organs (p. 346); urinogenital organs (p. 347); reproduction (p. 349); development
(p. 352).

Sub-class Crossopterygii (Choanichthes) . 353

Classification (p. 354).
Order Rhipidistia 354
Sub-orders Osteolepidoti (p. 354); Cml.acanthini (Actinistia) (p. 356).
Order Dipnoi (Dipneusti, Dipneumones) 361
Sub-orders Monopneumona (p. 364); Dipneumona (p. 365).
Ezample of the Sub-class.-Protopterus tBthiopicus 365
Endoskeleton (p. 367); alimentary canal and associated structures (p. 369);
respiratory system (p. 370); blood-vascular system (p. 370); nervous system
(p. 374); general metabolism and excretion (p. 375); reproduction (p. 376); de-
velopment (p. 377).
General Organisation of Dipnoi 379

CLASS AMPHIBIA 381

Ezample of the Class.-A Frog (Rana) 389
External characters (p. 389); endoskeleton (p. 391); alimentary canal and associ-
ated structures (p. 400); respiratory system (p. 403); blood-vascular system (p.
404); lymphatic system (p. 410); nervous system (p. 410); organs of special sense
(p. 412); endocrine glands (p. 415); urinogenital organs (p. 415); reproduction
(p. 416); development (p. 418).
Interrelationship of Amphibian Groups 421
Sub-class Apsidospondyli 421
Super-order Labyrinthodontia 422
Order Ichthyostegalia 423
Order Rhachitomi 424
Order Stereospondyli 425
Order Embolomeri . 426
Order Seymouriamorpha 427
Super-order Salientia (Anura, Batrachia) 428
Sub-class Lepospondyli 430
Order Aistopoda 430
Order Nectridia 430
Order Microsauria (Adelospond:vli) 431
Order Ph:vllospondyli 431
Order Urodela (Caudata) . 432
Order Apoda (G:vmnophiona, Caecilia). 434
General Organisation of Amphibians: External characters (p. 435); exoskeleton
(p. 437); endoskeleton (p. 438); musculature (p. 442); alimentary canal and
associated structures (p. 442); nervous system and organs of special sense (p. 447);
endocrine system (p. 450); urinogenital system and osmoregulation (p. 450);
reproduction and development (p. 452).

CLASS BEPTILIA 457

EJ:ample of the mass.-A Lizard (Lacerta) 462
Exoskeleton (p. 463); endoskeleton (p. 463); alimentary canal and associated
structures (p. 471); blood-vascular system (p. 474); respiratory system (p. 477);
brain (p. 478); endocrine glands (p. 481); organs of special sense (p. 481); urino-
genital system (p. 482).
Interrelationship of Reptilian Groups 484
XIV CONTENTS
Sub-class Anapsida
Order Cotylosauria
Sub-orders Captorhinomorpha (p. 4S6); Diadectomorpha (p. 4S6).
Order Chelonia .
Sub-class Ichthyopterygia
Order Mesosauria
Order Ichthyosauria .
Sub-class Synaptosauria 492
Order Protorosauria . 492
Order Sauropterygia . 492
Sub-orders Nothosauria (p. 492); Placodontia (p. 493); Plesiosauria (p. 493).
Sub-class Lepidosauria. 495
Order Eosuchia . 495
Order Rhynchocephalia 495
Squamata 496
Sub-orders Lacertilia (p. 496); Ophidia (Serpentes) (p. 497).
Sub-class Archosauria . 49S
Order Thecodontia 499
Order Crocodilia (Loricata) . 499
Order Pterosauria (Pterodactyl&) 502
Dinosaurs 504
Order Saurischia 505
Sub-orders Theropoda (p. 505); Sauropoda (p. 507).
Order Ornithischia 507
Sub-orders Ornithopod& (p. 5oS); Stegosauria (p. 510); Ankylosauria (p. 511);
Ceratopsia (p. 5 I I l .
Sub-class Synapsida 5I 3
Order Pelycosauria 5I3
Order Therapsida 514
Order Ictidosauria 5I 7
General Organisation of Recent Reptiles: External features (p. 5IS); integument
and exoskeleton (p. 522); endoskeleton (p. 524); alimentary canal and associated
structures (p. 533); respiratory system and voice (p. 540); blood-vascular system
(p. 54 I); nervous system (p. 542); organs of special sense (p. 543); endocrine
glands (p. 54S); excretion (p. 54S); reproduction (p. 549); development (p. 55 I).
CLASS AVES 555
Example of the Class.-The Common Pigeon (Columba Iivia, var. domestica) 565
External characters (p. 566); exoskeleton (p. 567); endoskeleton (p. 57 I);
muscular system (p. sSo); locomotion (p. 5S2); alimentary canal and associated
structures (p. 5S3); respiratory and vocal organs (p. 5S5); thyroid, parathyroid,
and thymus glands (p. 5S9); blood-vascular system (p. 5S9); lymphatic system
(p. 592); nervous system and hypophysis (p. 592); organs of special sense (p. 594);
excretory and reproductive organs, and adrenal glands (p. 596).
General Organisation of Birds 59S
Sub-class Archeornithes 599
Order Archeopterygiformes 599
Sub-class Neornithes . 6o2
Super-order Odontognathe . 603
Orders Hesperornithiformes (p. 6o4); Ichthyornithiformes (p. 6os).
Super-order Paleognathe . 6o6
Orders Struthioniformes (p. 6o7); Rheiformes (p. 6o7); Casuariiformes (p. 6o7);
Apterygiformes (p. 6oS); Dinornithiformes (p. 6oS); ]Epyornithiformes (p. 6o9);
Tinamiformes (p. 6o9).
Super-order Impenne 610
Order Sphenisciformes 6I o
Super-order Neognathe 6I2
Orders Gaviiformes (p. 6I3); Podicipitiformes (p. 6I3); Procellariiformes (p. 6I3);
Pelicaniiformes (p. 6I4); Ciconiiformes (p. 6I4); Anseriformes (p. 6I4); Falconi-
 CONTENTS XV

formes (p. 614); Galliiormes (p. 615); Gruiformes (p. 615); Diatrym.iiormes (p.
616); Charadriiformes (p. 616); Columbiformes (p. 617); Psittaciformes (p. 617);
Cuculiformes (p. 617); Strigiformes (p. 618); Caprimulgiformes (p. 618); Apodi-
formes (Micropodiformes) (p. 618); Coliiformes (p. 618); Trogoniformes (p. 619);
Coraciiformes (p. 619); Piciformes (p. 619); Passeriformes (p. 62o).
General Organisation of Neornithes: External characters (p. 62o); pterylosis (p.
626); endoskeleton (p. 628); alimentary canal and associated structures (p. 636);
respiratory and vocal organs (p. 638); circulatory organs (p. 639); nervous system
and organs of special sense (p. 639); excretion (p. 643); reproduction (p. 644);
development (p. 646).

CLASS MAMMALIA 652

Example of the Class.-The Rabbit (Oryctolagus wniculus) 654
External characters (p. 654); skeleton (p. 655); crelom (p. 666); alimentary canal
and associated structures (p. 666); blood-vascular system (p. 670); lymphatic
system (p. 675); respiratory system (p. 676); nervous system (p. 678); spinal
cord, cranial and spinal nerves (p. 682); autonomic nervous system (p. 682);
organs of special sense (p. 684); urinogenital organs (p. 684); development (p. 686).
Sub-class Prototheria . 687
Order Triconodonta . 687
Order Symmetrodonta 687
Order Monotremata . 688
Skeleton (p. 696); visceral anatomy (p. 700).
Sub-class Allotheria 701
Order Multituberculata 701
Sub-class Theria 703
Infra-class Pantotheria (Trituberculata) . 703
Order Dryolestoidea . 703
Order Docodonta 703
Infra-class Metatheria (Marsupialia, Didelphia) 704
Super-families Didelphoidea (p. 711); Borhyrenoidea (p. 712); Dasyuroidea (p. 712);
Perameloidea (p. 715); Crenolestoidea (p. 715); Phalangeroidea (p. 716).
Endoskeleton of Metatheria (p. 719).
Infra-class Eutheria (Placentalia, Monodelphia) 723
Introduction (p. 723).
Cohort Unguiculata 726
Order Lipotyphla 726
Skeleton of the Lipotyphla (p. 730).
Order Menotyphla 731
Order Primates . 734
Sub-order Prosimii 736
Infra-orders Lemuriformes (p. 737); Lorisiformes (p. 738); Tarsiiformes (p. 739).
Sub-order Anthropoidea 741
Super-family Ceboidea (Platyrrhina). 743
Super-family Cercopithecoidea. 743
Super-family Hominoidea . 744
Families Pongidre (p. 744); Bominidre (p. 745).
Skeleton of the Primates (p. 748).
Order Dermoptera 753
Order Chiroptera 754
Sub-orders Megachiroptera (p. 756); Microchiroptera (p. 756).
Skeleton of the Chiroptera 758
Order Treniodontia 76o
Order Tillodontia 76o
Order Edentata . 76o
Sub-orders Palreanodonta (p. 761); Xenarthra (p. 761).
Skeleton of Xenarthra (p. 765).
XVI CONTENTS
Order Pholidota. 772
Cohort Glires 774
Order Rodentia . • 77 4
Skeleton of the Rodentia (p. 778).
Order Lagomorpha 779
Cohort Mutica 779
Order Cetacea . 78o
Sub-orders Archeoceti (p. 782); Odontoceti (p. 783); Mysticeti (p. 784).
Skeleton of Cetacea (p. 785); internal adaptations of Cetacea (p. 789).
Cohort Ferungulata 79 r
Super-order Fere 791
Order Carnivora 791
Sub-orders Creodonts (p. 792); Fissipedia (p. 793); Pinnipedia (p. 797).
Skeleton of the Carnivora (p. 798).
Super-order Protoungulata 8o1
Order Condylarthra 8o2
Order Tubulidentata . 8o4
South American' Ungulata' Bos
Order Litoptema So5
Order Notoungulata . 8o6
Order Astrapotheria . Bo7
Super-order Penungulata (Subungulata) 8o7
Order Hyracoidea 8o8
Order Embrithopoda . 8ro
Order Pr£>boscidea 811
Orders Pantodonta (Amblypoda) and Dinocerata 8r8
Order Pyrotheria 8zo
Order Sirenia 8zo
Skeleton of Sirenia (p. 823).
Super-order Mesaxonia 824
Order Perissodactyla . 825
Families Equide (p. 826); Brontotheriidm (Titanotheridm) (p. 831); Chalicotheriidm
(p. 832).
Sub-order Ceratomorpha . 833
Super-family Tapiroidea. 8 33
Super-family Rhinocerotoidea. 834
Super-order Paraxonia 836
Order Artiodactyla 836
Sub-orders Suiformes (p. 837); Tylopoda (p. 838); Ruminantia (p. 839).
General Organisation of Mammalia: Integument and general external features
(p. 841); endoskeleton (p. 85o); alimentary canal and associated structures (p.
862); blood. vascular system (p. 881); lymphatic system (p. 883); respiratory
system (p. 884); nervous system (p. 884); organs of special sense (p. 887); endo-
crine organs (p. 888); urinogenital system (p. 888); breeding seasons (p. 896);
development and associated phenomena (p. 898); post-natal behaviour in
marsupials (p. 907).
REFERENCES 909
General (p. 909); Protochordates (p. 914); Agnathans (p. 915); Placoderms and
Fishes (p. 917); Amphibians (p. 920); Reptiles (p. 921); Birds (p. 921); Mammals
(p. 923).
INDEX . 927

TABLES
I. Ages (in million of years) of geological periods
II. Relationship of somites, muscles, nerves and visceral arches
III. Divisions of hypophysis
IV. Composition of milk •
 ILLUSTRATIONS
SUB-PHYLUM ADELOCHORDATA
CLASS ENTEROPNEUSTA Balanoglossus, Whole animal, Fig. I (p. 6); in burrow, Fig. 2
(p. 7); median sagittal section, Fig. 3 (p. 8); branchial region, Fig. 4 (p. 9); development,
Fig. 5 (p. 12); Fig. 6 (p. 13); tornaria larva, Fig. 7 (p. 14).
CLASS PTEROBRANCHIA Cephalodiscus, Colony, Fig. 8 (p. 15); zooid, Fig. 9 (p. x6);
visceral relationships, Fig. Io (p. 17); AtubaTia, Fig. II (p. 18); RhabdopleuTa, Colony,
Fig. 12 (p. x8); male zooid and crenrecium, Fig. 13 (p. 19).

SUB-PHYLUM TUNICATA (UROCHORDATA)

CLASS ASCIDIACEA Ascidia, whole animal, Fig. 15 (p. 23); branchial sac, Fig. 16 (p. 24);
pharyngeal region, Fig. 17 (p. 24); circulation and reproduction, Fig. 18 (p. 25); nerve
ganglion and associated parts, Fig. 19 (p. 27); nervous system, Fig. 20 (p. 28). Ascidian
development: Figs. 21-25 (pp. 29-33). Composite ascidians: Botf71llus, Fig. 26 (p. 35); a
colonial zooid, Fig. 27 (p. 35).
CLASS TBALIACEA Doliohtm, whole animal, Fig. 28 (p. 37); development, Figs. 29-30
(p. 38); asexual zooid, Fig. 31 (p. 39). P1fJ'osoma, Luminous colony, Fig. 32 (p. 40); colonial
individuals, Fig. 33 (p. 40). Salpa, Asexual form, Fig. 34 (p. 41); oozoid, Fig. 35 (p. 42);
nervous system, Fig. 36 (p. 43); development, Fig. 37 (p. 43).
CLASS LARVACEA OikopleuTa, Animal in 'house', Fig. 38 (p. 45); AppendiculaTia,
visceral relationships, Fig. 39 (p. 45).

SUB-PHYLUM ACRANIA
CLASS CEPBALOCHORDATA BTanchiostoma (Amphioxus), Whole animal in sand, Fig. 40
(p. 48); ventral and lateral veins, Fig. 41 (p. 48); visceral relationships, Fig. 42 (p. 50); general
anatomy, Fig. 43 (p. 51); pharyngeal region, Fig. 44 (p. 54); blood vascular system, Fig. 45 (p.
55); nephridium, Fig. 46 (p. 56}; solenocytes, Fig. 47 (p. 57); nervous system, Fig. 48 (p.
58); development Figs. 49-56 (pp. 6o-66).

SUB-PHYLUM CRANIATA (VERTEBRATA)

Introductory Examples: General and Skeletal. 'Ideal' vertebrate, external view, Fig. 57
(p. 79); epidermis, Fig. 58 (p. 82); placoid scale, Fig. 59 (p. 83); cosmoid scale, Fig. 6o (p. 83);
ganoid scale, Fig. 61 (p. 84); myomere patterns, Fig. 62 (p. 86); visceral relationships, Fig. 63
(p. 87); vertebral column in embryo, Fig. 64 (p. 89); segmentation, Fig. 65 (p. 89); cranial
elements in embryonic Salmo, Fig. 66 (p. go); cartilaginous skull, Fig. 67 (p. 92); bony skull,
Fig. 68 (p. 95); pelvic fins, Fig. 69 (p. 98); pectoral fins, Fig. 70 (p. 99); tetrapod girdle~.
manus, and pes, Fig. 71 (p. wo); dermal and buccal armour; placoid scale and teeth, Fig. 72
(p. !OJ).
Respiratory System. Pharyngeal region, including branchiogenic organs, Fig. 73 (p. ro6);
pulmonary lobule, Fig. 74 (p. 107); lungs and swim-bladder, Fig. 75 (p. 108).
Blood-vascular System. In fishes, Fig. 76 (p. 109); course of circulation, Fig. 77 (p. uo);
circulation in embryonic air-breather, Fig. 78 (p. II3); cardiac circulation in amphibian and
crocodile, Fig. 79 (p. u4); relationship of blood and lymph capillaries, Fig. So (p. II6).
Nervous System and Special Senses. Brain and proximal part of cord during development and
maturity, Fig. 81 (p. u8); autonomic system, Fig. 82 (p. ug); sympathetic and spinal nerves
and cord, Fig. 83 (p. 120); fibre, motor end-plate and muscle, Fig. 84 (p. 121); cord and con.
stituent structures, Fig. 85 (p. 123); cranial and anterior spinal nerves, Fig. 86 (p. 129); lateral-
line system in bony, Fig. 87 (p. 136) and cartilaginous fishes, Fig. 88 (p. 137); nasal and buccal
cavity in various vertebrates, Fig. 89 (p. 138); eye (macroscopic structures), Fig. 90 (p. 139);
retinal structures, Fig. 91 (p. qo); orbit, Fig. 92 (p. 143); audio-equilibration: evolutionary
stages, Fig. 93 (p. 144); semi-circular canals and lagena, Fig. 94 (p. 145).
xvii
xviii ILLUSTRATIONS
Endocrine System. Hypothalamico-hypophysial relationships, Fig. 95 (p. 149); hypophysia
arrangements, Fig. 96 (p. 150); secretory apparatus of testis, Fig. 97 (p. 152).
Urinogenital System. Development and relationships, Fig. 98 (p. 153); kidney circulation
and disposal, Fig. 99 (p. 154); nephron, Fig. IOo (p. 155); and probable stages of evolution,
Fig. 101 (p. 157); development, Fig. 102 (p. 161).

SUPER-CLASS AGNATHA
CLASS EUPBANERIDAJamoytius, Fig. 103 (p. 167).
CLASS HETEROSTRACI Pteraspis, Fig. 104 (p. 168); Drepanaspis, Fig. Io5 (p. 17o).
CLASS ANASPIDA Rhyncholepis, Fig. 106 (p. I]I).
CLASS OSTEOSTRACI llemicyclaspis, Fig. IO] (p. I]I); Kiae·raspis, cranial anatomy, Fig
108 (p. 173); Lanarkia, Fig. 109 (p. 174).
CLASSESPETROMYZONTIAand MYXINOIDEAPetromyzon, whole animal, Fig. 110 (p. 175);
buccal funnel, Fig. III (p. 177); integumental and epidermal cells of My;rine and fishes, Figs.
II2-II3 (pp. I78-I79); Petromyzon, trunk region, Fig. 1I4 (p. 18o); skull, Fig. II5 (p. 181);
skull and branchial basket in adult, Fig. II6 (p. 182); skull during metamorphosis, Fig. 1 I ] (p.
I82); head region, Fig. uS (p. 183); Ammocrete (Geotria), head region, Fig. II9 (p. r86J;
Petromyzon, brain, Fig. I2o (p. 188); olfactory and nasa-hypophysial ducts, Fig. 121 (p. 189);
development, Fig. I22 (p. 190); Lam.petra, eye and adjacent structures, Fig. 123 (p. 19I);
Petromyzon, cloacal region, Fig. 125 (p. 193); development, Fig. 126 (p. 195); Lmnpetra:
(development, Fig. 127 (p. 196); My;rine and Eptatretus, anterior regions, Fig. I28 (p. 198);
My;rine skeletal and visceral structures, Fig. I29 (p. 199); skull of adult, Fig. 130 (p. zoo); of
embryo, Fig. I3I (p. 2or); membranous labyrinth, Fig. 132 (p. 202).

SUPER-CLASS GNATHOSTOMATA
CLASS PLACODERMI Dinichthys, whole animal, Fig. 133 (p. 2o6); Climatius, head, Fig.
134 (p. 207); Diplacant11us, pectoral girdle, Fig. 135 (p. 208); Acanthodes, skull, Fig. 136
(p. 209); head region, Fig. 137 (p. 210); Coccosteus, skeleton, Fig. 138 (p. 21 1); Jltacro-
petalichthys, skeleton, Fig. 139 (p. 212); head and pectoral shield of Lumispis and Macro-
]Jetalichthys, Fig. qo (p. 213); Macropetalichthys, pectoral girdle, Fig. 141 (p. 213);
Pterichthyodes, skeleton, Fig. 142 (p. 214); Bothriolepis, whole animal, Fig. 143 (p. 215);
Gemuendina, whole animal, and trunk, Figs. I44-145 (pp. 216-217); Palreospondylus,
skeleton, Fig. 146 (p. 218).
CLASS ELASMOBRANCHII Carcharodon, jaws and teeth, Fig. 147 (p. 22o); Cladoselache.
whole animal, Fig. qS (p. 223); Ctenacanthus, whole animal, Fig. I49 (p. 223); Pleura-
canthus, skeleton, Fig. 150 (p. 224); lleterodontus, whole animal, Fig. I5I (p. 225); denti-
tion, Fig. 152 (p. 226); Chlamydoselachus, Fig. 153 (p. 227); Pristis and Aetobatis
illustrating adaptation and radiation, Fig. I54 (p. 228).
Example of Class: Dogfish. Whole animal, Fig. 155 (p. 229); vertebral column, Fig. I 56 (p. 231);
skull and visceral arches, Fig. I 57 (p. 232); visceral arches, Fig. 158 (p. 234); pectoral arch and
fin, Fig. 159 (p. 235); pelvic arch and fin, Fig. 160 (p. 236); visceral structures, Fig. 161 (p. 238);
Respiration and circulation: branchial sac, Fig. 162 (p. 239); head and branchial circulation,
Fig. 163 (p. 240); general circulation, Fig. 164 (p. 242); Nervous system: brain, Fig. 165
(p. 244); brain ventricles, Fig. 166 (p. 246); brain and spinal nerves, Fig. 167 (p. 247); dorsal
root cranial nerves, Fig. 168 (p. 248). Endocrine and urinogenital elements: adrenal, Fig. 169
(p. 252); male and female urinogenitalia, Fig. 170 (p. 253); kidney and urinary sinus of female,
Fig. 171 (p. 254); egg-case of dogfish, Fig. 172 p. 254).
SUB-CLASS BRADYODONTI Representative whole animals, Fig. I 73 (p. 257); Chimwra,
vertebral column, Fig. 174 (p. 259); skull, Fig. 175 (p. 260). Callorhunchus, skull, Fig. I 76
(p. 26I); brain, Fig. 177 (p. 262); urinogenital system, Fig. I]S (p. 263); embryo within egg,
Fig. I79 (p. 264).
General Organisation oi Elasmobranchii. Skeletal structures: Scumnorhinus, spinal column,
Fig. I8o (p. 267); Uroloplms, endoskeleton, Fig. I8r (p. 268); lleptranchias, skull, Fig. I82
(p. 269). Torpedo, Electric organs, Fig. 183 (p. 271); Carcharodon, Mechanism of eye
accommodation, Fig. I84 (p. 273); Cetorhinus, Urinogenital system, Fig. 185 (p. 277);
development, Figs. I86-190 (pp. 279-28I).
CLASS OSTEICHTHYES SUPER-ORDER CHONDOSTREI Palreoniscus, whole animal and
skeleton, Fig. 191 (p. 286); llelichthys, whole animal and skeleton, Fig. 192 (p. 287); Aci-
}Jenser, whole animal, Fig. I93 (p. 288); pre-caudal vertebro:e, Fig. 194 (p. 289). Chondros-
teus, skull and pectoral girdle, Fig. I95 (p. 289); Polypterus, whole animal, Fig. 196 (p. 290);
SUPER-ORDER HOLOSTEI. Dapedius, whole animal, Fig. 197 (p. 291); Lepiso.deus, whole
animal, Fig. I98 (p. 292); Amia, whole animal, Fig. I99 (p. 293). SUPER-ORDER TELEOSTEI,
Leptolepis, whole animal and skull, Fig. zoo (p. 294).
 ILLUSTRATIONS xix
Example of Sub-class: Sahno fario, Trout. Whole animal, Fig. 201 (p. 296); Tilapia,
whole animal and parental care, Fig. 202 (p. 297); Sahno. vertebr::c, Fig. 20J (p. J01).
Teleostei, caudal skeleton, Fig. 204 (p. J02); skull and relationship with pectoral girdle, Fig.
205 (p. J04); Salmo, skull of juvenile, Fig. 206 (p. J07); dermal fin-ray and its supports,
Fig. 207 (p. J07); pectoral girdle and fin, Fig. 208 (p. J08); pelvic fin and skeleton, Fig 209
(p. Jog); skeletal and visceral relationships, Fig. 210 (p. Jio); brain and cranial nerves,
Fig. 2II (p. J1J); brain and adjacent structures, Fig. 212 (p. J14); Teleostei, eye and optic
nerve, Fig. 2IJ (p. J16); Sahno, audio-equilibration, Fig. 214 (p. J17); kidney and ad-
jacent structures, Fig. 215 (p. J18).; development, Figs. 216-217 (p. Jig).

General Organisation of Actinopterygians.-Teleostei, optic adaptation, Fig. 218 (p. J22); exo-
skeletal and reproductive adaptations, Fig. 219 (p. J2J); bioluminescence and angling mech-
anisms, Fig. 220 (p. J24); morphogenetic adaptation to bottom-living, Fig. 221 (p. J26); bio-
luminescence in Argyropelecus, Fig. 222 (p. J28); tactile and other specialisations, Fig. 22J
(p. J28); Actinopterygian scales (cycloid and ctenoid), Fig. 224 (p. J29)- Acipenseroidea,
skull, Fig. 225 (p. JJO); Polypterini, skull, Fig. 226 (p. JJI). Teleostei, deglutition specialisa-
tions, Fig. 227 (p. JJ2); Polypterini, fins, Figs. 228-229 (p. JJJ); Teleostei, electric organs,
Fig. 2JO (p. JJ4); dental specialisation in Sargus, Fig. 2JI (p. JJ6); extra-branchial respir-
ation in Clarias, Fig. 2J2 (p. JJ8); in Anabas, Fig. 2JJ (p. JJ9)- Bolostei, swim-bladder
in Lepisosteus, Fig. 2J4 (p. J40). Teleostei, swim-bladder and audio-equilibration in a cyp-
rinoid, Fig. 2J5 (p. J41); gas gland, oval and blood supply, Fig. 2J6 (p. J42). Bolostei, brain,
Fig. 2J7 (p. J44l; Teleostei: brain and labyrinth, Fig. 2J8 (p. J45l- Bolostei: urinogenital
systems, Figs. 2J9-240 (p. 348); Teleostei: ectoparasitism and reproduction, Fig. 241 (p.
J50); viviparity, Fig. 242 (p. 351); Bolostei, development, Fig. 24J (p. J53)-

SUB-CLASS CROSSOPTERYGU
Order Rhipidistia: Osteolepis, whole animal and skull, Fig. 244 (p. 355); Undina, whole
animal, Fig. 245 (p. J57); Latimeria, whole animal, Fig. 246 (p. 358); head and sensory
organs, Figs. 247-248 (p. J59)-
0rder Dipnoi: Lung-fishes, whole animals and distribution, Fig. 249 (p. J61); Dipterus,
whole animal, Fig. 250 (p. J63)-
Example of Sub-class.-Protopterus rethiopicus, whole animal, Fig. 251 (p. J65); skull,
shoulder girdle, and fin, Fig. 252 (p. 367); Dentition, Fig. 253 (p. J68); alimentary canal and
associated structures, Fig. 254 (p. J69); lung cf. that of Polypterus, Fig. 255 (p. J71); blood
vascular and respiratory systems, Fig. 256 (p. J72); visceral relationships, Fig. 257 (p. J7 3);
brain, Fig. 258 (p. J74l; ::estivation, Fig. 259 (p. 375); urinogenital system, Fig. 26o (p. 376);
development, Figs. 261-262 (pp. 377-J78).

CLASS AMPHIBIA Fish/amphibian transition: shoulder girdle and fore-limb, Fig. 263 (p.
382).
Example of Class: Rana, endoskeleton, Fig. 264 (p. 392); beny skull, Fig. 265 (p. 393);
cartilaginous skull of tadpole, Fig. 266 (p. 394); pectoral girdle, Fig. 267 (p. 397); pelvic
girdle, Fig. 268 (p. 398); musculature, Fig. 269 (p. 399); visceral relationships, Fig. 270 (p.
401); abdominal viscera, Fig. 271 (p. 402); cardiac anatomy and circulation, Fig. 272 (p. 405);
arterial system, Fig. 273 (p. 406); venous system, Fig. 274 (p. 408); brain, pituitary and
cranial nerves, Fig. 275 (p. 41 r); eye and optic nerve, Fig. 276 (p. 413); urinogenital system,
Figs. 277-278 (pp. 416-417); development, Fig. 279 (p. 419).

SUB-CLASS APSIDOSPONDYLI
Labyrinthodontia: tooth, Fig. 280 (p. 422); skulls and vertebrre, Fig. 281 (p. 422); lchthyo-
stega, whole animal, Fig. 282 (p. 42J); cranium, Fig. 283 (p. 424); Paracyclotosaurus,
whole animal, Fig. 284 (p. 425); skulls of Palreogyrinus and the fish Osteolepis showing
evidence of fish/amphibian transition, Fig. 285 (p. 427); Seymouria, illustrating mosaic
evolution, Fig. 286 (p. 429).

SUB-CLASS LEPOSPONDYLI
'Horned' skull, and characteristic vertebra, Fig. 287 (p. 431); Ophiderpeton, skeleton, Fig.
288 (p. 4J1); Order Urodela: Andrias, Witness of the Deluge, Fig. 289 (p. 432); permanent
larv::c, Fig. 290 (p. 433); Salamandra salamandra, Fig. 291 (p. 434). Order Apoda: Cephalic
and genital structures of Crecilia, Fig. 292 (p. 4J4)-

General Organisation of the Amphibia.-Urodela: shondrocranium of Proteus, Fig. 293 (p.

4J8); skull of Salamandra atra, Fig. 294 (p. 4J9); Apoda: skull of lchthyophis, Fig. 295
(p. 440); Urodela: shoulder girdle and sternum, Fig. 296 (p. 441); pelvic girdle, Fig. 297 (p.
442); Anura: vocal apparatus, Fig. 298 (p. 443); Urodela: circulation and respiration, Fig.
XX ILLUSTRATIONS
299 (p. 44S); venous system, Fig. 300 (p. 446); vibration exteroception, Fig. 301 (p. 448);
Anura: lateral line in Xenopus, Fig. 302 (p. 449); melanophores in Xenopus, Fig. 303 (p.
4SO); Urodela: urinogenitalia, Figs. 304-3os (pp. 4So-4sr); Anura: amplexus and parental
care in Gast'f'otheca ma'f'supiata, Fig. 306 (p. 4S3); ovoviviparity and pseudo-placentation
in Pipa do'f'sige'f'a, Figs. 307-308 (pp. 4S4-45S); Apoda: reproduction and development in
lchthyophis, Fig. 309 (p. 4S6).
CLASS REPTILIA.
Skeleton of anapsid 'stem-reptile', Labidosaurus, Fig. 310 (p. 4S8).
Example of Class: Lace'f'ta, vertebne, Fig. 3II (p. 463); pectoral arch and sternum, Fig. 312,
(p. 46s); skull, Fig. 313 (p. 466); carpus, Fig. 314 (p. 469); pelvis, Fig. 3IS (p. 470); tarsus,
Fig. 316 (p. 471); visceral relationships, Figs. 317-318 (pp. 472-473); heart and large vessels.
Fig. 319 (p. 47S); cardiac structure and circulation, Fig. 320 (p. 476); brain and cranial nerves.
Fig. 321 (p. 479); brain and hypophysis, Fig. 322 (p. 48o); audio-equilibration, Fig. 323 (p,
481); urinogenitalia, Figs. 324-32S (pp. 482-483); skull structure of reptiles, Fig. 326 (p. 484),
SUB-CLASS ANAPSIDA
Chelonia, whole animal, Fig. 327 (p. 487).
SUB-CLASS ICHTHYOPTERYGIA Ophthalmosau...us, whole animal, Fig. 328 (p. 490); loco-
motory specialisations, Fig. 329 (p. 491).
SUB-CLASS SYNAPTOSAUB.IA Elasmosau...us, whole animal, Fig. 330 (p. 493).
SUB-CLASS LEPIDOSAURIA B.hynchocephalia, Sphenodon, whole animal, Fig. 331 (p.
496); Squamata ,C'f'otalus, whole animal, Fig. 332 (p. 498).
SUB-CLASS ARCHOSAURIA C'f'ocodylus, Fig. 333 (p. sao); B.hamphorhynchoidea, skull and
teeth, Fig. 33S (p. so4); Saurischia and Ornithischia: Representative dinosaurs, Fig. 336
(p. sos); pelvic structure, Fig. 337 (p. so6); Saurischia, axial skeleton of Tyrannosaurus,
Fig. 338 (p. so6): respiratory adaptations in dinosaurs, Fig. 339 (p. so8) ; Ornithischia, endo-
skeleton of Iguanodon, Fig. 340 (p. sag) : dentition in unrelated herbivores, Fig. 341 (p.
sro); brain, hypophysis and spinal enlargements in Stegosau'f'us, Fig. 342 (p. su).
SUB-CLASS SYNAPSIDA Pelycosauria; Dimet'f'odon, whole animal, Fig. 343 (p. sr2); Eda·
phosau'f'us, vertebral spine, Fig. 344 (p. SI3); Therapsida, Kannemeye'f'ia, skeleton, Fig. 34S
(p. 514); pectoral girdle, Fig. 346 (p. srs). Cynodontia, skull, Fig. 347 (p. sr6); Gorgonopsia,
skull, Fig. 348 (p. sr6); Lycrenops, whole animal, Fig. 349 (p. sr7); Ictidosauria, skull, Fig. 3so
(p. SI7).
General Organisation of Recent Reptilia: Lacertilia, arboreal specialisation in Chamreleon;
Fig. 351 (p. 519): Fossorial specialisations in legless lizard, Pygopus, Fig. 352 (p. 520);
Sphenodon, vertebrre, Fig. 3S3 (p. S24); Python, vertebrre, Fig. 3S4 (p. S24): C'f'ocodylus
and Sphenodon, endoskeleton, Figs. 3S5-356 (p. S2S); C'J'ocodylus, vertebrre, Fig. 3S7 (p.
S26); Chelonia, exo- and endo-skeleton, Figs. 3S8-3S9 (p. S27); Natriz, skull, Fig. 360 (p.
s28); Sphenodon, skull, Fig. 361 (p. 529); Chelonia, skull, Fig. 362 (p. s3o); Crocodylus,
skull of adult and juvenile, Figs. 363-364 (p. S31); Chelonia, tarsus, Fig. 36S (p. S32); Alli-
gato·r, carpus, Fig. 366 (p. S32); pelvis, Fig. 367 (p. S33); Crocodylus, Fig. 368 (p. S33);
Crotalus, rattlesnake fangs, poison apparatus, Figs. 369-370 (pp. S34-S3S); Elapid dental
specialisations, Fig. 371 (p. S36); Crocodylus, heart and great vessels, Fig. 372 (p. S42);
Alligator, brain and cranial nerves, Fig. 373 (p. S43); Lacertilia, vomeronasal organ, Fig. 374
(p. S44); Squamata, comparison of lacertilian and aphid ian eye, Fig. 37S (p. S4S); Sphenodon
pineal apparatus, Fig. 376 (p. s46); Crotali.nm, special senses, Fig. 377 (p. S47); Ophidia,
bilateral copulatory organs, Fig. 378 (p. sso); Alligator, development, Fig. 379 (p. SS2);
Lacerta, development, Fig. 380 (p. SS3l·
CLASS AVES.
Epigamic plumage, Fig. 381 (p. s61); sexual and aggression display, Fig. 382 (p. s62).
Example of Class: Columba Iivia, Fig. 383 (p. s66); contour feather, Fig. 384 (p. s68): down-
feather and its formation, Fig. 38s (p. s7o); pterylosis, Fig. 386 (p. S71); internal strutting and
pneumatism in bone, Fig. 387 (p. S72); endoskeleton, Fig. 388 (p. S72); cervical vertebra, Fig.
389 (p. S73); synsacrum, Fig. 390 (p. S74); skull, Fig. 391 (p. S7S); Neornithes, skull, Fig. 392
(p. S76); hyoid apparatus, Fig. 393 (p. S76); columella, Fig. 394 (p. S77); fore-limb, Fig. 39S
(p. S77l; manus, Fig. 396 (p. s78); pelvic girdle, Fig. 397 (p. S78); hind-limb, Fig. 398 (p. S79);
pes, Fig. 399 (p. s8o); musculature, Fig. 400 (p. s81); visceral relationships, Fig. 401 (p. s84);
trachea, bronchi, and lungs, Fig. 402 (p. s86); air-sac relationships, Fig. 403 (p. s87) : lungs and
air-sacs, Fig. 404 (p. s88); heart and large vessels, Fig. 405 (p. s8g); blood vascular system;
Fig. 406 (p. sgo); brain and cranial nerves, Fig. 407 (p. S92); brain, Fig. 408 (p. S93); eye,
Fig. 409 (p. S94); audio-equilibration, Fig. 410 (p. S95); male urinogenitalia, Fig. 41 I (p. 596),
female urinogenitalia, Fig. 412 (p. 596); cloacal region, Fig. 413 (p. S97).
 ILLUSTRATIONS xxi

SUB-CLASS ARCBJEORNITHES
Archreopteryz (and the reptile Rhamphorhynchus, Fig. 414 (p. 599); Archreopteryz
(Berlin specimen in situ), Fig. 415 (p. 6oo); skull, Fig. 416 (p. 601); manus, Fig. 417 (p. 601).

SUB-CLASS NEORNITHES
Hesperornis and lchthyornis, Fig. 418 (p. 603); skeletons, Figs. 419-420 (pp. 6o4-605);
Dinornis and Apteryz, Fig. 421 (p. 6o8); Eudromia (Tinamu), Fig. 422 (p. 6o9); Ap-
tenodytes and parental care, Fig. 423 (p. 6u); Map: adaptation and geographical distribu-
tion, Fig. 424 (p. 612); Diatryma, Fig. 425 (p. 616).
General Organisation of Neornithes: Rostral and associated specialisations, Fig. 426 (p. 621);
clawed wings of, Opisthocomus, Fig. 427 (p. 622); pedal specialisations, Fig. 428 (p. 623);
adaptation to lily lagoons, Fig. 429 (p. 624); toilet comb (of bittern, Botaurus, Fig. 430 (p.
625); Casuarius, feather construction, Fig. 431 (p. 627); avian sternum and pectoral articula-
tion, Fig. 432 (p. 629); Apteryz, skull, Figs. 433-434 (pp. 630-631); Anas, skull, Fig. 435
(p. 631); Psittacidlle, cranio-facial hinge, Fig. 436 (p. 632); Apteryz, shoulder girdle, Fig. 437
(p. 633); avian embryonic fore-limb showing digits, Fig. 438 (p. 634); Apteryz, pelvic girdle,
Fig. 439 (p. 634); hind-limb and pes, Fig. 440 (p. 635); Cecinus (woodpecker), feeding
specialisations, Fig. 441 (p. 636); Collocalia (swiftlet), salivary glands, Fig. 442 (p. 637);
Grus (crane), vocal specialisations, Fig. 443 (p. 638); Cyanochen (goose), vocal specialisations,
Fig. 444 (p. 639); Botaurus (bittern), adaptation to reed-beds, Fig. 445 (p. 641); avian tactile
bristles, Fig. 446 (p. 643); development, Figs. 447-451 (pp. 646-650).

CLASS MAMMALIA
Example of Class: Oryctolagus cuniculus (Rabbit). Vertebr<e, Fig. 452 (p. 656); skull, Figs.
453-454 (pp. 658-659); jaw musculature, Fig. 455 (p. 662); fore-arm and carpus, Fig. 456 (p.
664); sacrum, Fig. 457 (p. 665); pes, Fig. 458 (p. 666); head, neck, and thorax, Fig. 459 (p.
667) ; nasal region, Fig. 460 (p. 668); visceral relationships, Fig. 461 (p. 669); heart and great
vessels, Fig. 462 (p. 671); blood vascular system, Fig. 463 (p. 673); cardiac region and thor-
acic duct (of Homo), Fig. 464 (p. 676); larynx, Fig. 465 (p. 677); thoracic relationships, Fig.
466 (p. 677); brain, Figs. 467-468 (pp. 678-679); urinogenitalia, Fig. 469 (p. 684); internal
genitalia of female, Fig. 470 (p. 685); placentation, Fig. 471 (p. 686).

SUB-CLASS PROTOTHERIA
Priacodon, skull, Fig. 472 (p. 687); Ornifhorhynchus, whole animal in habitat, Fig. 473
(p. 689); Tachyglossus, whole animal in habitat, Fig. 474 (p. 69o); Ornithorhynchus,
cephalic anatomy, Fig. 475 (p. 691); buccal apparatus, Fig. 476 (p. 6g1); manus and pes, Fig.
477 (p. 692); poison apparatus, Fig. 478 (p. 693); eggs, Fig. 479 (p. 693); mammary glands,
cloaca and tail, Fig. 480 (p. 694); dentition, Fig. 481 (p. 695); Tachyglossus, cephalic anatomy,
Fig. 482 (p. 695); manus and pes, Fig. 483 (p. 696); Ornithorhynchus, endoskeleton, Fig. 484
(p. 697); Tachyglossus, skull, Fig. 485 (p. 698); Ornithorhynchus, scapula, Fig. 486 (p.
699); Tachyglossus, female urinogenitalia, Fig. 487 (p. 700); reptilian and monotreme
affinities shown in male urinogenitalia, Fig. 488 (p. 701).

SUB-CLASS ALLOTHERIA
Order Multituberculata: Treniolabidire, skull, Fig. 489 (p. 702).

SUB-CLASS THERIA
Order Dryolestoidea: Amphitherium, lower jaw, Fig. 490 (p. 703).
Map: animal distribution, continental connections, Fig. 491 (p. 705); Map: faunal regions,
Fig. 492 (p. 706).
Order Marsupialia: Tree-kangaroo, Dendrola.gus, Fig. 493 (p. 707); Water Opossum,
Chironectes, Fig. 494 (p. 7II); Tiger 'Cat', Dasyurops, Fig. 495 (p. 713); Tasmanian
'Wolf', Thylacinus, Fig. 496 (p. 714); Marsupial 'Mole', Notoryctes, Fig. 497 (p. 715);
Pygmy Glider, Acrobates, Fig. 498 (p. 717); Diprotodon, skeleton, Fig. 499 (p. 718); Noto-
therium, skull and dentition, Fig. soo (p. 718); Thylacoleo, skull, Fig. 501 (p. 718); Kan-
garoo, Macropus, atlas, Fig. 502 (p. 719); skeleton, Fig. 503 (p. 720); Wombat, Phascolomys,
skull, Fig. 504 (p. 720); Marsupial ' Cat'; Dasyurus, skull, Fig. 505 (p. 721); Rock Wallaby,
Petrogale, skull, Fig. 506 (p. 721); Possum, Tricl10surus, hind-leg and pes, Fig. 507 (p.
722) ; Macropus, pes, Fig. soB (p. 723).
Order Lipotyphla: Solenodon, Fig. 509 (p. 728); Golden Mole, Cllrysochloris, Fig. 510 (p.
729); Tenrec, Centetes, skull, Fig. SII (p. 730); Mole, Talpa, sternum and girdle, Fig. 512
(p. 730); fore-arm and manus, Fig. 513 (p. 731).
Order Menotyphla: Tree-shrews, Tupaia, Fig. 514 (p. 732); Ptilocercus, skull, Fig. 515 (p.
733).
xxii ILLUSTRATIONS
Order Primates: Le1nur, Fig. s16 (p. 737); skull and dentition, Fig. S17 (p. 738); Pero-
dicticus, Fig. s18 (p. 739); Tarsius, Fig. S19 (p. 740); skull and dentition, Fig. s2o (p. 741);
faces of Platyrrhine (Ateles) and Catarrhine (Macaca) compared, Fig. S21 (p. 742); Gorilla,
Fig. S22 (p. 74S); jaw structures of Gorilla and llo1no compared, Fig. S23 (p. 746); llo1no,
skull, Fig. S24 (p. 749); skeletons of llo1no and llylobates compared, Fig. S2S (p. 7S1);
Papio, carpus, Fig. S26 (p. 7S2); Feet of llo1no, Gorilla and Pongo, Fig. S27 (p. 7S3).
Order Dermoptera: Colugo, Cynocephalus, Fig. s28 (p. 7S4).
Order Chiroptera. Fruit-bats, Pteropus Fig. S29 (p. 7SS); feeding adaptations of Micro-
chiroptera, Fig. S30 (p. 7S7); Pteropus, skeleton, Fig. S3I (p. 7S9).
Order Edentata: Fairy Armadillo, Chla1nyphorus, Fig. S32 (p. 762); Three-banded armadillo
Tolypeutes, Fig. S33 (p. 763); defensive mechanism, Fig. S34 (p. 764); Glyptodon, skeleton,
Fig. S3S (p. 764); Mylodon, skeleton, Fig. S36 (p. 76s); Anteaters, Myr1necophaga, Fig. S37
(p. 766) and Ta1nandua, Fig. s38 (p. 767); Three-toed sloth, Bradypus, Fig. S39 (p. 768);
whole skeleton, shoulder girdle, manus, pes, skull, Figs. S40-S44 (pp. 769-770); Dasypus,
skull, Fig. S4S (p. 770); lUyrJnecoplwga, skull, Figs. S46-S47 (pp. 770-771); Dasypus,
shoulder-girdle, Fig. S48 (p. 771); pelvis and sacrum, Fig. S49 (p. 772).
Order Pholidota. Pangolin, Manis, Figs. sso-ss1 (p. 773).
Order Rodentia. Rattus, jaw musculature, Fig. SS2 (p. 776); Molar dentition in rodents,
Fig. SS3 (p. 777).
Order Cetacea: Whale-bone (Balmnoptera) and toothed (Orcinus) whales, Fig. SS4 (p.
780); skull of Protocetus, Fig. SSS (p. 782); Basilosaurus, skeleton; Fig. SS6 (p. 782);
evolution of blow-hole, Fig. SS7 (p. 782); Neobalmna, baleen and tongue sieve mechanism,
Figs. ss8-SS9 (pp. 78s-786); Balmna, vertebr<e, Fig. s6o (p. 786); porpoise, Plwcmna, skeleton,
Fig. s61 (p. 787); Balmnoptera, sternum, Fig. s62 (p. 788); dolphin, Globicephala, skull,
Fig. S63 (p. 788); hyperphalangy, Fig. s64 (p. 789).
Order Carnivora: Creodont and fissipede dentition compared, Fig. s6s (p. 793); Dentition in
modern carnivores, Fig. S66 (p. 794); Cheetah, Acinony;r, Fig. S67 (p. 79S); dentition of
Sabre-tooth (S1nilodon) and true Cat OUetailurus) compared, Fig. s68 (p. 796); Seal, Phoca,
skeleton, Fig. S69 (p. 798); Canis, dog, skull, Fig. S70 (p. 799); Felis, tiger, auditory bulla,
Fig. S71 (p. 799); Ursus, bear, auditory bulla, Fig. S72 (p. 8oo); carpus, Fig. S73 (p. 8oo).
Order Condylarthra: llyopsodus, skull, Fig. S74 (p. 8o2); Phenacodus, Fig. S7S (p. 8o3).
Order Tubulidentata: Aardvark, Orycteropus, Fig. s76 (p. 8o4).
Order Litopterna: Diadiaphorus, Fig. sn (p. Sos).
Order Notoungulata: To;rodon, jaw, Fig. s78 (p. 8o6); llomalodotl1erium, skeleton, Fig. S79
(p. 8o6).
Order Astrapotheria: Astrapotherium, skeleton, Fig. 58o (p. 8o7).
Order Hyracoidea: Rock Hyrax, Procavia, Fig. s81 (p. 809); skull and dentition, Fig. s82
(p. 810).
Order Embrithopoda: Arsinoitheriu1n, Fig. s83 (p. 8u).
Order Proboscidea: Hairy mammoth, Elephas, Fig. s84 (p. 812); Lo;rodonta, African
elephant, skull pneumatisation, Fig. s85 (p. 8r3); Moeritheriu1n, skull and dentition, Fig.
s86 (p. 814); Palmo1nastodon, skull and dentition, Fig. 587 (p. 81S); Deinotherium, Fig.
588 (p. 816); skull and dentition, Fig. s89 (p. 817); Dentition of African (Lo;rodonta) and
Indian (Elephas) elephants compared, Fig. S90 (p. 817).
Order Pantodonta. Coryphodon, Fig. S91 (p. 818).
Order Dinocerata. Uintatheriu1n, Fig. S92 (p. 819); dentition of Uintatheriida, Fig. S93
(p. 82o).
Order Sirenia: Manatee, Trichechus, Fig. S94 (p. 821); Dugong, skeleton, Fig. S9S (p. 822);
Trichechus, skull and dentition, Fig. 596 (p. 823).
Order Perissodactyla: Phylogeny and distribution of horses (Equid<e), Fig. S97 (p. 827); limbs
and teeth, Fig. S98 (p. 828); Brontops, Fig. S99 (p. 832); Tapirus, and young, Fig. 6oo (p.
834); Baluchitheriu1n, Fig. 6o1 (p. 83s).
Order Artiodactyla: llippopotamus, Fig. 6o2 (p. 838).
General Organisation of Mammalia: Hair, Fig. 6o3 (p. 841); tactile vibriss<e, Fig. 6o4 (p. 842);
hair-tracts, Fig. 6os (p. 843); claw structure, Fig. 6o6 (p. 84s); pes of Tarsius, Fig. 607 (p.
846); cutaneous glands and olfactory signals, Fig. 6o8 (p. 847); territory establishment in
Ungulata, Fig. 609 (p. 848); mammalian skull (relationships of principal bones), Fig. 6ro (p.
8s2); tympanic structure, Fig. 61 r (p. 8s3); hyoid apparatus, Fig. 612 (p. 8S4); cranial modi-
fications, Fig. 613 (p. 8ss); primitive plantigrade limbs of Carnivora, Fig. 614 (p. 8s7); con-
vergence in cursorial adaptations, Fig. 61s (p. 8s8); paraxonic carpus and tarsus, Fig. 616
 ILLUSTRATIONS XXlll

(p. 859); mammalian limb structures, Figs. 6r7-624 (pp. 86o-86r); tooth structure, Fig. 625
(p. 863); deciduous and permanent dentition of Canis, Fig. 626 (p. 864); Homodont dentition
of dolphins (in Lagrenorhynchus), Fig. 627 (p. 865); marsupial dentition (in Perameles),
Fig. 628 (p. 865); Cope-Osborn theory of tritubercular origin of molars, Fig. 629 (p. 866);
skull and dentition of Phascolarctos, Fig. 630 (p. 869); dentition of Macropus, Fig. 631
(p. 87o); skull and dentition in Sarcophilus, Fig. 632 (p. 87o); dentition of Didelphys, Fig.
633 (p. 871); dentition of Orycteropus, Aardvark, Fig. 634 (p. 871); dentition of Sus, pig,
Fig. 635 (p. 872); skull and dentition of Equus, horse, Fig. 636 (p. 873); fcetal jaw and teeth of
Right Whale, Balrenoptera, Fig. 637 (p. 874); pharyngeal region of Man, Fig. 638 (p. 877);
rumination and digestion in Eutherians, Fig. 639 (p. 878); marsupial digestion, Fig. 640 (p. 879);
brain of Tachyglossus (Spiny Ant-eater), Petrogale (Rock Wallaby), Ornithorhynchus,
(Platypus) Macropus (Kangaroo), Kogia (Whale), Figs. 641-646 (pp. 885-886); female exter-
nal genitalia of Lemur, Fig. 647 (p. 890); female urinogenitalia in l\fetatheria, Fig. 648 (p. 891);
in Eutheria, Fig. 649 (p. 892); external genitalia in Crocuta crocuta, Spotted Hyama, Fig.
65o; (p. 893) internal genitalia in C. crocuta, Fig. 651 (p. 894); development and placentation
in Eutheria, Figs. 652-656 (pp. 900-904), Metatheria, Figs. 657-658 (p. 906); relative size of
blastula in eutherian and monotreme, Fig. 659 (p. 907); mammary fcetus of Macropus, Fig.
66o (p. 907).
 PHYLUM CHORDATA
INTRODUCTION

T HE chordates exhibit an astonishing diversity of form, physiology, and

habit. The phylum includes such apparently unrelated animals as, for
example, sea-squirts and Man. The creatures which fall into this great
group are characterised essentially by the possession, at some stage of their life,
of one or more of the three fundamental structures which, along with other
features, appear to reveal their common ancestry. These structures are as
follows:
r. The chorda dorsalis or notochord. From this structure the phylum takes
its name. It occurs in the embryo of most chordates as a long, flexible cord of
specialised, vacuolated cells extending from head to tail along the dorsal mid-
line. It lies between the alimentary canal and the dorsal cord of the central
nervous system. In lower chordates, such as the Tunicata and Cephalo-
chorda, the notochord· is developed directly and unmistakably from the endo-
derm and, in the Adelochorda, a notochord-like structure remains permanently
in continuity with that layer. In the Craniata, however, its origin is not so
definite : it may originate from cells that are not obviously of endodermal
derivation. Sometimes it is enclosed in a firm sheath and forms a stiff, but
elastic, structure which supports the neighbouring single, hollow, fluid-filled,
non-ganglionated nerve-cord. In the Craniata, with a few exceptions among
lower forms, the notochord is replaced more or less completely in the adult
by a segmented cartilaginous or bony axial structure, the vertebral column.
In vertebrates, traces of the notochord often remain in the adult as vestiges
of gelatinous material in the spaces between the centra of the replacing
vertebrre.
2. The branchial clefts. Visceral clefts constitute another almost universal
feature of the Chordata. They occur as a paired series of perforations leading
from the pharynx, an anterior section of the alimentary canal, to the lateral
surface of the body. Within these clefts are the gills in many aquatic animals.
In the creatures equipped with lungs, branchial clefts or branchial grooves are
always found in the embryo. In the adults of vertebrates, the branchial
VOL. II. B
2 ZOOLOGY
apparatus is sometimes, as we shall see, converted to endocrine and other
functions. In the more lowly members of the Chordata the branchial apparatus
is used also as a feeding mechanism.
3· The dorsal, tubular, fluid-filled central nervous system, anteriorly differenti-
ated into a brain in advanced forms, is another structure common to most
chordates in the larval or later stages of their development.
The Chordata are ccelomate animals. In the lower sub-phyla the ccelom
develops in essentially the same way as in the Echinodermata, the Chcetognatha,
and the Brachiopoda, i.e. by direct outgrowth from the archenteron. In the
Craniata, this enteroccelic origin of the body cavity is no longer definitely
traceable, although in some cases indications of it may be detected.
In the Tunicata the body is not segmented, though faint indications of serial
repetition of parts are visible in certain groups. In the Adelochorda there is a
division of the ccelom into three parts, each occupying a definite region of the
body. This has led sometimes to the view that these animals are tri-segmented.
In the Cephalochorda and Craniata there are numerous segments (seep. r6r).
There has been considerable controversy concerning the period and mode of
origin of the Chordata. Lower chordate forms have been discovered in Cam-
brian strata (see Table r, p. 3). Indisputable vertebrate relics (e.g. fish-scales)
are known from Ordovician deposits more than 400 million years old. No
general agreement has been reached, but today more and more people tend to
accept Garstang's suggestion that the Chordata may have evolved from free-
swimming auricularian larvce partly by means of pcedomorphosis. This is
evolutionary change involving the retention, in the sexually mature descendant,
of characters that occurred in younger stages of the ancestor. It is now con-
sidered certain that pcedogenesis (reproduction in a pre-adult form) has been
a powerful influence in the elaboration of at least some animal types.
'Ontogeny' wrote Garstang in 1922 (in reference to this and the so-called
Biogenetic 'Law' of Haeckel) 'does not recapitulate Phylogeny: it creates it'.
Clues to the origin of vertebrates have been sought by men ever since the
dawn of evolutionary thought. It was natural to seek them among the inverte-
brates, both the living and the extinct, and various possibilities have been
suggested. However, the adult forms of most, if not all, diploblastic or
triploblastic invertebrates are so specialised as to general form, and especially in
regard to the situation and structure (with imposed physiological limitations)
of their individual organ systems, that it is difficult or impossible to imagine
them as ancestral to the Chordata.
Towards the end of the last century Garstang and Willey looked for possible
chordate progenitors in the youthful stages of various invertebrate animals.
They produced suggestive information that the evolution of ancestral chordate
forms from the larvce of echinoderm-like animals is by no means impossible.
Although echinoderms and chordates exhibit striking similarities in early
 PHYLUM CHORDATA 3
TABLE I. Ages in millions of years of geological periods and epochs, as estimated approximately,
by various workers (modified after Young). 1

Time
Schuchert Com pro- since Sediment
Zeuner Bullard Romer Holmes and mise begining thickness
Dunbar adopted of period (ooo ft.)

Pleistocene I 5 - I - I I 4
Pliocene - I5 - I5 - 9 10 I3
Miocene 30 25 - 20 - I5 25" 21
Oligocene - IS - IS - 10 35 I5
Eocene - I6 - 20 - I5 50 14
Palaeocene (40) - - - - 10 6o -
Cretaceous 40 76 50 50 So 6o IZO 64
Jurassic 40 23 35 30 35 30 150 20
Triassic (40) 30 35 40 25 30 ISo 25
Permian 35 15 25 30 40 30 210 I3
Carboniferous 50 47 85 6o 70 6o 270 40
Devonian (35) 44 so 40 40 30 300 37
Silurian 30 IS 30 30 40 30 330 IS
Ordovician so 47 go so 70 go 420 40
Cambrian 6o 47 70 100 go go SIO 40
I

development, and agree in other features as well, the adult forms of the various
star-fishes and their allies are organised with such peculiarity and complexity
that at first sight such a hypothesis might seem utterly invalid. But if we
follow Garstang's proposition that a series of changes may have taken place in
primitive echinoderm larvce that were not unlike today's auricularians, we have
a succession of pictures that must carry weight with anybody who approaches
the problem without prej).ldice. Garstang pointed out that the ciliated bands
and underlying nervous tissue of a larval form might become accentuated and
fused to form the forerunner of the dorsal neural tube. Such a view is free
from the powerful objections that can legitimately be raised against attempts to
consider that a dorsal nervous system might be derived from the nervous tissues
of any adult invertebrate and, in addition, it is not at variance with the principle
of neurobiotaxis, which insists that a concentration of nervous material evolves
in the area of maximum environmental stimulation. Among other changes,
it was suggested that the ciliated adoral band feeding-mechanism could become
the endostyle which is a characteristic of the protochordates (see p. 24). It
will be recalled, too, that in the auricularian a pore brings the body cavity into
communication with the external environment, and that this pattern is repeated
in various lower chordates. 'If the nervous system and endostyle are formed
in the way suggested, all that is required to turn the echinoderm larva into a
chordate is the formation of the notochord and the piercing of the gill-slits' (de
Beer). It has to be recognised that such hypothetical changes are radical, and
1 The term Liassic (or Lias) is often applied to a richly fossiliferous blue limestone deposit in
the Lower Jurassic. In North America the Carboniferous is divisible into upper Mississippian
and lower Pennsylvanian periods.
4 ZOOLOGY
no convincing explanation of their development has as yet been given. It is,
however, logical tentatively to assume that the modified larval form proposed
above could become sexually mature and that the neotenous animal which
resulted may have been the ancestral stock from which today's chordates
evolved.
Neither has any general agreement been reached as to the most appropriate
classification within the Phylum Chordata. In the present volume its members
will be considered in the following broad grouping :

Sub-phyla Adelochordata (Hemichordata)

Tunicata (Urochordata)
Acrania (Cephalochordata)
Craniata CVertebrata)
Members of the first three of the above groups are usually referred to as the
Protochordata, although, as we shall see, all authorities do not unquestionably
admit the Adelochorda as chordate animals. Some hold that they should
occupy a phylum to themselves. It has to be remembered that although the
notochord is clearly homologous in the Tunicata and Acrania, such homology
has certainly not been established with complete assurance in the Adelochorda.
However, in the Adelochorda there occurs an endoskeletal element which
develops as an outgrowth from the antero-dorsal part of the gut into the pro-
boscis, and this proboscis diverticulum may perhaps be a truly notochordal
structure. Again, all protochordates except Rhabdopleura (Adelochorda)
(p. I9) possess lateral pharyngeal perforations at some stage of development.
However, in other Adelochorda such perforations not only occur, but also
undergo multiplication by a characteristic method, involving the dorsal down-
growth of dividing 'tongue-bars'. An identical means of multiplication is
found in Acrania, and traces of the method may be discerned in young stages of
certain Tunicata. The nerve cord is constant in its origin in all chordates in
that it arises from the median dorsal ectoderm. In Adelochorda, the Ptero-
branchia lack a neuroccele, but Enteropneusta possess, in part of the dorsal
nerve cord, either a neuroccele or vestigial cavities. Again excepting the
Adelochorda, all chordates (and only chordates) possess at least some elements of
a true tail which, without viscera, nevertheless contains muscular, nervous, and
notochordal elements. (In the Craniata, including Man, it contains vertebrre.)
In addition to the morphological evidence of affinity briefly presented
above, it is noteworthy that chordates utilise in their muscles a phosphagen
that is a compound of the amino-acid creatine and not, as in the case of most
invertebrates studied, a compound of arginine. Both forms of phosphagen
have been reported in the Adelochorda, Tunicata and certain of the Echino-
dermata. At the same time, too much should not be made of this, since the
creatine compound has recently been demonstrated in many polychretes,
 PHYLUM CHORDATA 5
whilst other polychretes possess arginine compounds. There is up to the present
little other biochemical evidence of possible affinities.

SUB-PHYLUM ADELOCHORDATA (HEMICHORDATA)

T HE Adelochorda fall into two groups. The first, the Class Enteropneusta,
is composed of worm-like, simply-organised burrowing marine animals
such as Balanoglossus, Glossobalanus, Ptychodera, and Saccoglossus. The
second group, the Class Pterobranchia, includes only three genera, Cephalodiscus,
Rhabdopleura, and Atttbaria. These, too, live in the sea. They are colonial
and sedentary. The two classes have in common the peculiar structure of the
stomochord (presumed notochord), the division of the body into three regions
(which are sometimes looked upon as representing three segments), and certain
other features.

CLASS ENTEROPNEUSTA
Balanoglossus, 1 the so-called acorn-worm, may be used as a convenient
type.
External Characters and Cc:elom.-Balanoglossus (Fig. r) is a soft-bodied,
cylindrical, superficially worm-like animal. Much of its surface is ciliated.
The size varies extremely in the different species, some being only z or 3 centi-
metres long while other species grow to a length of zi metres. It is divisible
into three regions. In front there is a large muscular, generally ovoid proboscis
(prob.), with a small axial cavity. Immediately behind the proboscis and en-
circling its base is a prominent fold-the collar (col.). The third region or trunk
is long and nearly cylindrical, but somewhat depressed. The degree of
flattening varies between species.
Balanoglossus lives in the sea, usually in shallow water, burrowing in sand
or mud by means of its proboscis. One species has been found swarming on
the surface of the sea. Numerous glands in the integument secrete a viscid
matter to which grains of sand adhere in such a way as to form a fragile
temporary tube. The proboscis (Fig. 3, p.) has thick muscular walls and its
cavity the proboscis-cmlom (p. c.) opens to the exterior usually by a single minute
aperture-the proboscis-pore (p. p.). Occasionally two such apertures are
present. In some species the proboscis-pore does not communicate with the
proboscis-crelom, but terminates blindly, and may send off a narrow tubular
diverticulum which opens into the neurocrele. The narrow posterior part or
'neck' of the proboscis is strengthened by a layer of cartilage-like or chondroid
tissue, which supports the blood-vessels. The collar is also muscular. It may
1 The name Balanoglossus is used here as a general designation rather than as a strictly
generic name.
6 ZOOLOGY

pre-or.
cil. organ _ _ prob.

col.

me d.
n.

br. ap .

oes. reg:"

anus int . reg.

FIG. I.-Order Enteropneusta, Family Harrimanidre, Protoglossus. P. koehleri. br. ap.

branchial apertures; br. reg. branchial region; col. collar; hep. reg. hepatic region; int. reg.
intestinal region; med. dar. gr. median dorsal groove between left and right genital ridges; med.
dar. n. median dorsal nerve; res. reg. resophageal region; oocytes (in genital sacs); pre-or. cil.
organ. pre-oral ciliary organ; prob. proboscis; vent. l. mus. ventral longitudinal muscle; *posterior
limit of hepatic region. (Drawn by C. Burdon-Jones.)
 PHYLUM CHORDATA 7
contain one or two ca:lomic cavities. When two are present , the right and left
cavities are separated from one another by dorsal and ventral mesenteries.
They are completely cut off from the proboscis-cavity. The collar cavity (c. c.)
and also that of the proboscis (p. c.) are crossed by numerous strands of con-
nective tissue which give the region a spongy appearance. The collar-cavity
communicates with the exterior by a pair of collar-pores, and short ciliated
tubes (canals) leading into the first gill-pouches.

F1c. 2.- --Balanoglossus: The animal in its burrow. Locomotory power is concentrated in
the proboscis, collar a nd ventral cilia ry band. As Saccoglossus burrows, fine muddy sand is
swallowed. From this organic material is extracted. Inorganic and organic residues resembling
worm-castings are voided through the anus. (Redrawn aft er C. Burdon-Jones.)

On the dorsal surface of the anterior part of the trunk is a double row of
small pores- the branchial apertures (Fig. I , br. ap.). Each row is situated in
a long furrow. These pores increase in number during growth. In some
species the most anterior are overlapped by a posterior prolongation of the
collar called the operculum. A pair of longitudinal genital ridges extends
throughout a considerable part of the body behind and, in the region of the
branchial apertures. They contain the internally situated gonads. These
ridges are so prominent in some of the genera as to form a pair of wing-
like lat eral folds (Fig. 4, g.) , but in other genera they are absent. Behind the
branchial region in some genera are two rows of prominences formed by the
hepatic cceca. The trunk is irregularly ringed. This annulation, which is
entirely superficial and does not correspond to an internal segmentation, is most
8 ZOOLOGY
strongly marked posteriorly. The crelom of the trunk is divided into two
lateral closed cavities by a vertical partition (dorsal and ventral mesenteries, d. m
and v., Fig. 4).

FIG. 3--Balanoglossus:
Saccoglossus. Median sagittal
section of anterior end showing
general details of anatomy.
c. c. collar-crelom; c. e. ciliated
epithelium; c. m. circular
muscle; c. s. cardiac sac; c. t. s.
collar-trunk septum; d. l . m.
dorsal longitudinal muscle; d. 11.
dorsal nerve thickening ; d. n. f.
dorsal nerve fan; d. r. p. dorsal
respiratory pharynx; d. s. dor-
sal sinus or 'heart'; d. 11. dorsal
blood vessel; e. n. p. epidermal
nerve plexus; g. gill-pore; g.b.
gill-bar; gl. glomerulus; g. p.
gill-pouch; i. r. m. inner ring
musculature of collar; l . m .
longitudinal muscle band;
l. m. c. longitudinal muscles of
collar; m . mouth; n. c. neuro-
cord and neurocrele; o. r. m.
outer ring musculature of collar;
p. proboscis; p. c. proboscis-
crelom; p. d. buccal diver-
ticulum; p. p. proboscis pore;
p. o. preoral ciliary organ;
p. o. n . nerve thickening as-
sociated with ciliary organ;
p. sk. proboscis skeleton; t. b.
tongue bar; v. n. ventral nerve
thickening; v. n . p. ventral
nutritive pharynx; v. s. ventral
septum ; v. t. l. ventro-lateral
muscle band of left side of trunk.
(Drawn by C. Burdon-Janes.)

g. g.p.

Alimentary Canal and Associated Structures.-The mouth (Fig. 3, m.) is

situated ventrally at the base of the proboscis, within the collar. Into the
dorsal half of the anterior portion of the alimentary canal open the internal
gill-slits. Each of these is in the form of a long narrow U, the two limbs
separated by a narrow process-the tongue-bar (t. b.)-containing a prolonga-
tion of the body-cavity. In most of the Enteropneusta the internal gill-slits
 PHYLUM CHORDATA 9
lead into gill-pouches. These in turn communicate with the exterior by the gill-
pores. In the genus Ptychodera (Fig. 4), however, there are no gill-pouches;
the U-shaped gill-slits lead directly to the exterior. The bars between the gill
slits have skeletal supports each of which is composed of a number of parts.
Each consists of a dorsal basal portion and three long narrow rods. One rod
is median and two are lateral. The median one, which is bifurcated at the
end, lies in the septum or interval between two adjoining gill-sacs. The two
lateral rods lie in the neighbouring tongues. In most species a number of trans-
verse rods-the synapticulce-
connect together the tongues and
the adjoining septa, and are sup-
ported by slender processes of the
skeleton.
The remainder of the ali-
mentary canal is a nearly straight
tube, giving off, in its middle
part, paired hepatic cceca, which
t
(mainly in the Ptychoderidre)
bulge outwards in the series of
external prominences already
mentioned. Posteriorly it termi-
nates in an anal aperture situated
at the posterior extremity of the FIG. 4.-Balanoglossus: Ptychodera. Trans-
body. In the posterior part of verse section of the branchial region in: b. branchial
part of alimentary canal; b. c 3 , ccelom of trunk;
its extent in some Ptychoderidre d. m. dorsal mesentery; d. n. dorsal nerve; d. v. dor-
the intestine presents a ventral sal vessel; e. epidermis with nerve layer (black) at
its base; g. genital wing; g. p. branchial aperture
median ridge-like outgrowth of (gill-pore) encroached upon by tongue (I) ; l. lateral
m. longitudinal muscles; o. digestive part
its epithelium-the pygochord. septum; of pharynx; r. reproductive organ; t. tongue; v.
Throughout its length the intes- ventral mesentery and ventral vessel; v. 1z. ventral
nerve. (From Harmer, after Spcngcl.)
tine lies between the dorsal and
ventral divisions of the vertical partition. These act as mesenteries.
Food particles are driven backwards towards the mouth by the action of
cilia on the proboscis. Some are swept directly into the mouth with the so-
called respiratory current. This is caused mainly by lateral cilia on the sides
of the gill-bars. These drive water outwards through the branchial apertures
and so more fresh water is continually drawn in through the mouth to replace
that which is driven out. Other particles of food, silt and sand are entrapped
by mucus secreted from the surface of the proboscis. This mucus, with its con-
tents, is drawn into the alimentary canal, partly by the respiratory current and
partly by the action of cilia which line the buccal cavity and the gut generally.
Immediately anterior to the mouth, on the postero-ventral surface of the
proboscis, is a special ciliary organ (Fig. 3, p. o.), equipped with sensory cells.
IO ZOOLOGY
A sample of the particles which enter the mouth passes over the surface of this
organ. The sand or mud, which is taken into the gut, is eventually passed out
at the anus as castings. These can be seen on the surface of the sand in a form
resembling that of earth-worm castings (Fig. 2).
A series of pores (gastro-ctttaneous pores), variously arranged in the different
genera, connect the alimentary canal with the surface. They are unciliated, but
probably represent vestigial gill-openings, retained, as passive drainage chan-
nels, at the posterior limit of the pharynx which has been modified to perform
muscular movements, concerned with compacting the ingested sand and food.
Proboscis or Buccal Diverticulum (Stomochord). The dorsal wall of the
part of the digestive canal immediately following the mouth gives off a diverti-
culum (Fig. 3, p. d.) that runs forward some distance into the basal part of the
proboscis after giving off a short ventral branch. The diverticulum contains a
narrow lumen, and its wall is composed of a single layer of long and very narrow
cells, each of which contains a vacuole. This layer of cells forming the wall of
the diverticulum is continuous with the epithelium of the digestive canal
itself, the cells being somewhat modified by the presence of the vacuoles. The
diverticulum, because of its structure, and its relations, is sometimes regarded
as representing the notochord of the typical Chordata. The restricted form of
the notochordal structure may be related to the animal's principal mode of
locomotion. There is little doubt that a notochord extending right along the
body would be disadvantageous in an animal which moves by thrusting out its
proboscis and next drawing forward its body since it would hinder changes in
length. There is no trace of the flexural movements associated with the
presence of the notochord in typical chordates. Cilia also help Balanoglossus
in its locomotion.
In close relationship with the diverticulum on its ventral surface is the
remarkable proboscis-skeleton (p. sk.). This consists of a median part of an
hour-glass shape, and with a tooth-shaped process, bifurcating behind into two
flattened bars which lie on either side of the buccal cavity.
Blood-vascular System.-This has dorsal and ventral longitudinal trunks.
The dorsal vessel (d. v.) lies above the stomochord, and ends anteriorly in a sinus,
the dorsal sinus or heart (d. s.). This is situated in the posterior part of the
proboscis, in close contact with the stomochord. From the posterior part of
the sinus is given off a vessel which bifurcates to supply the proboscis. In
communication with the sinus in front are a number of vessels forming a
plexus, the glomerulus (gl.). This is a complex organ, possibly excretory,
situated at the anterior end of the proboscis diverticulum. From the posterior
end of the glomerulus on each side there passes backwards an efferent vessel
which breaks up into a plexus. The two plexuses communicate with the
ventral vessel enabling venous return. A closed cardiac sac (c. s.) envelops
the dorsal side of the sinus. Its ventral wall is equipped with muscle fibres,
 PHYLUM CHORDATA II

and it has been observed to contract rhythmically, assisting the circulation

of the blood. Its muscles are too feeble, however, to have more than a local
effect. Similar fibres, presumably muscular, are associated with the dorsal and
ventral vessels throughout the length of the trunk. These vessels lie in the
dorsal and ventral mesenteries. The fibres, which run more or less vertically
in contact with the mesenteries, are formed by the adjoining mesoderm
cells.
Nervous System.-This in some ways resembles that of the Echinodermata.
It consists of dorsal and ventral strands (d. n., v. n.) which extend throughout
the length of the body. These are merely thickenings of a layer of nerve~
fibres which extends over the entire body in the deeper part of the epidermis
(Fig. 3). Here and there are giant nerve-cells. The part of the dorsal strand
which lies in the collar (neurocord) is detached from the epidermis. It contains
a larger number of the giant nerve-cells than the rest. In some species it
encloses a canal, the neuroccele, opening in front and behind. In other species
the neuroccele is a closed canal or is represented vestigially by a number of
separate cavities (n. c.). At the posterior extremity of the collar the dorsal and
ventral strands are connected by a ring-like thickening, and there is a thickening
around the base of the proboscis. There are no organs of special sense unless
the preoral ciliary organ (see p. 9) has this function, but certain cells on the
epidermis of the proboscis and on the anterior edge of the collar may be sensory
in character. It has been shown that the dorsal and ventral nerve-cords are
conduction pathways. If one, or particularly both, are severed, and the
proboscis is stimulated, the contraction of the trunk is interfered with.
Isolated fragments of the body of Balanoglossus, however, will move reflexly
away from tactile stimuli or light.
Locomotion.-This is brought about chiefly by the epidermal cilia (Sacco-
glossus). The cilia of the trunk are capable of synchronised reversal. Ciliary
movement is reinforced by peristaltic contraction of the longitudinal muscles
which are under the control of the main longitudinal nerve-cords. The animal
burrows essentially by peristalsis, and these movements are controlled by the
dorsal nerve-cord of the proboscis.
Reproductive Organs.-The sexes are separate, and often differ in shape and
colour. The ovaries and testes are simple or branched saccular organs arranged
in a double row along the branchial region of the trunk and farther back. They
open to the exterior, each by a single pore.
Development.-This varies in different species. In some, e.g. Saccoglossus.
it is comparatively direct, the larvre feeding on yolk and not differing markedly
from the adults in form. Fertilisation is external. Segmentation is complete
and fairly regular, resulting in the formation of a blastula, which is at first
rounded, then flattened. On one side of the flattened blastula an invagina-
tion takes place. The embryo at this stage is covered with short cilia, with
12 ZOOLOGY
a ring of stronger cilia. The aperture of invagination closes and the ecto-
derm and endoderm become completely separate. Hatching generally take~
place at about this stage and the larva swims away, elongates, and a trans-
verse groove (gr.) appears (Fig. 5): the mouth is formed by an invagina-
tion in the position of the groove. The anus is developed in the position
formerly occupied by the blastopore. Before the mouth appears, a diverti-
culum is separated off anteriorly from the archenteron. Its cavity gives rise

a.t.
I

ar.
b.gr.

100JL
FIG. 5.- Balanoglossus: Development. Left: Embryo of Saccoglossus korsti at hatching stage
(36 hours old). Right: The same in optical section (fixed and cleared). a. t. apical tuft; ar.
archenteron ; b. gr. blastopore groove; gr. first annular groove; p. c. proboscis ccelom; t. telotroch.
(After Burdon· Jones.)

to the crelomic cavity of the proboscis. The paired crelomic cavities of the
collar and of the trunk are formed as outgrowths, either from the posterior wall
of the proboscis cavity, or directly from the wall of the archenteron. By the
appearance of another transverse groove (Fig. 6) the body of the embryo
becomes divided into anterior, middle, and posterior parts. These are the
beginnings of the proboscis, the collar, and the trunk respectively. The
branchial region is marked off by the appearance of a pair of apertures, the
first pair of branchial slits or gill-pores (g. p.). Other pairs subsequently
develop behind these.
 PHYLUM CHORDATA 13
In the species that undergo a metamorphosis the embryo assumes a larval
form termed Tornaria (Fig. 7). This stage is somewhat like an echinoderm
larva; in fact, it was not until the adelochordate life-history became known
that it was conclusively proved that the ciliated tornaria was not a larval
echinoderm. It has a looped ciliated band, sometimes lobed, sometimes pro-

FIG. 6.- Balanoglossus: Development. Creeping larva of Saccoglossus horsti at six days.
L eft: Dorsal view, showing median dorsal reduction of telotroch and continuity of trunk
crelom into tail.
Right: Lateral view, showing forward inclination of grooves, complete tri-metamerism of body
and postero-ventral extension of t elotroch.
a. anus ; c. collar; c. g. collar groove; g. p. giJI-pore; h. g. hepatic region of gut ; int. intestine;
p. proboscis; p. c. proboscis crelom ; p. g. proboscis groove ; ph. definitive pharynx and resophagus ;
l. c. c. left colla r crelom; p. a. t. post-ana l tail rudiment ; p. c. o. pre-oral ciliary organ; tr. trunk;
t. c. trunk crelom. (After Burdon-Jones.)

duced into tentacles, which follows a winding course over most of the preoral
surface and has a postoral loop round the mouth. This is termed the circum-
oral ciliated band (c. b.). Its function is to collect the food , which consists of
any minute organisms and organic particles that may be suspended in the sur-
rounding water. Further posteriorly, a ring of large cilia encircles the body.
This telotroch (t.) is the main locomotor organ. Some forms have an additional
ring of smaller cilia round the posterior (anal) end. At the anterior end, in the
middle of the preoral lobe, is an ectodermal thickening, the apical plate (a. pl.),
I4 ZOOLOGY
containing nerve-cells and eye-spots, and, like the apical plate of a trochophore,
constituting the nerve-centre of the larva. This disappears in the adult.
There is a short alimentary canal with mouth and anus.
At metamorphosis the ciliated bands are lost, the preoral part of the body
becoming uniformly ciliated to form the proboscis. A constriction separates
this from the collar. The trunk gradually elongates and the gill-slits begin to
appear. In some species the neurocord is formed by the separating off of
the deeper portion of the ectoderm along the middle line. In others it is

a.

(a) (b)
FIG. 7.-Balanoglossus: Development. Tornaria larva (r-r·s mm) of Glossobalanus. L eft :
Lateral and Right: Ventral view: a. anal aperture; a. t. apical tuft of cilia; a. pl. apical p late ;
c. b. circumoral ciliated band; c. c. region in which the collar crelom develops; c. s. cardiac sac;
g. p. position in which gill-pouches develop on the phary nx; i. intestine; m. mouth; p. c. probo.
scis crelom; ph. pharynx; p. p. proboscis pore; s. stomach; t. telotroch; t. c. region in which
trunk crelom dev elops. (Drawn by C. Burdon-Jones.)

formed by a longitudinal infolding of the whole thickness of the layer, which

becomes cut off to fom1 a medullary plate with its edges overlapped by the
adjacent ectoderm.
CLASS PTEROBRANCHIA
Three living genera-Cephalodiscus, Rhabdopleura and Atubaria-occur.
They are colonial and sedentary, but are nevertheless closely related to the
Enteropneusta. They resemble them in having the body divided into three
parts or regions-a proboscis with a proboscis-cavity, a collar with two collar-
cavities communicating with the exterior by a pair of collar-pores, and a trunk
with two distinct lateral cavities. They also have a structure resembling a
notochord with the same relations to the nervous system as in Balanoglossus.
 PHYLUM CHORDATA IS
They all differ from Balanoglossus firstly in having the alimentary canal
bent on itself, so that the anal opening is situated not far from the mouth and
secondly, in the presence of arms bearing tentacles arising from the collar.
Cephalodiscus, moreover, has only a single
pair of gill-pores. Rhabdopleura lacks such
openings, their places being taken, apparently,
by a pair of ciliated grooves. Cephalodisws
and Rhabdopleura live in associations or
colonies secreting a common case or invest-
ment.
Cephalodiscus has an investment (Fig. 8)
in the form of a branching gelatinous struc-
ture, which is in some species beset with
numerous short filiform processes, and con-
tains a number of tubular cavities (with ex-
ternal openings) occupied by zooids. The
latter (Fig. 9) are not in organic continuity,
so that, though enclosed in a common invest-
ment, they do not form a colony in the sense
in which the word is used in describing the
hydroid zoophytes. They have a feature in
common with such a colony in that they
multiply by the formation of buds; but these
become detached before they are mature.
With the collar-region (c. ) is connected a
series of usually eight to sixteen 'arms' (t. p.).
Each of these is beset, except in the case of
the male of one species, with numerous very FrG. 8.-0rder Pterobranchia,
fine pinnately-arranged tentacles. Each arm Family Cephalodiscidre. Cephalo-
di.~cus. Small portion of a gelatinous
contains a prolongation of the collar-cavity. colony of C. dodecalophus probably
The proboscis (Fig. ro, pr.) is a shield-shaped measuring a bou t 6" x g". The main
stems, bea ring branches w ith filiform
lobe overhanging the mouth. Its cavity processes, vary in breadth from 5 to
15 mm., according to the degree of
communicates with the exterior by two pro- flattening of the ccencecium. The
boscis-pores (p. p.). The cavities of the collar m ain trunks of a colony are attached
to stones, sponges or to some other
communicate with the exterior by a pair of solid substratum. The entire colony
tends to lie horizontally rather than
ciliated passages opening by the collar-pores. erect. (After Macintosh.)
Behind the collar-region on each side is a
small area in which the body-wall and that of the pharynx are coalescent.
This area is perforated by an opening, the gill-pore (g. p.). Great numbers of
cilia occur on the arms and proboscis. A nerve-strand, dorsal ganglion, or
collar-cord, containing nerve-fibres and ganglion-cells, is situated on the dorsal
side of the collar deep in the epidermis (c. n.). It is prolonged on to the
16 ZOOLOGY
dorsal surface of the proboscis and the dorsal surface of the arms. It is not
hollow. On the ventral side of this nerve-strand is a very slender cylindrical
cellular cord (p. d.) which is continuous behind with the epithelium of the
pharynx. This is homologous
with the diverticulum of Bal-
anoglossus, and perhaps homo-
logous with the notochord of
the Chordata.
Atubaria (Fig. II), described
by Sato as recently as 1936, re-
sembles Cephalodiscus in general
appearance, but lacks a protec-
tive investment. A . heterolopha
,.__ _ P'· was dredged from a depth of
200-300 metres a few miles from
Tokyo Bay when a number of
females were found attached to
a colony of an athecate hydroid.
It is free-living and about I·S
mm. long. Adults possess four
pairs of tentaculate plumes or
s. ' arms' arranged in two rows.
These are of different lengths.
The antero-internal pair are
longest, differing from the others
in being club-shaped (Fig. II).
Immature specimens lack arms,
bearing merely a single pair of
rod-like processes. The collar
region is short and thick, and
between collar and trunk there
are dorsal genital, and ventral
mouth apertures. The anal
aperture is below the genital
FIG. 9--Cepllalodiscus: Zooid. The zooid, with
budding stalk, has a total length of about 4 mm. opening. The trunk is per-
Numerous individuals occupy the structure shown in
Fig. 8. b. buds; c. collar; g. p. gill-pore; m. mout h; forated by a pair of oblique
pr. proboscis; s. stalk; I. p. tentaculate plumes. branchial slits (leading to the
(Drawn by C. Burdon-} ones. )
pharynx) and tapers basally,
with no attachment disc, into a long tail-like extremity. Atubaria
exhibits three cceloms : a single cavity in the proboscis, and paired cavi-
ties inside both collar and trunk. Those of the collar extend into the
arms.
 PHYLUM CHORDATA IJ
The stomochord (the so-called 'notochord') occurs as 'a slender tube, with
a small but distinct lumen, surrounded by about a dozen cells' (Komai). It
runs obliquely from the dorsal wall of the buccal cavity nearly to the centre of
the proboscis, where it ends blindly. The spacious buccal cavity bears a ridge
on its dorsal wall and internal to this ridge is a short median diverticulum.
The pharynx has folded walls p.p.
and is lined with ciliated cells.
The cesophagus is a funnel-
shaped sac which leads to a
large sac-like stomach occupy-
ing most of the trunk somite.
A pylorus separates the
stomach from the intestine
which ascends to the anus just
below the ovary, oviduct, and p.c.
genital aperture. The mode
of reproduction is unknown.
The cerebral ganglion lies on
the dorsal side of the proboscis
giving off anterior and pos- st.
terior nerve trunks.
Blood-vascular system.-
This includes a heart (Fig.
ro, h.), cardiac sac (c. s.),
dorsal and ventral vessels,
I.
and resembles that of the
Enteropneusta.
Reproduction.- Some
species of Cephalodiscus are
hermaphrodite. In most the FrG. Io. - Ceplwlodiscus: Visceral relationships. The
collar septum omitted t o show the circumcesophageal n erve
sexes are separate. The pos- ring. a. anal a perture; c. collar; c. c. colla r ccelom;
terior end of the body is c. n. collar cord; c. n. r. circumcesophageal nerve ring ;
c. s. cardiac sac ; d . b. v. dorsal blood vessel; gl. glomerulus ;
drawn out into an appendage g . a. genita l aperture; g. p. gill-p ore ; h . heart; i. intestine;
m. mouth; oe. cesophagus ; ov. ovary; p. c. proboscis
on which buds are developed ccelom; p. d. proboscis diverticulum; p. n. p. proboscis
(Fig. g) . A pair of ovaries nerve plexus; p. p. proboscis pore; pr. proboscis; s. stalk;
st. st omach; t. trunk; t. c. trunk ccelom; t. p. t entaculate
(Fig. ro, ov.) lie in the trunk- plumes; v. b. v. ventral blood vessel ; v. n. ventral nerve.
cavity, and there is a pair of (Drawn by C. Burdon-Janes.)
oviducts (ovd.) lined by elongated, pigmented epithelium. The development,
which is direct (without a free-swimming larval stage), takes place in passages
in the investment. In one species, the segmentation is complete, but unequal,
and a gastrula is formed by invagination. In another, the segmentation is
incomplete, and a gastrula is formed by delamination.
VOL. II. c
18 ZOOLOGY

a.

t.

FIG. II.- Order Pterobranchia, Family Cephalodiscidre. Atubaria. A. heterolopha, a free-

living pterobranch, without gelatinous investment (cf. Fig. 8). Left: Lateral view of left side.
Tentaculate plumes omitted. The total length of the animal with plumes is about 1·25 mm .
Right: Tentaculate plume, one of a pair from the anterior row. a. anus; c. collar; c. t. cut ends
of tentaculate plumes of which there are two rows of four; g. genital aperture ; g. p. gill-pore;
m. mouth ; pr. proboscis; s. stalk; I. trunk. (Drawn by C. Bunion-Jones, modified after Sato.)

p. r.z.
FIG. 12.- 0rder Pterobranchia, Family Rhabdopleuridre. Rlwbdopleur·a. A small portion
of a colony of R. normani showing the inter-relationship of the zooids and a branch carrying a
terminal bud and the buds that it has proliferated: br , bz, b3, young buds ; c. s. contractile stalk;
d. c. decumbent crenrecium; e. c. erect crenrecium; e. z. extended zooid; p. pectocaulus (or black
stolon); r. z. retracted zooid; I. b. terminal bud (or blastozooid). (Drawn by C. Burdon-Jones.)
 PHYLUM CHORDATA rg

Rhabdopleura (Fig. 12) occurs in colonies of zooids organically connected

together by a creeping stolon and enclosed in, though not in organic con-
tinuity with, a system of branching membranous tubes (d. c.). The collar
region bears a pair of hollow arms (Fig. 13), each with a double row of slender
tentacles (te.).
te.

F1c. IJ.-Rhabdopleura. Mature male zooid and orenrecium. The total length of zooid is
about r·s mm. (Drawn by C. Burdon-Jones.)
Right: Diagramma tic m edian sagittal section of the above with the gonad a nd duct omitted.
(After Schepotieff.)
a. arm ; an. anal aperture; c. ccencecium ; col. collar; col. n e. collar n erve; c. p. collar pore ;
c. s. cardiac sac; g. genita l aperture; int. intestine ; m . mouth ; ntc. proboscis diverticulum ; m.
cesophagus; pr. proboscis; pr. c. proboscis ccelom; ret. rectum; s. stalk or pedicle; st. stomach;
t. testis ; te. tentacles; tr. c. trunk ccclom ; v. n. ventral nerve.

There are two collar-pores (c. p.), each leading into a ciliated canal \vith an
internal funnel, and also a pair of proboscis-pores. The notochord and the
nervous system resemble those of Cephalodiscus. In Rhabdopleura the zooids
are of separate sexes, but the colony is hermaphrodite, although it is formed
by budding. A free-swimming larval stage occurs.
Cephalodiscus has been found at various widely separated localities in the
southern hemisphere. Species occur also off the coast of Japan and Korea.
20 ZOOLOGY
Some live in shallow water. None has been found at a greater depth than
about 300 fathoms. Eocephalodiscus occurred in the Upper Cretaceous.
Rhabdopleura has been found in widely separated localities (e.g. Shetlands and
South Australia). It seems doubtful if more than one species occurs.
Affinities.-Not all zoologists agree with the inclusion of the Adelochorda
among the chordates. Perhaps, too, the term Hemichorda is inadvisable,
since this implies the presence of half a true notochord, a question much in
dispute. Bateson was the first to attempt to homologise the foregut diverti-
culum of Enteropneusta with the notochord of the Tunicata, Acrania, and the
vertebrates, and although this concept was soon criticised by Spengel, it
nevertheless gained ground. There is to-day, however, a growing scepticism.
Komai has suggested that the stomochord may be homologous with the anterior
pituitary gland (p. 149). The Adelochorda is probably at least a greatly
modified branch from the base of the chordate tree. The gill-slits, with their
tongue-bars, and the dorsal tubular neurocord, are important pointers in this
direction. If the Adelochorda are chordates, it is of special interest that they
show many resemblances to the phylum Echinodermata. The tornaria larva,
for instance, is very like an echinoderm larva.
Kozlowski's work on graptolites (by the use of hydrofluoric acid on fossils
in a silicious matrix) has now revealed fine skeletal details that suggest that
these animals, formerly placed with the Hydrozoa or Ectoprocta, are perhaps
most closely allied to the Pterobranchia, and therefore may constitute a chor-
date class Graptolita. The tubular skeletons, composed of a succession of
concentric rings (each of two half-circles) appeared in the Middle Cambrian,
became universal in the Ordovician, and faded slowly and disappeared in the
Lower Carboniferous. Graptolites were colonial, a number of polyps living in
each theca budded from a stem or stipe. Five orders-Dendroidea, Tuboidea,
Camaroidea, Stolonoidea and Graptoloidea-have been erected.

T
SUB-PHYLUM TUNICATA (UROCHORDATA)
HREE main divisions of this ancient group have been recognised: (1) the
sessile Ascidiacea, (z) the Thaliacea, and (3) the Appendicularia. Each
has an organisation which, when taken as a whole, is 'significantly unique
in itself' (Berrill). The tunicates are marine. The Ascidiacea are all sessile,
whilst the other two groups are pelagic. They are especially interesting for
several reasons. They undergo a remarkable series of changes in the course of
their life-history. Some exhibit an alternation of generations quite as marked as
that existing among so many invertebrates. In most tunicates there is a
retrogressive metamorphosis almost, if not quite as striking as that which has
been described among the parasitic copepods or the Cirripedia. The sessile
form and ciliary feeding mechanism link the adult tunicates with the ptero-
branchs, Rhabdopleura and Cephalodiscus. Multiplication by budding, so com-
 PHYLUM CHORDATA 21

mon in the lower groups of invertebrates, but exceptional or absent in the higher,
is of very general occurrence in the Tunicata.
The free-swimming tunicate larva is unquestionably chordate in character.
In by far the greater number of cases it would be impossible by the study
of the adult alone to establish its relation with the Chordata. The affinities
of the Tunicata can be determined with certainty only when life-histories are
followed out, for the notochord and other significant structures are lost in the
later stages of metamorphosis. Although the solitary ascidians were observed
by Aristotle (who named them Thalia) it was not until the second half of the
nineteenth century that Kowalevsky established their chordate relationship.
CLASS ASCIDIACEA
The sessile ascidians have colonised practically every marine habitat from
rocky shore to ocean depths, but most species live in shallow coastal waters.
They are distributed from the equator to polar seas, and most families are
cosmopolitan. This widespread scattering of their kind is consequent upon the
fact that although the adults are stationary, the larv~ are well adapted for
distribution by ocean currents. The group can be tabulated briefly as follows:
CLASS ASCIDIACEA
Order Enterogona
Sub-order Aplousobranchia
Families Clavelinidre
Polycitoridre
Polyclinidre
Didemnidre
Sub-order Phlebobranchiata
Families Cionidre
Diazonidre
Perophoridre
Corellidre
Ascidiidre
Hypobythiidre
Agnesiidre
Order Pleurogona
Sub-order Stolidobranchiata
Families Styelidre
Pyuridre
Molgulidre
EXAMPLE OF THE CLASS.- THE ASCIDIAN
OR SEA-SQUIRT (ASCIDIA)
Sea-squirts occur often in large associations adhering to rocky shores.
Some species are found in great numbers attached to wharf-piles. When
22 ZOOLOGY
touched, the ascidian ejects with considerable force two fine jets of sea-water
which come from two apertures on its free end. The shape of the ascidian,
however, can be profitably studied only in the case of specimens that are
completely immersed in the sea-water, for specimens not so immersed always
undergo contraction (Fig. 14). In an uncontracted specimen the general shape
is that of a short cylinder with a broad base by which it is fixed to the rock.
The free end presents a large rounded oral aperture, and some little distance
from it on one side is a second atrial aperture of similar character. Experi-
ments with floating particles reveal that a strong current of water flows steadily
into the oral, and out of the atrial aperture. When the animal
is removed from the water both apertures become narrowed
(so as to be almost completely closed) by the contraction of
sphincters of muscular fibres which surround them. At the
/ same time the walls of the body contract. Streams of water
are forced through the apertures and the bulk of the animal
is considerably reduced.
Body-wall and Atrial Cavity.-The outer layer of the body-
wall is composed of a tough translucent substance forming a
thick test or tunic (Fig. 15, test). This consists largely of tunicin,
which X-ray diffraction analysis has identified with cellulose,
a characteristic plant product not perhaps otherwise found in
animals, except in some 'plant-like' Protozoa. The test of an
ascidian is frequently referred to as a cuticle. It is a cuticle
in the sense that it lies outside the ectoderm and is derived
FIG. 14.- 0rder
En terogona , from that layer in the first instance. There are, however,
Family Ascidiidre, cells contained in it, which may add to its substance in later
Ascidia. Entire
anima l seen from stages. They seem to be chiefly derived, not from the ecto-
the right side.
(After Herdman.) derm, but from the underlying mesoderm, from which they
migrate through the ectoderm to the outer surface. These
cells are found scattered through the substance of the test. Running through
it are also a number of branching tubes lined with cells, each terminal branch
ending in a little bulb-like dilation. The interior of each tube is divided into
two channels by a longitudinal septum, which, however, does not completely
divide the terminal bulb. Through these tubes (which are of the nature of
looped blood-vessels) blood circulates, passing along one channel, through the
terminal bulb, and back through the other channel.
When the test is divided (Fig. 15), the soft wall of the body or mantle (mant.)
comes into view. The mantle is suspended within the test and is usually
attached firmly to it only round the oral and atrial apertures. The mantle
(body-wall) consists of the ectoderm with underlying layers of connective-tissue
enclosing muscle fibres. It follows the general shape of the test, and at the
two apertures is produced into short and wide tubular prolongations, which are
 PHYLUM CHORDATA 23

known respectively as the oral and atrial siphons (Fig. 18, or. siph., atr. siph.).
These are continuous at their margins with the margins of the apertures of the
test, and round the openings are the strong sphincter muscles by which closure
is effected. In the rest of the mantle the muscular fibres are arranged in an
irregular network, crossing one another in all directions, but for the most part
either longitudinally or trans-
versely. Within the body-wall is
a cavity, the atrial or peribranchial
cavity (atr. cav.), communicating
with the exterior through the
atrial aperture. This cavity is
not a ccelom, for it is formed
entirely by involution from the
outer surface.
Pharynx.-The oral aperture
leads by a short and wide oral
passage (stomodt:eum) into a
chamber of large dimensions, the
pharynx or branchial chamber
(Fig. 15, ph.). This is a highly
characteristic organ of the Tuni-
cata. Its thin and delicate walls
are pierced by a number of slit-
like apertures, the stigmata (Fig.
18, stigm.), arranged in trans-
verse rows. Through these the
cavity of the pharynx communi-
cates with the atrial or peribran- FIG. 15.-Ascidia: Visceral relationships. The
greater part of the test and mantle has been removed
chial cavity, 1 which completely from that side so as to bring into view the relation of
surrounds it except along one these layers and of the internal cavities and the
course of the alimentary canal, etc. an. anus; atr.
side. The edges of the stigmata ap. atrial aperture; end. endostyle; gon. gonad;
are beset with numerous strong gonod. gonoduct; hyp. neural gland; hyp. d. duct of
neural gland; nzant. mantle; ne. gn. nerve-ganglion;
cilia. These drive currents of oes. ap . aperture of resophagus; or. ap. oral aperture;
ph. pharynx; stom. stomach; tent. tentacles; test,
water from the pharynx into the test; typh. typhlosole. (After Herdman.)
atrial cavity. The ciliary action
draws a current in through the oral aperture, and drives it through the stigmata
into the atrial cavity. From there it reaches the exterior through the atrial
aperture. The stigmata (Fig. r6) are elongated longitudinally. Groups of
from three to ten gill-slits are usually separated by strongly developed internal
longitudinal bars which are sometimes thicker and more prominent than the
1 A distinction is sometimes made between the lateral parts of this space (peribranchial cavities,
right and left) and the median unpaired (dorsal) part (atrial cavity, or cloaca). in which the two
peribranchial cavities coalesce, and which leads to the exterior through the atrial aperture.
 ZOOLOGY

horizontal or transverse bars that separate neighbouring rows. In all of these

bars run blood-vessels. Extending across the atrial cavity from the body-wall
to the wall of the pharynx are a number of
bands of vascular mesodermal tissue, the
p
lr connectives or trabecul<e (Fig. 17, vas. tr.).
It has already been mentioned that the
atrial cavity does not completely surround
the pharynx. This is owing to the fact that
the wall of the pharynx is united with the
mantle (Fig. 17).
Along the line of adhesion the inner sur-
FIG. r6.-A.scidia: Stigmata. Single
m esh of the bra nchial sac, seen from face of the pharynx presents a thickening in
inside. i. l. internal longitudinal bar ; the form of a pair of ventral longitudinal
l . v. longitudinal vessel; p. p'. papillre
projecting inwards from the branchial folds separated by a groove. This special-
bar; sg. stigma; tr. tra nsverse vessel. ised grooved thickening is the endostyle
(After Herdman.)
(end.).
The cells covering the endostyle are large and of two kinds-ciliated cells
and gland-cells. They are equipped with cilia at their free ends. These cilia
drive floating particles that come
within their influence outwards
towards the oral aperture. The
gland cells secrete and discharge
a viscid mucous matter. An-
teriorly the endostyle is con- bt.v
tinuous with a ciliated ridge
which runs circularly round the
anterior end of the pharynx. In
front of, and parallel with, this
circular ridge is another ridge of
similar character : these are the
peripharyngeal ridges. The groove
between them is the peripharyngeal
groove. Dorsally, i.e. opposite the
endostyle, the posterior peri-
pharyngeal ridge passes into a FrG. 17.-Ascidia: Pharyngeal region. bl. v.
median, much more prominent, blood-vessels; dors. lam. dorsal lamina; epi.
epidermis ; end. endostyle; gn . ganglion; hyp .
longitudinal ridge, the dorsal lamina neural gland; mus. muscular layer of wall of body ;
(dors. lam.), which runs along the tperibl'. peribranchial cavity; p h. pharynx ; test,
est; vas. tr. vascular trabeculre. (After Julin.)
middle of the dorsal surface of the
pharynx to the opening of the resophagus. In the living animal the lamina
is capable of being bent to one side in such a way as to form a deep groove.
The mucus secreted by the gland-cells of the endostyle is driven from it by the
 PHYLUM CHORDATA 25
ciliated cells and conveyed laterally round the inner surface of the pharyngeal
wall, mainly by the action of small frontal cilia, which are borne on the gill-
bars and project into the lumen of the pharynx. As the mucus passes across
the stigmata, it entangles microscopic organisms that are contained in the out-
going water and carries them round to the dorsal lamina. This bears cilia
\vhich drive the mucus and the
contained food posteriorly to
the opening of the cesophagus
through which they are In-
gested.
Some little distance in
front of the anterior periph-
aryngeal ridge, at the inner or lr.v
posterior end of the oral
siphon, is a circlet of delicate man.t
tentacles (Fig. r8, tent.).
Alimentary Canal.-The
cesophagus (Figs. 15, r8, (CS.)
leads from the pharynx (near
the posterior end of the dorsal
lamina) to the stomach (stom.),
which, together with the in-
testine, lies embedded in the
mantle on the left-hand side.
The stomach is a large fusi-
form sac with comparatively
thick glandular walls which
are said to secrete a strong
carbohydrate-splitting enzy- FIG. 18.-Ascidia: Circulation and reproduction.
The test and mantle have been rem oved. an. anus ; at1·.
me and weaker proteolytic cav. atrial cavity ; atr. siph. atrial siphon; br. car. branchia .
and lipolytic ferments as well. cardiac vessel; card. vise. cardio-visceral v essel; dors. v.
dorsa l vessel ; gonad. gonoduct; ht. heart; hyp. neural
Along its inner wall is a gla nd ; mant. mantle; ne. gn . n erve-ganglion; res.
or. si p h. oral siphon; ov. ovary; 1·eet. rectum;
thickening- the typhlosole. cesophagus;
stigm. stigmata ; stom. stomach ; tent. t entacles ; test.
There is no liver. A system test ; tr. v. transverse vessel; vent v. ventral v essel;
vise. br. viscera-branchial vessel. (From Herdman, after
of delicate tubules, the pyloric Perrier.)
gland, ramifies over the wall
of the intestine and is connected with a duct opening into the stomach. Little
is known of its significance : both digestive and excretory functions have been
suggest ed. The thin-walled intestine is bent around into a double loop and
runs forward to terminate in an anal aperture (an .) situated in the atrial cavity.
Blood Vascular System.-The system is well developed. The heart (Fig.
r8, ht.) is a simple muscular sac, situated near the stomach in the pericardium.
 ZOOLOGY
The last-named is entirely cut off from the surrounding spaces in which the
blood is contained. Its mode of pulsation is remarkable. The contractions
are of a peristaltic character, and follow one another from one end of the
heart to the other for a certain time. Then follows a short pause, and when the
contractions begin again they flow in the opposite direction. Thus the direction
of the current of blood through the heart is reversed at regular intervals.
There are no true vessels, the blood circulating through a system of channels
or sinuses devoid of epithelial lining, and of spaces or lacunre, forming a
hremocrele. In the description that follows, therefore, the word vessel is not
used in its strict sense. At each end of the heart is given off a large 'vessel'.
That given off ventrally, the branchia-cardiac vessel (br. car.), runs along the
middle of the ventral side of the pharynx below (externally to) the endostyle
and gives off a number of branches which extend along the bars between the
rows of stigmata. These branches give rise to smaller branches which pass
between the stigmata of each row. The vessel given off from the dorsal end
of the heart-the cardia-visceral (card. visc.)-breaks up into branches which
ramify over the surface of the alimentary canal and other organs. This system
of visceral vessels or lacunre opens into a large sinus, the viscera-branchial (vise.
br.). This runs along the middle of the dorsal wall of the pharynx externally
to the dorsal lamina, and communicates with the dorsal ends of the series of
transverse branchial vessels. In addition to these principal vessels there are
numerous lacunre extending everywhere throughout the body, and a number
of branches, given off both from the branchio-cardiac and cardio-visceral vessels,
ramify, as already stated, in the substance of the test.
The direction of the circulation through the main vessels differs according
to the direction of the contraction of the heart. When the heart contracts in a
dorso-ventral direction, the blood flows through the branchio-cardiac trunk to
the ventral wall of the pharynx and through the transverse vessels. After
undergoing oxygenation in the finer branches between the stigmata, it reaches
the viscero-branchial vessel, by which it is carried to the system of visceral
lacunre. From these it flows back to the heart through the cardio-visceral
vessel. When the contractions take the opposite direction, the course of this
main current of the blood is reversed. Because capillaries are absent, ascidian
blood and tissue fluid are intermingled. Lymphocytes, phagocytic macrophages
(including ingested substances), have been recognised, as well as vacuolated com-
partment cells (reserve food-containers} and other coloured and colourless cells.
Some but not all ascidians possess a green vanadium-containing pigment
which is carried in special vanadacytes and also flows free in the plasma. This
has been claimed, but not pJ;oved, to be a respiratory pigment. In fact, there
is some evidence that the oxidation potential of the pigment is too low for it to
function in this way. The exact method of respiration is still unknown. In
some ascidians niobium, and very little vanadium occurs (Carlisle).
 PHYLUM CHORDATA

Excretion seems to be carried out by the nephrocytes circulating in the blood.

These cells contain particles of urates and xanthine which are disposed of by
storage, in the form of concretions, in special excretory vesicles or renal organs.
Nervous System.-This is of an extremely simple character. There is a
single neural ganglion (Figs. 15, IJ, r8 and rg, ne. gn. , gn. and n. g.) which lies
between the oral and atrial apertures, embedded in the mantle. This is
elongated in the dorso-ventral direction, and gives off at each end nerves which
pass to the various parts of the body. In degang-
lionated individuals reflexes such as the so-called
'cross reflex' (whereby touching one siphon causes
the other to close before the one stimulated) and
body contraction are radically impaired.
Neural Gland.-This (Figs. rs, r8, hyp. ; Fig. rg,
gld., and Fig. 20, n. gl.) lies on the ventral side of
the neural ganglion and is sometimes considered to
be homologous with at least part of the hypophysis
in vertebrates. This is still a controversial subject,
though there appears to be good evidence that the
neural gland is of a secretory nature. A duct (Fig.
rg, dct., and Fig. 20, gl. d.) runs forward from it and
opens into the cavity of the pharynx ; the termi-
nation of the duct is dilated to form the ciliated
funnel, and this is folded on itself to form a pro-
minence, the dorsal tubercle, which projects into the Frc. Ig. -Ascidia: Dorsal
tubercle, nerve-ganglion, and
cavity of the pharynx: associated parts. From below.
Reproductive System.- The sexes are united. dct . duct of neural gland;
dors. lam. dorsal lamina; gld.
The ovary and the testis are situated close together neural gland; gn. ganglion;
dorsal tubercle; nv., nv.
on the left-hand side of the body in the intestinal hyp. nerves: periph. peripharyn-
loop. Continuous with the gonad is a duct-an geal ridge. (After Julin.)
oviduct or sperm-duct, as the case may be. This
opens into the atrial cavity close to the anus. If sex products from another
individual are drawn into the mouth, with the incurrent sea water, it is found
that some of the eggs or sperm become lodged in the ciliated funnel. It has
been suggested that the neural gland thereupon secretes a substance similar
to, or even identical with, the gonadotrophic hormone produced by the anterior
lobe of the pituitary of vertebrates (Fig. g6, p. rso). There is evidence that the
adjacent ganglion is sensitive to this hormone and responds by nervous stimula-
tion of the gonads to release their gametes. In this way adjacent individuals
may be stimulated to spawn at the same time, ensuring fertilisation. It is not
impossible that the anterior pituitary may have evolved from a' neural gland'
which was responsible for synchronisation of spawning in a somewhat ascidian-
like ancestor.
28 ZOOLOGY
Development and Metamorphosis.-ln the Ascidiacea fertilisation usually
takes place after the ova have passed out from the atrial cavity. But in a few
simple, and most (if not all) compound forms fertilisation takes place in the
atrium, or in the swollen, terminal portion of the oviduct, or in a special out-
growth of the atrium, serving as a brood-sac. After internal fertilisation the
zygotes may be discharged immediately (as in most species of Polycarpa),
retained until the 2- or 32-cell stage (P. pomaria), or discharged as blastula: or
gastrula:; or expulsion may be delayed until the tailed larval stage is attained.
Self-fertilisation is usually rendered impossible by ova and sperms becoming
mature at different times. Sometimes, however, both ripen simultaneously.

FrG. 2o.-Ascidia: Nervous system. Ante ro-dorsal aspect showing relationships with the
layers of the body; Left: in sagittal section; R ight: in transverse section. d. bl. s. dorsal
blood-vessel; d . l. dorsal lamina ; d. n. dorsal ne rve ; d . t. dorsal tubercle; eel. ectoderm; en. endo-
derm; e. p. br. epithelium o f peribranchial cavity; gl. d. duct of neural gland; l. v. points to the
ciliated epithelium covering a longitudinal vessel of branchial sac (pharynx); m . mantle; 11 .
n erve ; n. g . ganglion; n. gl. neural gland ; p. br. peribranchial cavity ; pp. b. periphargyngeal
band; spit. oral sphincter; 1., t'. test; tn. tentacle. (After Herdman.)

At the same time, simultaneous maturation of eggs and spermatozoa does not
necessarily imply self-fertilisation. Attempted in vitro fertilisations with
gametes of a single individual almost always fail.
A somewhat complicated series of membranes invests the ovum. The
maturing oocyte is enclosed in a layer of flat cells. These are the primitive
follicle-cells-derived in most species from undifferentiated cells of the ovary.
On the surface of this layer is developed a structureless basal membrane. The
follicle-cells increase by division and soon form a sphere of cubical cells. Certain
of the cells migrate into the interior of the sphere and form a layer on the
surface of the ovum. Others penetrate into the latter so as to lie in the super-
ficial strata of the yolk. The layer of cells on the surface of the ovum are t ermed
the test-cells (Fig. 2!, e) : they afterwards develop on the outer surface a thin
structureless layer, the chorion (d), and internal to them is formed a gelatinous
layer (x) through which the test-cells in a degenerated condition become
 PHYLUM CHORDATA 29
scattered. Meantime, external to the follicle-cells, between them and the basal
membrane, has appeared a layer of flattened epithelial cells. This, with the
basal membrane, is lost before the egg is discharged. In all the simple ascidians,
with the exception of the few in which development takes place internally, the
protoplasm of the follicle-cells (Fig. 21, c) is greatly vacuolated, so as to appear
frothy. The cells become greatly enlarged, projecting like papillre on the
surface and buoying up the developing ovum.
Segmentation is complete and approximately equal, but in the eight-cell
stage four of the cells are smaller and four larger. The smaller, situated on the
future dorsal side, are the beginning of the endoderm. The four larger form
the greater part, if not the whole, of the ectoderm. In the following stages the
ectoderm cells multiply more rapidly than the
endoderm, so that they soon become the
smaller. In the sixteen-celled stage the em-
bryo (Fig. 22, A) has the form of a flattened
blastula (placula) with ectoderm on one side
and endoderm on the other, and with a small
segmentation-cavity. The transition to the
gastrula stage is, in most ascidians, effected by
a process intermediate in character between
embolic and epibolic invagination ; in some
the invagination is of a distinctly epibolic FIG. zr.-Ciona: Development.
character. In the former case the ectoderm F ertilised egg from the oviduct after
the basal membrane and layer of flat-
cells continue to increase more rapidly than tened cells have been thrown off. c.
the endoderm. The whole embryo becomes follicle-cells; d. chorion ; e, test-
cells; f, ovum; x, gelatinous layer,
curved, with the concavity on the endodermal (From Korschelt and Heider, after
side, and the ectoderm extends over the endo- Kupffer.)
derm, the two layers coming to lie in close contact and the segmentation-cavity
thus becoming obliterated. The concavity deepens until the embryo assumes
the form of a saucer-shaped gastrula with an archenteron and a blastopore,
which is at first a very wide aperture extending along the whole of the future
dorsal side. The blastopore gradually becomes constricted (Fig. 22, B). The
closure takes place anteriorly, extending backwards. The opening is eventually
reduced to a small pore at the posterior end of the dorsal surface.
The embryo elongates in the direction of the future long axis. The dorsal
surface becomes recognisable by being flatter, while the ventral remains convex.
The ectoderm cells bordering the blastopore are distinguished from the rest by
their more cubical shape. These cells form the earliest rudiment of the
nervous system. They become arranged, as the blast opore undergoes con-
traction, in the form of a medullary plate on the dorsal surface. On the surface
of this plate appears a medullary groove. This is bounded by right and left
medztllary folds, which pass into one another behind the blastopore.
30 ZOOLOGY
The medullary folds grow upwards and inwards over the medullary groove.
They unite together (D), the union beginning behind and progressing forwards
in such a way as to form a canal, the neuroca:le, in the hinder portion of which
is the opening of the blastopore. In this process of closing-in of the medullary
groove, the fold which passes round behind the blastopore takes an important
part, growing forwards over the posterior portion of the canal. The blasto-
pore, thus enclosed in the medullary canal, persists for a time as a small opening

c D

Fw. 22.-Clavellina: Development. Early stages: A, flattened blastula; n, early gastrula;

C, approximately median optical section of more advanced gastrula in which the blastopore
has become greatly reduced and in which the f1rst rudiment of the notochord is discernible;
D , similar view of a later larva in which the m edullary canal has begun to be closed in posteriorly.
bl. p. blastopore; eel. ectoderm; end. endoderm; med. can. medullary canal; nerv. cells destined
to give rise to the nerve-cord ; n eur. neuropore ; nolo. notochord ; seg. cav . segmentat ion cavit y.
(A a nd B from K orschelt a nd Heider, after Seeligcr ; C and Dafter Van Beneden and Julin.)

-the neurenteric canal-by which the neurocrele and enteric cavity are placed
in communication. At the anterior end of the medullary canal, owing to its
incomplete closure in this region, there remains for a time an opening-the
neuropore (Fig. 23 , neur.)-leading to the exterior.
A notochord (Figs. 22 , C, D, 23 and 24, noto.) is formed from certain of
the cells of the wall of the archenteron along the middle line of the dorsal side.
These are arranged to form an elongated cord of cells which becomes com-
pletely constricted off from the endoderm of the wall of the archenteron, and
comes to lie between the latter and the medullary groove. Laterally, certain
 PHYLUM CHORDATA 31
cells of the endoderm divide to give rise to a pair of longitudinal strands of
cells-the rudiments of the mesoderm (Fig. 23, mes.). During this process of
mesoderm-formation, there are
no diverticula developed from
A
the archenteron. 7n€S

The embryo (Fig. 23, B)

now becomes pear-shaped, the
narrow part being the rudiment
of the future tail. As this
narrow portion elongates, the '
part of the enteric cavity which · ' ..

_.}
~

it contains soon disappears,

coming to be represented only
by a strand of endoderm cells.
This strand gives rise in the
middle to the backward exten-
sion of the notochord, laterally

---
to the mesoderm of the tail, and
8 med.can
ventrally to a cord of endoderm -~~ neur
~

cells continuous with the wall of .- ~

the enteric cavity in front. ''
'
The caudal region rapidly
increases in length. The an-
terior or trunk region, at first
round, becomes oval. At its
anterior end there appear three
processes of the ectoderm, the
rudiments of the adhesive papilla
(Fig. 24, adh.), organs by which
the larva subsequently becomes
fixed. The ectoderm cells at an
early stage secrete the rudiments
of the cellulose test. In the FIG . 23 .- Ciavellina: Development. Later stages:
A, approximately median optical section of a larva
caudal region, this forms longi- in which the medullary canal (neurocrele) has become
tudinal dorsal and ventral flaps enclosed throughout, communicating with the exterior
only by the neuropore at the anterior end and with the
which take on the function of archenteron by the neurenteric canal; B, larva with a
distinct rudiment of the tail a nd well-formed mesoderm-
unpaired fins. layer and notochord. Letters as in preceding figure;
The medullary canal be- in addition, mes. mesoderm. (After Van Beneden and
Julin.)
comes enlarged at its anterior
end. A vesicular outgrowth from this enlarged anterior portion forms the
sense-vesic!e (sens. ves.). The posterior narrow part forms the caudal portion of
the central nervous system (spinal cord). Masses of pigment in relation to the
32 ZOOLOGY
sense-vesicle early form the rudiment of the two larval sense-organs, otocyst
(or statocyst), and eye. The part behind this presents a thickened wall with
a narrow lumen. This is known as the ganglion of the trunk.
The embryonic alimentary canal consists of two regions, a wide region
situated altogether in front of the notochord, and a narrower portion situated
behind in the region of the notochord. The wider anterior part gives rise to
the pharynx. The posterior part gives rise to the cesophagus, stomach, and
intestine. The mouth-opening is formed shortly before the escape of the
embryo from the egg. An ectodermal invagination is formed at the anterior
end, and an endodermal diverticulum from the archenteron grows out to meet it.
Then the two coalesce and the oral passage is thus formed. The stomach and

FIG. 2-t .-Ascidia: Development. Free.swimming larva of Ascidia mammillata, lateral view.
adh. adhesive papill:e; ali. alimentary canal; atr. atrial aperture; cit. gr. ciliated diverticulum,
becoming ciliated funnel; end. endostyle; eye, eye; med. nerve.cord (ganglion of trunk); nato.
notochord; oto. otocyst; sens. ves. sense.vesicle; stig. earliest stigmata. (From Korschelt and
Heider, a fter Kovalevsky.)

intestine of the larva of most species have no lumen and the organism does not
feed.
The rudiments of the heart and pericardia! cavity first appear as a hollow
outgrowth from the archenteron. This outgrowth subsequently becomes
constricted off and involuted to form a double-walled sac. The inner layer of
the sac forms the wall of the heart, while the outer layer gives rise to the wall of
the pericardium.
The first beginnings of the atrial cavity appear as a pair of invaginations
of the ectoderm which grow inwards and form a pair of pouches, each opening
on the exterior by an aperture. There is a difference of opinion as to some
points in the history of these atrial pouches. It remains uncertain to what
extent the ectoderm and endoderm respectively share in the formation of the
atrial cavity. Spaces, into the formation of which the two ectodermal diver-
ticula at least largely enter, eventually grow round the pharynx and give rise
to the atrial cavity. Perforations, which become the stigmata, and which are
primarily two in number, place the cavity of the pharynx in communication
with the surrounding space. The two openings of the atrial pouches subse-
quently coalesce to form the single permanent atrial aperture.
 PHYLUM CHORDATA 33
It will be useful now, at the cost of a little repetition, to summarise the
various characteristics of the larval ascidian at the stage when it escapes from
the egg and becomes free-swimming (Fig. 24). In general shape it bears some
resemblance to a minute tadpole, consisting of an oval trunk and a long

. . .. ··_:
- ,·
:; ·.
, ' ... .

I
ht ! erul adh
strg

FIG. 25.- Ascidia: Develop-

ment. Metamorphosis of the free-
tailed larva into the fixed ascidian.
A, stage of free-swimming larva;
B , larva recently fixed; C, older
fixed stage. adh. adhesive papillro ;
air. atrial cavity; cit. gr. c iliated cil;f!r
diverticulum, becoming ciliated
funnel; end. endostyle ; hi. h eart;
med. ganglion of trunk; n. gn.
n erve-ganglion; nolo. notochord ;
or. oral aperture ; reel. rectum;
sens. ves. sense-vesicle; slig. stig-
mata; slot. stolon; I. tail. (From
Korschelt and Heider, after See-
liger.)

laterally-compressed tail. The tail is fringed with a caudal fin. This is merely
a delicate outgrowth of the thin test covering the whole of the surface. Running
through the delicate fringe is a series of strire, presenting somewhat the appear-
ance of fin-rays. In the axis of the tail there occurs the notochord (Fig. 25,
nato.), which at this stage consists of a cylindrical cord of gelatinous substance
VOL. II, D
34 ZOOLOGY
enclosed m a layer of cells. Parallel with this runs, on the dorsal side,
the narrow caudal portion of the nerve-cord. At the sides are bands of
muscle fibres. In the trunk the nerve-cord is dilated to form the ganglion
of the trunk, and, further forwards, expands into the sense-vesicle (sens. ves.)
with the otocyst or statocyst (Fig. 24, oto.) and eye. The enteric canal is dis-
tinguishable into pharynx, cesophagus, stomach, and intestine, but is not yet
functional since the mouth is covered by the continuous test. The pharynx,
however, is already equipped with an endostyle in its ventral floor, and also
with varying numbers of stigmata which pierce its walls. The atrial cavity
has grown round the pharynx, and opens on the exterior by a single aperture
only (atr.). The heart and pericardia! cavity have been formed. The period
in which the larva remains in this free-swimming stage varies between species.
In Botryllus the free-swimming period is from 90 to 180 seconds. In other
species it may last a month. Generally it lasts only a day or hvo. Then the
larva fixes itself by the adhesive papillre and begins the retrogressive meta-
morphosis by which it attains the adult condition.
The chief changes involved in the retrogressive metamorphosis (Fig. 25)
are the increase in the number of pharyngeal stigmata ; the diminution, and
eventually the complete disappearance, of the tail with the contained notochord
and caudal part of the nerve-cord ; the disappearance of the eye and the oto-
cyst ; the dwindling of the central part of the nervous system which gives rise
both to the adult ganglion and to the neural gland ; and the gradual develop-
ment of the reproductive organs. Thus, from an active free-swimming larva,
with complex organs of special sense, and provided with a notochord and well-
developed nervous system, there is a retrogression to the fixed inert adult, in which
all the parts indicative of affinities with the vertebrates have become aborted, except
the gill-slits, endostyle, and other parts of the feeding mechanism.
In some simple ascidians, and in the composite forms in which development
takes place within the body of the parent, the metamorphosis may be consider-
ably abbreviated. But there is always (so far as is known) a tailed larva
except in the genera M olgula and Pelonaia, in which the tailed stage is lacking
and only an obscure endodermal rudiment represents the notochord.

GENERAL ORGANISATION OF ASCIDIACEA

General Features.-Among the simple ascidians there is a considerable
degree of uniformity of structure, and little need be added here to the account
given of the example. Their shape varies a good deal : it is sometimes cylin-
drical, sometimes globular, sometimes compressed. They are usually sessile
and attached by a broad base, often with root-like processes, but in other cases
(e.g. Boltenia) elevated on a longer or shorter stalk. Most are solitary; but
 PHYLUM CHORDATA 35

some (the so-called social ascidians) multiply by budding, stolons being given
off on which new zooids are developed, so that associations or colonies are
formed. However, the connection
rst
between the zooids is not close, and
their tests remain distinct and po-iph. or.r ~n I
separate. The test varies con-
siderably in consistency, being
sometimes almost gelatinous,
transparent or translucent, some-
times tough and leathery. Occas-
ionally it is hardened by encrust-
ing sand-grains or fragments of
shells, or by spicules of carbonate
of lime. Calcareous spicules may
be developed in the substance of
the mantle. The apertures always
have the same position and re-

F IG. 27.-Composite Ascidian. A colonia l

zooid. The zooids are in clumps, as seen
in a vertical section of the colony. an. anus;
at. a trium ; at'. atrium of adjoining zooid;
c/. cloaca common to the two zooids ; en d .
endostyle ; gld. digestive gland; gn . nerve -
FIG. 26. - 0 rder Pleuro- ganglion ; ht. h eart; hyp. n eural gland; lang .
gona, Family Styelidre. languet s; mant. mantle; or. ap. oral aperture ;
BotryUus. A composite ov. ovary; periph. peripharyngeal band; ph.
ascidia n, B . violaceus. or. phary nx; reef. rectum; stom. stomach; te.
oral apertures ; cl. opening t estis ; tent. tentacles; tsl . t est, or common
of common cloacal chamber. gelatinous mass; v. d. v as deferens. (After
(After Milne-Edwards.) Herdman. )

lations, varying only in their relative prominence. The pharynx varies in its
size as compared with the rest of the internal organs, and in the position which
it occupies with regard to the various parts of the alimentary canal. It varies,
too, in the number and arrangement of the stigmata. The t entacles are some-
 ZOOLOGY
times simple, sometimes compound; and the dorsal lamina may or may not be
divided up into a system of lobes or languets (Fig. 27, lang.).
Composite Ascidians.-In the composite ascidians the zooids are embedded
in a common gelatinous mass formed of their united tests. The gelatinous
colony thus formed is sometimes flat and encrusting, sometimes branched or
lobed, sometimes elevated on a longer or shorter stalk. In certain forms the
gelatinous substance is hardened by the inclusion of numerous sand-grains.
The arrangement of the zooids differs greatly. Sometimes they occur
irregularly dotted over the entire surface without exhibiting any definite
arrangement. Sometimes they are arranged in rows or regular groups. In
Botryllus (Fig. 26) they form star-shaped, radiating sets around a common
cloacal chamber into which the atrial apertures of the zooids lead, while the
oral apertures are towards their outer ends. In essential structure the zooids
(Fig. 26) of such colonies resemble the simple ascidians. At one time it was
held that solitary and composite ascidians fell into two distinct Orders. Today
it is considered that the ability to bud is primitive in ascidians and that it has
been lost independently in several lines. Likewise, the solitary condition rs
secondary and has been acquired separately in different groups.

CLASS THALIACEA
The thaliaceans are free-swimming tunicates, sometimes simple, sometimes
colonial, and never provided with a caudal appendage in the adult condition.
The body is transparent. The test is a permanent structure. The muscular
fibres of the body-wall are arranged in complete or interrupted ring-like bands,
or held diffusely. The mouth and atriopore are situated at opposite ends of the
body. The pharynx has either hvo large, or many small, stigmata leading into
an atrial cavity which communicates with the exterior by the atrial aperture.
There is usually an alternation of generations of a most remarkable kind.
There may, or may not be a tailed larval stage.
Three orders-Doliolida, Pyrosomida, and Salpida-are recognised.

ORDER DouoLIDA (CvcLOMY ARIA)

The order is restricted to a single family, Doliolida:, made up of perhaps four

genera, Doliopsis ( = Anchinia), Doliolum (Figs. 28, 29), Dolioletta, and Dolio-
loides. These hermaphrodite animals are in general found in the phytoplank-
ton zone in tropical and sub-tropical seas. The species are commonest down to
200 metres, but 'nurse' forms have been dredged from 3,000 metres. They
are rarely found in shallow coastal water, but may occur in vast numbers in
suitable areas. They have been studied rarely alive, but nevertheless a good
deal has been learnt of their life-histories. The solitary sexual individual or
gonozooid is barrel-shaped. The test is transparent and delicate, and so it is
 PHYLUM CHORDATA 37
possible to see clearly the eight characteristic muscle-bands which completely
encircle the body (hence the name Cyclomyaria). Unlike the ascidians, the
oral and atrial apertures (Fig. 29, r. ap., atr. ap.) are at opposite extremities.
The relations of the various organs have undergone a corresponding modifica-
tion. The oral aperture leads into a wide pharynx (Fig. 28, 4) occupying at
least the anterior half of the body. The posterior wall of the pharynx alone
is usually perforated by stigmata. An endostyle (n) occurs, and a
peripharyngeal band ; but there is no dorsal lamina. The heart lies immedi-
ately posterior to the endostyle in the mid-ventral line. The gut-loop, of
variable shape from genus to genus, lies in the ventra-posterior region of the
body. The nervous system is composed of a dorsal ganglion which lies in the
third or fourth inter-muscular zone (3). Fibres from the ganglion radiate to

Fie. 28.-0rder Dol-

iolida, Family Doliolidaa,
Doliolutn. Gonozoid.
I. oral aperture; 2 . cili-
ated pit; J. ganglion and
nerves; 4· pharynx; 5·
mantle; 6. sense-cells; 7·
atrial aperture; 8. ovary;
9· intestine; IO. heart; II .
endostyle; I 2. testis; I 3 .
peripharyngeal band.
(From Young, after Neu-
mann.)

the eight muscle-bands. Branches from each principal fibre terminate in groups
of sensory cells (6).
Surrounding each aperture is a series of ten or twelve lobes-the oral and
atrial lobes-in which there are sense organs. The first and last of the muscular
hoops serve as sphincters for the two orifices. Doliolum moves through the
water by jet propulsion. The contraction of the muscle-bands drives water
backwards out of the atrial sac and so propels the animal forward.
The reproductive organs-testis (r2) and ovary (8)-are in the ventral
region. The eggs are discharged into the capacious cloacal aperture and escape
into the sea-water where, if fertilised, they develop independently. Develop-
ment appears to be not unlike that of the simple ascidians in all essential re-
spects. There is total segmentation, followed by the formation of an embolic
gastrula. The larva (Fig. 29) has a tail with a notochord (nato.) consisting of
about forty cells, and a body in which the characteristic muscular bands soon
 ZOOLOGY
make their appearance. This tailed larva becomes the asexual stage or
' nurse'. By and by the tail aborts, and two processes, one postero-dorsal, the
other ventral, known respectively as the cadophore (dors. st.) and the ventral
stolon (vent. st.), grow out from the body of the larva. On the stolon are formed
a number of slight projections or buds. These become constricted off, and in

FrG. zg.- Doliolwn: De-

velopment. Late stage in the
development of the tailed
larva. atr. ap. atrial aper·
ture; dors. st. cadophore; end.
endostyle; ht. heart ; ne. gn .
nerve-ganglion; nolo. noto-
c hord; or. ap. oral aperture :
vent. st. ventral stolon. (After
U ljanin.)

the form of little groups of cells, each consisting of seven strings of cells with an
ectodermal investment, creep over the surface of the body (Fig. 30, e, and
Fig. 31) till they reach the cadophore, to which they attach themselves after
multiplying by division. The cadophore soon becomes elongated, and the
bud-like bodies attached to it develop into zooids. As the long chain of
zooids thus established is further developed, the parent Doliolum (Fig. 30)
loses its stigmata, its endostyle, and its alimentary canal. At the same time

FIG. Jo.- Doliolum: De-

velopment. Lateral view of
asexual stage, showing the
early development of the buds.
at1·. ap. atrial aperture ; dors.
st. cadophore; e. embryos
passing over the surface from
the ventral stolon to the
cadophorc ; Itt. heart; ne. gn.
nerve-ganglion ; or. ap. oral
aperture; vent. st. ventral
stolon. (After Uljanin.)

the muscle-bands increase in thickness and the nervous system attains a higher
development, until the whole parent comes to play a part like that of the necto-
calyx of a siphonophore (Vol. I). Thus, its secondary function is to propel the
colony through the water by its contractions.
The zooids of the cadophore consist of two sets, differing from one another
in position and in future history-the lateral zooids and the median zooids. The
lateral zooids serve solely to carry on the nourishment and respiration of the
colony, and do not undergo any further development. Some of the median
 PHYLUM CHORDATA 39
buds, on the other hand, become detached and take on the special character
of phorozooids. When free, each phorozooid carries with it the stalk by means
of which it was attached to the stolon. On this
stalk there have previously become attached a
pu·ic
number of buds which are destined after a time
to be developed into the sexual zooids.
The succession of stages in the life-history of
Doliolum thus briefly sketched will be seen to
succeed one another in the following order: (1)
sexual form ; (2) tailed larva developed sexually
from (I); (3) first asexual form or 'nurse', the
direct outcome of (2) ; (4) second asexual form emb

(phorozooid) developed on the cadophore of (3)

from buds originating on the ventral stolon; (5)
the young stages of the sexual form (I) which are of
similar origin to (4) and which develop on its stalk.
Berrill should be consulted for a more com-
prehensive treatment of the Order.

ORDER PYROSOMIDA

The strikingly luminescent ('phosphorescent')

and distinctive Pyrosoma was discovered in the
early nineteenth century, but although its ana-
FIG. JI.-Doliolum: Asexual
tomy and embryology have been thoroughly zooid. Dorsal view of the posterior
investigated, little is yet known of its vital pro- aspect showing the course taken
by the buds (emb.) over the surface
cesses. Pyrosoma (Fig. 32) is a colonial tunicate, from the ventral stolon (vent. stat.)
to the cadophore stol.) and
the colony consisting of numerous individuals their growth on the(dors. latter. lat. bds.
forming a gelatinous tube that is shaped like a lateral buds; med. bds. median
buds; peric. pericardium. (.-\fter
cylinder. Some pyrosomes may grow to about 4 Barrois.)
feet long with a tube width of almost a foot. They
are common in warmer seas to a depth of 500 metres ; a few are abyssal. The
internal cavity, closed at one end and open at the other, serves as a common
cloaca for all the zooids. The oral apertures of the zooids are situated at
the outer surface of the cylinder, whilst the atrial apertures (Fig. 33, atr. ap.) of
the organisms open into the common cloacal cavity. Thus each animal sieves
a tiny stream of water from the exterior sea into the lumen of the cylinder. An
extensive stream of water is shot out of the open, posterior end of the cylinder
and the whole community moves forward by jet propulsion. Velocity is in-
creased by the narrowing of the aperture by means of a diaphragm. The colour
and texture of the community vary according to species. Although only one
genus-Pyrosoma-is generally recognised, it has been suggested that some
species should be separated into a second genus Pyrostremma.
 ZOOLOGY
The Pyrosoma zooids of a given colony are seemingly quite independent of
each other. Each individual is characterised by the possession of two body-
cavities-the pharynx and the atrium, the latter with two peribranchial exten-
sions. The gut and gonads lie near the mid-line. The heart and stolon are
situated directly behind the endostyle. Musculature is weak. Branchial and
atrial sphincters are present, in addition to circumoral muscles and paired trans-
verse atrial muscles. The latter connect with the corresponding muscles of
neighbouring zooids by means of t est fibres.

alr.ap
FIG. 33--Purosoma: Colonial indivi-
duals. atr. ap. atrial aperture; or. ap. oral
aperture; ph. pharynx; proc. processes of test
FIG. 32.- 0rder Pyrosomida, Family on outer surface of colony; slot. stolon on
Pyrosomidre, Pyrosoma. Left: Luminous which are developed buds giving rise to
colony of P. spinosum. Right: end view.
(Modified after Murray and Hjort.) new zooids; tent. tentacles. (After Herdman.)

The pharynx of Pyrosoma is divided. The prebranchial buccal cavity lies

anterior to the peripharyngeal bands. The branchial sac has the endostyle
extending throughout its length. The peripharyngeal bands are continuous
with the ciliated margins of the endostyle and unite dorsally behind the
ganglion. Dorsal languets are recognisable. The cesophagus is narrow and
distinct from the stomach. A pyloric gland occurs; its relationships are
generally similar to those observed in ascidians. Numerous elongated gill-
slits are present. On each side of the pharynx, immediately over the peri-
pharyngeal bands, lies a flat mass of specialised mesodermal cells enclosed
within a blood-sinus. These are strikingly luminous. The light is produced
by symbiotic bacteria which inhabit the cells and, as the bacteria invade the
eggs, light may be produced at all stages of the life-cycle of Pyrosoma. The
 PHYLUM CHORDATA

luminous organs are not innervated. At the same time, they become active
only as the result of a stimulus and the whole colony may light up together.
In seas where Pyrosoma is especially common the light may be sufficiently
powerful to enable print to be read. The luminescence is probably a protective
device giving warning of unpalatability, and it may have other functions as well.
In Pyrosoma development is direct, without a tailed larval stage, and
takes place within the body of the parent. The ovum contains a relatively
large quantity of food-yolk, and the segmentation is meroblastic. A primary
zooid (cyathozooid) is formed. This has a stolon which constricts, giving rise to
four buds (blastozooids), from which the colony develops by further budding.
The primary zooid (cyathozooid) begins to atrophy. At this stage the young
colony, composed of four blastozooids with the remains of the cyathozooid
enclosing a mass of yolk (the whole invested in a common cellulose test),
passes out from the brood-pouch in which it was developed and reaches the
exterior through the cloaca of the parent colony.

ORDER SALPIDA (DESMOMYARIA)

These remarkable animals (Figs. 34, 35) occur in almost every sea. They
are most numerous down to 200 metres, but some penetrate to depths as remote

FIG. 34.-Salpa: Oozooid. Asexual form of S . democratica in ventral view. atr. ap. atrial aper-
t.ue ; branch. dorsal lamina; end. endostyle; ht. heart; mus. bds. muscular bands; ne. gn. n erve.
ganglion; or. ap. oral aperture; proc. processes at the posterior end; sens. org. sensory organ
(ciliated funnel and languet); stol. stolon. (After Vogt and Jung.)

as r,soo metres. They are planktonic tunicates with a transparent, prism-

shaped, orcylindrieal body reaching from 8 to rgomm. (exclusive of appendages)
according to the species. They are propelled forward by the contraction of
muscle-bands which eject water posteriorly. The best-known genus is Salpa.
Every species is found in two forms : (r) the oozooid or solitary form , which
develops from an egg, is bilaterally symmetrical, and has a budding stolon
but no gonads; and (;2) the blastozooid or aggregate form , which develops from
a bud and has gonads but no stolon. Blastozooids are connected in groups
initially, but generally become separate later in life. They are asymmetrical
in form.
 ZOOLOGY
In the oozooid both branchial and atrial apertures (Figs. 34, 35) are wide
and situated at the opposite ends of the body. The test is relatively thin and
delicate. A series of seven hoop-like muscles almost, or quite, surrounds the
body. The oral muscles, and certain others, strengthen and control the closure
of the lower li]1. This acts as a valve to shut the mouth when the body-muscles
contract to expel water backwards and to propel the animal sharply forward.
The atrial muscles act as sphincters. The endostyle and the peripharyngeal
bands control the direction of the food-entangling mucus. The entrapped
food is drawn into the resophagus along with mucus strings which stream back-
wards from all round these bands, forming a conical net. Gill slits are repre-
sented only by a pair of large apertures on either side of a single median gill-

c.f hr
'

FIG. 35.- 0rder Salpida, Family Salpidre. Salpa: Oozooid. Lateral v iew of a section-which
is sagitta l (longitudinal, vertical, a nd m edian) in the ora l two-thirds, and oblique in the atria l third.
at. a trial cavity; b1·. branchia; c. c. ciliated crests on the edge of the branchia; c. f. ciliated funnel ;
d. l. dorsal lip; &nd. endost y le ; ey. ey e ; gl. digestive gland; gn. ganglion ; ht. heart; int. intestine;
lng. languet ; mo. m outh ; oe. resophagus; m. ap. resophageal aperture; p h. pharynx; pp.
peripharyngeal band; st. (right) stolon; st. (left) stomach; v. l. ventral lip. (After Delage and
Herouard.)

bar, which is really homologous with the dorsal lamina of other forms. The
pharyngeal and atrial cavities are incompletely separated by this obliquely
running bar. Together they constitute a single large chamber. The remainder
of the alimentary canal is divisible into resophagus, st omach, and gut; the
pyloric gland stretches from the stomach over the wall of the intestine. These
form a relatively small dark mass called the ' nucleus'. Behind the endostyle,
and in the region of the fifth and sixth body-muscle, lies the heart. From
between heart and endostyle arises the stolon. In the dorso-anterior region
there occurs a horseshoe-shaped eye (ey. ), which is developed from the dorsal
surface, and, associated with this, the dorsal ganglion (Fig. 36). This single
ganglion gives off nerves to the various parts of the body. A single large
tentacle, the so-called languet (Fig. 35, lng.) , extends dorsally into the buccal
cavity in front of the peripharyngeal bands.
 PHYLUM CHORDATA 43

The solitary form described above reproduces asexually. The .stolon

(which appeared while the oozooid was still an embryo) divides into seg-
ments, each of which becomes a sexual blastozooid. Chains of new individuals

ey
FIG. 36.-Salpa: Nervous
system. Diagrammatic la-
teral view of the ganglion
and neighbouring parts. at. hr.\ap
wall of atrial cavity; b1·. "~iiliiiiiiiiiiiiieiiilii~;;;;-~~0::;:::.;;;;;;:;;;;;:~---'\
branchia; br. ap. aperture / -
of branchia; c. c. ciliated a( .
crests of branchia; c. f.
ciliated funnel; ey. eye;
n. gl. gland (paired) that
may represent neural gland;
ph. wall of pharynx. (After
Delage and Herouard .)

are thus formed. These become detached from the asexual parent, and so the
aggregate phase arises.
In each individual blastozooid only one ovum forms in the ovary and one
testis develops. The t estis matures later than the ovum and the latter is
fertilised by a sperm from the testis
of an individual of an older chain.
Development is direct and takes
place within the body of the parent.
The developing embryo projects into
the branchial cavity. The nourish-
ment of the embryo (Fig. 37) is
effected by the formation of a
diffusion placenta through which a
close union is brought about between
the vascular system of the parent
and that of the embryo. The
FIG. 37.- Salpa: Development. Late stage
placenta of Salpa is partly formed (blastozooid) showing the placental connection
from follicle-cells and ectoderm- with the parent. atr. ap. atrial aperture; br.
branchia; cit. gr. ciliated groove; ebl. el:eoblast
cells of the embryo, partly from the (mass o f tissue probably representing a vestige of
cells of the wall of the oviduct. ms. the tail) ; end. endostyle ; n. gn. nerve-ganglion;
<esophagus ; or. ap. oral aperture; peric. peri-
Segmentation is complete. During cardium; pl. placenta; reel. rectum; stol. stolon;
stom. stomach. (From Korschelt and Heider,
segmentation there is a migration after Salensky.)
inwards of some of the cells of the
follicle and of the wall of the oviduct. These enter the segmenting ovum and
pass among the blastomeres. It is not certain what part these inwardly
migrating cells play in the development of the embryos. Possibly they act
44 ZOOLOGY
merely as carriers of nourishment, and are broken up and eventually completely
absorbed.
There is no tailed larval stage. The embryo develops the muscle-bands
and all the characteristic parts of the adult while still enclosed within the
body of the parent and nourished by means of the placenta. This sexually-
developed embryo, however, does not give rise to a form exactly like the parent,
but to one which differs from the latter in certain less important features and
notably in the absence of reproductive organs. The sexually formed embryo,
in other words, gives rise to an asexual solitary oozooid which escapes to the
exterior and then becomes free-swimming. After a time there is developed
a process or stolon (stol.), on the surface of which are formed a number of bud-
like projections. These increase in size as the stolon elongates, and each
eventually assumes the form of a sexual Salpa. The chain of zooids formed on
the stolon breaks off in lengths which swim about intact while reproductive
organs develop in the individuals.

CLASS LARV ACEA (APPENDICULARIA)

These extraordinary animals are transparent, free-swimming, oceanic
tunicates with a permanent tail supported by a skeletal notochord. Most of
them are extremely small with a trunk rarely more than I mm. long, and a tail
generally not more than three times the length of the trunk. A few exceptions,
however, occur, and, in particular, the deep-water Bathochorda:us (Oiko-
pleuridre) grows to the relatively enormous length of 3 inches. The test is
represented by a relatively large temporary envelope, the 'house', which is
formed with great rapidity, as a secretion from the surface of the ectoderm, and
frequently thrown off and renewed. The pharynx has only two stigmata, and
these lead directly to the exterior. There is no atrial or peribranchial cavity.
The principal ganglion gives off a nerve-cord with ganglionic enlargements run-
ning dorsally to the end of the tail. No budding occurs; development takes
place without metamorphosis. Appendicularians are widespread in their
distribution and are said to occur in Cambrian deposits. The class contains the
single Order Copelata. This has been divided into three families : Oiko-
pleuridre, Fritillariidre, and Kowalevskaiidre.
General Features.-ln shape the Larvacea are not unlike tadpoles, with a
rounded body and a lqng tail-like appendage attached to the ventral side. At
the extremity of the body most remote from the tail is the aperture of the
mouth. This leads into a comparatively wide pharynx (Fig. 39, ph.), in the
ventral wall of which (except in Kowalevskia) is an endostyle similar to that of
the simple ascidian, but comparatively short. Round the pharynx there run
obliquely two bands covered with strong cilia-the peripharyngeal bands, which
join a median dorsal ciliated band. On the ventral side of the pharynx there
 PHYLUM CHORDATA 45

are two ciliated openings-the stigmata (stig.), which communicate with the
exterior by short passages; the atrial canals, situated on either side behind the
anus. The axis of the tail is occupied
by a cylindrical rod-the notochord or
urochord (noto.).
In Oikopleura (Fig. 38) the 'house'
is a comparatively large structure
within which the animal is enclosed.
Undulatory movements of the tail
cause a current of water to flow in
through a pair of incurrent apertures
FIG. 38.- -0rdcr Copelata, Family Oiko-
and out through a single excurrent pleuridre. OiiWIJleura. The animal is en·
aperture. The former are closed by show closed in a secreted 'house'. The arrows
the course of the current. (From Herd-
lattice-works of fine threads, prevent- man, after Fol. )
ing the passage of any but the
smallest organisms. In the interior is an elaborate apparatus for filtering out
the minute organisms from the water as it passes through. The filtered plank-
ton is sucked into the mouth. Here the processes are not unlike those occurring
in the ascidians. Endostyle and peripharyngeal bands are present and a con-
tinuous cord of mucus-entangled prey
is drawn into the stomach (Fig. 39) .
The appendicularians show consider-
able diversity in the structure of the
stomach and digestive apparatus.
The intestine (int.) ends at the anus
(an.), which opens on the median
ventral line in front of the stigmata.
In most appendicularians the
heart consists of a simple, rapidly
beating sac, but in others (Fritillaria)
FIG. 39.- 0rder Copelata, Family Oiko- it is said to be composed of two highly
pleuridre. Appendicularia. From the right
side. an . anus ; ht. hea rt ; int. intestine ; ne. specialised cells which are equipped
nerve ; ne'. caudal portion of nerves; ne. gn' . with fibrillar projections extending
principal n erve-ganglion ; ne. gn.", ne. gn."' first
two ganglia of nerve of tail; nolo. notochord; and uniting. In Kowalevskia the
ces. <esophagus; or. ap. oral aperture; oto.
otocyst (statocyst) ; peri. bd. peripharyngeal heart is absent altogether.
band; ph. pharynx; tes. testis; stig. one of the Two ganglia constitute the essent-
stigmata; stom. stomach. (After Herdman.)
ial nervous units of the body. The
anterior cerebral ganglion (Fig. 39, ne. gn'.) is situated on the dorsal side of
the mouth. A large nerve runs forward from this ganglion and divides to form
an arch around the mouth; this supplies the sensory cilia of the buccal
region. The middle part of the ganglion contains a sensory vesicle with a
contained otolith. From the anterior ganglion there runs also a second large
 ZOOLOGY
nerve which joins the (proximal) caudal ganglion (ne. gn".), situated to the left
of the root of the tail. The tail is twisted through 90°, so that its dorsal edge,
with its nerve cord, is to the left, as in the tadpole larva: of many ascidians.
The tail muscles, which consist of two rows each of ten cells, are therefore
arranged so that each of these rows is above and below the notochord (though
morphologically on its right and left sides). The muscle cells are supplied
with nerves from a corresponding number of ganglionic enlargements of the
nerve cord. This appearance of segmentation is quite different in scale from
the segmentation of higher chordates, since it involves individual cells, not
somites. The generally minute bodies of Larvacea are composed of remarkably
few cells. The notochord, for instance, is formed from about twenty cells in a
single row, as compared with about forty such cells in the ascidian tadpole
larva.
In one species, Oikopleura dioica, individuals are either male, with a single
testis, or female, with a single ovary. All other appendicularians, so far as is
known, are hermaphrodite. In the posterior part of the trunk there occur a
single ovary and a single testis or paired testes. When sexual maturity occurs
the ripe ova leave the trunk by rupture of the body-wall, after which the
animal gradually dies. The testis has a duct to the exterior. The development
of the appendicularian is very like that of the ascidians, except that the former
seem to exhibit a precocious differentiation when the various developmental
stages are compared.
Affinities.-Several views have been advanced concerning the phylogenetic
significance of the tailed appendicularian, and its relationships with the other
tunicates and the protochordates in general. Perhaps the most satisfactory
hypothesis is that the adult appendicularian, so like the ascidian tadpole, is in fact
essentially a neotenic ascidian larva. In this view it is held that the develop-
ment of sexual maturity, along with other complexities in the larval form,
would lead to the elimination of the ancestral adult phase. Subsequent to the
above pcedomorphosis, other changes may have led to the development of the
forms that are living today.
It must be stressed, however, that several authorities believe that appen-
dicularians are merely primitive tunicates and a degenerate offshoot from
vertebrate stock. A third view is that they are neotenic doliolids which have
retained the tail, lost the cloaca, and converted the test into the appendicularian
'house'.
 PHYLUM CHORDATA 47

SUB-PHYLUM ACRANIA (CEPHALOCHORDATA)

T HE Acrania consists of a single class-the Cephalochorda-which was

once held to include two families : first, the Branchiostomidce, and
secondly, the Amphioxididce. The first contains two genera, Branchia-
stoma (which is usually known by the name of one of its sub-genera, Amphioxus)
and Asymmetron. The organisms once believed to belong to the second
' family ' are actually larval individuals of the genus Asymmetron.
The Acrania are immensely important in any consideration of the compara-
tive anatomy and systematic relationships of the Chordata. Amphioxus, for
example, possesses no separate head, brain, eyes, auditory apparatus, or jaws,
yet its nervous and blood vascular systems are clearly of the same general
pattern as those which we see more highly elaborated in the fishes, amphibians,
reptiles, birds, and mammals. Certain features of the digestive system, too,
have much in common with the digestive arrangements of vertebrates. On
the other hand, the apparatus of ingestion is strikingly similar to that of the
tunicates. At the same time, Amphioxus possesses excretory mechanisms
which, in part, resemble not those of the other protochordates, nor that of the
Echinodermata, but are, instead, surprisingly akin to those of the flatworms,
Mollusca and Annelida (Vol. I). Amphioxus exhibits a well-developed noto-
chord, hollow dorsal nervous system, and gill-slits in the adult as well as in
the larval form. Although seemingly primitive and helpless, the Branchio-
stomidre are in fact very successful : they occur in suitable habitats all over the
world. On the Chinese coast at least one species is sufficiently common to be
sold in bulk in the fish-market.

EXAMPLE OF THE SUB-PHYLUM.- THE LANCELET (AMPHIOXUS)

The remarkable little Lancelet, 1 Branchiostoma lanceolatum ( = Amphioxus
lanceolatus), which ranges from the Mediterranean to the North Sea, may be
taken as a suitable type. The lancelet swims actively, but is primarily a
sand-burrowing creature, and is therefore confined to shallow waters. It is a
small, almost transparent, superficially fish-like animal, and its length does not
exceed 5·8 em., or less than 2 inches. The body is compressed, elongated,
and, as the name suggests, pointed at both ends (Figs. 40 and 41). The
anterior two-thirds of the body is roughly triangular in transverse section,
having right and left sides inclined towards one another dorsally, and a convex
ventral surface. The posterior third is nearly oval in section, the right and left
sides meeting above and below in a somewhat sharp edge. Extending along

1 The name Lancelet is the diminutive of the Devon name for the Lance or Sand-eel (Ammo-
dytes). The use of the name Amphioxus as a generic term is, strictly speaking, unjustifiable since
Branchiostoma (given by Costa in 1834) antedates Yarrell's Amphioxus by two years.
 ZOOLOGY
the whole of the dorsal border is a median longitudinal fold, the dorsal fin
(dors. f.). This is continued round the posterior end of the body and extends
forwards, as the ventral fin (vent. f.), as far as the region where the oval trans-
verse section passes into the triangular. The portion of the continuous median
fold which extends round the pointed posterior extremity of the body is some-

FIG. 40.-0rdcr Branchiostomida, Family Branchiostomidre, Branchiostoma. Amphioxus

(B. lanceolatum) generally lives buried (except for part of the buccal apparatus) in the sandy floor
of the shallows. It is sometimes found in fine shell gravel. (Redrawn after H esse, Allee, and
Schmidt.)

FIG. 41.- Amphioxus. A, ventral, B, side view of the entire a nimal. an. anus. atrp. atria-
pore; cd. f. caudal fin; cir. cirri; dors. f. dorsal fin; dors. f. r. dorsal fin-rays ; gon. gonads; mtpl.
metapleure; myom. myomeres ; nch. notochord; or. hd. oral hood; vent. f. ventral fin ; vent. f. r.
ventral fin-rays. (After Kirkaldy.)

what wider than the rest, and may be distinguished as the caudal fin (cd.f.). In
the anterior two-thirds of the body there is no median ventral fin , but at the
junction of each iateral with the ventral surface is a paired longitudinal fold, the
metapleure (mtpl.), which extends forwards to the oral hood mentioned in the
next paragraph. It is important to realise, however, that these fins present none
of the structural elaboration that exists in the fins of the true vertebrate fishes.
 PHYLUM CHORDATA 49
Below the pointed anterior extremity is a large median aperture surrounded
by a frill-like membrane, the oral hood (or. hd.), the edge of which is beset with
numerous tentacles or cirri (cir.). The oral hood encloses a cup-shaped cavity
or vestibule, which communicates with the mouth (Fig. 43, mth.). On the wall of
the oral hood is a specially modified tract of the epithelium divided into fin&er-
shaped lobes. The cells of this tract, which is known as the wheel-organ, are
provided with long cilia. The metachronal waves of the cilia reminded
observers of the 'wheel organ' of rotifers, hence the name used. The move-
ments of these cilia collect food particles which fall out of the main stream.
Along the roof of the vestibule runs a ciliated groove-the groove of Hatschek.
Immediately in front of the anterior termination of the ventral fin, and partly
enclosed by the metapleures, is a rounded aperture of considerable size, the
atriopore (atrp.). A short distance from the posterior extremity of the body is
the anus (an.), placed asymmetrically on the left side of the central fin. The
post-anal portion of the body is distinguished as the tail.
Amphioxus ordinarily lives with the greater part of the body buried in sand,
only the anterior end with the expanded oral hood protruding. It also swims
quickly but erratically, and frequently lies on one side on the sand. It burrows
head or tail foremost, with great rapidity. A current of water, used in feeding
and respiration, is constantly passing in at the mouth and out at the atriopore.
Body-wall.-The body is covered with an epidermis (Fig. 42) formed of a
single layer of columnar epithelial cells, some of which are specialised as
nerve cells while some are unicellular glands. On the surface of the epidermis
is a cuticle perforated by pores. The epithelium of the buccal cirri presents
at intervals regular groups of sensory cells, some of them bearing stiff sensory
hair-like structures, others cilia. Beneath the epidermis is the dermis, formed
mainly of connective-tissue.
The muscular layer (my., myom.) is remarkable for exhibiting metameric
segmentation. It consists of a large number-about sixty-of muscle-segments
or myomeres, separated from one another by partitions of dense connective-
tissue, the myocommas, and having the appearance, in a surface view, of a series
of very open V's with their apices directed forwards (Figs. 41 and 43). Each
myomere is composed of numerous fiat, striated muscle-plates, arranged longi-
tudinally, so that each is attached to two successive myocommas. By means
of this arrangement the body can be bent from side to side with great rapidity.
The myomeres of the right and left sides of the body are not opposite to one
another, but have an alternate arrangement. A special set of transverse muscles
(Fig. 44 mt.) extends across the ventral surface of the anterior two-thirds of
the body, lying in the floor of the atrial cavity (see below).
Thus in the Lancelet we see for the first time in chordates a clear-cut body
segmentation and the consequent ability to swim actively by lateral body
flexion effected by the serial contraction of muscle blocks operating
VOL. II. E
so ZOOLOGY
synchronously under the control of a relatively elaborate central nervous system
(see p. 57). Another striking and characteristic feature of the muscular layer
of the body-wall is the immense thickness of its dorsal portion. In the higher
worms and many other invertebrate animals the muscles form a layer of
approximately equal thickness surrounding the body-cavity, which contains,
among other organs, the central nervous system. In vertebrates, on the other
hand, the dorsal body-wall is greatly thickened, and in it are contained both the
nervous system and the notochord.

FIG. 42.-Amphioxus: Visceral relationships. A, Pharyngeal region. a. dorsal aort.e; b.

atrium; c. notochord; co. ccelom; e. endostyle; g. gonad; kb. branchiallamell;e; kd. pharynx;
I. liver; my. myomere; n. nephridium; r. nerve cord; sn . spinal nerves; B, Intestinal region.
atr. atrium; ael. ccelom; d. ao. dorsal aorta. int. intestine; myom. myomere; nch. notochord;
nett . nerve-cord; s. int. v. sub-intestinal vein. (A, from Hertwig, after Lankester and Boveri;
B, partly after Rolph.)

Skeleton.-The chief of the skeletal or supporting structures of the Lancelet

is the notochord (Figs. 42 and 43, c., nch.), a cylindrical rod, tapering at both
ends, and extending along the whole length of the body in the median plane.
It lies immediately above the enteric tract and between the right and left
myomeres. It is composed of a peculiar form of cellular tissue known as
notochordal tissue, formed of large vacuolated cells extending from side to side
of the notochord, and having the nuclei confined to its dorsal and ventral
regions. Around these cells is a structureless layer, secreted by the cells,
enclosed in a notochordal sheath of connective-tissue, which is produced dorsally
into an investment for the canal enclosing the central nervous system. The
notochord, like the parenchyma of plants, owes its resistant character to the
vacuoles of its component cells being tensely filled with fluid. This produces a
condition of turgescence. The notochord has not the function of most endo-
skeletal structures, i.e., to act as a strut against the pull of voluntary muscles.
PHYLUM CHORDATA sr
52 ZOOLOGY
The mouth leads into the largest section of the enteric canal, the pharynx
(ph.), a high, compressed chamber extending through the anterior half of the
body. Its walls are perforated by more than a hundred pairs of narrow oblique
clefts, the gill-slits or branchial apertures (br. cl.), which place the cavity of the
pharynx in communication with the atrium (see below). The posterior end of
the pharynx leads to the tubular intestine (int.), which extends backwards
almost in a straight line to the anus.
On the ventral wall of the pharynx is a longitudinal groove, the endostyle
(Fig. 42, A, e.), lined by ciliated epithelium containing groups of gland-cells.
This is homologous with the endostyle in the ascidians. A somewhat similar
structure, the epipharyngeal groove, extends along the dorsal aspect of the
pharynx. Its sides are formed by ciliated cells, which, at the anterior end of
the groove, curve downwards, as the peripharyngeal bands, and join the anterior
end of the endostyle.
Just behind the anterior end of the intestine there arises ventrally a blind
pouch, the mid-gut diverticulum, the so-called liver (Fig. 43, lr.) extending for-
wards to the right of the pharynx. This is lined with glandular epithelium and
secretes digestive enzymes (a lipase and a protease) which are carried into
the lumen of the mid-gut by ciliary activity. It is also a place where fat is
stored.
The gill-slits (br. cl.) are long, narrow clefts, which slope antero-dorsally, so
obliquely that many always appear in a single transverse section. The clefts
are more numerous than the myomeres in the adult, but correspond in number
with them in the larva. Hence they are fundamentally metameric, but under-
go an increase in number ~s growth proceeds.
The branchiallamellte (Fig. 43, br. sep., Fig. 42, A, kb.). or portions of the
pharyngeal wall separating the clefts from one another, are covered by an
epithelium which is for the most part endodermal in origin, and is composed of
greatly elongated and ciliated cells. On the outer face of each lamella, however,
the cells are shorter and not ciliated. These are actually portions of the
epithelial lining of the atrium, and of ectodermal origin. Each lamella is
supported towards its outer edge by one of the branchial rods (Fig. 43, br. r.).
These are narrow bars united with one another dorsally by loops, but ending
below in free extremities which are alternately simple and forked.
The forked bars are the primary rods (br. r. r). Those with simple ends are
the secondary (br. r. 2) branchial rods. The lamellce in which they are contained
are similarly to be distinguished as primary lamellte (br. sep. r) and secondary
or tongue-lamellte (br. sep. 2). In the young condition, the .two clefts between
any two primary lamellce are represented by a single aperture. As develop-
ment proceeds, a downgrowth takes place from the dorsal edge of the aperture,
forming, as in Balanoglossus, a tongue which extends downwards, dividing the
original cleft into two, and itself becoming a secondary lamella. A further
 PHYLUM CHORDATA 53
complication is produced by the formation of transverse branchial junctions or
synapticulce, supported by rods connecting the primary septa with one another
at tolerably regular intervals.
Atrium.-The branchial clefts lead into a wide atrium which is a chamber
occupying most of the space between the body-wall and the pharynx (Figs. 42,
B, and 43, atr.). It is crescentic in section, surrounding the ventral and lateral
regions of the pharynx, but not its dorsal portion. It ends blindly in front. It
opens externally, behind the level of the pharynx, by the atriopore (atrp.).
It is continued backwards by a blind, pouch-like extension (atr.) lying to the
right of the intestine (Fig. 42, B, atr.). The whole cavity is lined by an atrial
epithelium of ectodermal origin.
Ingestion, Digestion, and Absorption.-In broad outline these take place as
follows : The cirri are turned inwards and form a grating when feeding, exclud-
ing sand. At the same time a stream of water, with its contained organic food
substances, is sucked into the mouth. This current, as in protochordates
generally, is principally due to the activity of the lateral cilia which line the
gill-slits. These beat outwards into the atrial cavity. The cilia of the wheel-
organ beat at right angles to its winding course, in the general direction of the
mouth. They cause fine particles, which fall out of the main stream entering
the mouth, to collect in shallow grooves between the lobes of the organ. Thence
these particles are gradually drawn into the mouth, wrapped in mucus. Within
the pharynx sheets of mucus (produced by gland cells in the ventral endostyle)
are driven posterodorsally across the gill-slits by the frontal cilia. These lie
on the surfaces of the gill-bars facing the pharyngeal lumen. The dorsal
movement of the mucus is assisted anteriorly by the cilia of the peripharyngeal
bands. As the excurrent water passes through the gill-slits, particles of food
which remain suspended in it are entrapped by the mucus. The resultant
mixture of slime and nourishment is directed into the epipharyngeal groove.
The cilia there beat posteriorly, conveying the food-cord into the intestine.
Digestion begins in the mid-gut, where the food is mixed, by ciliary activity,
with carbohydrate-, fat-, and protein-splitting enzymes. These are secreted
by the epithelium of the diverticulum and mid-gut. Food does not normally
enter the diverticulum. Absorption takes place in the hind-gut, and to a lesser
degree in the hinder part of the mid-gut. It is also possible that some absorp-
tion occurs in the diverticulum. There is evidence that intracellular digestion
also occurs at least in the hind-gut (Barrington). Most of the digestion occurs
ap"parently after the food and mucus cord has passed the ilio-colon ring.
This is a powerfully ciliated region (at the hind end of the mid-gut) which
imparts a rotary movement to the food-cord as it passes into the hind-gut, and
where the secretions of the mid-gut are admixed. The mechanical agitation
seems to be transmitted to the part of the cord still in the mid-gut, and so
possibly facilitates the action of enzymes in that area as well.
54 ZOOLOGY
Cmlom.-The relationship of the crelom with the peripheral structures is
easily grasped if we think of the whole body essentially as two tubes, firstly, an
outer ectodermal layer, and
fk secondly, an inner endodermal
alimentary canal. The space
between the two is the crelom,
and it is lined by crelomic
epithelium of mesodermal
ongm. During development
this simplicity can be seen
much more clearly. Owing
to the immense size of the
atrium, the crelomic body-
cu
cavity is much reduced. It is
represented, in the pharyngeal
-!{ region, by paired cavities (Fig.
42, A, co., Fig. 43, ccel., Fig.
44, sc.) lying one on either side
of the dorsal region of the
pharynx above the atrium,
and connected by narrow
canals in the primary branch-
ial lamellre (Fig 44, right side)
with a median longitudinal
oj---· space below the endostyle
+ .· (ec.). In the intestinal region
-rru. -· it entirely surrounds the intes-
tine, but is much reduced on
the right side, being displaced
FrG. 44.-Amphioxus: Pharyngeal region. Trans- by the backward extension
verse section, passing on the right through a primary, on
the left through a secondary branchial lamella. ao. dorsal of the atrium (Fig. 42, B, atr.,
aorta; c. dermis; ec. endostylar portion of ccelom; f.
fascia or investing layer of myomere; jh. compartment Fig. 43, atr'.). On the left side
containing fin-ray; g. gonad; gl. glomerulus (modified part a forward extension of it sur-
of branchial artery in relation to nephridium); k. branchial
artery; kd. pharynx; ld. combined atrial and ccelomic wall rounds the liver (Fig. 42, A, l.).
(ligamentum denticulatum); m. myomere; mt. transverse
muscle; n. nephridium; of. metapleural lymph-space; Separate cavities lie in the
p. atrium; sc. ccelom; si. ventral aorta; sk. sheath of metapleures.
notochord and nerve-cord; uf. spaces in ventral wall.
(From Korschelt and Heider, after Boveri and Hatschek.) Blood-vascular System.-
Although Amphioxus does not
possess a heart, its circulatory system is in general outline strikingly similar
to those that we see in increasing complexity in the vertebrate series.
Although the principal arteries have muscular walls, and the dorsal aorta has
an endothelial lining, most blood-vessels are very similar but, O\ving to certain
 PHYLUM CHORDATA 55
homologies with the more complex vessels of the Craniata, some of them are
called arteries and some veins.
Lying in the ventral wall of the pharynx, below the endostyle, is a median
longitudinal vessel, the ventral aorta (Fig. 44, si., Fig. 45, v. ao.). This is
contractile, and drives the blood forwards. From it are given off, on each side,
lateral branches, the afferent branchial arteries (Fig. 44, k. ; Fig. 45, aj. br. a.),
with small contractile dilations at their bases. These force blood through the
vessels of the branchial lamellre which communicate by cross-branches with
similar vessels (aj. br. a'.) in the secondary or tongue-lamellce. The blood is
exposed to what is probably a respiratory current while traversing these vessels.
The blood, however, carries no respiratory pigment and no proof of oxygenation
in the present situation has yet been obtained. The blood leaves the branchial
lamellre dorsally by efferent branchial arteries (ef. br. a.). These open on each
side into paired longitudinal vessels, the right and left dorsal aortm (d. ao.), lying

l
L; ".,. . .,.,,_.
ph
ofbr.a • ......,.
q/:hr.a ·u.ao €if.hr. a

FIG. 45.-Amphioxus: Blood vascular system. af. br. a. primary afferent branchial vessels;
af br. a'. secondary afferent branchial vessels; br. cl. branchial cleft; cp. intestinal capillaries;
d . ao. paired dorsal aort<E; d. ao' . m edian dorsal aorta; e. f br. a. efferent branchial arteries ; hep .
port. v. hepatic portal vein; hep . v. hepatic vein; int. intestine ; lr. liver ; p h. pharynx; s. int. v.
sub-intestinal vein; v. ao. ventral aorta.

one on either side of the epipharyngeal groove. Anteriorly both dorsal aortre
are continued forwards to the region of the snout. The right aorta is much
dilated. Posteriorly they unite with one another, behind the level of the
pharynx, into an unpaired dorsal aorta (d. ao'.). This extends backwards in the
middle line, immediately below the notochord and above the intestine.
The unpaired dorsal aorta sends off branches to the intestine. In the
intestinal walls, these break up to form a plexus of microscopic capillary-like
vessels. From these the blood is collected and poured into a median longi-
tudinal vessel, the sub-intestinal vein (Figs. 42, B, and 45, s. int. v.), lying
beneath the intestine. In this trunk the blood flows forwards, and, at the origin
of the 'liver', passes into a hepatic portal vein (hep. port. v.), which extends along
the ventral side of the 'liver' and breaks up into minute vessels in that organ.
From the 'liver' the blood makes its way into a hepatic vein (hep. v.), which
extends along the dorsal aspect of the digestive gland, and, turning downwards
and forwards, joins the posterior end of the ventral aorta.
It will be seen that the vascular system of Amphioxus consists essentially
VOL. II. E *
 ZOOLOGY
of (a) a dorsal vessel represented by the paired and unpaired dorsal aortre,
(b) a ventral vessel represented by the sub-intestinal vein and the ventral aorta,
and (c) commissural vessels represented by the afferent and efferent branchial
arteries and the intestinal capillaries. So far the resemblance to the vascular
system of annelids is fairly close, but two important differences are to be
noted. The blood in the ventral vessel travels forwards, whereas that in the
dorsal vessel travels backwards-the precise opposite of what occurs in worms.
The ventral vessel is broken up, as it were, into two parts, by the interposition in
its course of the minute vessels
of the 'liver'. Thus, all the
blood from the intestine has to
pass through that organ before
reaching the ventral aorta.
This passage of the intestinal
blood supply through the ves-
sels of the 'liver' constitutes
what is called the hepatic portal
system, and is eminently charac-
teristic of vertebrates (pp. II2,
245).
The blood is colourless, and
appears to contain no leucocy-
tes. It is not confined to the
true blood-vessels just de-
scribed, but occurs also in cer-
tain cavities or lymph-spaces.
The most important of these
are the cavities in the dorsal
and ventral fins containing the
FIG. 46.-Amphioxus: Nephridium. Of the left side fin-rays (Fig. 44,Jh.), and paired
with part of the wall of the pharynx. (From Willey,
after Boveri.) canals in the metapleures (of.).
It has been suggested that a
considerable proportion of the meagre amount of oxygen required by Amphioxus
may be taken into the blood-stream through these superficial areas.
Excretory Organs.-The excretory system of Amphioxus is of outstanding
interest in that, unlike the blood-vascular and nervous systems, it shows no
similarity whatever with that of the Craniata. Neither has it any affinity with
that of the Echinodermata.
The principal organs of excretion are about ninety pairs of peculiarly
modified nephridia (Fig. 43, nph.) situated above the pharynx and in relation
with the main crelomic cavities. Each nephridium (Fig. 46) is a bent tube
consisting of an anterior vertical and a posterior horizontal limb. The vertical
 PHYLUM CHORDATA 57
limb ends in a large group of solenocytes (flame-cells) (Fig. 47) . There are several
smaller groups on the horizontal limb. The organ thus closely corresponds to
the type of nephridium with closed inner end bearing solenocytes such as
occur in certain of the Polychceta and other invertebrate groups. On the
ventral surface of the horizontal limb, opposite a secondary branchial lamella,
is a single aperture bearing long cilia and opening into the atrium. This
corresponds with the nephridiopore or
external aperture of the typical nephri-
dium. Through this aperture to the
atrium are driven the waste nitrogenous
products that have been taken up by the
solenocytes. Goodrich has calculated
that, in a specimen containing some soo
solenocytes about 50ft long in each of its
perhaps 200 nephroids, the total excretory
length would be no less than 5 metres.
(The entire animal is less than 5 centi-
metres long.)
An excretory function has also been
assigned to a single pair of organs called
the brown funnels (Fig. 43, br. f.) , also
situated on the dorsal aspect of the
pharynx at its posterior end. Their wide
backwardly-directed ends open into the
atrium; their narrow anterior ends have
been said to communicate with the crelom.
Nervous System.- The central nervous
system of Amphioxus consists of a dorsal
nerve cord (Fig. 42, A, r; B, neu. Fig. 43,
sp. cd.) lying in the neural canal imme-
diate! y above the notochord. It is rough! y Showing FIG. 47.- Amphioxus: Solenocytes.
nuclei, long flagella and the
triangular in transverse section. An- open'ngs into the main excretory canal
which leads to the atrium. (From Young,
t eriorly it ends abruptly, some distance after Goodric h .)
behind the anterior end of the notochord.
Posteriorly it tapers to a point over the hinder end of the latter. It is traversed
by an axial cavity, the neuroccele (Fig. 43, cent. c.), connected with the mid-
dorsal region by a longitudinal cleft-the dorsal fissure. At the fore-end of the
nerve-tube the neurocrele becomes dilated, forming a considerable cavity, the
encephalocccle or cerebral ventricle (Fig. 43, en. ccc., Fig. 48, c.v.). A little behind
this the dorsal fissure widens out above to form a trough-like dorsal dilatation
(dil.) covered only by the delicate connective-tissue sheath which invests the
whole nerve-tube. The anterior end of the nerve-cord, containing these two
58 ZOOLOGY
cavities, may be considered as a primitive brain although it is not distinguish-
able externally from the remaining portion or spinal cord.
The anterior and dorsal region of the brain is produced into a small, hollow-
pointed pouch which comes into relation with the olfactory organ and is called
the median olfactory lobe. Embedded in the posterior and ventral walls of the
cerebral ventricle is a structure composed of small cells, the infundibular organ.
These cells are ciliated, and the organ is probably one of special sense, but this
has not been proved. In the young animal the cerebral ventricle opens up by
a neuropore (Fig. 48, B, np.) which subsequently closes.

0
FrG. 48.- Amphioxus:
Nervous system. A. brain
and cerebral nerves of a
young specimen; B, trans-
verse section through neuro-
pore; C, behind cerebral ven-
tricle; D , through dorsal
dilatation. ch. notochord ;
c. v. cerebral ventricle; dil.
dorsal dilatation; e. pigment-
spot; np. neuropore; olf.
Kiilliker's pit; I, II, cerebral
nerves. (From Willey, after
Hatschek.)

The nerve-cord is mainly composed of longitudinal nerve-fibres with

abundant nerve-cells mostly grouped around the neuroccele. At intervals
giant nerve-cells occur. These multipolar cells of immense proportional size are
connected with nerve-fibres of unusual thickness, termed giant fibres. The
peripheral nervous system is simple, but is on the same fundamental pattern
that occurs in the Craniata. From the dorsal nerve-cord are given off a dorsal
and a ventral nerve-root on each side to every segment. The roots are not
united. Each ventral root carries motor-fibres to a myotome. These motor-
fibres t erminate on muscle-fibres at motor end-plates like those of vertebrates.
The dorsal roots, on the other hand, pass between the myotomes. They
carry sensory fibres from the respective segment, and motor-fibres to the non-
myotomal muscles of the ventral sections of the body. Some dorsal roots carry
fibres which supply the gut-wall where, similar to the vertebrates, a nerve-
plexus occurs. Two pairs of dorsal roots-the cerebral nerves (Fig. 48, I and II)
 PHYLUM CHORDATA 59

-arise from the brain in front of the first myomere. The first comes from
the anterior extremity and the second from its dorsal region. These bring
impulses from the tentacles and sense-organs of the oral hood. There
are no corresponding ventral roots in the cephalic region. The walls of
the atrium, both ventral and parietal, contain a system of neurones
which, lying in or just below the epithelium, have connexions with the
central nervous system and may be of sensory as well as motor significance.
The presumed sensory fibres which in various regions appear to contribute
to the dorsal spinal roots are never aggregated into ganglia as in verte-
brates.
Organs of Special Sense.-At the level of the anterior end of the brain is a
narrow ciliated depression called Kolliker's pit (Figs. 43, 48). This opens
externally on the left side of the snout and was once thought to be an organ of
smell. It is not innervated. In the larva its cavity is in direct communication
with the neurocrele through the neuropore (np.).
An unpaired pigment-spot (e.) in the front wall of the brain is usually re-
ferred to as a median cerebral eye. However, there is no lens or other
accessory apparatus, and experimental evidence seems to show that this so-
called eye is not sensitive to light. In fact, it has been suggested that, in
this animal which lies with its anterior end protruding from the sand, the
function of the pigmented area is actually to prevent light from penetrating
vertically to the undoubted photoreceptors which occur further down the
nerve-cord.
There is no trace of auditory organs. The groove of Hatschek, on the roof
of the buccal cavity, has had sensory functions ascribed to it, but no proof has
been forthcoming. Lastly, the sensory cells on the buccal cirri probably give
these organs important tactile and perhaps other functions.
Reproductive System.-The sexes are separate, but apart from the repro-
ductive organs there is no difference between male and female. The gonads
(Figs. 42A, Fig. 43, gon., and 44, g.) occur as about twenty-six pairs of pouches
arranged metamerically along the body-wall. They project into the atrium
and largely fill its cavity. The inner or mesial face of each pouch is covered
by atrial epithelium which is pushed inwards by the growth of the gonads.
Within this, and completely surrounding the reproductive organ, is a single
layer of epithelium which is shown by development to be crelomic. Hence each
gonad is surrounded by a closed crelomic sac.
When ripe, the inner walls of the gonadal pouches burst. The ova or
sperms then make their way into the atrium and are thence (according to both
Kowalevsky and Hatschek) emitted through the mouth to the external water.
The laid eggs are covered by a thin vitelline membrane, to which a second, inner
peri-vitelline membrane is added, the substance of which is derived from droplets
in the protoplasm.
6o ZOOLOGY
Development.-After maturation (Fig. 49. A) and fertilisation in the
surrounding sea-water, the membranes separate from the developing egg, leav-
ing a wide space around it. Segmentation is complete, there being very little
yolk. It begins by a sagittal cleft dividing the egg into two (B), and is followed
by a second cleft, also meridional, at right angles to the first (D). Next, an
B
A D

F G
E

H K

FIG. 49.-Amphioxus: Development. Segmentation of the oosperm. D, the four-celled

stage from above; G, vertical section of H; I<, vertical section of the blastula stage / . (From
Korschelt and Heider, after Hatschek.)

approximately equatorial cleavage takes place, and so the embryo now consists
of eight cells (E), of which the four belonging to the upper hemisphere, distin-
guished by the presence of the polar bodies, are smaller than the lower four.
Apertures at the poles lead into a central cavity. Further meridional and
latitudinal divisions take place, and the embryo becomes a blasfttla (I, K). It
now encloses a spacious blastocrele. The cells on its lower pole (megameres) are
larger than the rest (micromeres). The polar apertures disappear owing to the
closer approximation of the cells.
 PHYLUM CHORDATA 6r

Invagination then takes place (Fig. so, A), due to the rapid multiplication
of the micromeres. The lower pole of the blastula is gradually pushed in until
the whole lower hemisphere is in complete contact with the upper hemisphere
and the blastoccele is obliterated (B). The gastrula thus formed is at first
basin-shaped, having a very wide blastopore, but its cavity (the archenteron)
gradually deepens, and the blastopore is reduced to a comparatively narrow
aperture (C). At the same time the aspects of the body are marked out. The
dorsal surface becomes flattened, the ventral convex. The blastopore marks
the posterior end and is distinctly dorsal in position. Cilia are developed from
the ectoderm cells and their vibration causes the embryo to rotate within its
membrane.
The ectoderm cells forming the median portion of the flattened dorsal
surface now become differentiated and sink below the rest, giving rise to the

A 8

FIG. 50.-Amphioxus: Development. Three stages in the formation of the gastrula. (From
Korschelt and Heider, after Hatschek.)

medullary plate (Fig. sr, A, mp.). The ordinary ectoderm cells on each side
of this plate rise up as a pair of longitudinal medullary folds (hb.), extend towards
the middle line, and unite (B, hb.) to cover the medullary plate. The latter
bends upwards at the sides and becomes trough-like instead of flat (C). The
two sides come in contact with one another above and so the plate is converted
into a tube, the nerve-cord (D, n.). This encloses a central canal, the neurocrele,
which continues dorsally into a narrow cleft. The medullary folds extend
behind the blastopore and unite: the result is that the blastopore now opens
into the neuroccele and becomes known as the neurenteric canal (Fig. 52, A, en.).
Anteriorly the folds remain apart up to a late period, so that the neuroccele
opens externally in front by a wide aperture, the neuropore (Figs. 53, and 54,
np.).
While the central nervous system is thus being formed, the wall of the
archenteron develops dorso-laterally a pair of longitudinal folds (* in Fig. SI,
A and B). The cavities in these, continuous with the archenteron, are the
beginnings of the enteroccelic system. Transverse folds appear and divide
the longitudinal folds into segments, with the result that the archenteron
comes to have appended to it dorso-laterally a paired series of offshoots, the
62 ZOOLOGY
enterocmlic or cmlomic pouches (Fig. 51, mk.), arranged metamerically. In this
way segmentation is established. At this period the embryo ruptures its
containing membrane and begins free existence. Before long the crelomic
pouches separate from the archenteron and take on the form of a series of closed
cmlomic sacs or somites (Fig. sr, C, D), lying between ectoderm and endoderm.
From the walls of these sacs the mesoderm is derived. Their cavities unite and

A B
hb
mp
ch

ml1

ak
ill -ik

c
D

FIG. 51.-Amphioxus: Development. Four stages in the development of the notochord,

nervous system, and mesoderm. ak. ectoderm; ch. notochord; dh. cavity of archenteron; hb.
ridge of ectoderm growing over m edullary plate; ik. endoderm; lh. ccelom; mk. ccelomic pouch;
mk 1 • parietal layer of mesoderm; mk2 • visceral layer ; mp. medullary plate; n . nerve-cord; ns.
mesoblastic somite; * = archent eron. (From Korschelt and Heider , after Hatschek.)

become the crelom. This is therefore an enterocmle like that of Sagitta and the
Echinodermata.
While the crelomic sacs are in course of formation a median groove appears
along the dorsal wall of the archenteron (Fig. sr, B, C, ch.). It deepens, its
inner walls unite, and it becomes a solid rod, the notochord (D, ch.), lying
immediately beneath the nerve-tube. The ordinary endoderm cells soon unite
beneath it and so shut it off from the archenteron. It will be seen that the noto-
chord, like the nerve-cord, never exhibits any trace of segmentation. At its
first formation it stops short of the anterior end of the archenteron: its final
extension to the end of the snout is a subsequent process.
 PHYLUM CHORDATA

New crelomic pouches are formed in regular order from before backwards,
the embryo at the same time elongating and becoming laterally compressed
and pointed fore and aft. At the anterior end the mouth (Fig. 53, m.) appears
v v

FIG. 52.- Amphioxus: De-

velopment. Embryo. A,
from the side; B, in horizon-
tal section. ak. ectoderm;
en. neurenteric canal; dk.
archenteron; ik. endoderm;
mk. mesodermal folds; n.
neuraltube; ud.archenteron ;
us. first crelomic pouch; ush.
crelomic cavity; V. anterior;
H. posterior end. (From
Korschelt and Heider, after
Hatschek.)

H If

FIG. 53.- Amp:!lioxus: Development. A, young larva ; B , anterior end more highly magnified.
c. provisional tail-fin; ch. notochord; en. neurenteric canal; d. enteric canal; h. crelom of head;
k . club-shaped gland; k'. its external aperture; ks. first gill-slit; m. mouth; mr. nerve-cord; n. p.
neuropore; sv. sub-intestinal vein; w. pre-oral pit. (From Korschelt and Heider, after Hatschek.)

on the left side of the body as a small aperture, which soon increases greatly
in size. On the ventral surface another small aperture, the first gill-slit (ks.),
makes its appearance, and soon shifts over to the ,right side. Here it forms a
direct communication between the pharynx and the exterior, like the stigmata
 ZOOLOGY
of the Appendicularia (Larvacea). There is at
present no trace of the atrium.
The anterior end of the alimentary canal has
meanwhile grown out into a pair of pouches, which
become shut off as closed sacs. Of these, the right
gives rise to the ccelom of the head (h.), and the left
to a depression called the preoral pit (w.), which
opens on the exterior and from which the groove
of Hatschek and the wheel-organ are afterwards
formed. The preoral pit also gives rise to Hatschek's
nephridium (Fig. 54, x.), a narrow ciliated tube
which opens into the anterior part of the pharynx,
and runs forwards to terminate blindly in the roof
of the oral hood. It disappears completely in the
t.):S"'~ adult.
·- ;:l :>.
bo~ On the floor of the archenteron in the neigh-
~ .::::
P-.8::::
bourhood of the mouth a depression appears and
2.- =
"'i::-o
ol gives rise to a structure known as the club-shaped
;:i e:! ~
gland (k.), which may be a modified gill-cleft.
·~ -e ~
<::! ;:l "'

Posteriorly the neurenteric canal closes and the

=
~ ~~
ol ol
~!ii ....l anus appears.
lit= ...
~~ !! We left the mesoderm in the form of separate
~
~~ paired somites, arranged metamerically in the dorsal
Q) 1-4..
> (/)
... ..,""0 region of the embryo. These increase in size, and
Cl)

~ - ~·a
--g~ :r: extend both upwards and downwards, each present-
il.;;11
ol ~ ol
ing a somatic layer (Fig. 51, D, mk. 1 ) in contact with
~E;t!
< bD the
C)
external ectoderm, and a splanchnic layer (mk. 2 )
'0~ in contact with the nervous system and notochord
....:~~
iii~ dorsally, and with the enteric canal ventrally. At
~..6
0;:> 0
a about the level of the ventral surface of the noto-
G3 U~
go; . ~
chord a horizontal partition is formed in each
~""' . ui~
ccelomic pouch (Fig. 51, D , C), separating it into a
dorsal and ventral portion. The dorsal section is
•• 11)+-'
fiJI ....... · -
::1 ;:...-;;;
.s~;; distinguished as the protovertebra or myotome (ns.),
Q, = -
.cl ""0 bD
8"'7 and its cavity as the myoccele or muscle-cavity. The
<o~ ventral section is called the lateral plate mesoderm
I_..,·-
...t-=E or splanchnotome, and its cavity forms a segment
.,., s-::!""0
0~ ·- "'O "t:: of the ccelom.
~ ~g. The lateral plates now unite with one another
<;:; = in pairs below the enteric canal, their cavities be-
coming continuous. At the same time the cavities of successive lateral plates
are placed in communication with one another by the absorption of their ad-
 PHYLUM CHORDATA

jacent (anterior and posterior) walls. In this way the cavities of the entire
series of ventral plates, right and left, unite to form the single unsegmented
ccelom of the adult, their walls giving rise to the ccelomic epithelium.
At the same time the cells of the splanchnic layer of the protovertebrre
become converted into muscular fibres, which nearly fill the myoccele, and give
rise to the myomere. The myocommas arise from the adjacent anterior and
posterior walls of the protovertebrre. An outpushing of the splanchnic layer,
at about the level of the ventral surface of the notochord, grows upwards
between the myomere externally and the notochord and nerve-tube internally.
From the cells lining this pouch the connective-tissue sheath of the notochord
and neural canal arises and perhaps also the fin-rays. From the parietal layer
of the protovertebrre is formed the dermis or connective-tissue layer of the
skin.

/
ap
/

F1c. 55.- Amphioxus: Development. Ventra l aspect of three larv<e showing the development
of the atrium. ap. atriopore ; k . gill-slit s; lf. left metapleural fold; m. mouth; 1j. right meta.
pleural fold; w. pre-oral p it. (From Korschelt and H eider, after Lankester and Willey.)

The larva increases in size, and becomes very long and narrow, with a
pointed anterior end and a provisional caudal fin posteriorly (Fig. 53, c). As
growth proceeds, new segments are added behind those already formed. The
notochord grows forwards to the anterior end of the snout, and the eye-spot
(au.) and olfactory pit appear. The latter is an ectodermal pit which com-
municates with the neuroccele by the still open neuropore (np.). The mouth
(m.) attains a relatively immense size and remains on the left side.
Additional gill-slits arise behind the one already mentioned. They all make
their appearance near the middle ventral line, and gradually shift over to the
right side. At first they correspond with the myomeres, so that the segmenta-
tion of the pharynx is part of the general metamerism of the body. Altogether,
fourteen clefts are produced in a single longitudinal series. Above, i.e. dorsal
to them, a second longitudinal series makes its appearance, containing eight
clefts, so that at this stage there are two parallel rows of gill-slits on the right
side of the body, and none on the left. But as growth proceeds, the first or
66 ZOOLOGY
ventral series gradually travels over to the left side, producing a symmetrical
arrangement, and at the same time the first slit and t he last five of the first or
definitely left series close up and disappear. Thus t he numbers are equalised
on the two sides. At first each gill-slit is simple, but before long a fold grows
down from its dorsal edge, and, extending ventrally, divides the single aperture
into two. This fold is the secondary or tongue-lamella. The original bars of
tissue between the undivided slits become the primary lamellce.
While the development of the gill-slits is proceeding, the atrium is in course
of formation. Paired longitudinal ridges, the metaplettral folds (Fig. 55, lf., rf.,

FIG. su.-Amphioxus: Development. Diagrammatic transverse sections of three larv<e to

show the development of the atrium. ao. aorta; d. intestine; f. fascia (layer of connective-tissue on
inner surface of myomere); fh. cavity for dorsal fin-ray; m. myomere; 12. nerve cord; p. atrium;
sf. metapleural fold ; sfh . metapleural lymph-space ; si. sub-intestinal vein; sk . sheath of
notochord and nerve-cord; sl. sub-atrial ridge; sp. crelom. (From Korschelt and Heider, after
Lankester and Willey.)

Fig. 56, sf.), appear on the ventral side of the body, behind the gill-slits, and
gradually extend forwards, dorsal to them. Their arrangement is very
asymmetrical in correspondence with that of the clefts themselves. On the
inner face of each fold, i.e. the face \vhich looks towards its fellow of the opposite
side, a longitudinal, subatrial ridge (Fig. 56, A, sl.) appears. The two sub-
atrial ridges meet and coalesce. A canal (B, p.) is formed immediately below
the ventral body-wall. This canal is the commencement of the atrium. It
is at first quite narrow, but gradually it extends upwards on each side (C, p.)
until it attains its full dimensions. It is at first open, both in front and behind.
The posterior opening remains as the atriopore, but the anterior opening be-
comes gradually shifted forwards as the fusion of the subatrial ridges proceeds
(Fig. 56, B and C). Finally it is completely closed. In this way the gill-slits
 PHYLUM CHORDATA

come to open, not directly on the exterior, but into a cavity formed by the
union of paired ridges of the body-wall, and therefore lined by ectoderm.
The mouth gradually passes to the ventral surface, and undergoes a relative
diminution in size. A fold of integument develops round it and forms the oral
hood, which is probably a stomodreum. The endostyle appears on the right of
the pharynx (Fig. 54, fl.}, and is at first rod-shaped, then ¥-shaped. Ulti-
mately the limbs of the V unite in the middle ventral line. The gill-slits
increase in number and become more and more vertically elongated. The
provisional caudal fin disappears. The gonads arise from the outer and ventral
regions of the protovertebrre in the form of pouches, which gradually assume
the permanent form previously described.
Affinities.-The Lancelet has had a chequered zoological history. Pallas,
the eighteenth-century naturalist who first studied it, considered it to be a slug,
and placed it among the Gastropoda. When its chordate character became
evident it was for a long time placed among the fishes as the type of a distinct
order. On further study it became obvious that an animal without skull,
true brain, heart, auditory organs, paired eyes, or kidneys, and with a pharynx
surrounded by an atrium, must be widely separated from the lowliest fish.
There was still, however, no suspicion of any connection between Amphioxus
and the Tunicata until the development of both was worked out and it was
shown that in several fundamental points (e.g., the formation of the nervous
system and the notochord) there was a close resemblance. The likeness is
further emphasised by the presence of an endostyle, an epipharyngeal groove
(dorsal lamina), and peripharyngeal bands, which are used in an essentially
similar method of endopharyngeal filter-feeding in both forms. Such a
feeding mechanism is not found in any other animals except Enteropneusta,
the ammoccete larva of cyclostomes (p. 196), and the larvre of some amphibians.
The mechanism occurs so widely in protochordates that it is generally
thought to be the primitive mode of feeding in the phylum Chordata.
Those chief exponents of the method, the ascidians, were once said to be
obviously degenerate, but are now regarded by some zoologists as essentially
primitive in habit, and as having evolved the tadpole larva as a dispersal phase
in the life-history. Predomorphosis (which probably gave rise to the Larvacea,
as we have seen) could then have produced the stock from which higher
chordates arose. 1 The most plausible suggestion yet made to account for the
1 Berrill holds that vertebrates may have evolved from a neotenous ascidian tadpole that
matured as a free-swimming organism exploiting the rich pastures of shallow continental seas.
(Thaliacians and appendicularians are the direct but modified survivors; the relationship of the
Adelochorda (p. 5) remains obscure.) The hypothetical vertebrate ancestor elaborated its sensory
and motor equipment and ascended coastal rivers, Amphioxus (p. 47) being a degenerate littoral
relic of an intermediate phase of evolution. Within the river systems there arose a relatively
simple unarmoured ostracoderm (p. 171), the vertebrate prototype.
Berrill discusses in detail questions of lower chordate relationships, and a brief introductory
chapter summarises speculations from the time of St. Hilaire ( 1818) until the present day (see
alsop. 213).
68 ZOOLOGY
changes in position of the gill-slits, and other organs, which occur during the
development of Amphioxus is that these are relics of the changes that might
have occurred, during metamorphosis, in a hypothetical ascidian ancestor.
Alternatively (and much less probably), Amphioxus may be a specialised and
degenerate descendant of the Agnatha (jawless vertebrate hagfishes, lampreys,
and their allies, p. 176). According to this view, the acquired specialisations
are the multiplication of the gill-slits and myomeres, the asymmetry, and the
complicated larval history. The supposed losses by degeneration include the
simplification of the brain, the absence of eyes, otic capsules, exoskeleton, and
generative ducts. Such losses are perhaps not impossible, but the presence of
true nephridia with flame-cells (structures otherwise unknown in the Chordata)
lends no support to the above hypothesis.
The evidence as to the real position of the Acrania is still insufficient, but
investigations have tended to bring them nearer to the Craniates than has often
been suspected. It is probable that Amphioxus and its allies resemble in many
fundamental respects the fish-like creatures that existed in Silurian seas, and it
is possible that the modern fishes of today evolved from animals of much the
same kind.
 GENERAL INTRODUCTION TO CRANIATE
(VERTEBRATE) FORMS

T HE Craniata includes the agnathans, fishes, amphibians, reptiles, birds,

and mammals, or, in other words, vertebrate animals with a skull, a
highly complex brain, a muscular heart of three or four chambers, and,
almost always (seep. 343), red blood-corpuscles.
The Craniata may be defined as Euchorda in which the notochord is not
continued to the end of the snout, but stops short beneath the fore-brain, some
distance from its anterior end. A skull is always present, and there are usually
paired limbs. The pharynx is of moderate dimensions, and is perforated by
not more than seven pairs of gill-slits except in some cyclostomes (p. 175).
The gill-pouches do not open into an atrium. The liver is massive, and not
obviously tubular. The renal tubules unite to form large paired kidneys and
open into ducts which discharge into or near the posterior end of the intestine.
The brain is complex, and there are at least ten pairs of cranial nerves.
Except in cyclostomes the spinal nerves are formed by the union of dorsal and
ventral roots. Paired eyes of great complexity, derived in part from the brain,
are present. There is a pair of auditory organs. There is typically a single
pair of gonads. The reproductive products are usually discharged by ducts
derived from the renal system. There is never a typical invaginate gastrula,
and the mesoderm arises in the form of paired longitudinal bands which sub-
sequently become segmented. The crelom is nearly always developed as a
schizocrele.
In spite of the obvious and striking diversity of organisation among Craniata
-between, for instance, a lamprey, a pigeon, and Man-there is a fundamental
unity running through the whole group, both in the general arrangement of the
various systems of organs and the structure of the organs themselves. The
range of variation in the whole of the classes included in the division is, in fact,
considerably less than in many single classes of invertebrates-for instance,
Hydrozoa or Crustacea. Hence it is convenient to begin with a preliminary
account of the Craniata as a whole. In this way needless repetition will be
avoided.
VOL. II. F
70 ZOOLOGY
An abbreviated classification of the group is as follows:
SUB-PHYLUM CRANIATA (VERTEBRATA)
Super-class Agnatha
Classes Euphanerida (Silurian)
Heterostraci (Pteraspida) (Ordovician-Devonian)
Anaspida (Silurian-Devonian)
Osteostraci (Cephalaspida) (Silurian-Devonian)
Petromyzontia (?-Recent)
Myxinoidea (?-Recent)
Super-class Gnathostomata
Classes Placodermi (Silurian-Permian)
Elasmobranchii (Devonian-Recent)
Osteichthyes (Devonian-Recent)
Amphibia (Devonian-Recent)
Reptilia (Carboniferous-Recent)
Aves (Jurassic-Recent)
Mammalia (Triassic-Recent)

CRANIATE TISSUES
The numerous cell-types containing the living, self-perpetuating protoplasm
(nucleus and cytoplasm) of the vertebrate body are aggregated in different
arrangements to form materials of the following principal kinds:
Epithelial tissue.-Simple (single-layered) epithelia: columnar (ciliated and
non-ciliated), cuboidal, squamous (pavement). Pseudostratified: colum-
nar (ciliated and non-ciliated). Compound (stratified) epithelia: stratified
columnar, stratified cuboidal, stratified squamous, stratified transitional.
Connective tissue. 1-Fibrous and cellular material embedded in an amor-
phous ground substance. It consists of yellow elastic fibres, white
collagenous fibres, reticular fibres; and mainly fibroblasts, histiocytes,
fat cells, and mast cells (mastzellen). It can be roughly subdivided into:
Loose connective tissue or areolar tissue (as in sub-cutaneous layers of the
integument); dense connective tissue (as in the sheaths surrounding
muscles and nerves).
Adipose tissue.-Usually considered as a separate tissue, but it is merely
connective tissue in which enough cells develop sufficient fat and are
aggregated together to form an apparently definite tissue.
Blood.-This consists of plasma, a watery fluid containing small amounts of
various salts, protein, lipid, and glucose. In this medium lie erythrocytes
(red blood cells or corpuscles); leucocytes (white blood cells) including
t In many histology textbooks the term connective tissue embraces adipose tissue, blood,
cartilage, and bone. In usage one generally employs the term in the manner referred to above.
 PHYLUM CHORDATA 71
polymorphs (mainly neutrophils, also eosinophils and basophils), mono-
cytes and lymphocytes; thrombocytes (nucleated spindle cells) typically in
sub-mammalian groups, or blood platelets, typically in mammals.
Lymph.-The fluid in the lymphatics. Essentially similar to plasma but
with greatly reduced protein content and containing lymphocytes.
Cartilage.-Hyaline. Fibrous. Elastic.
Bone. 1
Muscle.-Striated (skeletal, striped, somatic, or voluntary). Smooth
(visceral, plain, unstriped, or involuntary). Cardiac (heart).
Neural tissue.

The various individual organs and systems are composed, in varying

complexity, of the above classes of material which seem to originate as follows :
Ectodermal Derivatives.-Epithelium of the skin (epidermis) and its append-
ages, such as the hair, nails, feathers, sebaceous, sweat, and mammary and
other glands. Epithelium of the buccal and anal areas, including the various
salivary glands and epithelial derivatives such as the enamel of the teeth.
Epithelium of the lower part of the urethra. Epithelium of the nasal passages
and associated glands. Epithelium of the glands and canals and that of the
conjunctiva, crystalline lens, retina, and some of the intrinsic muscles of the eye.
Epithelium lining the external auditory meatus, the membranous labyrinth,
the spinal canal, the aqueduct, and the IVth, Illrd, and lateral ventricles of the
brain, as well as nerve-cells and nerve-fibres. The pituitary and pineal glands.
In addition, the derivatives of the neural crest are ectodermal in origin.
They include most of the branchial arch cartilages and parts of the trabeculre
cranii (at least in Amphibia); the ganglia of the spinal, and some of the
cranial, nerves; parts of the autonomic nervous system ; probably the pia
and arachnoid, and possibly the Schwann cells ; probably the adrenal medulla
and other chromaffin tissues ; parts of some of the teeth ; and the pigment cells.
Mesodermal Derivatives.-Dermis and most of the skeleton (except tra-
beculre of the chondrocranium jaws and gill arches). Dentine. All the
connective tissues, including blood. The spleen, lymph tissue, and peritoneum.
The adrenal cortex and the endothelial lining of the heart and blood-vessels.
Epithelium of the uriniferous tubules, ureters, tests (and ducts). Epithelium
of the ovary and Graafian follicles. Epithelium of the oviducts, uterus, and
the upper part of the vagina. Muscular tissues.
Endodermal Derivatives.-Epithelium of the alimentary canal from pharynx
to, and including, the rectum as well as that of all the glands opening into it,
including liver and pancreas. Epithelium of the Eustachian tube and tympanic
1 Stensio and 0rvig have suggested that there is no clear distinction between bone and dentine
in the earliest vertebrates and that the primitive exoskeletal element is not the placoid scale but
the lepidomorium. This consists of a bony core, the apical part of which is modified to dentine
and surrounded by enamel. The whole structure arises around a single looped blood vessel.
72 ZOOLOGY

cavity and that of the larynx, trachea, bronchi, and alveoli. Thyroid and para-
thyroid glands and the embryonic rudiment of the thymus. Epithelium of the
urinary bladder, female urethra as well as that of the upper parts of the male
urethra and associated glands.
The germ-layer theory, involving many of the above facts, was put forward
by von Baer in 1828. It states that : (1) in normal embryos the materials
from which the organs develop are arranged in three layers-ectoderm, meso-
derm, and f'ndoderm; and (z) in different animals homologous structures arise
from corresponding layers.
But, as de _Beer has pointed out, the second part of the theory cannot
nowadays be maintained. For example, the anterior thymus may form from
ectoderm and endoderm (Salmo and the marsupial Trichosurus), from endoderm
alone (Oryctolagtts and Homo), or from ectoderm alone (Sus and the mole, Talpa).
Numerous comparable examples have caused the germ-layer theory to fall into
disrepute among modern embryologists. Some structures, formerly thought
to be of mesodermal origin, are now known to be derived from the neural crest.
The existence of the germ layers, however, cannot be denied (even though
their structure may vary somewhat in different groups) and this makes it
difficult to abandon the concept entirely. Perhaps for the time being there
is little harm in this, provided that it is remembered clearly that the derivatives
of the germ layers are not necessarily homologous in different animals.

THE CRANIATE BODY

The internal tissues of vertebrates are covered and protected by an integu-
ment (p. 82) which varies remarkably in structure and function from one group
to another. The underlying tissues are moulded around, and mechanically
supported by, an endoskeletal framework of cartilage, bone, or both. These
support the more pliable soft tissttes, and sometimes form protective compart-
ments such as the cranium, vertebral canal, and thorax. They develop special
surfaces, including depressions and projections, for the attachment of muscles,
tendons, and ligaments. Many serve also as mobile, articulated levers which
allow the free movement of various parts of the body and of the animal as a
whole. Although bone, which is peculiar to vertebrates, is found at an early
stage in the evolution of the Agnatha (p. 166), it is convenient first to deal with
cartilage.
Cartilage is resilient, flexible, but to some degree rigid. It is developed from
embryonic mesenchyme and composed of cartilage cells (chondrocytes) enclosed
within small spaces (lacunre) and embedded in a matrix composed of connective
tissue fibres and amorphous ground substance. The amount and type of its
constituent fibres determine whether a cartilage is hyaline, elastic, or fibrous.
The last-named is predominantly composed of collagenous fibres and is ex-
 PHYLUM CHORDATA 73
tremely tough. It is found in joints and in parts of tendons (sinews) that
undergo friction against, or are inserted on, bone. Elastic cartilage is per-
meated with yellow elastic fibres, and occurs in areas of great flexibility (e.g.
epiglottis and mammalian pinna). Hyaline cartilage consists of amorphous
ground-substance laced with numerous collagenous fibres. This type of
cartilage is the most widespread form and may persist over the embryonic
period, or be replaced by bone (e.g. p. 95). Except over articular surfaces,
cartilage is covered by a dense connective tissue sheath or perichondrium.
The amorphous substance of cartilage is compression-resistant; the internal
collagenous fibres resist tension. Blood vessels penetrate the substance of the
larger cartilages by means of cartilage canals, but, in general, chondrocytes are
nourished by simple diffusion.
A cartilaginous structure has a consistency not unlike that of very firm
jelly. A jelly, if unsupported, would collapse under a weight, yet 'may resist
it successfully if wrapped in a piece of sacking whose fibres take up the upward
thrust due to the downward pressure of the weight . . . . The jelly-like, firm,
matrix [of cartilage] with contained cells resists compression, but the resist-
ance offered is strengthened by the collagen fibres of the perichondrium and
the matrix' (Murray). Cartilage can grow at any point within its bulk. It
is sufficiently strong, as we shall see, to support in water the bulk of a shark
between 40 and so feet in length. In cartilaginous animals there is sometimes
an impregnation of cartilage with mineral salts (p. 231), but this must not be
confused with true bone. There is some evidence that the calcium deposited
in this way interferes with the diffusion of nutrient material to, and gases from,
the chondrocytes. If mineralisation proceeds too far the chondrocytes tend
to die. Bone formation, as we will see below, is brought about by the calcifica-
tion of an intercellular substance without interfering with the nutrition of the
parent tissue.
Bone is a complicated, living tissue made up of cells and collagenous fibres in
an amorphous ground-substance impregnated with tricalcium phosphate and
other salts. The term 'bone matrix' is usually employed to refer to the whole
of the non-cellular constituents. The matrix is made up of numerous concentric
rings of thin lamellce through which run numerous anastomosing Haversian
canals admitting arterioles, venules, and nerve-fibres. Interstitial lamina
occupy the spaces between each such Haversian system. Within this inter-
stitial material are minute spaces (lacunce). Radiating canaliculi link the
lacunce with others and ultimately with a Haversian canal, although the
canaliculi of one 'system' do not apparently connect with another. An osteocyte
or bone-cell occupies each lacuna. These are thought to be derived from
osteoblasts, which, in the growing animal, are probably responsible for the
deposition of the inorganic components of bone. The bone is surrounded by
another specialised connective tissue, the periosteum, the inner part being a
74 ZOOLOGY

vascular osteogenic layer containing undifferentiated, potential osteoblasts.

The periosteum, too, is pierced by blood vessels, as is also the medullary or
marrow cavity, with its luematopmtic (blood-forming) tissue.
Bone-growth occurs partly by means of sub-periostial deposition of osseus
material (membrane bone) and partly by endochondrial ossification. Re-
modelling by resorption and by new bone formation subsequently takes place.
At the same time, ossification extends towards the still cartilaginous ends of the
bone, and, at the extremities themselves, secondary centres of ossification may
arise. This results in the formation of bony epiphyses, which, however, remain
separated from the shaft by thin, plate-like epiphysial cartilage. In fishes,
crocodilians, and chelonians the epiphysial region remains cartilaginous.
All viscera and other tissues are associated in greater or less degree with
connective tissue (Fig. 8o, p. n6). This acts as a secondary support in either
simple or specialised form. Connective tissue surrounds vessels as perivaswlar
sheaths ; nerve tissue as perineural sheaths ; and many glands as thickened
fibrous capsules. Around muscles it forms epimysia! sheaths. It forms also
intermuscular septa. Individual muscle fasciculi, forming the substance of
the muscle, are held together by delicate connective tissue sheaths or peri-
mysia. Bursm (small, fluid-containing, friction-reducing, and otherwise
protective sacs where tendons are in contact with each other, or with bones)
are formed of connective tissue lined with mesothelium. The tendons them-
selves (the function of which is to exert a concentrated pull at the musculo-
tendinous j~mction, see below) are composed almost exclusively of white
collagenous fibres. Such cords are formed of flexible and relatively non-elastic
tissue and are of great tensile strength and transmit the muscle-pull to the
structure on which the tendon is inserted. Fascia-both superficial and deep
-is connective tissue differentiated into sheets which in the case of superficial
fascia sometimes constitute a fat-containing panniculus adipostts (e.g. Cetacea,
Man). Superficial fascia, immediately below the connective tissue of the dermis
(p. n6), may carry sufficient adipose tissue to modify surface contours, and is
of resthetic and sexual selection significance in Man. Deep fascire lie in inti-
mate protective, and sometimes tensile, association with muscles, bones, and
other structures, and may merge with the periosteum of bone in surface areas.
Connective tissue forms also perichondrium, the sheath of hyaline cartilage
(p. 72), as well as cord-like ligaments which run from bone to bone, particularly
in the neighbourhood of movable joints. It forms also aponeuroses (if sheet-
like) as well as mesenteries and meninges.
In certain specialised situations ossifications (sesamoid bones) appear in
tendons (e.g. patella in certain mammals, osjalcijorme in moles (p. 731)). Such
structures are different from heterotopic bone arising adventitiously in
fibrous (e.g. scar) tissue. Different too are the visceral bones (p. 94): (1) the
os cordis in ungulates which persists in Man as a dense fibrous mass between the
 PHYLUM CHORDATA 75

atrioventricular and aortic apertures, (2) os palpebrce in the eyelid of Crocodilia,

and (3) the os penis (os priapi, baculum) which arises as a rod- or plate-like bone
(of very variable form) in the fibrous septum dividing the corpora cavemosa of
'a few insectivores' (Kingsley) and most, if not all, rodents, bats, carnivores,
and Primates (excluding Man). This bone can be massive: in the Walrus it
reaches a length of 23 inches and a weight of more than I lb. I2 oz.
Within the supporting framework of bone, cartilage, and connective tissue
lie the muscles, all of which possess in high degree the property of contracti-
lity. These are of three kinds: (I) skeletal or somatic (striated), (2) visceral
(unstriated, plain, or smooth), and (3) cardiac (heart), which exists in an obviously
syncytial arrangement and exhibits certain resemblances to the other two.
The two first-named are often spoken respectively of as 'voluntary' and 'in-
voluntary' muscles. In general, such terms are not inappropriate, but it
should be remembered that (for example) although the ciliary muscles control-
ling the curvature of the mammalian eye-lens are unstriped or 'plain', the eye
can be nevertheless focused at will.
Skeletal or striped muscle consists of numerous individual, elongated,
fibres which are supplied by rich vascular plexuses. Each fibre is enclosed by a
sheath or sarcolemma and consists of a finely granular cytoplasm (sarcoplasm)
containing many peripherally situated nuclei. Embedded in the sarcoplasm
are numerous fine fibrils (myofibrils) arranged longitudinally. Each myofibril
bears a regular series of transverse light and dark striations. These are
responsible for the characteristic striped appearance of each fibre. Fibres may
be red, white, or transitional, and both red and white fibres often occur in the
same muscle, especially in the Mammalia. The dark colour is due to the
relative abundance of myoglobin, or muscle hremoglobin, which is related to
blood hremoglobin, from which it receives the oxygen used in muscle meta-
bolism.
Functionally, striated muscle must be considered in relation to the nervous
and skeletal systems. Nerves activate muscles, and these then act upon
cartilage, bones, tendons, fascia, ligaments, skin, and other structures. The
origin and insertion of a muscle are respectively its relatively fixed and mobile
points of attachment. For example, in the gastrocnemius muscle of the leg the
proximal end is attached at its origin to the femur and the distal end at its
insertion on the foot. A muscle originates at its head, the main mass of its
fleshy fibres is the belly, and it inserts at its tail (but nowadays the last term is
seldom used). Muscles may insert by either fleshy or fibrous portions.
The arrangement of the constituent fibres of relatively simple muscles is
parallel to the direction of pull. Pennate muscles, on the other hand, have
the fibres arranged obliquely in relation to the attachment. In unipennate
muscles the fleshy fibres approach the insertion from one direction; in bipennate
elements there is a dual convergence; and in multipennate there is a multiple
 ZOOLOGY
convergence. Muscles may be named according to shape (e.g. deltoideus,
trapezius), structure (triceps, digastricus), situation (pectoralis, brachialis),
attachments (coracohumeralis, sternocoracoideus), action (flexor tarsi, abductor),
and other characteristics.
The essential function of striped or voluntary muscles is to approximate two
points, e.g. moving manus to mouth by flexing the forearm. Flexion bends;
it reduces the angle between two parts. Extension widens the angle, sometimes
rearranging limbs to the straight position. Protraction and retraction draw
bones forward and backward respectively. In relation to some structures
such terms can be synonymous with flexion and extension. Adduction and
abduction are the respective movements of a structure toward or away from an
axis (e.g. the midline of an animal). Elevators raise, and depressors lower a
part (such as the lower jaw). Pronation turns the palm downward (by rotation
of the radius in relation to the ulna), whilst supination arranges the palm in the
reverse position.
Muscular activity is carried out by the contraction of individual fibres on an
'aU-or-none' principle. A group of fibres innervated by a single nerve-fibre
is called a motor-unit, and gradation of the contraction depends on varying
the number of motor-units in action. No entire muscle acts alone. Along
with the prime-mover, synergists and antagonists operate in ancillary capacities
in carrying out what may be apparently the most simple skeletal re-arrange-
ment. The contraction-and even the tone-of muscle is maintained by motor
nerves, individual fibres of which terminate as motor end-plates applied to
individual muscle fibres (Fig. 84). In all animals, and in homiothermal
forms particularly, muscle activity produces heat, which may be conserved
by special devices (pp. 559, 78r, 883). In some mammals reflex shivering
contractions, resulting in compensatory heat production, occur when the body
temperature falls below an optimum degree.
Visceral, smooth, or plain muscles lack the characteristic strice of skele-
tal fibres. They are controlled by the autonomic nervous system (p. rrg).
Vertebrate animals (or their eggs) are thrust into the world by such muscles,
and thenceforward the circulation of the blood (with its contained food sub-
stances and oxygen), the digestion of food, the secretion by glands, the elimina-
tion of metabolites, and the maintenance of many other vital functions depend
on its routine, involuntary activity. Thus, for example, plain or smooth
muscle is found in the walls of the gut, lungs, blood vessels, bladder, genitalia,
and various glands. In these it is, among numerous other functions, con-
cerned with peristalsis and retroperistalsis, changes in the vascular tone and
the control of blood pressure, with the control of air passages (bronchi), ex-
cretory rates, thermoregulation (in birds and mammals), and with the discharge
of sexual products and so on. Visceral fibres are uninucleate, spindle-shaped
bodies which lack a sarcolemma and lie in intimate association in sheets, the
 PHYLUM CHORDATA 77
individual fibres perhaps bound together by delicate reticular tissue. Al-
though the activity of visceral muscles is essentially controlled by innervation
from the autonomic nervous system, local nerve plexuses occur, and there is
abundant experimental evidence of an intrinsic nervous mechanism in certain
areas (e.g. gut).
Cardiac muscle is confined to the heart, but in some forms it extends into
the roots of the great vessels (e.g. in the conus arteriosus of the dogfish, p. 240).
It is essentially similar to skeletal or striated muscle in structure, except that
the sarcolemma is much thinner; the nuclei are central in position; and the
fibres branch and possess conspicuous intercalated discs interposed among the
other fine transverse striations. The branched fibres lack obvious bound-
aries and were previously regarded as syncitial. Although the heart-beat is
myogenic in origin (p. rog), its frequency is under autonomic control (p. rrg).
Sympathetic nerves innervate the branched fibres; para-sympathetic (vagal)
terminations are restricted (in the higher mammals) to the atria and the atrio-
ventricular bundle (p. 882). All living tissues are connected with the rhyth-
mically pumping heart by means of a closed system of supplying arteries and
arterioles, and of draining venules and veins. Between arterial and venous
elements are capillary beds and tissue spaces (Fig. 8o). Only in the spleen,
liver, bone-marrow and some endocrine glands does the blood come into
closer contact with the general tissues.
Arteries get progressively narrower, and are required to withstand less
blood pressure, the further they go from the heart. As they progressively
bifurcate, their total cross-section is increased, and there is a corresponding
reduction in circulation rate. The elastin and collagen content of arteries
varies greatly with situation and diameter. In cross-section, an artery is
lined by an endothelium, which is sheathed by a lamina elastica, in turn sur-
rounded by a tough fibrous tunica media (of elastin, collagen, and muscular
fibres), and by an outermost tunica externa (principally of collagen material).
By the time arteries narrow to a diameter of about o·r-2 mm. the tunica media
is almost solely muscular, and they are now arterioles, innervated by the
autonomic system. Their tone can be altered, thus regulating the blood supply
to skin and viscera. Arterioles empty into even narrower, thin-walled capil-
laries, each perhaps composed of a fragile perivascular layer of connective tissue
and an endothelial lining. In lower craniates these, too, are contractile. In
all groups these link up to form a capillary network, the richness of which varies
according to the nutritive and concomitant requirements of their situation.
Between the capillaries and the other tissues lie tissue spaces containing
tisstte fluid. Across these must pass gaseous, nutritional, excretory, and other
materials. The capillary walls are highly permeable to all but the largest mole-
cules, i.e. proteins. These are retained in the capillary, and exert an osmotic
pressure into the capillary, whereas movement outward from the capillary is
 ZOOLOGY
determined by the excess hydrostatic pressure over the osmotic pressure at its
arteriolar end. At the venule end, hydrostatic pressure is less than osmotic
pressure, and so water and its solutes (e.g. metabolites) move back in again.
Some fluid passes into lymph capillaries (lymphatics) as lymph which contains
some protein, and much the same inorganic components as blood. It contains
also lymphocytes, but lacks red cells and blood platelets, which cannot penetrate
the blood capillary walls. By means of lymph hearts or valves (according to
the type of animal), the fluid is at length forced back into the general circulation,
the disposition of the conducting vessels and their entry into the venous system
varying from group to group.
Meanwhile, altered capillary blood flows into venules, the walls of which are
strengthened by a definitive sheath of connective tissue. Venules in tum unite
to form veins. These are lined with endothelium, but, although veins have
basically the same structure as arteries, the remaining layers are often difficult
to distinguish. There is a great diminution of both elastic and muscular tissue.
In higher craniates, venous return in many veins is facilitated by the presence of
valves that prevent back-flow. Blood can be forced forward against gravity,
and pooling avoided, by the movement of adjacent skeletal muscles and other
means, including, in animals with closed thoracic respiration, the negative
intra-thoracic pressures.
Arteries and veins usually run in close proximity to each other, and in
certain situations a small artery or an arteriole, by means of a special neuro-
muscular mechanism, delivers blood directly into a neighbouring venule, thus
short-circuiting the capillary network and forming an arterio-venous anastomosis.
Such anastomoses appear to be best developed in situations where metabolic
activity is intermittent (e.g. gut), or where a relatively unimpeded circulation
will better subserve thermoregulation (e.g. skin, membranous wings, etc.) in
homiothermous animals (p. 883).
In the lower craniates in particular (and in higher groups as well) wide
blood sinuses, sheathed only in endothelium, occur at definite points. These
may replace capillaries and allow of a uniquely intimate relationship between
blood and tissue, and the performance of special functions in liver, spleen, bone
marrow, and adrenal glands. In certain lower craniates (seep. 243) sinusoidal
dilations occur in the great veins and are part of a specialised mechanism of
venous return.
We have seen that the above fundamental tissues, and (as is obvious) the
various living components of structures such as skin, teeth, receptor organs,
and the numerous viscera, must all possess a nerve supply. A brief outline of
the nervous system is given on pp. n6-135).
External eharaeters.-The body (Fig. 57) is bilaterally symmetrical, elon-
gated in an antero-posterior direction, and usually more or less cylindrical.
It is divisible into three regions: (r) the head, which contains the brain, the
 clf.t
A
----- d.f.!l.
e

:::~~
\ cf

e . lir.a.1 --- ------ vf "0

e .br.a.7 l.t ::q
pelf ...-:::
t""'
c:::
~
B n
::q
0
:;d
na.---... t:J
;:...

·. ;:
vf
m.'lh

FIG. 57 .-Sub-phylum Craniata: External views oi an " ideal" vertebrate. ab . p. abdomina l pore; au. position of audi·
t ory organ; an . a nus; c. f. cauda l fi n ; d. f. 1, fi rst, a nd d . f. 2, second d orsal fin ; e. paired eye ; e. br. a. I , firs t , and e. hr. a.
7 . seventh gill-cleft; f. I. fore -limbs ; h. l . hind-lim b; I. l. la teral line ; mth. mouth; n a. nasal aperture; p et. f pectoral fin;
pn. e. p ineal organ ; pv. f. pelvic fin ; v. f. ventra l fi n. The dotted line in A indicates a cont inuous median fin.
'l
\0
Bo ZOOLOGY
chief sensory organs, and the mouth and pharynx; (2) the trunk, to which
the ccelom is confined, and which contains the principal digestive, circulatory,
excretory, and reproductive organs ; and (3) the tail, or region situated
posteriorly to the ccelom and anus. Viscera do not extend into the tail. At
the same time, it should be remembered that functionally the expression may
refer in fishes and fish-like animals to pre-anal parts that nevertheless undergo
lateral undulations (p. 86). Between the head and trunk there is frequently
a narrow neck, into which the ccelom does not extend. In aquatic vertebrates
the tail is normally of great size, not marked off externally from the trunk.
In mammals it is usually greatly reduced in diameter, and has the appearance
of a mere unpaired posterior appendage.
The mouth (mth.) is generally a transverse aperture placed at or near the
anterior end of the head. Near it, either dorsal or ventral in position, are the
paired nostrils or anterior nares (na.). In various widely unrelated fishes and
fish-like vertebrates there have arisen internal nares by which the paired
olfactory organs communicate with the buccal cavity (see also Fig. 8g, p. 138).
A peculiar arrangement exists in myxinoids. An unpaired terminal nostril
leads into a duct that opens into the pharynx. Through this passes the
respiratory current (p. 199). This water enters the nostril, crosses the olfactory
epithelium, and traverses the pharynx and finally the gills. This specialisation is
of course not homologous with the paired structures that have occasionally
arisen among teleosts of burrowing habit (e.g. stargazer, Astroscopus, and certain
echelid and ophichthid eels), nor are any of the above arrangements phylogeneti-
cally related to the internal nares of lung-fishes (p. 361). Although the internal
nares of the Dipnoi (secondarily lost in the Ccelacanthini, p. 356) are of course
homologous with those subserving respiration in the Amphibia (p. 412), and
in tetrapods generally, there is no good evidence that they were utilised in
breathing by ancestral crossopterygians. Certainly modern lung-fishes appear
to respire essentially by gulping in air through the buccal cavity (p. 365).
While restivating too, Protopterus breathes through the mouth. It is not
improbable that internal nares served initially as adjuncts to the olfactory
organs, making for a more rapid and efficient testing of the environment than
do the blind respiratory sacs characteristic of fishes in general. Other air-
breathing fishes (e.g. Periophthalmus, Clarias, p. 338, Polypterus, p. 290) do not
possess internal nares.
The eyes (e) are paired and vary greatly in structure and efficiency. On the
dorsal surface of the head there is sometimes more or less indication of a vesti-
gial median eye or pineal organ (pn. e.) (see pp. 189, 345). Posterior to the
paired eyes are the auditory organs (au.), the position of which is indicated in
the higher forms by an auditory aperture.
On the sides of the head, behind the mouth, are a series of openings, the
gill-slits or external branchial apertures (e. br. a. 1-7). They are rarely more
 PHYLUM CHORDATA 8r

than seven in number, and in air-breathing forms disappear more or less

completely in the adult. In the higher fishes a fold, the operculum (Fig. 205,
op; p. 304), articulates with the hyoid arch immediately in front of the first
gill-slit and extends backwards, covering the branchial apertures. In the
larvce of urodeles there occurs a somewhat similar opercular flap. In frogs the
structure covers the external gills and gill-slits, and even the site of the fore-
limbs. In the placoderms (p. 205) the operculum is supported by the
mandibular arch.
On the ventral surface at the junction of the trunk and tail is the anus
(an.). Distinct urinary and genital apertures, or a single urinogenital apertttre,
are sometimes found either in front of or behind the anus. More commonly
the urinary and genital ducts open into the termination of the alimentary canal,
or cloaca, so that there is only a single egestive opening, known as the cloacal
aperture. On either side of this there may be a small abdominal pore (ab. p.)
(of doubtful function) communicating with the ccelom.
In fishes and some amphibians (p. 380) the trunk and tail are produced in the
middle dorsal line into a vertical fold or median fin, which is continued round
the end of the tail and forwards in the middle line to the anus. This continuous
fin frequently becomes broken up into distinct dorsal (d. f. I and 2), ventral
(v. f.), and caudal (cd. f.) fins, which may assume very various forms. In the
higher classes all trace of median fins disappears (cf., however, analogous
structures in Iclztlzyosaurus (p. 491) and the Cetacea (p. 78o)).
Fishes also possess paired fins. Immediately posterior to the last gill-
slit is a more or less horizontal outgrowth, the pectoral fin (pet. f.), while a
similar but smaller structure, the pelvic fin (pv. f.), arises at the side of the
anus.
In all Craniata above the fishes, i.e. from the Amphibia upwards, the paired
fins are replaced by fore- and hind-limbs (f. l., h. l.), each consisting of three
divisions-upper-arm (brachium), fore-arm (antebrachium), and hand (manus)
in the one case; thigh and shank (leg), and foot (pes) in the other. Both hand
and foot normally terminate in five fingers or digits. The pentadactyle limb thus
formed is an obvious adaptation to land-life and is characteristic of tetrapods.
The paired fins, or limbs (as the case may be), are almost the only lateral
appendages possessed by vertebrates, if we except the barbels possessed by
certain fishes (p. 338) and 'balancers' of urodele larvce.
Body-wall and Internal Cavities.-The body is covered externally by
a skin consisting of two principal layers, an outer and stratified epithelial layer,
the epidermis (Fig. 58), derived from the ectoderm of the embryo, and an inner
or connective-tissue layer, the dermis or corium of mesodermal origin (p. 71).
In terrestrial vertebrates (p. 385) the superficial layers (stratum corneum) are
dead, flattened, and keratinised (horny). The outermost cells are constantly
abraded and replaced from below. In fishes (Fig. 58) a keratinised layer
82 ZOOLOGY
probably never occurs. New cells for the replenishment of the outer layers are
produced by division of cells in the Malpighian layer (stratum Malpighii), which
lies at the base of the epidermis. In the Agnatha (p. 164) and the true fishes
the connective tissue-fibres of the dermis almost all run parallel to the surface,
and do not form a felt-like mass of fibres running in all directions, as in
mammals.
Although the epidermis of fish is usually many layered, exceptionally it may
be only two cells thick, as in many Syngnathidre. The basal layer always
consists of undifferentiated epidermal cells, although here and there throughout
the epidermis may usually be found wandering cells (wanderzellen) which are

p.

=----
FIG. 58.-Integument: Epidermis of fish. Tench (Tinea) . Vertical section: c. c. club cells;
d. c. t. deep connective tissue; m . c. mucous cells; p. c. pigment cells; sc. scale (see also Fig. 112,
p. 178). The club cells do not occur very near the surface but are nevertheless lost when the
epidermis is abraded. Although there is some flattening of the surface cells, keratinisation does
not usually occur in fishes. (Drawn by G. H. 0. Burgess.)

capable of amceboid movement and enter into the epidermis from the under-
lying connective tissue. Just above the basal layer there often occur the
earliest stages in the development of the various types of secretory cell. Keratin
occurs only in certain specialised epidermal structures (e.g. lamprey teeth and
possibly the 'pearl organs' of cyprinids). Secretory glands are very rare in
fishes, but secretory gland cells are always found. Mucous cells occur in the
epidermis of all fishes. These are usually of the goblet-cell type with a basal
semi-lunar nucleus and a theca of cytoplasm surrounding the secretion (Fig. 58),
but in some fishes (and agnathans) other types of mucous cell are found (as for
instance in Myxine, Fig. 197). The lubricating secretions from mucous or
slime cells are variously held to (1) reduce surface drag, (2) produce a surface
 PHYLUM CHORDATA

difficult for predators to grip, (3) prevent the lodgment of organisms, and (4) assist
in the control of osmosis. There is reason to believe that the two last-mentioned
functions are important.
The skin of vertebrates is
rarely naked and is usually
provided with protective struc-
tures of various kinds, such as
scales, bony plates, feathers, b
or hair. Scales of the fishes
are of different shapes, such as
cycloid, ctenoid, or rhomboid,
but it is the structure rather
than the shape that is of im-
portance and of value in classi- 0
fication. There are three main FIG. 59.- Integument: Placoid scale. Median dorsal
denticle scale of Raja blanda. A , left side view; B,
kinds of scales in fishes : the section, enlarged; b. basal plat e; d. dentinal tubules;
placoid, the cosmoid, and the o. opening of p ulp cavity; s. p. proj ecting spine; tr.
trabecu lar dentine. (After Goodrich. )
ganoid. The placoid scale
(Fig. 59). with an ectodermal cap of enamel and a body of dentine surrounding
a pulp-chamber, has essentially the same composition as a tooth. In the

c. .... IU.

.
'.

.
'
'

'

!I·
FIG. 6o.-lntegument: Cosmoid scale. A, piece of a thick transverse section enlarged; B,
section throu gh the hind edge, enlarged; C, outer view of a complete scale. Carboniferous
rhipidistian M egalichthys. ac. a nterior region covered by next scale; c. large vascular cavity ; ch.
chamber o f cosmine layer; dt. cana liculi of cosm ine ; g . t hin outermost layer; h. irregular vascula r
canals; i. isopedine layer; o. opening of chamber of cosmine layer on surface; pc. pulp cavity from
which canaliculi radia t e ; vc. vertical canal leading to vascula r cavity. (After Goodrich .)

elasmobranchs, in which the placoid scale is characteristic, there 1s a clear

transition from the scales on the body to the t eeth on the Jaws. They lie
superficially and, if lost, can be continually replaced.
 ZOOLOGY
The cosmoid scale (Fig. 6o) lies deeper in the dermis, and consists of three
layers. The outer cosmine layer is formed of an acellular substance somewhat
like dentine. This contains vascular spaces and fine radiating-tubules. On the
outer surface it has a thin layer of the hard, glossy vitrodentine. The middle
vascular layer is formed of bone perforated by numerous anastomosing canals for
the blood-vessels. The bottom isopedine layer consists of severallaminre of bone
lying parallel to one another, through which at intervals run canals for blood-
vessels passing to the vascular layer. Cosmoid scales are not shed, and during
growth expand by the addition of cosmine round the edge of the upper surface

FIG. 6x.-Integument: Ganoid scale. A, diagrammatic and enlarged view of a piece of scale
from the carboniferous pai<eoniscoid Plectrolepis ( = Eurynotus). B, outer view of a complete
scale; C. transverse section of a scale. a. a nterior covered region; ap. articulating process; c.
fine canaliculi of cosmine layer ; g. ganoine layer; h. system of horizontal canals; i. isopedine
layer; o. opening of v ertical canals on the outer surface; p. exposed posterior shiny part of the
scale; s. outer surface; vc. v ertical canal. (After Goodrich.)

and by addition to the isopedine layers below. It is possible that the vitro-
dentine covering of the scale can be resorbed and replaced at intervals during
the life of the fish, but not all authorities are agreed upon this point. Cosmoid
scales are found only in the crossopterygians (p. 353), but in living forms much
of the characteristic structure of the scale has been lost.
The ganoid scale (Fig. 6r) is composed of layers of acellular ganoine on the
upper surface which pass into layers of isopedine below. Between these two
layers are two more. The upper resembles cosmine and the lower one the
vascular layers (see above), but these are not universally present and may be
much reduced. The growth of the ganoid scale differs from that of the cosmoid
in that fresh layers are laid down on the top of the scale as well as on the under-
side. This type of scale and its derivatives is confined to the Actinopterygii
 PHYLUM CHORDATA ss
(p. 283). A superficially similar type of scaling is found in acanthodian fishes
(p. 20J).
Scales and bony plates occur in the higher vertebrata, but their relation
to those of the fishes is still obscure. In reptiles and birds the scales and
feathers, and in mammals the hair, are epidermal in origin and are formed
of a tough, horny, sulphur-containing protein termed keratin. Details of the
structure are given in the accounts of the respective classes.
Beneath the skin is the muscular layer. The homology of the lateral
musculature of all fish-like chordates, from the Acrania (p. 47) to teleost fishes
(p. 239), is well established. The action of this musculature causes lateral
undulation of the body and is much the same in each group, although consider-
able differences in the degree of efficiency (as measured by speed and agility
in locomotion) are manifest. Such muscles contract serially. In association
with an extensive and serial connective-tissue system, they act in relation to
the flexible, yet resistant central notochord (see below). There is fossil evidence
that optimal locomotion was probably achieved by fish-like chordates in the
early Palreozoic (p. 3), and the general pattern of musculature has not changed
significantly since. Forms existing to-day exhibit three principal types of
lateral muscle architecture: (r) amphioxine, embracing the Acrania, (2)
cyclostomine, including recent agnathous chordates, and (3) piscine, comprising
the cartilaginous fishes (Elasmobranchii) and the bony fishes (Actinopterygii).
(The external appearance of the lateral musculature of representatives of
each of these groups, with individual elements offset, is shown in Fig. 62.)
In detail, the lateral layer consists of zigzag muscle-segments or myomeres,
separated from one another by partitions of connective tissue, or myocommas,
and formed of longitudinally disposed muscle-fibres. The myomeres are not
placed at right angles to the long axis of the body, but are directed from the
median vertical plane outwards and backwards, and are at the same time
convex in front and concave behind, so as to have a cone-in-cone arrangement
(Fig. 63, C). Each myomere, moreover, is divisible into a dorsal (d. m.) and
a ventral (v. m.) portion. In the higher groups this segmental arrangement,
though present in the embryo, is lost in the adult, the myomeres becoming
converted into more or less longitudinal bands having an extremely complex
arrangement.
All three types of lateral musculature mentioned above possess the common
function of bending the animal from side to side, achieving locomotor force as
mentioned above. In each instance pull is exerted obliquely to the notochord
(Fig. 63) and the long axis of the body. 'The myomeres pull at once latero-
craniad and latero-caudad to bend the body from its position of equilibrium, or
rest, into concavity. If continued, this movement would lead to a condition of
instability, in which the vectors of the forces applied would pass across the
axis of bending and the body would be distorted' (Nursall). However, this
VOL. II. G
86 ZOOLOGY

2
c

3 4

FrG. 62.-Musculature: Myomere pattern of fishes and fish-like chordates.

A. Amphioxine: (r) lateral view of Amphioxus; (2) lateral view of an individual myomere;
(3) horizontal sections through myomere.
B. Cyclostomine: (r) lateral view of Petromyzon; (2) lateral view of an individual myomere;
(3) horizontal sections through myomere.
C. Piscine: (1) lateral view of Squalus; (2) lateral view of an individual myomere from body
region.
D. Piscine: (r) lateral view of Perea ; (2) lateral view of an individual myomere from body
region; (3) lateral view of a myomere from caudal peduncle; (4) horizontal sections through myo-
mere of caudal peduncle. (All drawings are semi-diagrammatic.) (After Nursall.)
 1-0
::r:
>-<:
t-<
c::::
::;::::
(")
::r:
0
:;o
Cj
>
~
p,.· in-t
FIG. 63.- Visceral relationships: "Ideal" craniate. A . sagittal section ; B, transv erse section of the h ead; C, of the trunk;
D, of the tail. al. bl. allantoic bladder; an. a nus; au. auricle ; b. d. bile-duct; br. f. branchial filaments ; buc. c. buccal cavity; c. a.
conus arteriosus ; cd. a. caudal a rtery; cd. v. caudal vein ; ccel. ccelom; en . centrum ; crb . cerebellum; crd. v . cardinal vein; c. s. c.
cerebrospinal cavity; d. ao. dorsal aorta ; dien. diencepha lon ; d. f . dorsal fin; d. m . dorsal muscles; e. br. a. external bra nchial
aperture; f. r. fin-ray; g. b. gall-bladder ; gl. glottis; gon. gonad; g. p. gill-pouch ; h. a. hremal arch ; h. c. hremal canal ; i . br. a.
internal branchial apertures; int. intestine; lg. lung; lr. liver ; l. v. lateral vein; m . muscles; m . b. mid -brain; nzed. obl.
medulla oblongata; mes. mesentery; ms. n . d. mesonephric duct; ms. np!t. mesonephres; mth. m outh; mt. n. d. m etanephric
duct; mt. nph. metanephros; n. a. neural arch; nc!t. n otochord ; p. a. g. post-anal gut ; pc. pericardium ; ph. pharynx; pn.
pancreas; pn. b. pineal body; P- n. d. pronephric duct ; pn. e. pineal sense-organ; p . nph. pronephros; pr. peritoneum,
parietal layer, and pr'. visceral layer ; p rosen. prosencephalon ; pty. b. pituitary body ; pty. s. pituitary sac ; r. sub-peritoneal
rib; r'. intermuscular rib ; sk . skull; sp. c. spinal cord ; spl. spleen; st. stom ach; s. v. sinus venosus ; thd. thyroid ; t. p .
transverse process ; v. ventricle ; v. ao. ventral aorta; v. m. v entral muscles; vs. b. visceral bar. en
-....:)
88 ZOOLOGY
is prevented by the length of the body and its resistance to compression. At
the same time, restorative forces are applied on the opposite (now convex)
side of the body and these supplant the original bending movement. Thus the
animal becomes concave on the other side. Actual locomotion is achieved by
the reaction with the water of lateral waves set up by the rapid serial contrac-
tion of myomeres antero-posteriorly along the sides of the body. The
myomeres are attached to the centra, neural and hcemal spines of the vertebrae
and to the inter-muscular myocommas. Longitudinal ligamentous (p. 74)
secondary attachments, formed by the apposition of deep myosepta, sometimes
occur, especially in selachians. In the myosepta of some species there occur
intermuscular bones (p. 94).
In the trunk the muscular layer encloses the crelom (Fig. 63, A and C, ca:l.).
The muscular layer, as in Amphioxus, is not of even diameter throughout, but
is greatly thickened dorsally, so that the ccelom is, as it were, thrown towards
the ventral side. Its dorsal portion, moreover, is excavated by a canal, the
neural or cerebrospinal cavity (c. s. c.), in which the central nervous system is
contained, and the anterior portion of which is always dilated, as the cranial
cavity, for the brain. Thus a transverse section of the trunk has the form of a
double tube. In the head, neck, and tail (B, D), the ccelom is absent in the
adult, and the muscles occupy practically the whole of the interval between
the skin and the skeleton. In the tail, however, there is found a hcemal canal
(h. c.) containing connective-tissue, and representing a virtual backward
extension of the ccelom. This canal contains the principal caudal artery and
vein. The fins, or fore- and hind-limbs, are moved by longitudinal muscles
derived from those of the trunk. All the voluntary muscles of Craniata are
striated.
The ccelom is lined by peritoneum (C, pr.), a membrane formed of an outer
layer of connective-tissue, a middle muscle layer, and an inner layer of ccelomic
epithelium bounding the cavity, and thus forming the innermost layer of the
body-wall. In fishes the ccelom is divided into two chambers: a large abdominal
cavity containing the chief viscera, and a small forwardly-placed pericardia!
cavity (A, pc.) containing the heart, and lined by a detached portion of peri-
toneum known as the pericardium. In mammals there is a vertical muscular
partition, the diaphragm, dividing the ccelom into an anterior chamber or
thorax containing the heart and lungs, and a posterior chamber or abdomen
containing the remaining viscera. In birds a somewhat similar but less
massive, oblique septum occurs behind the liver and anterior to the other
viscera.
Skeleton.-The hard parts or supporting structures of Craniata fall into two
categories-the exoskeleton and the endoskeletcn. The exoskeleton consists of
bony or horny deposits in the skin, and may be either epidermal or dermal,
or both, but is never cuticular, like the armour of an arthro:pod or the shell of a
 PHYLUM CHORDATA 8g

mollusc. The epidermal exoskeleton is always formed by the cornification or

conversion into horn of epidermal cells, and may take the form of scales,
feathers, hairs, claws, nails, horns,
and hoofs. The dermal exo-
skeleton occurs as either bony or
horn-like deposits in the dermis,
such as the scales and dermal fin-
rays of fishes, and the bony armour
of a sturgeon, crocodile, or arma-
dillo.
The endoskeleton presents an
immense range of variation in the
different classes and orders. As
in Amphioxus, the axis of the
entire skeletal system is formed by
the notochord (Fig. 64, nch.), an
elastic rod made of peculiar vacuo-
lated cells (Figs. 64, 65, nch.) and
covered by a laminated sheath (sh.
nch.) with an external elastic metn-
brane (el. m.) around it. The FIG. 64. Vertebral column: Craniate embryo.
whole sheath is, in the Craniata, a c. c. central canal; el. m . external elastic membrane;
h. r. hremal ridges; n. c. neural arch; nell. noto-
cuticular product of the superficial chord ; nc/1. c. notochordal cells ; p. c. t. perichordal
tube ; sll. nell. sheath of notochord; sk. c. skeleto·
notochordal cells (nch. c.) , being genous cells migrating into notochordal sheath; sk. I.
developed as a secretion from their skeletogenous layer; sp. cd. spinal cord. (Modified
from Balfour and Gadow.)
outer or free surfaces. The noto-
chord lies in the middle line of the dorsal body-wall between the cerebrospinal
cavity above, and the crelom below. It is developed as a median rod of cells

FIG. 65.- Vertebral column:

Segmentation. c. n . t. perichordal
tu he ; h. r. hwmal ridge; h. t.
hremal arch; i . v. f. intervertebral
foramen; n . t. neural arch; n ch.
notochord. The dotted lines in-
dicate the st•gmentation into ver-
tl'bne.

detached, in the neurula, from the continuous chordo-mesoderm sheet of the

gastrula. Posteriorly it extends to the end of the tail, but in front it always
stops short of the anterior end of the head, ending near the middle of the brain
immediately behind the compound pituitary body (Fig. 63, A , pty. b.). The
 ZOOLOGY
extension of the nervous system in front of the notochord is one of the most
striking differences between the Craniata and the Acrania (p. 47). In the
latter the notochord is uniquely prolonged to a considerable distance beyond
the anterior end of the nerve-tube during the elongation of the larva (p. 62).
In the majority of living Craniata the notochord is a purely embryonic
structure, and all but the anterior end of it is replaced in the adult by the
vertebral column. The cells of mesoderm (sclerotome) surrounding the noto-
chord become concentrated around the sheath and give rise to the skeletogenous
layer (Fig. 64, sk. l.), some ofthe cells of which
(sk. c.) may migrate through the elastic mem-
brane into the sheath itself. In this way the
notochord becomes surrounded by a cellular
investment which soon takes on the structure
of cartilage, and may be called the perichordal
tube (Fig. 64, p. c. t., and Fig. 65, c. n. t.).
The skeletogenous layer also grows upwards,
and gives rise to an inverted tunnel of
cartilage (p. 72), the neural arch (n. c., n. t.),
...........
enclosing the cerebro-spinal cavity and con-
I ! nected below with the perichordal tube ; and

:(
· ../ to paired hamal ridges (h. r.) of cartilage
au.c standing out from the sides of the perichordal
tube into the muscles. In the region of the
tail these unite below to enclose the hamal
I. canal (h. c.) already mentioned. Actually,
nck
FrG. 66.- Cranial elements: Em-
however, the vertebral column thus consti-
bryo Salmon. From above. au. c. tuted is from the first more or less broken up
auditory capsule ; nch. notochord; pc.
parachordal; pty. position of pituitary into segments, and in the higher forms is
body; tr. trabecula. (From a model replaced by a chain of bones, the vertebra.
by Ziegler.)
These begin a short distance behind the
anterior end of the notochord and extend to the extremity of the tail.
A vertebra consists essentially of the following parts : (I) a centrum or
body (Fig. 63, C, en.) lying below the spinal canal in the position formerly
occupied by the notochord and perichordal tube, and arising either in the
skeletogenous layer proper, or in the notochordal sheath after its invasion by
skeletogenous cells; (z) a neural arch (n. a.) which springs from the dorsal
surface of the centrum and encircles the neural canal, representing a segment
of the neural tube; and (3) a pair of transverse processes (t. p.) which extend
outward from the centrum among the muscles and represent segments of the
hremal ridges. To these are often attached ribs which extend downwards in
the body-wall, sometimes between the dorsal and ventral muscles (r'.}, some-
times immediately external to the peritoneum (r.). Jn the anterior part of the
 PHYLUM CHORDATA

ventral body-wall a cartilaginous or bony sternum or breast-bone may be

developed. Thus the anterior or thoracic region of the ccelom is enclosed in an
articulated bony framev;ork formed of the vertebral column above, the ribs
at the sides, and the sternum below. The ribs in these circumstances become
segmented each into two parts, a dorsal vertebral rib, articulating with a
vertebra, and a ventral sternal rib with the sternum. The former is usually
ossified; the latter remains as cartilage. In the tail there is frequently a
hamzal arch (Fig. 63, D, h. a.) springing from the ventral aspect of the centrum
and enclosing the ha:mal canal. Thus the line of centra in the fully-formed
vertebral column occupies the precise position of the notochord. The neural
arches encircle the spinal portion of the cerebrospinal cavity; the transverse
processes, ribs, and sternum encircle the ccelom ; and the ha:mal arches similarly
surround the ha:mal canal or vestigial ccelom of the tail. As we ascend the
craniate series we find every gradation from the persistent notochord of the
lampreys and hags, through the imperfectly differentiated vertebra: of sharks
and rays, to the complete bony vertebral column of the higher forms.
The vertebra: are equal in number to the myomeres, but are arranged
alternately with them, the fibrous partition between two myomeres abutting
against the middle of a vertebra, so that each muscle-segment acts upon
two adjacent vertebra:. Thus, the myomeres are metameric or segmental
structures, and the vertebra: are intersegmental.
In connection with the anterior end of the notochord, where no vertebra:
are formed, there are developed certain elements of the skull or cephalic
skeleton. This structure is eminently characteristic of the whole craniate
division, and gives the group its name. The skull makes its first appearance
in the embryo in the form of paired cartilaginous plates, the parachordals
(Fig. 66, pc.), lying one on each side of the anterior end of the notochord (nch.),
and thus continuing forward the line of vertebral centra. In front of the
parachordals are developed a pair of curved cartilaginous rods, the trabeculce
(tr.). These underlie the anterior part of the brain as the parachordals underlie
its posterior part. Their hinder ends diverge so as to embrace the pituitary
body (pty.). Cartilaginous investments are also formed around three prominent
paired organs of special sense : I. a pair of olfactory capsules round the organs of
smell; 2. optic capsules round the organs of sight; and 3· auditory capsules
(au. c.) round the organs of hearing and equilibrium. The optic capsule,
which may be either cartilaginous or fibrous, remains free from the remaining
elements of the skull in accordance with the mobility of the eye. It constitutes,
in fact, the sclerotic or outer coat of that organ. The olfactory capsules
are usually formed in relation to the trabecula:, and are continuous with
those structures from an early stage. The auditory capsules in some cases
arise as outgrowths of the parachordals, in others as independent cartilages,
each of which, however, soon unites with the parachordal of its own side. As
 ZOOLOGY
development proceeds, the trabeculre and parachordals become fused into a
single basal plate (Fig. 67, B, b. cr.) underlying the brain. The skull-floor thus
formed gives off vertical upgrowths on each side or, in some groups, the side
wall cartilage may be of independent origin. The vertical components close
in above to a greater or less extent, and so give rise to a more or less com-
plete cranium or brain-case enclosing the brain and the organs of olfaction
and hearing, and furnishing open cavities or orbits for the eyes.
In the continuous solid cranial box thus formed, certain definite regions
can be distinguished. These are firstly a posterior or occipital region, formed
from the parachordals, united or articulated with the anterior end of the
vertebral column, and presenting a large aperture, the foramen magnum
FIG. 67-~rtilagin­
OUS skull. A . Left side ;
B . in sagittal section.
au. cp. auditory capsule;
b. br. I-5. basi-branchials;
b. cr. basis cranii; b. hy.
basi-hyal; c. br. cerato-
branchial; c. hy. cerato-
hyal; ep. br. epi-bran-
chial; ep. hy. epi-hyal;
jon. fontanelle; for. mag.
foramen magnum; h. br.
hypo-branchia l; h. hy.
hypo-hyal ; hy. m. hyom-
andibular; lb. I - 4. lab-
ial cartilages; mck. c.
Meckel's cartilage; m.
eth mesethmoid; nv. I -
Io, foramina for cranial
nerves; olf. cp. olfactory
capsule; pal. qu. palata-
quadrate; ph. br. pharyn-
gobranchial; qu. quad-
rate; r. rostrum; s. t.
pituitary fossa or sella
turcica.

(Fig. 67, B , for. mag.). Through this the spinal cord is continuous with the
brain. There is secondly an auditory region formed by the two prominent
auditory capsules (A, au. cp.). Thirdly, there is a trabecular region, including
all the rest. The last-named is again divisible into an interorbital region,
between the orbits or eye-sockets ; an olfactory region, constituted by the
olfactory capsules (olf. cp.) and by a median vertical plate, the mesethmoid
(B, m. eth.), which separates them from one another; and a pre-nasal region or
rostrum (r.) extending forwards from the mesethmoid and forming a more or
less well-marked anterior prolongation of the cranium. The cavity for the
brain (B) extends from the foramen magnum behind to the olfactory region in
front. Its floor is formed from the basal plate of the embryo and is called the
basis cranii (b. cr.). Its roof is always incomplete, for it contains one or more
apertures or fontanelles (Jon.). These are closed only by membranes. Their
retention is due to the imperfect union above the side-walls.
 PHYLUM CHORDATA 93
In the walls of the brain-case are apertures or foramina through which pass
the cranial nerves. The most important such apertures are as follows : The
olfactory foramina (nv. I) are situated at the anterior end of the cerebral cavity,
one on each side of the mesethmoid. Through these run the first cranial or
olfactory nerves. The optic foramina (nv. 2) occur in the interorbital region
and are traversed by the second cranial or optic nerves. The trigeminal
foramina (nv. S) lie anterior to the auditory capsule and allow egress to the
fifth or trigeminal nerves. The auditory foramina (nv. 8), in the inner wall of
the auditory capsule, carry the short eighth (auditory) nerves. The vagtts
foramina (nv. 10), carrying the large wandering tenth or vagus nerve, lie
immediately posterior to the auditory capsule.
In addition to the elements of the brain-case-parachordals, trabeculre,
and auditory capsules-there enter into the composition of the skull another
set of elements called visceral bars. These are cartilaginous rods formed just
outside the endodermal lining of the pharynx between the gill-slits. They
thus encircle the pharynx like a series of paired half-hoops (Fig. 63, B, vs. b.).
The corresponding right and left bars become united with one another below
by an unpaired cartilage (Fig. 67, A, b. br.), forming a visceral arch. The
unpaired ventral parts may unite successive arches with one another in the
middle ventral line, thus giving rise to a more or less basket-like visceral
skeleton. The visceral skeleton has a segmental arrangement, being arranged
in an antero-posterior series, whereas in the cranium there is no clear indication
of segmentation. There is, however, no exact correspondence between the
segments of the visceral skeleton and the metameres. The visceral arches
vary in number from four to nine. The foremost is distinguished as the man-
dibular arch, and lies just behind the mouth. The second is called the hyoid
arch. The rest are known as branchial arches because they support the gills in
aquatic forms.
In all Craniata except the Agnatha the mandibular arch becomes modified
into jaws for the support of the mouth. Each mandibular bar divides into a
dorsal and a ventral portion called respectively the palato-quadrate cartilage
(Fig. 67, A, pal. qu.) and Meckel's cartilage (mck. c.). The palata-quadrates
grow forwards along the upper or anterior margin of the mouth, and may unite
with one another in the middle line, forming an upper jaw. Meckel's cartilages
similarly extend along the lower or posterior margin of the mouth and unite
in the middle line, forming the lower jaw. The quadrate (qu.), or posterior end
of the palata-quadrate furnishes an articulation for the lower jaw, and often
acquires a connection with the cranium, thus serving to suspend the jaws from
the latter. Thus each jaw arises from the union of paired bars. The final
result is two unpaired transverse structures, one lying in the anterior, the other
in the posterior margin of the transversely elongated mouth, and moving in a
vertical plane.
94 ZOOLOGY

The hyoid bar usually becomes divided into two parts: a dorsal, the
hyomandibular (hy.m.), and a ventral, the hyoid cornu. The latter is again
divisible from above downwards into segments called respectively epi-hyal
(ep.hy.), cerato-hyal (c.hy.), and hypo-hyal (h.hy.). The median ventral element
of the arch, or basi-hyal (b.hy.), supports the tongue. 1
Hyoid and mandibular are 'visceral' bones, developing in association
with the alimentary tract. Other 'visceral' bones are the os cordis in the heart
of some ungulates, and the diaphragmatic ossification in camels. The bacuhtm
(os penis, os priapi), an ossification in the fibrous septum between the corpora
cavernosa (p. 8go) and the intermuscular bones of many fishes (p. 301), also
come into this category (pp. 74, 88).
In early bony fishes-e.g. the Acanthodii (p. 207)-the jaws are attached
by ligaments to the neurocranium. The hyoid arch, instead of being modified
to support the upper and lower jaws, is as complete as are the posterior arches.
The gill-slit lying in front of the hyoid arch, instead of being reduced to a spiracle
which may even be closed and disappear, is fully functional as a complete gill.
This type of jaw suspension is autodiastylic. When the hyomandibular becomes
attached to the hinge of the upper and lower jaws, and supports them so that
the attachment to the skull is lost, the suspension is termed hyostylic. This is
the form of attachment of the majority of fishes. In some early forms, and in
a few living fishes, there is an attachment to the skull as well as to the hyoman-
dibular. This condition is termed ampltistylic. It appears that in f1shes the
autodiastylic attachment is the most primitive. The amphistylic condition
was derived from it. The hyostylic condition is the most recent. It is
probable that the last-named condition has been arrived at independently in
the later Chondrichthyes and bony fishes.
Finally, the upper jaw, instead of merely lying against the skull and attached
only by ligaments, as in the autodiastylic phase, may become fused with it.
This condition is termed autosystylic. Two groups of fishes (Dipnoi and Holo-
cephali) have the jaws attached to the skull and no hyoid suspension, but the
condition in each case is different. The Holocephali have a complete hyoid
arch which is free from the cranium, a condition which, according to different
interpretations, may be an approach to the autodiastylic condition or a
secondary modification such as is shown by the skates (p. 226). To this kind
of suspension the term ltolostylic has been given. In the Dipnoi and all tetra-
pods the hyoid arch becomes broken up and the hyomandibular attached to
the skull, where it soon enters into the service of the ear region. This condition
is usually termed autostylic. As this term, however, really covers more than
one condition it is now divided into autosystylic and holostylic.
The branchial arches become divided transversely into dorsoventral seg-
1 Jarvik's reconstruction of the head of Eusthenopteron suggests that many structures in the
neurocranium are derived from the dorsal components of the three pro·otic visceral arches.
 PHYLUM CHORDATA 95

ments called respectively pharyngo-branchial (ph.br.), epi-branchial (ep.br.),

cerato-branchial (c.br.), and hypo-branchial (h.br.). The visceral skeleton thus
acquires the character of an articulated framework which allows of the dilata-
tion of the pharynx during swallowing and the inspiration of water and of its
more or less complete closure at other times (Fig. 68).
In connection with, and always superficial to, the rostrum, olfactory
capsules, and jaws, are frequently found labial cartilages (lb. 1-4). These
sometimes attain considerable dimensions.
In certain fishes (such as elasmobranchs) the cartilages of the skull and
vertebral column become more or less encrusted by a superficial granular

Q .. m/<A<c:TH
f'.Af/PH
CPA

"
B.5PH
I'I'IUH

~VO

FIG. 68.-Bony skull. A. Sagittal section; B . transverse section of occipital region; C, of

parietal region; D, of frontal region; E, of ethmoidal region. Cartilaginous parts are dotted ;
replacing bones are marked in thick type, investing bones in italics. M ck. C. Meckel's cartilage;
Nv. I - IO, foramina for crani al nerves; r. rostrum; s. I. sella turcica or pit uitary fossa. Re-
placing bones- AL.SPH. pterosphenoid ; ART. articular; B .BR. basi-branchial; B .HY . basi-
hyal; B.OC. basi-occipital ; B .SPH. basi-sphenoid; C.BR. cerato-branchial; C.HY. ceratohyal;
E C. ETH. lateral ethmoid; EP.BR. epi-branchial ; EP.HY. epi-hyal; EP.OT. epi-otic ; EX.OC.
ex-occipital ; H.BR. hy po-branchial; H .HY. hypo-hyal ; HY.M. hy omandibular ; M.ETH.
mesethmoid; OP.OT. opisthotic; OR.SPH. orbito-sphenoid; PAL. pa latine ; PH.BR. pharyngo-
branchial; PR.OT. pro-otic; PR .SPH. pre-sphenoid; PTG. pterygoid; QU . quadrate; S.OC.
supra-occipita l. Investing bones-DNT. d entary; FR . frontal ; MX. maxilla ; NA . nasal;
PA. parietal ; PA.SPH. para-sphenoid; PMX. premaxilla; SQ. squamosal ; VO. , v.o. vomer.

deposit of lime-salts, giving rise to calcified cartilage (p. 73). In all the other
forms true ossification t akes place, the cartilaginous skull becomes complicated,
and to a greater or less extent replaced, by distinct bones. Of these there are
two kinds, replacing or 'cartilage' -bones and investing or 'membrane ' -bones.
Replacing bones may begin by the deposition of patches of bony matter within
the cartilage (endochondral ossification).
As development proceeds, the bone first formed (primary bone) is mostly
replaced by secondary bone. Interstitial lamellre (or lamina, see p. 73) are
the persisting primary bone. The perichondrium is transformed to peri-
osteum, giving rise to a subjacent collar of bone. This is perichondral or
periosteal ossification.
The bones in question are usually said to be preformed in cartilage, i.e.
g6 ZOOLOGY
they replace originally cartilaginous parts. In the case of investing (membrane)
bones, centres of ossification also appear, in constant positions, in the fibrous
tissue outside the cartilage. They may remain quite independent of the original
cartilaginous skull and its replacing bones, so as to be readily removable by
boiling or maceration; or they may eventually become, as it were, grafted on
to the cartilage, in which case all distinction between investing and replacing
bones is lost in the adult. The investing bones are to be looked upon as
portions of the exoskeleton which have retreated from the surface and acquired
intimate relations with the endoskeleton.
The replacing bones have a very definite relation to the regions of the
cartilaginous cranium. In the occipital region four bones are formed, sur-
rounding the foramen magnum. These are a median ventral basioccipital
(Fig. 68, A and B, B.OC.), paired lateral exoccipitals (EX.OC.), and a median
dorsal supraoccipital (S.OC.). In each auditory capsule three ossifications
commonly appear. These are a pro-otic (A, PR.OT.) in front, an opisthotic
(OP.OT.) behind, and an epiotic over the arch of the posterior semicircular
canal of the ear (see below). In front of the basioccipital a bone called the
basisphenoid (A and C, B.SPH.) is formed in the floor of the skull. The
basisphenoid appears in the position of the posterior ends of the trabeculre,
and bears on its upper or cranial surface a depression, the sella turcica (s. t.),
for the reception of the pituitary body. Connected on either side with the
basisphenoid are paired bones, the latero- or ptero-sphenoids, which help
to furnish the side-walls of the interorbital region. In the Mammalia, their
place is taken by the alisphenoids. The basisphenoid is continued forwards
by another median bone, the presphenoid (A and D, PR.SPH.) with which
paired ossifications, the orbitosphenoids (OR.SPH.), are connected and com-
plete the side-walls of the inter-orbital region. The basioccipital, basi-
sphenoid, and presphenoid together form the basis cranii of the bony skull.
A vertical plate of bone, the mesethmoid (M.ETH.), appears in the posterior
portion of the cartilage of the same name. The outer walls of the olfactory
capsules may be ossified by paired ectoethmoids (EC.ETH.).
So far the cranial cavity has its hinder region alone roofed over by bone,
viz. by the supraoccipital : for the rest of it the replacing bones furnish floor
and side-walls only. This deficiency is made good by two pairs of investing
bones. These are the parietals (PA.), formed immediately in front of the supra-
occipital and usually articulating below with the pterosphenoids, and the
frontals (FR.), placed in front of the parietals, and often connected below with
the orbitosphenoids. A pair of nasals (N A.) are developed above the olfactory
capsules and immediately in advance of the frontals. Below the base of the
skull two important investing bones make their appearance, the vomer (vo.)-
which may be double-in front, and the parasphenoid (PA.SPH.) behind.
The result of the peculiar arrangement of replacing and investing bones
 PHYLUM CHORDATA 97
just described is that the brain-case, in becoming ossified, acquires a kind of
secondary segmentation, being clearly divisible in the higher groups, and
especially in the Mammalia, into three quasi-segments. These are: (r) the
occipital segment (B), formed by the basioccipital below, the exoccipitals at
the sides, and the supraoccipital above 1 ; (2) the parietal segment (C), formed
by the basisphenoid below, the alisphenoids laterally, and the parietals above ;
and (3) the frontal segment (D), constituted by the presphenoid below, the
orbitosphenoids on either side, and the frontals above. It must be observed
that this segmentation of the cranium is quite independent of the primary
segmentation of the head, which is determined by the presence of myomeres
and by the relations of the cerebral nerves.
The cranial bones have constant relations to the cerebral nerves. The
olfactory nerves (A, Nv. I) pass out one on either side of the mesethmoid, the
optic nerves (Nv. 2) through or immediately behind the orbitosphenoids,
the fifth nerves (Nv. 5) through or immediately behind the pterosphenoids, and
the tenth nerves (Nv. Io) through or immediately in front of the exoccipitals.
It will be seen that a clear distinction can be drawn between the primary
cranium, chondrocranium, or neurocranium (formed by the fusion of the para-
chordals, auditory capsules, and trabeculre, and consisting of an undivided
mass of cartilage more or less replaced by bones). and the secondary cranium or
osteocranium, which is modified by the super-addition of investing bones.
A similar distinction may be drawn between the primary and secondary
jaws. The primary upper jaw, or palato-quadrate, becomes ossified by three
chief replacing bones on each side, the autopalatine (A, PAL.) in front, then the
autopterygo£d (PTG.), and the quadrate (QU.) behind. The last-named furnishes
the articulation for the lower jaw or mandible. In the higher classes the
front of the primary upper jaw is not a distinct cartilaginous structure, and
the palatine and pterygoid are developed as investing bones. The secondary
upper jaw is constituted by two pairs of investing bones, the premaxilla (PMX.)
and the maxilla (MX.), which in bony skulls furnish the actual anterior boun-
dary of the mouth, the primary jaw becoming altogether shut out of the gape.
The proximal end of the primary lower jaw ossifies to form a replacing bone, the
articular (ART.), by which the mandible is hinged. The rest of it remains as
a slender, unossified Meckel's cartilage (Mck. C.), which may disappear entirely
in the adult or its symphysial ends may sometimes ossify as mentomeckelian
bones. The secondary lower jaw is formed by a variable number of investing
bones, the most important of which is the dentary (DNT.). In the Mammalia
the dentary forms the entire mandible, and articulates, not with the quadrate,
but with a large investing bone formed external to the latter, and known as the
squamosal (SQ.).
I With the occipital segment in many fishes are amalgamated one or several of the most
anterior vertebrre.
g8 ZOOLOGY
In the hyoid arch a replacing bone, the hyomandibular (HY.M.), appears
in the cartilage of the same name, and ossifications are also formed in the
various segments of the hyoid cornua (EP.HY., C.HY., H.HY., B.HY.) and of
the branchial arches (PH.BR., EP.BR., C.BR., H.BR., B.BR.). In the air-
breathing forms both hyoid and branchial arches undergo more or less complete
atrophy, the whole gill-bearing apparatus becoming reduced mainly to a small
hyoid bone (which supports the tongue) and the stapes of the middle ear.

FIG. 6g.-Pelvic fins. Three stages in

development. In A, the anterior pterygio·
phores on the right side (Rad.) have united to
form a basal cartilage (Bas.); in B the
basalia (Bas.) are fully formed and are uniting
at* to form the pelvic girdle; inC the pelvic
girdle (G.) is fully constituted, and at t has
segmented from the basalia on the right side.
Cl. cloacal aperture. (After Wiedersheim.)

The skeleton of the median fins is formed of a single row of cartilaginous

rays or pterygiophores (Fig. 6g), lying in the median plane, and primitively
more numerous than the vertebrre. They may ossify, and be supplemented
by dermal fin-rays, of varying composition, developed in the dermis towards
the free margin of the fin. The latter are clearly exoskeletal structures.
Both pectoral and pelvic fins are supported by pterygiophores or radialia
(Fig. 6g, Rad.). The basal or proximal ends of these are articulated with stout
cartilages, the basalia (Bas.), which are often replaced by bones, that serve to
strengthen the fin at its point of union with the trunk.
The structure of the paired fins varies in different groups of fishes (Fig. 70).
It is supposed that they arose from an originally continuous fold along each
side of the body from which the fins became constricted. This view is to some
extent supported by embryological evidence and by the condition of the paired
 PHYLUM CHORDATA 99
fins of Cladoselache (p. 222), which, being without a posterior notch, have the
appearance of being the remains of a previously continuous fin. The rows of
spines on each side of the acanthodians (Fig. 134) and of the agnathous Anaspida
(Fig. 106) are likewise suggestive of a continuous fold. When such a fin as
that of Cladoselache (pleurorhachic type) became free from the body-wall,
it would consist of an axis of basalia and a fringe of radialia on the preaxial
side. When more radials appeared on the postaxial side, as in Pleuracanthus,

FrG.7o.-Pectoral fins. A, Cladoselache; B, Cladodus; C, Pleuracanthus; D,a 'shark'; E,

Neoceratodus; F, a primitive Actinopterygian; G, a teleost. The dotted lines suggest possible
lines of junctional evolution.

the fin reached the condition known as the archipterygium (mesorhachic type).
This type of fin is also found in the crossopterygians and dipnoans (p. 353). In
the majority of Chondrichthyes (p. 219) the fin has three basalia, termed the
pro-, meso-, and meta-pterygium, respectively, and a number of radialia arranged
around them. The fin of the Polypterini (p. 290) has a somewhat similar
appearance, though probably of a different origin. The Actinopterygii
(p. 282) show a further modification, the basalia being lost and the radials
reduced to small ossicles which lie within the body-wall, and not in the fin web.
Besides the basals and radials, the fins are all strengthened in the free part by
fin-rays, which are of several kinds : horny, fibrous, or modified scales.
100 ZOOLOGY
In all classes above the fishes the paired fins are, as we have seen, replaced
by five-toed or pentadactyle limbs (seep. 382). These are supported by bones,
probably to be looked upon as greatly modified pterygiophores, and obviously
serially homologous in the fore- and hind-limbs. In the proximal division of

FIG. 71.-Tetrapod girdles, manus, and pes. A, pectoral, B, pelvic girdles, and C, generalised
skeleton of hand and foot. actb. acetabulum; gl. glenoid cavity; p. cor. pro-coracoid; SCP.
scapula; CL. clavicle; COR. coracoid; HU humerus; of shoulder blade. FE. femur; IL. ilium;
IS. ischium; PU. pubis. Bones of the fore-limb are underlined. Note that the astragalus is not
(as shown in the figure for comparative purposes) composed of two separate elements (seep. 859).

each limb there is a single rod-like bone. In the forelimbs this is the humerus
(Fig. 71, HU.). In the hind-limb it is the femur (FE.) or thigh-bone. In the
middle division there are two elongated bones, an anterior, the radius, and a
posterior, the ulna, in the fore-limb; an anterior, the tibia, and a posterior,
the fibula, in the hind-limb. Next follow the bones of the hand and foot,
 PHYLUM CHORDATA IOI

which are again divisible into three sets. In the fore-limb these consist of carpals
or wrist-bones, metacarpals or hand-bones, and phalanges or finger-bones. In
the hind-limb they consist of tarsals or ankle-bones, metatarsals or foot-bones,
and phalanges or toe-bones. The carpals and tarsals consist typically of three
rows of small nodules of bone or cartilage, the proximal row containing three,
the middle two, and the distal five elements.
The three proximal carpals are called respectively radiale, intermedium,
and ulnare, those of the middle row the first and second centralia, those of the
third row the five distalia, the separate elements being distinguished by
numbers, counting from the anterior or radial edge of the limb. In the tarsus
the bones of the first row are known respectively as tibiale, intermedium, and
fibulare, those of the second row as centralia, and those of the third as distalia.
The metacarpals and metatarsals are five rod-like bones, one articulating
with each distale : they are followed by the phalanges, of which each digit
may have from one to five. The first digit of the fore-limb is distinguished as
the pollex or thumb, that of the hind-limb as the hallux or great toe. The
fifth digit of each limb is the minimus.
In connection with the paired appendages are formed supporting Umb-
girdles. These occur in the portions of the trunk adjacent to the appendages
and serve for the articulation of the latter. In the embryonic condition they
are continuous with the basalia and are probably ingrowths of the primitive
fin-skeleton (Fig. 6g). The shoulder-girdle or pectoral arch has primarily the
form of paired bars, which may unite in the middle ventral line so as to form an
inverted arch. Each bar-i.e. each half of the arch-furnishes a concave or
convex glenoid surface (Fig. 71, gl.) for the articulation of the pectoral fin or
fore-limb, and is thereby divided into two portions-a dorsal or scapular region,
above the glenoid surface, and a ventral or coracoid region below it. The
coracoid region is again divisible, in all classes above the fishes, into two
portions: an anterior, the pre-coracoid (p. cor.), and a posterior, the coracoid
proper. Each of these regions commonly ossifies-a replacing bone, the
scapula (SCP.), appearing in the scapular region, another, the coracoid (COR.),
in the coracoid region, while in relation with the pre-coracoid is formed a bone,
the clavicle (CL.), largely or entirely developed independently of pre-existing
cartilage.
The constitution of the hip-girdle, or pelvic arch, is very similar. It consists
originally of paired bars, which may unite in the middle ventral line, and are
divided by the acetabulum (Fig. JI, actb.), the articular surface for the pelvic
fin or hind-limb, into a dorsal or iliac region, and a ventral or pubo-ischial
region, the last named being again divisible, in all classes above the fishes, into
an anterior portion, or pubis, and a posterior portion, or ischium. Each region
is replaced in the higher forms by a bone. The pelvic girdle thus consists of a
dorsal ilium (IL.) serially homologous with the scapula, an antero-ventral
VOL. II. H
102 ZOOLOGY

pubis (PU.) which is homologous with the pre-coracoid and clavicle, and a
postero-ventral ischium (IS.) which is homologous with the coracoid. The
long bones of the limbs are divisible each into a shaft, and proximal and distal
extremities. When ossification takes place the shaft is converted into a tubular
bone, the cartilaginous axis of which is absorbed and replaced by vascular
hrematopretic (blood-forming) marrow. The extremities become simply cal-
cified in the lower forms, but in the higher a distinct centre of ossification may
appear in each, forming the epiphysis, which finally becomes ankylosed to the
shaft.
Alimentary Canal and Associated Structures.-The alimentary canal begins
at the mouth and is divisible into buccal cavity (Fig. 63, A, buc. c.), pharynx
(ph.), resophagus, stomach (st.), and intestine (int.), the latter often communi-
cating with the exterior by the posterior aperture of a cloaca, a chamber which
also receives the urinary and genital ducts. The buccal cavity is developed
from the stomodreum of the embryo. The proctodreum gives rise to a very
small area in the neighbourhood of the anus, or, when a cloaca, is present, to
the external portion of the latter. All the rest of the canal is formed from the
mesenteron, and is therefore lined by an epithelium of endodermal origin.
In fishes and in the embryos of higher forms the pharynx communicates with
the exterior by the gill-slits (i. br. a, I-7). It communicates with the stomach
by the resophagus. The stomach (st.) is usually bent upon itself in the form
of a U. The intestine (int.) is generally more or less convoluted, hence the
stomach and intestine are together considerably longer than the enclosing
abdominal cavity. In the embryo the intestine is sometimes continued back-
wards into the hremal canal by an extension called the post-anal gut (p. a. g.),
which may perhaps indicate that the anus has shifted forwards in the course of
evolution.
The epithelium of the buccal cavity is usually many-layered, like that of
the skin, of which it is developmentally an in-turned portion. The pharynx
and resophagus have also a laminated epithelium, but the rest of the canal is
lined by a single layer of cells underlaid by a layer of connective-tissue, the
deeper part of which is called the sub-mucosa. Epithelium and connective
tissue together constitute the mucous membrane. The mucous membrane of the
stomach and intestine usually contains close-set tubular glands. The digestive
and other functions of these glands will be dealt with in individual animal
types. Outside the mucous membrane are layers of smooth (plain or unstriped)
muscle, usually an internal circular and an external longitudinal layer. Exter-
nally the intra-crelomic portion of the canal is invested by peritoneum formed
of a layer of connective-tissue next the gut and a single-layered crelomic
epithelium facing the body-cavity.
In connection with the alimentary canal certain very characteristic struc-
tures are developed. In the mucous membrane of the mouth calcifications
 PHYLUM CHORDATA I03

appear and form the teeth which, homologous with placoid scales (p. 83), usually
occur in a row along the ridge of each jaw, but may be developed on the roof of
the mouth, on the tongue, and even in the pharynx. A tooth is usually formed
of three calcified tissues-dentine, enamel, and cement (see footnote, p. JI).
The bulk of the tooth is made up of dentine (Figs. 72, 59), which occurs in three
forms. Hard dentine consists of a matrix of animal matter strongly impreg-

FIG. 72.-Dermal and buccal armour: Placoid

scales and permanent teeth. Left: Incisor of
mammal (Homo). The root is buried deep in the
jaw-bone. The pulp-cavity is surrounded by a
layer of odontoblasts (not arrowed) and contains
vascular and nervous tissue that enters through
the small apical foramen . R ight: Placoid scale of
selachian . The basal plate (of dentine) is em -
bedded in the stratum vasculare of the dermis and
receives it as pulp. Septa of t he vascular layer connect with the sub-cutaneous tissue
overlying the muscles. (Not to scale.) (Redrawn after Hamilton, et al., and Kendall.)

nated with lime-salts and permeated by delicate, more or less parallel, tubules
containing organic fibrils. V asodentine is permeated with blood-vessels, and
consequently appears red and moist in the fresh condition. Osteodentine
approaches bone in its structure and mode of development. The free surface
of the tooth is usually capped by a layer of enamel, a dense prismatic
structure containing not more than 3 to 5 per cent. of animal matter. It is,
therefore, the hardest tissue in the body. The cement coats that portion
of the tooth which is embedded in the tissues of the jaw, and sometimes forms
a thin layer over the enamel; it has practically the structure of bone. At
104 ZOOLOGY

the inner end of the tooth there is frequently an aperture leading into a
cavity filled in the fresh condition by the tooth-pulp, which is abundantly
supplied with nerves and blood-vessels. For tooth development, see p.
862).
In some fishes the scales or elements of the dermal exoskeleton imperceptibly
pass into the teeth over the ridges of the jaws, and are identical with them in
structure so that there can be no doubt as to the homology of the two. Teeth
are. in fact, portions of the exoskeleton which have migrated from the skin
into the buccal cavity, and even into the pharynx, and have there increased
in size and assumed special functions.
In terrestrial Craniata buccal glands open by ducts into the mouth. The
most prominent of these are the racemose salivary glands, which secrete a fluid
that in some animals contains an enzyme capable of converting starch into
sugar. There are also two larger, and highly characteristic, digestive glands
(liver and pancreas) in the abdominal cavity, both developed as outpushings of
the intestine, but differing greatly from one another in their fully developed
state. The liver (Fig. 63) is a dark-red organ of relatively immense size and
manifold functions. Among these are the secretion of bile (which emulsifies
fats), and the formation of glycogen (animal starch), which, after being stored
in liver-cells, is discharged to the blood in the form of sugar. The liver
is formed of a mass of polyhedral cells with numerous minute intercellular
spaces which receive the bile secreted from the cells and from which it passes
into the hepatic ducts. The pancreas (Fig. 63, A, pn.) is a racemose gland
that secretes pancreatic juice, the various enzymes of which act upon proteins,
sugars, and fats. The ducts of both glands usually open into the anterior end
of the intestine. That of the liver (b. d.) generally (but not always) gives off a
blind offshoot ending in a capacious dilatation, the gall-bladder (g. b.). In
this bile is stored. We thus have one or more hepatic ducts conveying the bile
from the liver and meeting with a cystic duct from the gall-bladder, while from
the junction a common bile-duct leads into the intestine. When a common
duct carries both bile and pancreatic juice it is a hepato-pancreatic dttct.
Another important and characteristic organ in the abdomen of Craniata is
the spleen (spl.), a blood-storage organ of variable size and shape, attached to
the stomach by a fold of peritoneum. It has no duct, being connected with
the general circulation. It is formed of a pulpy substance containing numerous
erythrocytes, many of them in process of disintegration. Dispersed through
the pulp are masses of leucocytes (formed by splenic components of the
reticulo-endothelial system) which pass into the general circulation.
The parathyroid and thymus glands are each formed in connection with the
alimentary canal. The thyroid (thd.) is developed as an out-pushing of the floor
of the pharynx which becomes shut off, and forms an endocrine organ of con-
siderable size. Its final position varies considerably in the different classes.
 PHYLUM CHORDATA 105

It is probably homologous with the endostyle of the Tunicata and of Amphioxus,

which is an open groove on the ventral side of the pharynx (p. 24). This
view is supported by the condition of the parts in the larval Lamprey (p. 196).
The thymus is developed from the epithelium of the gill-pouches, usually the
3rd and 4th. In the adult it may take the form of a number of separate gland-
like bodies lying above the gills, or may be situated in the neck, or even in
the thorax. The thymus is essentially a lymphoid body and probably not an
endocrine gland. The endocrine parathyroids also form by budding from the
3rd and 4th gill-pouches (Fig. 73).
The whole intra-abdominal portion of the alimentary canal, as well as the
liver, pancreas, spleen, and, indeed, all the abdominal viscera, are supported
by folds of peritoneum, called by the general name of mesentery (Fig. 63, C,
mes.) and having the usual relation to the parietal and visceral layers of the
peritoneum. Besides providing support, and allowing an essential degree of
mobility in contractile (e.g. gut) and other structures, mesenteries provide
bridges along which blood vascular and nerve connections can reach the viscera.
Organs of Respiration.-Two principal kinds are found in Craniata: water-
breathing organs or gills, and air-breathing organs or lungs. Both may occur
in the same species. Gills arise as a series of paired pouches of the pharynx
which extend outwards, or towards the surface of the body, and finally open
on the exterior by the gill-slits already noticed. Each gill-pouch thus com-
municates with the pharynx by an internal and with the outside water by an
external branchial aperture. It is separated from its predecessor and from its
successor in the series by stout fibrous partitions, the interbranchial septa.
The membrane forming the anterior and posterior walls of the pouches is
raised into a number of horizontal ridges, the branchial filaments, which are
abundantly supplied with blood coursing through thin-walled capillaries. A
current of water entering at the mouth passes into the pharynx, thence by the
internal gill-slits into the gill-pouches, and finally makes its way out by the
external gill-slits, bathing the branchial filaments as it goes. The exchange of
carbon dioxide for oxygen takes place in the blood-vessels of the branchial
filaments, which are, therefore, the actual organs of external respiration (Fig. 73).
As already mentioned, the walls of the pharynx are supported by the
visceral arches, which surround it like a series of incomplete hoops, each half-
arch or visceral bar being embedded in the inner or pharyngeal side of an inter-
branchial septum. Thus the visceral arches alternate with the gill-pouches,
each being related to the posterior set of filaments of one pouch and the anterior
set of the next. In the higher bony fishes (e.g. a trout) the interbranchial septa
become reduced to narrow bars enclosing the visceral arches, so that a double
set of free branchial filaments springs from each visceral bar and constitutes
a single gill. Thus an entire gill or holobranch (Fig. 73) is the morphological
equivalent of two hal£-gills-hemibranchs or sets of branchial filaments
 106 ZOOLOGY

pharyngo· water current

tympanic tube entering mouth

c 0
FIG. 73.-Pbaryngeal region: Brancbiogenic organs. A. Embryonic. Ventral diagrammatic
view of original sites and relationships of branchiogenic endocrine (p. 147) and other structures.
Part of the wall is removed to show the interior of the pharyngeal pouches. B. Adult, selachian .
Diagram, showing relationship of mouth (lacking valve, cf. C, D below) and spiracle (xst gill
pouch, see p. 94) and the direction of the respiratory current through the gill-clefts (with
elongated inter-branchial septa, see Fig. 153. p. 227). C and D. Adult, teleost. The xst pouch
becomes a functional gill. Note role of oral valve and opercula in respiration. The cesophageal
valve is not shown. (Modified from Boyd, et al., and Storer.)

belonging to the adjacent sides of two consecutive gill-pouches. On the other

hand, a gill-pouch is equivalent to the posterior hemibranch of one gill and
the anterior hemibranch of its immediate successor.
In some Amphibia (p. 381) water-breathing organs of a different kind are
found. These are the external gills. They are developed as branched out-
growths of the body-wall in immediate relation with the gill-bars. 'Except
for their point of origin there is little difference between external and internal
 PHYLUM CHORDATA !07

gills. The tissues entering into their formation are probably the same'
(Noble).
Lungs are found almost universally in Craniata from the Dipnoi (lung-
fishes) (p. 361) upwards. They are developed as a hollow outpushing from the
ventral wall of the embryonic fore-gut or anterior part of the alimentary

respiratory
bronchiole
branch of
pulmonary artery
artery

vein

capillary network
in alveolar wall 3 pleural layers
(with elastic networks)
FIG. 74.- Respiratory system: Portion of a pulmonary lobule in Man. Nerves omitted. (From
Maximov and Bloom, after Braus.)

canal. This passes backwards and upwards, usually dividing into right and
left divisions, finally coming to lie in the dorsal region of the crelom. The
inner surface of the single or double lung thus formed is raised into a more or
less complex network of ridges and so increases the vascularised surface exposed
to the action of the air. In the higher forms, the ridges, increasing in number
and complexity, and uniting with one another across the lumen of the lung,
convert it into a sponge-like structure. Within, an immense system of terminal
 ro8 ZOOLOGY
bronchioles carry inspired air into the minute pouch-like alveoli, the walls of
which are invested with capillaries. Here gaseous exchanges occur (Fig. 74).
The respiratory epithelium is, of course, endodermal. Since (except in birds)
the lungs are blind sacs, special mechanisms are necessary for renewing the
air contained in them. These are described in the different animal types (e.g.
p. 371, cf. p. 587). Auxiliary respiratory
~iZkLL~·-··-·-r.L organs occur in various groups (e.g. pp.
E a..c. 338, 339).
\U\1!\II\I\llllll\t!\l,\\tll"'''\\\\\\\\\\\\\\mm\\\\\\W\\l\\\\)\\)\111!\\\llDt
In many bony fishes, but not in elasmo-
_ ~:~gggnw~;t@:·-~··--·r.z. the position occupied in air-breathers by
branchs, there occurs, in approximately
D
· a.c. ~=liinJuul\:\7-»
..'.·· the lungs, a structure called the swim-,
I!I!!I\UI!I!jiQtl!!ll!l!!l!!!l!l!l!!l!!!l\!r!!II!\I!II!\I!!!!!I!!U\!IIIII\\!!\\111
!J.· ,· air-, or gas-bladder (Fig. 75). This organ
is often considered to be homologous with
c ~1111111!1/!!("!11('/lr!l!!ll'llf(f!!lrlt!!!(!!!Wl!l!l!flll.
/ ..··· a.c. · the lung, but the structures may, in fact,
have evolved independently. The swim-
g." ·. . l.l ~~-r.l bladder arises in teleosts (p. 293) as an
outgrowth from the dorsal or lateral walls

·~ ...
of the foregut.
In the lung fishes (p. 361) and the
Polypteridre (p. 290) the lungs, on the
other hand, arise as down-growths from
A ~...gfiiiiif!!it""'":~;;,,""uul!!f
the pharyngeal floor (p. 371). Both struc-
g: l.l. tures are innervated by the autonomic
FrG. 75.-Lung and swim-bladder in nervous system (p. rr9), but their nervous
Osteichthyes. From left side. A, primitive
symmetrical arrangement; B, Polypterus; connections are different. The lungs are
C, Neoceratodus; D, Physostomatous Tele. supplied by right and left pulmonary
ost; E, Physoclistic Teleost; a.c. alimen-
tary canal; g. glottis; 1.1. left lung; r.l. arteries which arise from the last pair of
right lung. (From Goodrich, after Kerr.)
epibranchial arteries. In the adult, the
swim-bladder receives its arterial blood from the dorsal aorta.
The swim-bladder may be either open or closed. In teleosts it is essentially
a hydrostatic organ (p. 339), but may be adapted, with the development of
alveoli, to respiratory function in inhabitants of swamp-waters of low oxygen
content. The swim-bladder can also exist as a pressure register or as an acces-
sory hearing organ, or as an apparatus of sound-production or resonation (p.
340). In some species, especially those which live in the ocean depths, the
swim-bladder may be reduced, or even lost, or sometimes converted to fat-
storage function.
Blood-vascular System.-This attains a far higher degree of complexity than
in any of the groups previously studied. Its essential features can be under-
stood from a general description of the circulatory organs of fishes.
The heart (Figs. 63 and 76) is a muscular organ contained in the pericardia!
 PHYLUM CHORDATA

sus, opens into the atrium.

The atrium opens into the
ventricle. They do not,
however, lie in a straight
line, but in a zigzag
fashion, so that the sinus
and atrium are dorsal in
position, the ventricle
ventral. Usually a fourth
chamber, the conus arte-
riosus (c. art.), is added in
front of the ventricle.
The various chambers are
separated from one an-
other by valvular aper-
tures, varying between
groups, which allow blood
to flow in one direction
only, viz., from behind for-
wards-i.e. from sinus to
atrium, atrium to ventri-
de, and ventricle to conus.
The heart is made of
cardiac muscle which is
particularly thick and
strong in the ventricle.
It is lined internally by
endothelium and covered
externally by the visceral
layer of the pericardium.
The myogenic nature
of the heart-beat was first
demonstrated in poikilo-
thermous vertebrates. In
these the heart-beat origi-
nates in the sinus venosus
and spreads through the cardiac tissue to other regions. Admixed with the
specialised contracting tissue are numerous ganglia and nerve fibres, but the
actual wave spreads through the muscle. In chelonians a coronary nerve runs
VOL. II. H*
IIO ZOOLOGY
in a narrow bridge of cardiac tissue between sinus and ventricle. If this nerve
is cut, the transmission wave and cardiac rhythmicity is not interfered with.
If the bridge of cardiac muscle is severed, and the coronary nerve left intact,
each chamber still contacts, but arhythmically. In Amphibia and reptiles
the auricular and ventricular musculature is continuous across the auriculo-
ventricular groove, and no specialised bundle of fibres is necessary to transmit
the excitation wave. If the cardiac tissue between auricle and ventricle is
clamped, the ventricular beat is stopped but later it may begin again spon-
taneously (myogenically).
Springing from the ventricle (or from the conus when that chamber is
present) and passing directly forwards in the middle line below the gills, is a
large, thick-walled, elastic blood-vessel, the ventral aorta (Figs. 63, B, and 76

FIG. n.-Blood-vascular system. Course of the circulation in a fish. Vessels containing

aerated blood red, those containing non-aerated blood blue, lymphatics black. B, capillaries of
the body generally; E, of the alimentary canal; G, of the gills; K , ofthe kidneys; L, of the liver;
T, of the tail. a. br. a. afferent branchial arteries; au. atrium; c. a. conus arteriosus; d. ao.
dorsal aorta; e. br. a. efferent branchial a rteries;. h. p. v. hepatic portal vein; h . v . h epatic vein; lc.
lacteals ; ly . lymphatics ; pr. cv. v. precaval veins ; r. p. v. renal portal veins ; s. v. sinus venosus;
v. ventricle; v. ao. ventral aorta. The arrows show the direction of the current. (After Parker.)

v. ao.). At its origin, which may be dilated to form a bulbus arteriosus, are
valves so disposed as to allow of the flow of blood in one direction only, viz.
from the ventricle into the aorta. It gives off on each side a series of half-
hoop-like vessels, the afferent branchial arteries (a. br. a.), one to each gill.
These vessels ramify extensively, and their ultimate branches open into a
network of capillaries (Fig. 77, G). These have walls formed of a single layer
of epithelial cells, which permeate the connective-tissue layer of the branchial
filaments, and have therefore nothing between them and the surrounding
water but the epithelium of the filaments. The blood, driven by the contrac-
tions of the heart into the ventral aorta, is pumped into these respiratory capil-
laries, and there exchanges its superfluous carbon dioxide for oxygen. It
then passes from the capillaries into another set of vessels which join with
one another, like the tributaries of a river, into larger and larger trunks, finally
uniting in each gill, into an efferent branchial artery (e. br. a.). The efferent
arteries of both sides pass upwards and discharge into a median longitudinal
 PHYLUM CHORDATA III

vessel, the dorsal aorta (d. ao.), situated immediately beneath the notochord or
vertebral column. From this trunk, or from the efferent branchial arteries,
numerous vessels, the systemic arteries, are given off to all parts of the body.
The most important of these are the carotid arteries (Fig. 76, c. a.) to the head,
the subclavian (scl. a.) to the pectoral fins, the cmliac (cl. a.) and mesenteric
(ms. a.) to the stomach, intestine, liver, spleen, and pancreas, the renal (r. a.)
to the kidneys, the spermatic (sp. a.) or ovarian to the gonads, and the iliac
(il. a.) to the pelvic fins. After giving off the last-named the aorta is continued
as the caudal artery (cd. a.) to the end of the tail.
With the exception of the capillaries, all the vessels described in the pre-
ceding paragraph, including the dorsal and ventral aortre, are arteries. They
are firm, elastic tubes, which do not collapse when empty and usually contain
but little blood in the dead animal. They carry the blood from the heart to
the body generally.
The systemic arteries branch and branch again into smaller and smaller
trunks (arterioles) and finally pour their blood into a capillary network (Fig. 77,
B, K, and T) with which all the tissues of the body, except epithelium and
cartilage, are permeated. In these systemic capillaries occurs internal or
tissue respiration. The blood parts with its oxygen and nutrient constituents
to the tissues, and receives from them the various products of destructive
metabolism-carbon dioxide, water, and nitrogenous waste. The systemic,
like the respiratory, capillaries are microscopic, and their walls are formed of
a single layer of epithelial cells.
We saw that the respiratory capillaries are in connection with two sets of
vessels, afferent and efferent. The same applies to the systemic capillaries,
with the important difference that their efferent vessels are not arteries, but
thin-walled, non-elastic, collapsible tubes or venules. These receive the de-
oxygenated blood from the capillaries, and unite into larger and larger trunks,
the veins, which finally open into one or other of the great veins, presently to
be described, by which the blood is returned to the heart. As a general rule
the vein of any part of the body runs parallel to its artery, from which it is at
once distinguished by its wider calibre, by its dark colour (due to the contained
bluish-purple blood seen through its thin walls), by being gorged with blood
after death, by the complete collapse of its walls when empty, and by its
possession of valves. In some cases the veins become dilated into spacious
cavities called sinuses; but sinuses without proper walls (such as occur in
many of the Invertebrata) are never found in the Craniata except in individual
organs (liver and spleen). The veins from the head join to form large, paired
anterior cardinal (jugular) veins (Fig. 76, j. v.) which pass backwards, one on
each side of the head, and are joined by the prominent posterior cardinal veins
(crd. v.) coming from the trunk. Each jugular unites with the corresponding
cardinal to form a large precaval vein (Ductus Cuvieri) (pr. cv. v.) which passes
II2 ZOOLOGY
directly downwards and enters the sinus venosus. The blood from the tail
returns by a caudal vein (cd. v.), lying immediately below the caudal artery in
the hremal canal of the caudal vertebrre (Fig. 63, D). On reaching the crelom
the caudal vein forks horizontally, and the two branches either become
directly continuous with the cardinals or pass one to each kidney under the
name of the renal portal veins (Fig. 76, r. p. v.). In the kidneys they break up
into capillaries (Fig. 77, K), their blood mingling with that brought by the
renal arteries and being finally discharged into the cardinals by the renal veins
(Fig. 76, r. v.). Thus the blood from the tail may either return directly to the
heart or may go by way of the capillaries of the kidneys. In the latter case
there is said to be a renal portal system, the essential characteristic of which is
that the kidney has a double blood-supply, one of oxygenated blood from the
renal artery, and one of deoxygenated blood from the renal portal vein. In
other words, it has two afferent vessels, an artery and a vein, and the latter is
further distinguished by the fact that it both begins and ends in capillaries
instead of beginning in capillaries and ending in a vein of higher order.
The blood from the gonads is returned to the cardinals by veins called
spermatic (sp. v.) in the male, ovarian in the female. That from the paired fins
takes, in what appears to be the most typical case, a somewhat curious course.
On each side of the body there is a lateral vein (lat. v.), running in the body-wall
and following the course of the embryonic ridge between the pectoral and
pelvic fins. It receives, anteriorly, a subclavian vein (scl. v.) from the pectoral
fin, and posteriorly an iliac vein (il. v.) from the pelvic fin, and in front pours its
blood into the precaval.
The veins from the stomach, intestine, spleen, and pancreas join to form
a large hepatic portal vein (h. p. v.), which passes to the liver and there breaks
up into capillaries. The portal system among other functions conveys ingested
food substances from the intestine to the liver. Thus the liver has a double
blood-supply, receiving oxygenated blood by the hepatic artery (h. a.), a branch
of the creliac, and deoxygenated but food-laden blood by the hepatic portal
vein (Fig. 77, L). In this way we have a hepatic portal system resembling the
renal portal system, both in the double blood-supply and in the fact that the
afferent vein terminates, as it originates, in capillaries. After circulating
through the liver the blood is poured, by hepatic veins (h. v.), into the sinus
venosus. The hepatic portal system, unlike the renal portal system, is of
universal occurrence in the Craniata.
In the embryo there is a sub-intestinal vein, corresponding with that of
Amphioxus, and lying beneath the intestine and the post-anal gut. Its
posterior portion becomes the caudal vein of the adult, its anterior portion one
of the factors of the hepatic portal vein.
To sum up: The circulatory organs of the branchiate Craniata consist of
(a) a muscular force-pump, the heart, provided with valves and driving the
 PHYLUM CHORDATA II3

blood into (b) a set of thick-walled elastic, afferent vessels, the arteries and
arterioles, from which it passes into (c) a network of microscopic vessels or
capillaries which permeate the tissues, supply-
ing them with oxygen and nutrient material
and receiving from them carbon dioxide and
other waste products. From the capillary
network the blood is carried off by (d) the
venules and veins, thin-walled, non-elastic
tubes which return it to the heart. Thus the
general scheme of the circulation is simple :
The arteries spring from the heart, or from
arteries of a higher order, and end in capil-
laries. The veins begin in capillaries and end
in vessels of a higher order or in the heart.
Actually, however, two factors complicate the
system : Firstly, the interposition of the gills
in the course of the outgoing current, as a
result of which we have arteries serving as ffC'----
both afferent and efferent vessels of the re-
spiratory capillaries. The efferent arteries --A
take their origin in those capillaries after the
manner of veins. Secondly, by the inter-
position of two very large and important
organs, the liver and the kidney. in the course
of the returning current, as a result of which
we have veins acting as both afferent and
efferent vessels of the hepatic and renal
capillaries. The afferent vessels of both
organs end in capillaries after the fashion of
arteries. FrG. 78.-Blood-vascular system:
Embryo of an air-breathing cran-
In the embryos of the higher, or air- iate.. A. dorsal aorta, and auricle;
breathing, Craniata, the circulatory organs A b. aortic arches; Acd. caudal artery;
All. allantoic arteries; Am. vitelline
agree in essentials with the above description. arteries; B. ventral aorta; c, c 1 •
The most important difference is that, as no carotid arteries; D. precaval veins;
Ic, E. iliac arteries; HC. cardinal
gills are present, the branches of the ventral veins; KL. gill-clefts; RA, S, 5 1 .
roots of dorsal aorta; Sb. subclavian
aorta do not break up into capillaries, but arteries; Sb 1 . subclavian veins; V.
pass directly into the dorsal aorta, forming ventricle; VC. jugular vein; Vm.
vitelline veins. (After Wiedersheim.)
the aortic arches (Fig. 78, Ab.). With the
appearance of the lungs, however, a very profound change occurs in the blood-
system. The last aortic arch of each side now gives off a pulmonary artery
(Fig. 79, Ap.) to the corresponding lung, and the blood, after circulating through
the lung capillaries, is returned by a pulmonary vein (lr.) directly into the left
II4 ZOOLOGY
side of the heart and not into an ordinary system vein of higher order. In the
atrium a vertical partition is developed, separating a left auricle (A 1), which
receives the oxygenated blood from the lungs, from a right auricle (A), into
which is poured the deoxygenated blood of the sinus venosus. In crocodiles,
birds, and mammals (B) a further important cardiac specialisation is found:
the ventricle also becomes divided into right and left chambers. So we get a
four-chambered heart with right and left auricles and right and left ventricles.
At the same time the conus arteriosus and sinus venosus cease to exist as dis-
tinct chambers. The left auricle receives oxygenated blood from the lungs and
passes it into the left ventricle, whence it is propelled through the system. The
right auricle receives deoxygenated blood from the system, and passes it into
the right ventricle to be pumped into the lungs for aeration. Thus the four-
chambered heart of the higher verte-
B
brates is quite different from that of
A a fish. In the latter the four cham-
bers-sinus venosus, atrium, ven-
tl tricle, and conus arteriosus-form a
single longitudinal series, whereas in

, a mammal, for instance, the four

chambers constitute practically a
double heart, there being no direct
communication between the auricle
and ventricle of the right side, or re-
FIG. 79.-Blood-vascular system: Cardiac spiratory heart, and those of the left
circulation. A, an amphibian; B, a crocodile. side, or systemic heart. The modifica-
A . right auricle; A 1 .left auricle ; Ap. pulmonary
artery; lr. pulmonary vein; R A . aortic arches ; tions undergone by the arteries and
V . ventricle ; V1 . left ventricle; v, v. and V e,
Ve. pre- and post-cavals. (After Wiedersheim.) veins in the higher Vertebrata will be
considered under the various classes.
It has been generally believed that the complete division of the heart of
birds and mammals into arterial and venous components has evolved indepen-
dently, but recent studies bring this point into debate. It is not impossible
that the inter-ventricular septum may have arisen when the tetrapod ancestors
were still fishes and been secondarily lost in recent Amphibia. In the Dipnoi
(p. 361), a specialised group far from the main line of vertebrate descent, the
heart is almost completely divided. Although it is impossible to derive the
modern tetrapod condition from that of the Dipnoi, it may be reasonable to
suppose that the interventricular partition is homologous in both lung-fishes
and tetrapods. If this is true, the original divided condition must have been
present in early amphibians, replaced by a secondary septum in the sauris-
chian reptiles (which gave rise to the birds), and seemingly retained in the
therapsids (and so became the interventricular septum in the Mammalia)
(Foxon).
 PHYLUM CHORDATA IIS
In addition to the circulations through the respiratory and systemic
hearts (called sometimes the lesser and greater circulations) there is a circulation
within the heart itself. This is the coronary circulation by which the cardiac
tissues are nourished and supplied with oxygen and their excretory products
removed. With the exception of the amphibians, the coronary circulation
consists of blood vessels which ramify throughout the walls of the heart
chambers. The venous blood draining from the heart tissues is collected and
returned to the right atrium by the coronary sinus.
The coronary circulation in the amphibians is much reduced and supplies
only the conus arteriosus ; the thin-walled ventricle is nourished by blood
passing through into the general circulation.
The blood of Craniata is almost always red (cj., however, p. 343). The red
respiratory pigment (luemoglobin) is not dissolved in the plasma, as in most
red-blooded invertebrates, but is confined to erythrocytes (red blood-corpuscles)
(p. 70), which float in the plasma. These are usually flat oval discs, the
centre bulged out by a large nucleus (nu.), but in mammals they are bi-
concave non-nucleated, and usually circular. The number and size of red
cells vary among animals in regard to altitude and oxygen pressure. Leucocytes
(white cells) of various kinds also occur (p. 71). The colour of the blood
varies with the amount of oxygen taken up by the hremoglobin. When
thoroughly aerated it is of a bright scarlet colour, but assumes a bluish-purple
hue after deoxygenation. Oxygenated blood is usually found in arteries, and
is often spoken of as arterial blood. Deoxygenated, darker blood is usually
found in veins and is called venous. But it must not be forgotten that an artery,
e.g. the ventral aorta or the pulmonary artery, may contain venous blood,
and a vein, e.g. the pulmonary vein, arterial blood. The distinction between
the two classes of vessels does not depend upon their contents, but upon their
relation to the heart and the capillaries and upon the structure of their walls
(pp. 78, 107).
In addition to the blood-vessels the circulatory system of Craniata contains
lymph-vessels or lymphatics (Fig. 77, ly.). In most tissues (Fig. So) there is a
lymphatic network of lymph-capillaries, interwoven with, but independent of,
the blood-capillaries. From this network lymphatic vessels pass off, and finally
discharge their contents into one or other of the veins. Many of the lower
Craniata possess spacious lymph-sinuses surrounding the blood-vessels ; and
there are communications between the lymphatics and the crelom by means
of minute apertures or stomata. The lymphatics contain lymph, which is
essentially blood minus its red cells and most of the plasma proteins. Lymph
drains from the tissues: it passes into the lymph capillaries, and thence into
the lymphatics. These are efferent vessels, conveying the fluid from the
capillaries to the venous system.
The lymphatic nodes or glands which occur in the course of the vessels
n6 ZOOLOGY
contain phagocytic cells, and also add lymphocytes to the lymph whence they
enter the blood stream. The lymphatics contain valves which prevent back-
flow of lymph towards the veins when the vessels are compressed by the
surrounding tissues, such as muscle. In some groups there are muscular
dilations in some of the lymphatics. These lymph-hearts contract rhythmically
and actually propel the lymph forwards towards the venous system. The

s.g.

s.M.

d. (p.)

b. cap. (a.) ly. cap. b. cap. (v.)

FIG. So.-Circulatory system: Relationship of blood and lymph capillaries below epidermis of
mammal. Blood and its contents flow in at the arterial end of the capillary (b. cap. (a.)), and leave
at the venous end (b. cap. (v.)). Arrows indicate fluid exchanges between a blood capillary, the
surrounding tissues, and an adjacent lymph capillary (ly. cap.). Other structures are: d. (p).
dermis (papillary layer; below, and not shown, is the reticular layer); s. c. stratum corneum;
s. g. stratum granulosum; s. M. stratum Malpighii. (Modified after Ham.)

lymphatics of the intestine have an important function in the absorption of

emulsified fats, and are known as lacteals.
Nervous System.-The nervous system attains in the higher vertebrates a
structural complexity and functional importance which are probably without
parallel in the animal kingdom. It is a system specialised to co-ordinate the
activities of the animal in relation to changes in the internal and external
environment, such activities including not only the contraction of muscles which
may result in overt movement, but also processes in the cardio-vascular,
respiratory, digestive, and other systems which are classified as visceral. In
its most general sense its function may be said to be that of maintaining a con-
 PHYLUM CHORDATA II7
dition of equilibrium not only between the internal organs, tissues, and fluids
of the body, but also between the body as a whole and its external environment.
To perform these functions the nervous system is equipped with afferent
or sensory connections which carry information in the form of nervous impulses
from organs which are sensitive to external events (exteroceptive organs such
as the eyes and ears) or to internal events such as changes of tension in muscles,
ligaments, and in the walls of viscera, or of the chemical composition of the body-
fluids. Nerves capable of responding to these internal events are classified as
interoceptive, and include proprioceptors signalling internal body movements
and visceroceptors registering mechanical changes in viscera and blood-vessels.
Receptor organs, whether extero- or intero-ceptive, vary in relation to special
needs. For example, the eyes of nocturnal animals are often larger than those
of related diurnal species, since there is a need for greater light-gathering power.
Again, those of hunting animals usually look forward (as in the cat), whereas
those of hunted animals (e.g. the mouse) look laterally, and thus provide a
wider field of vision.
As a complement to the afferent connections, efferent or motor connections
are also present by which effector organs such as muscles and glands can be
stimulated to perform their characteristic function. In addition, some
mechanism is necessary in all more complex animals, by which the information
derived from varied receptor organs can be integrated, and by which the
activity of the effector organs can be co-ordinated, so as to result in behaviour
which is appropriate to the situation which the receptors have revealed.
One other general characteristic of the nervous system must be mentioned.
All living tissue is altered to some extent, either temporarily or permanently,
by activities which occur within it. In nervous tissue such alterations are
particularly important, especially in the higher vertebrates, where they con-
stitute the physical basis of the process of adaptation so far as behaviour is
concerned, or, more simply, the physical basis of learning. The nervous system
may be altered by stimuli which pass through it, so that on a subsequent occa-
sion it does not react in exactly the same way to similar, or even identical
stimuli. At present one can only speculate concerning the physical nature of
the change which has occurred, but that a change is there, is shown by the
altered reaction obtained. This power of change, the plasticity of nervous
tissue, is particularly characteristic of certain parts of the nervous system in
the higher vertebrates.
Anatomically the nervous system can be divided into central and peripheral
parts (Figs. 8r, 82). The peripheral nervous system consists mainly of afferent
and efferent nerve-fibres, gathered together in nerves which conduct to and from
the central nervous system in which the integrating and co-ordinating mechanisms
are situated. The latter is formed, of course, by the brain and spinal cord.
Nerve-fibres are processes (axons) of nerve-cells which in the case of the afferent
 H
H
00

m.'b h .b

pn.en
sp.c.
c
~---n h ' /2~-~- ·-;;,.
N
ol~l o .l . . 0
!:/' r.if pl

s
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~
_:_llj (' ".. opl.l H
.,.,. ;; '}))_ l . ·. . . .
~~~;._-~-:::'c'~; ;-:;~7.-, -:-"'~ . .:....£!!! :_·~-,~,, ~D~
' ~ 0~~~·\·P·~~
.~ e· ..
IJ.Pl. di.coe . . inf .. 'l ·' z. '
cr.crb
pt
FIG. 8r.-Nervous system: Brain and proximal part of spinal cord during development and maturity. A, first stage, side view,
the cavity indicated by a dotted line; B, second stage; C, side view of fulJy.formed brain with undivided prosencephalon; D, the
same in sagittal section; E, transverse section of prosencepha lon; F, of diencephalon; G, of mesencephalon; H , of medulla
oblongata ; I, side view of brain with cerebral hemispheres; K, sagittal section of the same; L, dorsa l view, the cavities exposed
on the right side; al. pl. alar, or dorsolateral wall; b. pl. basal or ventro-late ral wall; cblm. cerebellum; c. c. central canal; cr. crb.
crura cerebri; c. h. cerebral hemispheres; c. s. corpora striata; di . coe. third ventricle; dien. diencephalon; ep. coe. cerebellar
ventricle; f. b. fore.brain; f. m. foramen of Monro; h. b. hind·brain; inf. infundibulum; l. iter; l. t. lamina terminalis ; m. b.
mid-brain; m. coe. mesoccele; med. obl. medulla oblongata; mt. coe. fourth ventricle; olf. l. olfactory lobe; opt. coe. optic ventricle:
opt. l. optic lobes; o. th. optic thalami; pa. coe. lateral or first ventricle; pal. pallium; pn. b. pineal body (epiphysis); pn·. e. pineal
eye (parietal organ); prs. coe. prosoccele; prs. ·en. telencephalon; pt. pty. pituitary body ; rh. coe. rhinocceles; sp. c. spinal cord.
 PHYLUM CHORDATA

mesencephalon

superior
cervical
ganglion
inferior
cervical
ganglion

splanchnic
nerves

lumbar
splanchnic
nerves
51

Preganglionic }
______ Postganglionic Sympathetic fibres

••.............. Preganglionic }
-•-•-•- Pos t gangtomc I. . Parasympathetic fibres
FIG. 82.-Nervous system: Distribution of sympathetic and parasympathetic nerves in mamma
(Man). (Modified after Kunz.)
!20 ZOOLOGY
fibres are situated in small swellings or ganglia on the nerves close to the point
where they join the central nervous system (Fig. 83). The cells whose
processes form the efferent or motor nerves are situated within the central
nervous system, but there is an important exception to this statement. Viscera
such as the heart, blood-vessels, intestine, etc., and glands, receive their motor

Viscera
FIG. 83.-Nervous system: Relationship of the sympathetic sntem to the spinal nerves and
cord. The main visceral fibres are shown on the right side and, for contrast, the somatic fibres on
the left. Afferent fibres are indicated by interrupted lines and efferent fibres by continuous lines.
(From LeGros Clark, after Cunningham.)

or efferent innervation from a part of the peripheral nervous system which is

called autonomic, and which consists of two divisions, the sympathetic and para-
sympathetic (see below). The fibres which connect with the muscle of a viscus,
or the secretory epithelium of a gland, are the processes of cells situated in
ganglia outside the central nervous system. On each side of the vertebral
column there is a series of such ganglia, two for each segment of the body (Fig.
83). These are known as the ganglia of the sympathetic subdivision of the
autonomic nervous system, and they are linked longitudinally to form the two
 PHYLUM CHORDATA 121

sympathetic chains of ganglia. Although these ganglia may possess some

degree of autonomy, they are controlled by the central nervous system through
preganglionic fibres, which in mammals arise from cells in the thoracic part of
the spinal cord.
It will be seen therefore that the sympathetic innervation of a viscus is
effected through two nerve-fibres, the one preganglionic, originating in the
central nervous system, the other postganglionic, originating in a ganglion
and ending in the viscus. It follows that the impulse carried by the pre-
ganglionic fibre must be passed on to the postganglionic fibre through a
synaptic junction in the ganglion. Since preganglionic fibres branch and

Frc. 84.-Nervous System: A medullated fibre terminating at its motor end-plate on a striated
muscle. m. myelin sheath ; n . neurilemma; s. n . nucleus of Schwann cell; sa. sarcolemma.
(From Lc Gras Clark, after Gutmann and Young.)

ramify widely, many postganglionic fibres may receive impulses from one
preganglionic, so that a widespread response may be obtained from a single
impulse. This form of innervation is in sharp contrast to that of skeletal
muscle, where only one nerve-fibre arising within the central nervous system
is involved and where the response is limited to the particular part of a muscle
connected to that fibre (Fig. 84).
In addition to the sympathetic chain of ganglia, small and often micro-
scopic collections of nerve-cells are found irregularly scattered anterior to the
dorsal aorta and in the walls of such viscera as the heart and intestine. These
are known as parasympathetic ganglia. The fibres which arise from them also
innervate viscera, but their action is usually antagonistic to that of the sym-
pathetic system ; for example, when stimulation of the sympathetic fibre leads
to contraction, stimulation of the parasympathetic causes relaxation. The
parasympathetic ganglia are also connected to the central nervous system by
preganglionic fibres which travel in cranial or sacral nerves from the brain and
spinal cord respectively.
The nerves supplying viscera, both afferent and efferent, with their ganglia,
are often referred to as the visceral nervous system, and are distinguished from
the afferent and efferent nerves which supply skin and skeletal muscle which are
122 ZOOLOGY
called somatic. The distinction between somatic and visceral is not always
clear-cut and there are several nerves whose classification in these terms is
uncertain or controversial. This applies particularly to the afferent nerves.
On the efferent side, the presence of a peripheral synaptic junction in a sym-
pathetic or parasympathetic ganglion is a criterion for distinguishing the
visceral from the somatic type of innervation, although it is not applicable in
quite every case. As will be seen, the terms visceral and somatic are also
applied within the central nervous system to regions which have predominant
connections with these subdivisions of the peripheral system ; but again,
attempts to place all parts of the central nervous system in one category or the
other have not been entirely satisfactory.
The nervous system of all Chordata first appears as a groove in a strip of
thickened ectoderm on the dorsal aspect of the embryo. The groove deepens,
its lips fuse, and it separates from the surface as a dorsal longitudinal neural
tube, the cavity of which is called a neuroccele. So far the agreement of the
Craniata with the lower Chordata is complete, but a fundamental advance is
seen in the fact that at a very early stage the anterior end of the neural tube
undergoes a marked dilatation and forms the rudiment of the brain, the rest
becoming the spinal cord (Fig. 8r). Very soon constrictions appear in the
dilated part and divide it into three bulb-like swellings or vesicles, the fore-
brain, mid-brain, and hind-brain, the latter continuous with the spinal cord.
Functionally there appears to be a primary relationship of the fore-brain
with olfaction, the mid-brain with vision, and the hind-brain with the auditory
and vestibular apparatus, and also, in aquatic vertebrates, with the sensory
inflow from the lateral line organs (p. 135). It is probable that the develop-
ment of special sense organs at the cranial end of the animal is causally related
to the formation of the three vesicles of the brain in this situation.
Structurally the spinal cord is the least differentiated part of the central
nervous system. It remains as a longitudinal tube, but the walls thicken greatly
and the neurocrele is reduced to a narrow central canal lined by columnar
ciliated epithelium or ependyma. In the thickened walls nerve-cells and a spe-
cial variety of supporting cells known as neuroglia develop. The latter, mainly
of ectodermal origin, perform many of the functions of mesodermal connective
tissues in other parts of the body.
The nerve-cells or neurones consist of a cell body containing the nucleus
and bearing numerous fine branching processes or dendrites. There is in
addition a single process, the axon, which usually gives off only a few branches
and often travels for a considerable distance from the cell body. The axons
that are covered by a lipoid sheath of myelin are said to be myelinated or
medullated (Fig. 84). Somatic and preganglionic visceral axons are generally
medullated, whereas post-ganglionic visceral axons are not.
Nervous impulses are received on the dendrites or on the surface of the cell
 PHYLUM CHORDATA 123

body. Normally conduction in neurones occurs only in one direction, i.e. from
the dendrites to the cell body, and then away from the cell body along the axon.
Neurones are thus said to exhibit polarity. The axon loses its sheath and
becomes naked just before its termination, so that the impulse which it carries
can be passed through a synapse to the dendrite or cell body of another neurone,
or through a specialised end-plate to a muscle-fibre (Fig. 84). It is clear that

POSTERIOR
median sulcus
central canal
(In grey commissure)

posterior horn
(of grey matter)

white
matter

FrG. ss.-Nervous system: Spinal cord and the relationships ol its constituent structures.
Above: Transverse section under low power showing distribution of grey and white matter.
Below: High-power diagrams revealing their contents. (After Ham.)

many axons (such as those which pass from the spinal cord to muscles in the
feet) must be of very great length in some animals.
When the spinal cord is cut transversely it can be seen with the naked eye
to consist of grey matter which is situated around the central canal and has a
A or H shape in section, the whole of the superficial part of the cord consisting
of white matter (Fig.85). The grey matter contains cell bodies, dendrites, and the
terminations of the axons which are making connections with them, and, of
124 ZOOLOGY
course, the beginnings of the axons arising from the cell bodies. The latter
enter the white matter, which is formed of axons running for varying distances
cranially or caudally as the tracts of the spinal cord. Its whiteness is due to the
myelination of these axons. It is clear that the grey matter is the tissue in
which interconnections between neurones occur and enable the functions of
integration and co-ordination to be carried out. The white matter consists of
conducting pathways between different segments of the spinal cord and between
the spinal cord and the brain.
The spinal nerves are the only means by which the spinal cord is connected
to the receptor and effector organs of the periphery. There is one on each side
in each segment of the body, often referred to as a 'mixed spinal nerve', since
it contains both afferent and efferent fibres. Each nerve divides as it enters
the canal in the vertebral column into two roots, dorsal and ventral. The
dorsal root carries all the afferent fibres and has on it the ganglion previously
referred to : its fibres enter the dorsal horn of grey matter in the cord. The
ventral root consists of efferent fibres which are the axons of cells in the ventral
horn of grey matter. In the most primitive living vertebrates (lampreys)
(p. 176) the roots do not join. The afferent and efferent fibres run indepen-
dently to their destinations, and mixed spinal nerves are not formed.
It is customary to classify nerve-fibres as somatic and visceral according to
the structures they supply. The efferent or motor-fibres supplying the striated
muscle developed from somites are classified as somatic motor, and the sympathe-
tic and parasympathetic pre- and post-ganglionic fibres through which the
smooth muscle and glands of viscera are supplied as visceral motor. The
afferent fibres are similarly classified: 'somatic sensory' fibres are those supply-
ing the skin and sense organs on the surface of the body and all structures
developed from somites; 'visceral sensory' fibres supply viscera, i.e. structures
developed from lateral plate mesoderm and from endoderm. It must be
admitted, however, that in the case of the afferent fibres this classification is
not entirely satisfactory. There are many afferent nerves where some justifica-
tion could be found for placing them in either category.
In all the higher vertebrates visceral motor-fibres leave the spinal cord in
the ventral roots of the spinal nerves with the somatic motor-fibres. In
cyclostomes and in some fish they are found also, and perhaps entirely in some
cases, in the dorsal roots, which do not therefore consist only of afferent
fibres in these animals. The functional categories 'somatic sensory', 'visceral
motor', etc., in which nerve-fibres can be classified are usually referred to as
the components of the nerve. A mixed spinal nerve will normally contain
representatives of all the four components so jar mentioned.
Cranially the spinal cord is continuous with the most caudal of the three
cerebral vesicles, the hind-brain. The part directly continuous with the spinal
cord is the medulla oblongata or myelencephalon. Here the side walls of the
 PHYLUM CHORDATA 125

neural tube thicken and fold outwards to form the floor of an enlarged part of
the central canal called the 4th ventricle of the brain. Towards the cranial
end the hind-brain narrows as it joins the mid-brain at the isthmus, and
becomes flexed so as to show a ventral convexity. This cranial end of the
hind-brain is sometimes distinguished as the metencephalon, and it is in this
region that the cerebellum develops in all vertebrates (Fig. 8r).
The same grey and white matter is found in the hind-brain as was seen in
the spinal cord. In the former, however, the grey matter is broken up into
more or less discrete masses forming the nuclei of cranial nerves, etc., while the
white matter consists chiefly of ascending and descending tracts connecting
different parts of the brain with the spinal cord. It is joined by a series of
nerves which supply the mouth, pharynx, and branchial arches. Since the
last-named are respiratory in function, and the heart and arterial arches are
closely related to them both anatomically and functionally, the principal
centres controlling cardio-vascular and respiratory functions are found in the
hind-brain. This applies to air-breathing as well as aquatic vertebrates, since
lungs are also derivatives of the pharynx (p. 107) and the heart always develops
first below the floor of the pharynx, though subsequently it may be displaced
caudally into a thorax.
In addition to the branchial arch nerves, the hind-brain receives the sensory
inflow from the inner ear (vestibular, from the semicircular canals, and auditory)
and in aquatic vertebrates from the lateral line system. These sensory systems
will be described more fully later. Here it is necessary to say only that, apart
from audition, they are mainly concerned with the orientation of the body in
space, i.e. with equilibration. Important centres are therefore found in the
hind-brain for the control of position and posture, and it is from ~hese that
the cerebellum, largely concerned in similar functions, develops.
The form of the cerebellum varies greatly in different vertebrates. In its
simplest form (e.g. in living Agnatha and Amphibia) it consists of a thin band
of nervous tissue which bridges over the 4th ventricle at the cranial end of the
hind-brain (metencephalon), and most of its grey matter is spread superficially
to form a cortex. In fishes, birds, and mammals it is a much larger and more
conspicuous structure. The roof of the caudal part of the 4th ventricle
remains as a thin and non-nervous layer from which a secretory apparatus
called a choroid plexus develops. This secretes a watery fluid (very like physio-
logical saline) into the cavity of the ventricle (cerebrospinal fluid).
The mid-brain (mesencephalon), immediately cranial to the hind-brain, is
less extensively modified from the form of the original neural tube. It becomes
bent (the mid-brain flexure) so as to present a dorsal convexity which is known as
its roof or tectum. Here bilateral swellings (into which fibres of the optic
nerves grow) are found in all vertebrates. These are the optic lobes. Two
additional swellings are present in mammals which receive fibres from the
VOL D. I
!26 ZOOLOGY
auditory centres of the hind-brain. The only motor nerve nuclei in the mid-
brain are those which control the movements of the eyeball, the size of the pupil,
and the mechanism of accommodation. These are situated more ventrally in
the tegmentum, and, ventral to them, are found bundles of fibres (particularly
well developed in mammals) which connect the fore-brain with lower levels
of the nervous system; these are the crura cerebri. The neuroca::le of the
mid-brain generally remains small as the aqueduct connecting the 4th ventricle
of the hind-brain with the ventricles of the fore-brain; in some forms it
extends into the optic lobes as optic ventricles.
The hind-brain and mid-brain together constitute the brain-stem, w~ich,
in spite of variations due to differences in the form of the cerebellum and a
number of associated structures, preserves a very stable pattern throughout
the vertebrate class. It can be thought of as a continuation cranially of the
spinal cord, modified by such factors as the inflow of optic and auditory im-
pulses, and the development of special motor mechanisms not present in the
spinal cord. The nerves attached to the brain-stem, with the exception of the
optic nerve, are in many respects comparable with segmental spinal nerves.
The fore-brain, at the cranial end of the neural tube, is more varied in form
and also fundamentally different in structure from the spinal cord. It receives
impulses directly only from the olfactory nerves and the small nervus ter-
minalis, neither of which is comparable with a spinal nerve. It lies entirely
cranial to the notochord and has been called the archencephalon, in contrast to
the deuterencephalon, which lies dorsal to the notochord and includes both
brain-stem and spinal cord. At a very early period of development the archen-
cephalon becomes bent ventrally over the end of the notochord. This is the
primary cerebral flexure and, though permanent, it soon becomes obscured by
other changes in this region and is not noticeable in the adult. These two
regions, archencephalon and deuterencephalon, show not only fundamental
differences in adult structure and in their relationship to the notochord, but
important embryological differences as well.
The most cranial part of the fore-brain becomes differentiated as the telen-
cephalon or end-brain vesicle, characterised by its connection on either side
with the bilateral olfactory apparatus. The caudal part, between the telen-
cephalon and the mid-brain, is the diencephalon. The telencephalon possesses
thick walls and a thin roof and floor. Its walls are joined by the olfactory
nerves in regions differentiated as the olfactory bulbs which, in many vertebrates,
are drawn out to become pedunculated (the olfactory peduncles). Peduncles and
bulbs are hollow, containing a prolongation of the neurocrele of the telen-
cephalon. In nearly all vertebrates the lateral walls of the telencephalon also
bulge outwards and forwards to form the bilateral cerebral hemispheres. These
contain a cavity, the lateral ventricle, which remains in connection with the
cranial end of the neurocrele through an interventricular foramen on either side.
 PHYLUM CHORDATA !27

This evagination of the cerebral hemispheres is very incomplete in cyclo-

stomes (p. 175) and most fishes. In some of the latter, especially teleosts
(p. 239), the evagination of the hemisphere is accompanied by a marked
eversion of the dorsal part of the lateral wall which grows downwards and
becomes combined with the ventral part of this wall to form a massive collec-
tion of nervous tissue in many ways comparable with the basal ganglia in the
hemispheres of tetrapods. The whole structure is covered by a thin non-
nervous membrane which is the stretched roof of the telencephalon.
In lung-fishes (p. 36r) and in tetrapods the evagination of the hemispheres
is much more complete, and no eversion takes place. The dorsal part of the
hemispheres remains comparatively thin as the pallium, the region in which the
cerebral cortex of reptiles (p. 478), birds (p. 592), and mammals (p. 678) will
develop ; the ventral part becomes massive as the basal ganglia. It should be
pointed out that in primitive vertebrates (cyclostomes, fishes, amphibia) the
whole hemisphere is dominated by connections from the olfactory bulbs, and
is sometimes referred to as an olfactory lobe ; in some reptiles and mammals
and in all birds, olfactory dominance becomes much reduced.
A small part of the telencephalon remains unaffected by the evagination of
the hemisphere and median in position. It is bounded cranially by the lamina
terminalis, which marks the morphological cranial end of the brain. Its cavity
is continuous caudally with that of the diencephalon, so that a median or third
ventricle is formed. This third ventricle communicates laterally on each side
through the interventricular foramina (of Monro) with the lateral ventricles
of the hemispheres, and caudally through the aqueduct of the mid-brain with
the fourth ventricle of the hind-brain.
As already stated, the diencephalon is a median part of the neural tube
between the telencephalon cranially and the mid-brain caudally. Masses of
nerve-cells develop in its walls, which form the thalami (right and left). These
serve largely as relay stations in the course of ascending tracts conveying sen-
sory impulses from the brain-stem and spinal cord to the hemispheres.
These sensory impulses are mainly exteroceptive from the surface of the
body and proprioceptive from muscles, ligaments, etc., so that the thalamus
has been classed as a somatic sensory centre. It is also connected, however,
with a well-developed mass of nervous tissue in the floor of the diencephalon,
the hypothalamus, which is concerned with the integration of various visceral
and metabolic activities. A funnel-shaped diverticulum, the infundibulum,
grows ventrally from the hypothalamus and comes into relationship with an
upgrowth from the roof of the embryonic mouth cavity or stomodceum. From
these two outgrowths the anterior and posterior lobes of an important endo-
crine gland, the pituitary or hypophysis, are formed (Fig. Sr). This is always
situated immediately cranial to the notochord between the trabeculce of the
chondrocranium, and retains throughout life its relationship, which is functional
!28 ZOOLOGY
as well as developmental, with the hypothalamus. In both bony and carti-
laginous fishes a further outgrowth from the infundibulum forms the saccus
vasculosus, a structure of uncertain function, but possibly sensitive to pressure
transmitted from an aquatic environment.
The roof of the diencephalon remains thin and a choroid plexus, similar in
structure and function to that of the 4th ventricle, develops in it. Cranially,
where it extends into the telencephalon, it forms a saccular outgrowth, the
paraphysis; caudally, just before the mid-brain is reached, the pineal apparatus
develops. This is represented in mammals by the pineal organ, which appears
to be vestigial and possibly functionless. In some vertebrates, e.g. cyclo-
stomes, and many reptiles, two outgrowths are formed in this region (pineal
and parapineal), one or both of which may be sensitive to light and show many
of the structural features of an eye (see pp. r89, 345).
The brain of vertebrates is enclosed in the cranial cavity of the skull
(neurocranium) and the spinal cord in the canal formed between the neural
arches and centra of the vertebrce. The whole cerebrospinal cavity is lined
by a tough fibrous membrane, the dura mater. The connective tissue between
the dura mater and the nervous system differentiates into two much thinner
membranes, the outer arachnoid and the inner pia mater, which is everywhere
closely applied to the nervous tissue. Between them is the subarachnoid space
filled with cerebrospinal fluid secreted by the choroid plexuses ; this forms a
protective fluid cushion around the brain and spinal cord. Further, by its
continuous secretion and resorption into the blood-stream it helps to remove
metabolites from the nervous tissue. The dura mater, arachnoid, and pia mater
are referred to as the meninges or membranes of the nervous system.
The cranial nerves of vertebrates are numbered I to XII, but this mode of
designation dates from a period when little was known in detail of the morpho-
logy and functions of the nerves in question. The nervus terminalis, which was
discovered later, is not included in the series. There is no doubt that in the
light of modem knowledge a more satisfactory classification could be devised.
The old system is based only on superficial topographical knowledge of the
human and mammalian brain and naturally leads to difficulties when applied
to vertebrates as a whole. It is still, however, the system in common use, and
is employed as a basis for the description which follows.
It has already been noted that most of the cranial nerves are comparable
with spinal nerves, or, more accurately, with the roots of spinal nerves, and
show evidence of a similar segmental arrangement. This does not apply to
three of them, the nervus terminalis, the olfactory (Ist), and the optic (lind).
These must be described individually (Fig. 86).
The nervus terminalis is a minute nerve distributed to the mucous membrane
of the nasal sac; a few nerve-cells are scattered along its course and it joins
the fore-brain near the ventral end of the lamina terminalis. It may contain a
<: Acoustico- Supratemporal Occipital
0 Superficial Deep
Ophthalmic Abducens latera/is Latera/is
r' Ophthalmic
.... v
!""

'"d
::I:
~

~
~
("')
::I:
0
:::0
tl
>
~

Palatine Mandibular Pretrematic

VII v VII
Deep ophthalmic nerue ............IID. Other nerues .......................................... -
Othcr trigeminal branches...:nmmmn Spiracle and gill slits ..........................e::)
FIG. 86.- Nervous system: Cranial and anterior spinal
Facial nerve ...............................~ Profundus and gasserian ganglia.. :::.
... nerves. (Modified after Norris and H ughes.) Latera/is...................................... Geniculate ganglion .............................. :::':";~
* Occipital............ ........................ ::::::==. Brain and spinal cord.........................J"" ·J
....
N
1.0
130 ZOOLOGY
few sensory fibres, and possibly autonomic efferent fibres to blood-vessels, but
little is known of its functions.
The Ist (olfactory) nerve consists of a number of fila olfactoria which run
from the olfactory receptors of the nasal sac to the olfactory bulbs of the telence-
phalon. Though an afferent nerve, it possesses no ganglion. Its fibres are
the processes of the receptor cells. In these respects it is unique among all the
nerves of the vertebrate body. In many air-breathing vertebrates, notably the
Reptilia, a specialised organ, containing receptors of the olfactory type, is
found in the naso-buccal region. This is the vomeronasal organ or organ of
Jacobson. It is innervated by a special branch of the olfactory nerve (pp. 137,
481).
The lind (optic) nerve runs from the retina of the eye back to the brain.
The retina is part of the optic vesicle, which, at an early embryonic stage,
developed as an outgrowth from the diencephalic part of the neural tube.
Morphologically it is therefore a part of the brain, and the 'nerve' which grows
from it is comparable with a tract in the central nervous system rather than
with a peripheral nerve. Although the optic vesicle is a derivative of the fore-
brain the optic nerve, after decussating in a chiasma, grows into the tectum of
the mid-brain in all vertebrates below the Mammalia. In mammals the decus-
sation is incomplete and the majority of fibres enter a part of the thalamus
called the lateral geniculate body.
The remaining cranial nerves fall into two groups: (1) somatic motor nerves,
in general comparable with the ventral roots of spinal nerves, and (2) branchial
arch nerves, comparable with the dorsal roots.
Somatic Motor Nerves. These are nerves which supply striated muscle
developed from somites: the IIIrd (oculomotor), IVth (trochlear), VIth
(abducens), and XIIth (hypoglossal}. The IIIrd and IVth nerves arise from
the mid-brain and the VIth from the cranial end of the hind-brain. All three
supply muscles which move the eyeball; and the IIIrd also contains fibres which
supply smooth muscle within the eyeball (e.g. the constrictor muscle of the
pupil). The IIIrd nerve is therefore said to possess a visceral motor in addition
to its somatic motor component. The IVth (trochlear) nerve is exceptional,
since it is the only cranial nerve to emerge from the dorsal aspect of the brain-
stem. The XIIth (hypoglossal) nerve arises from the ventral surface of the
medulla and supplies the hypobranchial musculature, including the muscles
of the tongue. It probably represents the ventral roots of several nerves, since
the muscles supplied are developed from several somites. A hypoglossal
nerve cannot be recognised as a cranial nerve in cyclostomes and other primi-
tive vertebrates, where it is probably represented by the ventral roots of the
most cranial spinal nerves.
Branchial arch nerves. This group contains the Vth (trigeminal}, VIIth
(facial}, IXth (glossopharyngeal), Xth (vagus), and Xlth (spinal accessory)
 PHYLUM CHORDATA 131

nerves. The spinal accessory is not recognised as a separate nerve below the
tetrapods, but is represented by a part of the vagus. The VIIIth (vesti-
bular and auditory) nerve and the lateral line nerves will also be described
with the branchial arch nerves, since they are closely associated with them.
Their position is somewhat equivocal, for they have no functional relationship
to the branchial arches or their derivatives.
Apart from the VIIIth, these nerves all have a close relation to the pharyn-
geal arches and the branchial clefts between them. They all contain afferent
fibres, from the skin (somatic afferent) and from the mucous membrane of the
mouth and pharynx (visceral afferent), and they all possess ganglia like those
of the dorsal roots of spinal nerves. Some of the visceral afferent fibres come
from taste receptors and are classed as special visceral afferent. In addition,
nearly all these nerves contain visceral efferent fibres, some of which supply
glands and smooth muscle like visceral efferent fibres elsewhere. Others sup-
ply the striated muscle of the pharynx, cesophagus, and visceral arches (includ-
ing those of the larynx in air-breathing vertebrates). Though striated, these
muscles are developed from lateral plate mesoderm and not from somites (p.
r6r). They are specialised, however, in relation to the movements of the jaws,
swallowing, respiration, and phonation, and the nerves supplying them are
classed as special visceral efferent nerves.
It will be seen that the nerves in this group are all complex and may contain
as many as five components : somatic and visceral afferent, special visceral
afferent, visceral efferent, and special visceral efferent. The presence of efferent
fibres has not been demonstrated in the dorsal roots of spinal nerves in most
vertebrates, but they are probably present in the primitive chordate, Amphioxus
(p. 47), in cyclostomes (p. 175), and perhaps in fishes (pp. zrg, 282). Their
presence in this group of cranial nerves is therefore an example of the persis-
tence of a primitive feature in nerves which are in many ways highly specialised.
In general the anatomical pattern of branchial arch nerves is as follows :
they possess a main branch which passes posterior to the branchial cleft (post-
trematic) supplying both skin and muscle and a smaller but similar branch in
front of it (pretrematic). There is usually a dorsal cutaneous branch and another,
the pharyngeal branch, which passes medially to the mucous membrane of the
roof of the pharynx. All these branches are not present in every nerve, just
as every component listed is not present in all of them.
The trigeminal (Vth) is large and emerges from the cranial part of the
hind-brain close to the VII nerve. It soon enters the large semilunar or Gas-
serian ganglion, from which its three main branches ari£e: ophthalmic, maxil-
lary, and mandibular.
The ophthalmic branch consists of two parts : the ramus ophthalmicus
profundus and the ramus ophthalmiczts superficialis. The R. ophthalmicus pro-
fundus passes deeply through the orbit, supplies sensory fibres to the eyeball,
132 ZOOLOGY

and ends in the skin of the end of the snout. There are reasons for believ-
ing that this deep ophthalmic branch was originally a separate cranial nerve,
representing the dorsal root belonging to a premandibular somite, the ventral
root to which is the oculomotor nerve. The superficial ophthalmic branch is
the dorsal cutaneous, the maxillary the pretrematic, and the mandibular the
post-trematic branch, as in a typical branchial arch nerve. The gill-slit is
represented by the mouth, the gill of course being absent. The special visceral
efferent fibres all run in the mandibular or post-trematic branch and supply
the muscles which move the jaws. It may be called the nerve of mastication.
There is no pharyngeal branch. The ventral root corresponding to this nerve
is probably the IVth (trochlear), supplying the superior oblique muscle in the
orbit.
The facial (VIIth) is the next in this series. It is a fairly typical branchial
arch-nerve, belonging to the hyoid arch. The sensory ganglion on its root is
the geniculate ganglion. Its main or post-trematic branch is the hyomandibular
nerve, which supplies the muscles of the hyoid arch. In mammals these
migrate widely to form the superficial muscles of facial expression.
The chorda tympani branch of mammals may represent the pretrematic
branch, but is more probably the representative of the internal mandibular
nerve of fishes, which is post-trematic in position. It supplies the mucous
membrane of the tongue with taste or special visceral sensory fibres and in tetra-
pods, visceral efferent fibres to salivary glands. The true pretrematic branch
is probably the prespiracular nerve of fishes and is reduced or absent in tetra-
pods.
The pharyngeal branch is represented by the palatine nerve of fishes, which
runs medially above the palato-quadrate bar; it is the greater superficial
petrosal nerve of mammalian anatomy, and contains visceral afferent and efferent
fibres. The dorsal cutaneous branch is small and may be absent. This is
certainly true of tetrapods, but in fishes sensory fibres are more numerous in
the VIIth nerve than in higher vertebrates, particularly those serving the taste
receptors, which may be distributed widely over the surface of the body.
The IXth or glossopharyngeal is the nerve of the third visceral arch. It is
a typical branchial arch nerve with a sensory ganglion, pre- and post-trematic
branches, and a pharyngeal branch. A dorsal cutaneous branch is small or
absent.
Its afferent fibres come from the mucous membrane of the pharynx and in
mammals, where it is the principal nerve of taste, from the posterior part of the
tongue. It also receives afferent fibres from the part of the internal carotid
artery in mammals which is developed from the third arterial arch and forms
the carotid sinus (p. 674). These, together with other afferents from chemo-
receptors in the carotid body, are important in cardiovascular reflexes for the
adjustment of blood pressure. Although these fibres from the carotid sinus
 PHYLUM CHORDATA 133
and carotid body have been investigated chiefly in mammals, similar fibres are
probably present in other vertebrates.
The Xth or vagus is a large nerve which emerges from the medulla oblongata
by several roots which lie in series with the IXth nerve. It probably represents
the combined nerves of all the visceral arches caudal to the third. It supplies
these arches with the characteristic pharyngeal, pre- and post-trematic branches.
There is also a small dorsal cutaneous branch. In the air-breathing tetrapods
these branches are represented by pharyngeal and laryngeal nerves, all of which
contain special visceral efferent and visceral afferent fibres. The vagus also
supplies the heart and swim-bladder (pp. 108, 361) or lungs (pp. 107-108),
structures which develop from the pharynx or in close relation to it. The
pulmonary branches come from the trunk of the vagus, the pneumogastric
nerve which continues caudally to supply the oesophagus, stomach, and much
of the intestine. It is this wide distribution to viscera which has earned the
name 'vagus' or 'wanderer'. The visceral efferent fibres to the heart, lungs,
and alimentary canal are classed as 'parasympathetic' and are functionally
antagonistic to the sympathetic supply to these viscera in mammals. The
vagus also contains numerous visceral afferent fibres which serve important
reflexes.
The Xlth or spinal accessory is probably the nerve of the last (5th or 6th)
visceral arch. It is not present in fishes, where it is represented by the most
caudal rootlets of the vagus. In mammals, where it was first described, it
consists of a cranial and a spinal part. The cranial part is incompletely separ-
ated from the vagus, and supplies motor fibres to the larynx which are distri-
buted in the laryngeal branches of the vagus. The spinal part arises by a
series of roots from the cervical part of the spinal cord, independent of the roots
of the spinal nerves, and enters the skull and joins the cranial part. Its fibres
are distributed to certain muscles of the neck and shoulder girdle which are
developed, at least in part, from lateral plate mesoderm related to the caudal
end of the pharynx.
The VIIIth or acoustic nerve has been left to the last: there is some doubt
if it should be included in the branchial arch series. It is a nerve of special
sense, which in tetrapods supplies the inner ear. The latter consists of a
vestibular part (the semicircular canals, utricle and saccule) concerned with
the appreciation of position and movement of the head, and the lagena (Fig.
94) or, in mammals, the more complex cochlea (Fig. 93), sensitive to auditory
vibrations. The nerve therefore consists of vestibular and auditory subdivisions.
The inner ear, however, is probably a highly specialised member of the class of
receptors known as lateral-line or neuromast organs, widely distributed in
characteristic patterns on the head and body of aquatic vertebrates (pp. 136,
137, 395). These are sensitive to movement and possibly to vibration of low
frequency in the surrounding water, and, since they are superficial in position
134 ZOOLOGY
TABLE II. Organisation of the head.

NERVE
SOMITE. MUSCLE. VISCERAL
ventral root. dorsal root. ARCH.

rst or premandibular Superior, inferior, IIIrd, Oculo- Ramus ophthal- Trabecul::e of

and anterior motor micus p~ofun- chondrocran-
!internal) rec- dus of V ium
tus, and inferior
oblique of eye

2nd or mandibular Superior oblique IVth, Trochlear Vth, Trigeminal rst, Mandibular
of eye (excluding the
Ramus ophthal-
micus profun-
dus)

3rd or hyoidean Posterior (extern- VIth, Abducens VIIth, Facial 2nd, Hyoid
al) rectus of eye

4th (the somite dis- - - IXth, Glosso- 3rd (or the rst
appears) pharyngeal Branchial)

5th and several sub- Hypobranchial XIIth, Hypo- Xth, Vagus 4th and subse-
sequent somites and tongue glossal Xlth, Spinal quent visceral
muscles accessory arches

It should be noted that the first three somites, all of which give rise to extrinsic muscles of
the eye, are all pre-otic in position. The numbering of the post-otic somites is uncertain, since
one and perhaps more than one disappear without giving rise to muscles; the number certainly
varies in different vertebrates.

and exteroceptive in function, they are classed as somatic sensory. The nerves
supplying these organs enter the central nervous system in close association
with the VIII th nerve as an anterior and posterior lateral line nerve. The pos-
terior lateral line nerve gives branches to the IXth and Xth cranial nerves and
is distributed to the lateral line organs in their territory. The branch to the
vagus is particularly large, and soon leaves the trunk of the vagus as what is
called its lateral line branch. This runs caudally throughout the body supply-
ing the lateral line proper. The anterior lateral line nerve, like the posterior,
does not run independently to the organs it supplies, but joins branches of the
VIIth and Vth nerves, particularly the hyomandibular branch of the former
and the ramus ophthalmicus superficialis of the latter.
The lateral line nerves and the VIIIth nerve are sometimes known as the
acoustico-lateral system, of which only the VIIIth nerve persists in air-breathing
vertebrates. The fact that the lateral line nerves join the branchial arch nerves
and are distributed with them has led to the idea that a lateral line or special
somatic sensory component may have been present in all segmental nerves at
an early evolutionary stage. This probably applies only to cranial nerves and
gives some justification for the classification of the acoustico-lateral with the
branchial arch nerves. It must be pointed out, however, that the sensory
ganglia of the acoustico-lateral system develop from ectodermal placodes,
which differ in position from those associated with the ganglia of other sensory
 PHYLUM CHORDATA I35
cranial nerves, and this might be used as a justification for classifying them
separately.
That the somatic motor cranial nerves may correspond with the ventral
roots of spinal nerves and the branchial arch nerves with the dorsal roots has
already been indicated (p. 130). There is also evidence that these cranial
nerves are segmentally arranged in relation to somites in the same way as
spinal nerves, although, owing to specialisations which result from the pre-
sence of special sense organs, jaws, and branchial arches, this is not obvious in
the adult body. The position is somewhat clearer in the embryos of the lower
Craniata, and the accompanying table is based mainly on evidence from this
source.
Sensory Organs.-These vary in form and disposition from group to group.
Some are internal-e.g. kina!sthetic receptors which apprise the central nervous
system of the degree of muscle tension and, in collaboration with sensory com-
ponents of the inner ear (p. 145), enable the tetrapod to maintain the position
it desires. Most sensory organs are distributed on the external surface of the
animal and, directly connected with the central nervous system, bring to it
information concerning the external environment.
The study of skin receptors has been mostly carried out on Man, and for
the past fifty years a theory of punctate sensibility has been generally accepted.
This held that four primary modalites-touch, warmth, cold, and pain-
operated within the 'law' of specific nervous energies (see Weddell). This
concept has been seriously questioned both on physiological and histological
grounds. It is now known that various kinds of sensory discrimination are
possible in localised regions in the absence of the various cutaneous end-
organs postulated by the punctate hypothesis. This is strikingly apparent in
the cornea, which is supplied only with free nerve-endings and which, it was
formerly believed, was sensitive only to pain. It is now known that touch,
warmth, and cold stimuli can be appreciated there as well.
In fishes and some amphibians, characteristic sense-organs known as the
neuromast-organs or organs of the lateral-line occur. Extending along the sides
of the trunk and tail is a longitudinal streak, due to the presence either of an
open groove or of a tube sunk in the skin, and continued on to the head
in the form of branching grooves or canals (Figs. 87, 88). These organs, and
certain others in the form of pits or of unbranched canals, are lined with
epithelium. Here occur groups of sensory cells-neuromasts-which are pro-
duced at their free ends into hair-like processes. Neuromasts are innervated
by lateral line nerves (see pp. 136, 449), and, at their first appearance in the
embryo, are distinct, segmentally arranged patches of sensory epithelium in
intimate connection with the ganglia of cranial nerves. Cutaneous sense-
organs of the lateral-line system, having at first a metameric arrangement,
also occur in the aquatic Amphibia. The neuromast organs enable the animal
 ZOOLOGY

op.
.·/

ie.
P~- ~ oc.
II

FIG. 87.- Lateral-line system: Bony fishes.

I. The relation of the lateral-line canal to the scales on the body of Perea jluviatilis. A,
longitudinal section ; B, scales and canal seen in side view. a. bridge of scale covering the canal;
as. anterior region of scale ; ep. epidermis; lc. latera l-line ca na l; ln. lateral-line nerve; 11. nerve to
sense organ; op. ext ernal opening of canal; p s. posterior edge of scale; so. sense organ in canal.
II. The head of Amia calva showing the system of lateral-line canals and their nerve supply
(from Allis). b. buccal branch of facial nerve; g. dorsal branch of glossopharyngeal; h. hyo-
mandibular branch of facial ; ll. lateral-line of trunk; lv. latera l-line of vagus; md. mandibular
canal; oc. occipital ca nal ; p. pit organs on body ; po. post-orbital canal; ps. pit organs on the
head. sob. sub-orbital canal; soc. supra-orbital canal ; sof. superior ophthalmic branch of facial.
t. temporal canal.
 PHYLUM CHORDATA 137
to detect vibrations in the water of too low a frequency to form a sound
capable of perception by the ear.
Substances suspended in surrounding water or dissolved by saliva are
tested by special taste-buds \vhich are composed of groups of narrow, rod-shaped
cells. In fishes these are widely distributed in the mouth and branchial
cavities, but also on the outer surface of the head, and in some species over
almost the whole surface of the body. In higher Craniata they have become
chiefly restricted to the moist epithelium of the tongue and soft palate, and are
supplied mainly by branches of the glossopharyngeal.

FIG. 88.- Lateral-line system: Carti-

laginous fish. Pteroptalca vale11tiennii in
···sc. dorsal view showing t he great development
of latera l line organs. de. endolymphatic
openings; pv. pectoral fi n ; plv. pelvic fi n ;
>C. spine ; sp. spiracle. (After Goodrich.)

The olfactory organ is typically a sac-like invagination of the skin of the

snout, anterior to the mouth (Fig. 8g). It communicates with the exterior
by an external nostril. It is paired in all Craniata, except cyclostomes, in which
there is a single olfactory sac, supplied, however, by paired olfactory nerves (p.
130). The sac is lined by the olfactory mucous membrane or Schneiderian
membrane, the epithelium of which contains peculiar, elongated sensory cells.
The free ends of these cells are often produced into hair-like processes and the
olfactory nerve-fibres consist of processes which run from the base of the
cell to the olfactory bulb. In the Dipnoi (p. 361) and all higher groups, the
posterior end of each sac communicates with the cavity of the mouth by an
aperture called the posterior nostril, and an analogous communication occurs
in the case of the unpaired organ of the hag-fishes (p. 197).
In many air-breathing vertebrates there is formed an offshoot from the
olfactory organ, which, becoming separated, forms a distinct sac lined with
olfactory epithelium and which may or may not open into the mouth. This is
the vomeronasal or Jacobson's Organ, by which some animals are able to smell
 ZOOLOGY

Choana
Homo s~p1ens

Maxilloturb.

Choana

Olf. sac. Sp1nal cord Pose. chamber

Oral cavity Pharynx

Naso-hypophyslal Peuomyzon (/uviatilis Choana
canal orr. sac. Spinal cord

-
; ; . ;..- .
.
. ~Ch,do _ _- Gut
do"
-~..------
.. . : >

Oral cavity Nasopharyngeal canal

Myxine glutmosa
Acipenser sturio
Ant. nostri l .....
l,.......
H .
I· . ~ • Olf. sac.
t.• •. ....;

( Post. nostril )
Scy/Jium canicula
FIG. 89.-0lfactory apparatus: Nasal cavity and its relationship to the buccal cavity in
vertebrate types. In Lacerta (a lizard) , Oryctolagus (rabbit), and Homo (Man) t he nasal septum has
been removed to display the nasal chambers. The underside of the snout in Scyllimn (a dogfish)
is drawn with the nasal skin fold on the right bent back to show the relationship of the nostrils
to the mouth. (From Allison, partly after Matthes.)
 PHYLUM CHORDATA 139
substances held in the mouth. The apparatus is supplied by a separate branch
of the olfactory nerve. The organ occurs only in tetrapods, being first seen
in the Amphibia (p. 381). It reaches a high degree of efficiency in snakes and
at least some lizards (p. 543) , but is probably of minor significance in chelonians
and the Crocodilia. In all birds and in most mammals it occurs during develop-
ment. In the mammalian embryo it is evident as a distinct groove on the
lower medial aspect of each nasal cavity; the duct, however, appears to be cut
off from the nasal apparatus in 'higher' groups. In monotremes the organ
remains readily identifiable in the adult and is equipped with a turbinal process
projecting into its lumen.
The paired eye (Fig. 90), of varying but more or less globular shape, lies
in the orbit. It con-
sists essentially of the
light-sensitive retina,
connected by the optic
tract to the brain, en-
closed within a capsule
and equipped with ap-
paratus for collecting
light and focusing it
upon the retina.
The external part of
the capsule, the sclerotic,
consists of dense fibrous
tissue which may con-
tain cartilage and in
some cases plates of
bone. Where it is ex- FIG. go.-Eye: Macroscopic structures in mammal. (After Dakin.)
posed to the light the
dense fibrous tissue is replaced by a peculiar variety of connective tissue which
is transparent ; this part of the capsule is called the cornea, and is covered on
both its outer and inner surfaces with epithelium. The whole external coat
has the character of a spherical case, mostly opaque (the sclerotic). but with
a circular transparent window (the cornea) . The curvature of the cornea is
not the same as the sclerotic; almost flat in fishes, it bulges outwards in
terrestrial vertebrates.
Lining the sclerotic is the second coat, the choroid, consisting of highly
vascular connective tissue, pigment cells, and in certain situations smooth
muscle fibres. The choroid is in intimate contact with the sclerotic, but at the
corneal margin it is continued as the anterior surface of a membrane, the iris,
which is separated by a space (the anterior chamber of the eye) from the cornea.
In this region its degree of pigmentation gives the characteristic colour to the
140 ZOOLOGY
eye, and it is perforated by a circular or slit-like aperture, the pupil. In most
vertebrates the size of the pupil can be varied by two sets of smooth muscle-
fibres in the iris : radiating fibres which dilate, and circular fibres which con-
strict the pupil. Just behind the iris, and close to its peripheral margin, the
choroid is thrown into radiating folds which project into the eyeball and con-
tain smooth muscle-fibres, of importance in the mechanism of accommodation.
These folds are the ciliary processes.
The retina (Fig. gr) lines the surface of the choroid and extends forwards over
the ciliary processes to form the posterior surface of the iris ; it ends at the

pigment
epithelium

layer of rods
and cones

outer nuclear
layer
rod granules

. outer pleKiform layer

fibre of MUller
amacrine cells
inner nuclear /eyer
diffuse
amacrine cell .. ·---
centrifugal fibre

inner pleKi form layer

FIG. 91.-Vision: Retinalstmcture. In general, the retina of diurnal animals is rich in cones.
That of nocturnal animals possesses a predominance of rods making for high sensitivity to light
with a sacrifice of acuity and colour vision. Some animals (e.g. certain bats) apparently possess
no cones; others (e.g. many reptiles, some mamma ls) no rods. (Redrawn from Gray , after
Cajal.)

pupillary margin. Only the posterior part of the retina (the pars optica) is
light-sensitive ; this extends almost to the ciliary processes and ends along an
irregular line, the ora serrata. The part of the retina which extends over the
ciliary processes (pars ciliaris) and on to the inner surface of the iris (pars
iridica) is not light sensitive and is comparatively simple in structure.
Throughout the retina consists essentially of two layers. The outer of these
is a single layer of cuboidal or low columnar heavily pigmented cells, the pigment
layer. The inner layer is also a simple cuboidal epithelium in the pars iridica
(where it is pigmented) and in the pars ciliaris of the retina. In the pars
optica, behind the ora serrata it becomes highly complex, containing the
 PHYLUM CHORDATA 141

light-sensitive elements and nerve-cells and fibres which form the beginning
of the pathway by which optic stimuli are conveyed to the brain. The light-
sensitive elements are highly specialised cells which from their shape are classi-
fied as rods or cones. They are placed perpendicular to the surface in intimate
contact with the outer pigment layer already referred to; their inner ends
connect with small bipolar nerve-cells which in turn connect with larger nerve-
cells (the ganglion cells) still nearer the inner surface of the retina. From the
ganglion cells arise fibres which are at first situated on the inner surface of the
retina; they all converge on a small circular area, usually to the nasal side of
the posterior pole of the eyeball, called the optic disc, and here they pierce the
coats of the eyeball to become the optic nerve. It will be noted that in order
to reach the light-sensitive elements adjacent to the outer pigmented layer,
light-rays must pass through the layers of nerve-fibres, ganglion cells, and bi-
polar cells. A retina of this type is called 'inverted' and is characteristic of
vertebrates. In some invertebrates (e.g. cephalopods) eyes superficially similar
to the vertebrate eye may be found, but the light-sensitive elements form the
innermost layer of the retina, which is therefore of fundamentally different
construction.
The biconvex lens lies immediately behind the iris. It is formed from
layers of fibres, each derived from a single cell, and is enclosed in a capsule.
It is attached round its periphery by a suspensory ligament to the ciliary pro-
cesses. In mammals this ligament is under tension, which can be relaxed when
the ciliary muscle contracts and draws the ciliary processes closer to the lens.
This allows the lens to assume a more nearly spherical shape, thus shortening
its focal lengths and producing accommodation for near vision. Different
mechanisms which serve the same function have been developed in other
vertebrate classes.
The space between the cornea and the iris is called the anterior chamber of
the eye ; it communicates through the pupillary opening with a narrow space
between the front of the lens and the iris, the posterior chamber. Both cham-
bers are filled with a watery fluid, the aqueous humour. The greater part of
the cavity of the eye, bounded in front by the lens and the ciliary processes
and for the rest of its extent by the pars optica of the retina, is filled with
a transparent jelly-like substance, the vitreous body. The cornea, aqueous
humour, lens, and vitreous body together constitute the dioptric apparatus,
which focuses an image of external objects on the retina. The iris is a dia-
phragm by which the amount of light which enters can be regulated.
Among many nocturnal forms (e.g. domestic cat), and some animals living
in muddy waters, the receptivity of the eye is enhanced by a reflecting plate,
the tapetum lucidum. Generally this lies in the choroid. It varies con-
siderably, and has arisen independently in different groups. It may be formed
of silvery connective tissue, or a layer of guanin granules or other material.
 ZOOLOGY

The tapetum often reflects a characteristic colour, e.g. the 'fiery' eye of the
Crocodilia.
The mode of development of the eye is as characteristic as its structure.
At an early stage of development a hollow outgrowth-the optic vesicle-is
given off from each side of the fore-brain. It grows out and touches the ecto-
derm at the side of the head. This ectoderm becomes thickened and invagin-
ates. Eventually it forms a closed sac and separates from the rest of the
ectoderm. The sac is the rudiment of the lens; it has thick walls and a small
cavity. Meanwhile the optic vesicle has also invaginated and become converted
into a two-layer optic cup. Its cavity, originally continuous with the neuroccele,
becomes obliterated. The invagination of the vesicle to form the cup does not
take place symmetrically. It occurs obliquely from the external (posterior)
and ventral aspect of the vesicle, so that the optic cup is incomplete along one
side where there is a cleft-the choroid fissure. This is afterwards closed by the
union of its edges. The outer layer of the optic cup becomes the pigment-
ary layer of the retina. From its inner layer the rest of that tissue, includ-
ing the rods and cones, is formed. The stalk of the optic cup occupies, in the
embryonic eye, the place of the optic nerve, but the actual fibres of the nerve
are formed as backward growths from the nerve-cells of the retina to the
brain.
Mesoderm grows in between the developing lens and the external ectoderm.
From this the main substance of the cornea and its inner or posterior epithelium
are formed. The adjacent ectoderm becomes the external epithelium or con-
junctiva. Mesoderm also enters the optic cup through the choroid fissure and
becomes the vitreous body. Lastly, the mesoderm immediately surrounding
the optic cup is differentiated to form the choroid, the iris, and the sclerotic.
Thus the paired eye is derived from both mesoderm and ectoderm. The sub-
stantia propria of the cornea, the sclerotic, choroid and vitreous body are meso-
dermal. The superficial epithelium of the cornea, and the lens, are derived from
the surface ectoderm directly; the retina and optic nerve are derived from the
central nervous system and are therefore indirectly ectodermal in origin.
The eyeball is moved by six muscles (Fig. 92). Four of these arise from
the inner wall of the orbit, and pass, diverging as they go, to their insertion
round the equator of the eye. One of them is dorsal in position, and is called
the superior rectus (sup. rect.), a second ventral, the inferior rectus, a third
anterior, the anterior or internal recttis, and a fourth posterior, the posterior or
external rectus (post. rect.). The usual names (internal and external) of the two
last-named muscles originate from their position in Man, in which, because the
eye looks forwards instead of outwards, its anterior surface becomes internal,
its posterior surface external. The two remaining muscles usually arise from
the anterior region of the orbit, and are inserted respectively into the dorsal
and ventral surface of the eyeball. They are the superior and inferior oblique
 PHYLUM CHORDATA 143
muscles. The eye muscles are remarkably similar throughout the vertebrates:
few structures have remained more 'conservative' (Gilbert).
The Apparatus of Audition and Equilibration. The receptor organ con-
cerned with these functions is the membranous labyrinth, situated on either

FIG. 92.-0rbit: Neural and muscular relationships in a cartilaginous fish. (Dogfish.) ant. rect.
anterior rectus; cereb. cerebellum; cil. a. , cil. p. anterior and posterior ciliary nerves; e.p .a.
efferent pseudobranchial artery ; ep. epiphysis; g. cil. ciliary ganglion ; inf. obl. inferior oblique ;
lam. lamina terminalis of cerebrum; 0. V. and VI I, superficial ophthalmic branch of trigeminal
and facial; obl. sup. superior oblique; olf. b. olfactory bulb; opt. l. optic lobe; post. rect. posterior
rectus; r. op. prof. ramus ophthalmicus profundus of trigeminal; r . s. sensory root of ciliary
ganglion; sup. rect. superior rectus; that. thalamus; II to X, cranial nerves. (After Young.)

side of the hind-brain in all vertebrates; it is often called the inner ear, especially
in tetrapods, in which a middle and external ear may also be present (Fig. 93).
The latter is concerned with the collection of sound vibrations, and the former
with their transmission to the inner ear, where they are translated into nervous
impulses carried by the VIII th cranial nerve to the brain. It is, of course, the
144 ZOOLOGY
membranous labyrinth, or inner ear, which is the essential receptor organ, and
it serves a double function : it is sensitive to vibrations which can be appre-
ciated as sound, and it is sensitive also to movements of the head and to the

FrG. 93.- Audio-equilibration: Series of evolutionary stages in vertebrate types. A. Cross-

section of h alf of a fish skull in the ear region. The ear structures consist only of the deep-lying
sacs and semicircular canals. B . An amphibian. The hyomandibular bone (hm) of the fish is
pressed into service as a sound transmitter, the stapes (s); the first gill slit, the spiracle (sp) ,
becomes the Eustachian tube (eu) and the middle-ear cavity (me) , while the outer e nd of the
spiracle is closed by the tympanic membrane (tm) . C. A mammal -like r eptile. The stapes
passes close to two skull bones (q, quadrate ; a, articular) which form the jaw joint. D. Man (the
ear region only, on a larger scale) . The two jaw-j oint bones have been pressed into service as
accessory ossicles, the malleus (m) and the incus (i). E . A primitive lan d animal and a mammal-
like reptile to show the relation of the eard rum t o the jaw joint. At first in a notch high on the
side of the skull (the otic notch) occupying the place of the fish spiracle, it shifts in mammal-like
reptiles to the jaw region. In mamma ls the jaw com es to be formed of one bone only (d, the
dentary), and the bones of the jaw-joint region are freed to act as accessory hearing organs; oe,
tube of outer ear. (After Romer.)

position of the head in relation to the gravitational pull. These functions are
located in different parts of the labyrinth and are served by the auditory and
vestibular divisions of the VIIIth nerve respectively. These divisions are
always present, although the nerve is often referred to simply as the auditory
nerve.
 PHYLUM CHORD ATA 145
Embryologically the inner ear is formed from a localised thickening of
ectoderm, the otic placode, on either side of the hind-brain. This placode be-
comes depressed to form a flask-shaped otic vesicle, which in most vertebrates
loses its connection with the surface ectoderm. The otic placode lies in series
with similar placodes from which lateral line organs (p. 135) are developed;
the inner ear which develops from it is therefore thought to represent a highly
specialised lateral line organ and to belong to the acoustico-lateral system.
At first simple, the otic vesicle soon becomes divided by a constriction into
a dorsal utriclus and a ventral sacculus which
remain in communication throughout life and
are imperfectly differentiated from each other
in primitive vertebrates. From the utricle three
semicircular canals are formed. These lie in
planes at right angles to one another : one
anterior, one posterior (both vertical), and one
external and horizontal. The canals remain
in communication with the utricle, but as the
anterior and posterior canals have their adjacent
limbs united, there are in all only five openings. ap
Each canal is dilated at one end to form an
ce
ampulla, which is situated anteriorly in the
anterior and external canals and posteriorly in
the posterior canal (Fig. 94).
FIG. 94.-Audio-equilibrium. aa.
The ventral compartment, or saccul·us, gives ampulla of anterior canal, ae. of
off a blind pouch, the lagena, which in mam- horizontal canal, ap. of posterior
canal; ass. apex of superior u t ricu -
mals becomes elongated and coiled to form lar sinus; ca. anterior, ce. horizontal,
the cochlear duct. The saccule and lagena (or cp. posterior semi-circular canal;
cus. canal uniting sacculus with
cochlear duct) are concerned with auditory utriculus ; de. endolymphatic duct;
l. lagena ; rec. utricular recess; s.
function ; the utricle and semicircular canals sacculus ; se. endolymphatic sac;
with sense of position and movement. The sp . posterior utricular sinus ; ss.
superior utricular sinus; u . utri-
membranous labyrinth is filled with fluid, culus. (After Wiedersheim.)
the endolymph, and becomes surrounded by the
cartilage or bone of the auditory capsule, a part of the chondrocranium (p. 97) .
The cartilage, or bone, adapts itself to the form of the membranous laby-
rinth, and the complex cavity so formed is called the bony labyrinth. It presents
a large excavation (the vestibule) for the utricle and saccule, and tunnel-like pas-
sages for the semicircular canals and lagena. It is from the spirally coiled
form of the latter that the term cochlea ('snail') is derived. The membranous
labyrinth fits loosely in the bony labyrinth, so that it is separated from its
capsule in most regions by a narrow perilymphatic space. This communicates
with the subarachnoid space around the brain by means of the aqueductus coch-
learis and is filled with perilymph which is very like cerebrospinal fluid (p. 125).
VOL. II. K
146 ZOOLOGY

The actual sensory receptors are patches of neuro-epithelial cells which are
found in particular situations in the,wall of the membranous labyrinth. The
cells are flask-shaped, with long hair-like processes which project into the
endolymph in the cavity of the labyrinth; the terminal filaments of the VIIIth
nerve ramify round their bases. The neuro-epithelial cells are surrounded by
supporting cells, and are found in the cristce in the ampullre of the semicircular
canals, in the maculce of the utricle and the saccule, and in the lagena or cochlear
duct. In the latter they form the receptive element of the organ of Corti in
mammals. Each group of epithelial cells has its hair-like processes in contact
with, or embedded in, a gelatinous mass, which, particularly in the macula of
the utricle, contains crystals of calcium salts and forms an otolith or otolithic
membrane. Otoliths are very conspicuous in fishes where they are of varying
size and number. Owing to their weight, changes in position alter the stresses
transmitted from the otoliths to the hair-cells, and it is probable that this con-
stitutes the stimulus which is appreciated as 'position sense'. Similar effects
can be produced by movements of the head, owing to the inertia of the otoliths,
or by vibration in the endolymph, the latter constituting a stimulus which
can be appreciated as sound.
The membranous labyrinth is the only auditory apparatus possessed by
fishes (except for such accessory apparatus as the W eberian ossicles found in
many teleosts; p. 342). The change from water to land, when the first tetra-
pods arose, necessitated important additions to deal with vibrations conveyed
through air instead of the denser medium of water. The most obvious of these
is the ear-drum or tympanum, a membrane on or near the surface of the body,
and separated from the otic capsule and labyrinth by a cavity developed as
a diverticulum of the pharynx-probably the spiracular pouch. This cavity
is the middle-ear.
Vibrations are transmitted across the middle ear from the ear-drum to the
labyrinth, or inner ear, by a bony rod, the columella, in amphibians, reptiles,
and birds. The columella represents the upper element of the hyoid arch
(the hyomandibula), which is no longer required for jaw-suspension owing to
the acquisition of the condition of autostyly (p. 94). The outer ends of the hyo-
mandibula (columella) becomes embedded in the tympanum and the inner
end fits into an opening in the otic capsule, the fenestra ovalis. In mammals
the columella is found as a small stirrup-shaped bone, the stapes, which still
fits into the fenestra ovalis; two other bones, however, the incus and the malleus,
have been added to connect it to the tympanum, so that vibrations are trans-
mitted by a chain of three ossicles instead of by a single bone. Both the incus
and the malleus are derived from the jaw skeleton, the incus being the quadrate
and the malleus the hinder end of Meckel's cartilage, the articular. These
bones have been freed from their original function of providing a joint for the
lower jaw by the development of a new articulation between the dentary and
 PHYLUM CHORDATA 147
the squamosal bones (p. 97). It is probable that the ossicular chain is a more
efficient mechanism than the single columella for the transmission of vibra-
tions to the inner ear, possibly by increasing their force and diminishing their
amplitude, but the details of its mode of action are not entirely clear. In
mammals the ear-drum has sunk below the surface, with which it remains
connected by a tubular external auditory meatus. A pinna is also developed
and serves to reflect sound vibrations into the meatus. The pinna and meatus
together constitute the external ear (Fig. 93, p. 144).
The evidence, both palreontological and embryological, for the morpho-
logical changes which have occurred in the bony structures associated with the
ear, is remarkably complete. The evolution of the ear-drum and middle ear is
less well understood.
Endocrine Organs.-As recently as the end of the last century the animal
body was considered essentially as a group of complicated, little-understood
organs, the functions of which, however, were probably explicable in terms of
the co-ordinating and integrating nervous system (p. rr6). Despite the earlier
work of Bordeau, Claude Bernard, Berthold, Brown-Sequard, and many others,
it was not until 1902 that the work of Bayliss and Starling promoted a general
acceptance of the idea of chemical reflexes. Largely due to the work of Pavlov,
it was at the turn of the century generally considered that pancreatic secretion,
for example, was entirely under neural control. Bayliss and Starling now
showed that even after denervation of the mammalian pancreas, the organ
still secreted when acid entered the small intestine from the stomach. Injec-
tion of hydrochloric acid into the general circulation did not stimulate pan-
creatic outflow, so Bayliss and Starling ground up intestinal mucosa, injected a
weak acid filtrate into the general circulation, and observed a flow of pancreatic
JUICe. They called the activating mucosal substance secretin and described it
as a hormone.
In 1925 Mellanby showed that purified secretin mainly stimulated the out-
flow of water and bicarbonate from the pancreas, whereas stimulation of the
vagus produced a juice rich in enzymes. Later, in 1942, there was discovered
a second hormone, pancreozymin. This stimulates enzyme secretion, so that
duodenal hormones can cause the production of normal pancreatic juice in the
absence of vagal activity. Normally the nervous action of the vagus (p. 133)
initiates secretion, often anticipating the arrival of food in the duodenum: the
slower activating endocrine action becomes dominant later.
After the discovery of secretin, several organs, whose functions had been
only vaguely surmised, were shown to be partly or wholly endocrine in function
(see below). Leaving neurohumors and other hormone-like substances out of
consideration, hormones are in general produced by highly vascular aggrega-
tions of specialised epithelial cells which, lacking ducts, pour their secretions
as trace substances into the bloodstream, by which they are carried throughout
 ZOOLOGY
the body. Their precise mode of action is unknown. Hormones act on
particular tissues (target organs) and cause specific effects. The target organ
may be another endocrine gland which itself may be in turn caused to secrete,
and thus reflexly influence other target organs. Sometimes the increased
activity of the secondarily stimulated endocrine gland will result in a hormonal
'feed-back' that inhibits the action of the initial mover in the series. Thus, for
example, the anterior pituitary gland produces gonadotrophic hormones which
activate the gonads, and these in turn produce sex hormones which, along with
other special effects (e.g. the changes occurring at puberty), to some degree
control also the subsequent activity of the anterior pituitary. Such a reciprocal
relationship occurs also between anterior pituitary and the thyroid gland in
some vertebrates. Again, the hormones of two entirely different glands some-
times operate in a balanced synergism (e.g. ce!jtrogen from the ovary and pro-
gesterone from the corpus luteum together prepare the mammalian uterus for the
implantation of the fertilised egg even while it is still descending the oviduct).
Endocrine glands occur in the protochordates (p. 27) and are already
established in the general vertebrate pattern in the Agnatha (p. 176). Gener-
ally, if not always, individual glands have arisen by the functional and structural
metamorphosis of tissues that previously subserved other ends. For a pictorial
example of this see Fig. 97, p. 152. Thus, too, the thyroid (for example) has
developed (in phylogeny) from the endostyle (p. 52) of ancestral chordates.
The thyroid is of special interest in that it can be demonstrated that its homo-
logue in the Lamprey arises in much the same way during ontogeny, i.e. in the
developmental histology of the individual (p. 197). Likewise, each season the
male bird develops a fresh complement of secretory interstitial cells from un-
specialised connective tissue. Certain fishes and amphibians likewise season-
ally develop lob1tle-boundary cells, of secretory function, from fibroblasts (Fig.
97)·
Work on the comparative biochemistry and evolution of hormones is still
in its infancy. Individual hormones which differ comparatively little in their
chemical structure are capable of the most strikingly diverse effects. Again,
hormones that are (in the present state of knowledge) indistinguishable may
cause quite different effects in various classes of animals: for example, the
differential influence of adenohypophysial prolactin in birds and mammals
(see below). The evolution of the endocrine system has involved the differen-
tiation of chemical substances and special means of producing them on one
hand, and of specific tissue responses on the other. However, it must not be
thought that the hormone is simply a 'key' to 'unlock' a response. Most tissue
reactions (e.g. lactation) require a plurality of hormonal influences. Further,
alien substances will sometimes bring about endocrine effects (e.g. the advent
of comb-growth and spermatogenesis after the administration of sulphonamide
to diseased chickens).
 PHYLUM CHORDATA 149
There is some evidence that hormones have been evolved from metabolites.
Pituitary, parathyroid, and pancreatic hormones are prot eins ; thy roxin (from
the thyroid) and adrenalin (from the adrenal medulla) are derivatives of the
amino-acid tyrosine. The gonads, corpus luteum, and the adrenal cortex (or
interrenal tissue) produce steroid hormones.
Individual endocrine glands are briefly dealt with later under the various

FIG. 95.-Neuro-endocrine system: Hypothalamico-hypophysial relationships in a bird (Pigeon,

Colum ba). A . optic chiasma; B . d iencephalo n ; C. m edulla oblongata; D . d orsum sellre ; E.
osseus floor of sella. I . interna l carotid artery ; 2 . inferior hypophysial a rt ery ; J. a nterior ramus
and 4· post erior ramus of interna l carot id ; 5 · infu ndibula r a r t ery ; 6. prim ary capillary p lexus on
t he eminentia; 7· p ortal vessels ; 8. seconda ry plexus in pars distalis ; 9· capillar y bed in pars
n eura lis ; IO . interna l ophthalmic artery. (The capilla r y nets ar e simplified , a nd t h e peri-hypo-
physia l veins omitted.) (From Marshall , m odified after Wingstra nd .)

classes. Their functions are best known in hom<eothermic animals with which
t he following summary is mostly concerned :
Pituitary gland (hypophysis). The pit uitary is connected with the hypo-
thalamus by the infundibulum (Figs. 95, g6) and lies in t he sella tttrcica, an
excavation of the basisphenoid. 'No other single structure in the body is so
doubly prot ected, so centrally placed , so well hidden ' (Cushing). The hy po-
phy sial portal system connect s the pars dist alis (see T able III, p. 151) with
the hy pothalamus. The flow is in the direction of the gland t o which is
ISO ZOOLOGY
carried a neurohumor that influences the anterior lobe of the pituitary. The
production by the hypophysis of hormones which influence other organs
stimulated the epigram of Long that it is the 'conductor of the glandular
orchestra'. It must be remembered, however, that certain target organs
within the 'orchestra' have the capacity to modulate the activity of the
'conductor' and, further, that after hypophysectomy at least some of them
(thyroid, adrenal cortex) continue to function at reduced level.

A FISH: An AMPHIBIAN: A REPTILE:

teleost , Eupomotis frog. Rana turtle, Chrysemys

p.d.

A BIRD: A MARSUPIAL : A EUTHERIAN:

passerine, Passer opossum. Didelphis Man. Homo
FIG. 96.-Bypophysial arrangements: Vertebrate types. There is considerable variation
within each animal class. p.d. pars distalis ; p.i. pars intermedia; p.t. p ars tuberalis; n.z. neural
zone. (Modified after Green.)

The pituitary differs in the arrangement of its constituent parts from group
to group as shown in Fig. 93. The pars distalis secretes gonadotrophins of
which the follicle stimulating hormone (F. S . H.) activates ovarian follicles or,
as the tubule-ripening hormone (T. R. H.) , stimulates spermatogenesis in the
testis tubules. Luteinising hormone (L. H.) is also gonadotrophic in function,
causing the luteal reaction in the ovary that results in the formation of its
corpus luteum (or corpora lutea). As the interstitial cell stimulating hormone
(I. C. S. H.) it activates the interstitial Leydig cells in the male.
The adrenocorticotrophic hormone (A. C. T. H.) influences the cortical tissue
of the adrenal gland (or its homologue) and the thyrotrophic hormone (T. S. H.)
 PHYLUM CHORDATA I5I

TABLE III. Division of the hypophysis. (After Green, modified from Rioch, Wislocki, and O'Leary.)
Major Divisions Subdivisions
I. Adenohypophysis---- (a) Pars distalis
~~b~) glandular \
(b) Pars tuberalis - - - - - - - - - - - - Together =
(Derived from hypophysial
Rathke's pouch) stalk
(c) Pars intermedia----..__
----..__ __ Together =
/ . posterior lobe
2. Neurohypophysis--- (a) Lobus nervosus /
(syn. Neural lobe

l
\ or Infundibular
(Derived from process)
diencephalon) \
\ Infundibular
·. ...------------- stem
(b) Infundibulum--.______
(syn. Neural ~- ,---·
stalk) ~Median eminencej
of tuber
cinereum

stimulates the thyroid gland. The lactogenic hormone influences the mammary
gland (after its initial stimulation by restrogen) and, as prolactin, leads to
broodiness, certain forms of maternal behaviour, the production of crop-milk
in pigeons (p. 583) and brood-patches in many widely diverse species (p. 560).
The somatotrophic or growth hormone (S. T. H.) influences growth in some
classes. Disfunction may lead to giantism or dwarfism. This hormone has
not been proved to exist in birds. Hypophysectomy in birds, however, does
cause dwarfism, but growth can be restored by prolactin administration.
The 'posterior lobe' (Fig. g6) is in direct neural communication with the
brain. From it have been extracted vasopressin (A. D. H.) of essentially anti-
diuretic, but with some vaso-constrictor function, and oxytocin, which aids
milk ejection, and uterine contraction at term. The work of E. and B. Scharrer,
Bargmann, and others suggests that posterior lobe hormones are secreted in the
hypothalamus. They may be stored, and perhaps modified, in the posterior
pituitary before final discharge.
The pars intermedia is absent in birds and mammals but is prominent in
lower vertebrates. Intermedin causes melanophore-expansion (p. 450). How-
ever, a substance with the properties of intermedin (when administered to
frogs and reptiles) has been extracted from the pars distalis of birds.
Thyroid gland. Formed as a pharyngeal diverticulum, this gland is en-
capsulated by fibrous connective tissue and contains vesicles that are lined
with specialised epithelium which holds thyroid colloid. The epithelial cells
withdraw iodine from the bloodstream. After combination with the amino-
acid tyrosine, and further modifications, there is formed the hormone thyroxin
which has a stimulating effect on body metabolism in general and specific
effects in various classes of animals (see e.g. pp. 421, 456).
 ZOOLOGY

Parathyroid glands. These minute bodies are developed from the third and
fourth gill-pouches and produce parathormone which controls the distribution
of blood calcium and phosphates. For ttlttmobranchial glands, see p. 347·
Adrenal gland. These are paired compound
glands of two physiologically and anatomically
distinct parts-the medullary tissue and the
cortex. The latter interdigitates with the former
in Amphibia, Reptilia, Aves, and monotremes
and is homologous with the inter-renal glands
of sub-amphibian forms. The medulla arises
from neural crest ectoderm (as do also the
sympathetic nerve trunks) and, under sym-
pathetic control, secretes adrenalin, which
prepares the animal for fight or flight. Thus
adrenalin, among numerous other effects,
increases cardiac rate and vasoconstriction,
elevates blood sugar level, and relaxes the
bronchii to increase lung ventilation. In
general, parasympathetic stimulation causes
antagonistic effects. Chromaffin tissue is found
in various viscera in fishes (seep. 380).
The 'cortical' tissue produces steroid hor-
mones of three principal kinds: (r) mineralo-
corticoids concerned with the retention of sodium
and the release of potassium in the kidney,
(z) glttcocorticoids important in the regulation
of blood-sugar levels and glycogen deposition,
and (3) gonadoids, perhaps of relatively little
FrG. 97 .-Secretory arrangements:
Vertebrate testis. The interstitial importance under normal conditions.
Leydig cells (of connective tissue
origin) (above) form the typical verte- Gonads. Interstitial cells occur in the testes
brate pattern (Agnatha to Mammalia) (Fig. 97) and produce the steroid testosterone
except (below) in certain fishes and
non-anuran amphibians where there (male sex hormone) which has profound effects
has been a shift in hormone pro-
duction to specialised connective on sexual characters and behaviour. The testes
tissue cells (boundary cells) in the produce also restrogens. In the ovary cestro-
walls of the seminiferous tubules.
(After Marshall and Lofts.) genic celts of various kinds produce female sex
hormones (restrogens). These influence sexual
characters and behaviour. Androgens, too, are formed in the ovary and are,
incidentally, responsible for comb-growth in the hens of domestic fowl.
Corpus Iuteum. This is a temporary endocrine organ formed by the multi-
plication of luteal cells in the empty follicle after ovulation. These produce
progesterone which is important in the maintenance of pregnancy.
Pancreas. Apart from its exocrine function (p. ro4) this gland is studded
 PHYLUM CHORDATA I 53
internally with endocrine islets of Langerhans, which produce the hormone
insulin, which partly controls blood-sugar level. A high blood-sugar level
stimt.lates the secretion of insulin which lowers blood-sugar by promoting its
A

B 9.9,-d

J~~
p.nph

c 77~-S.n.d

0
p .n .d
\

v. e

FIG. 98.- Urinogenital system: Development and relationships. A . development of pronephros

and pronephric duct; B, atrophy of pronephros, d evelopment of mesonephros; C, differentiation
of pro- and mesonephric ducts ; D . development of metanephros; male type ; E, female type.
al. bl. allantoic bladder; cl. cloaca ; gon. gonad ; int. intestine ; m. c. Bowman's capsule; ms. n . d.
meso-nephric duct; m s. nph. mesonephros; mt. n. d. m etanephric duct; mt. nph. metanephros;
nst. nephrostome; ov. ovary; p. n . d. and sg. d. pronephric duct ; p. nph. pronephros; t. testis ;
v. e. vasa efferentia.

storage (as glycogen in liver and muscles), increasing peripheral utilisation, and
preventing its over-production from ingested fats and protein (see alsop. 146).
For placental hormones seep. 905 and for pineal function in lower verte-
brates, pp. r8g, 345, 347· The lymphoid thymus has been often claimed, without
154 ZOOLOGY

convincing evidence, to possess endocrine functions. The same is true of

brown fat, which some have believed to be an endocrine 'hibernation gland'.
Brown fat occurs as a lobular, and superficially gland-like tissue. It is 'histogen-
etically and physiologically different from ordinary yellow or white adipose
tissue (Fawcett), but has since been found also in widely unrelated, non-hibern-
ating mammals (e.g. Rattus and the rhesus monkey, 111acaca).
Urinogenital Organs.-In all Craniata there is so close a connection between
the organs of renal excretion and those of reproduction that the two systems
are conveniently considered together
as the urinogenital organs.
Speaking generally, the excretory
organ consists basically of three parts,
all paired and situated along the dorsal
wall of the crelom : the pronephros
('fore-kidney') (Fig. g8, A, p. nph.),
the mesonephros ('mid-kidney') (ms.
nph.), and the metanephros ('hind-
kidney') (mt. nplt.). Each of these is
provided with a duct, the pro- (p. n. d.),
meso- (ms. n. d.), or meta-nephric (mt.
n . d.) duct, which opens into the cloaca.
The gonads (gon .) lie in the crelom
suspended from its dorsal wall by a
fold of peritoneum. In some cases their
products are discharged into the crelom
FIG. 99.-Excretory system: Circulation and and make their exit by genital pores,
disposal in mammalian kidney. The kidney is
the viscus elegantissimum of the early ana- but more usually the pronephric duct
tomists. The arrow n ear the capsule (cap)
shows the relationship of the nephron (Fig. 100) in the female assumes the functions of
with cortex (cor) and medulla (med). Myriad an oviduct and the mesonephric duct
collecting tubules open into the pelvis from
which the urine is conveyed by the ureter (ttr) in the male those of a spermiduct.
to the storage bladder. (Modified after Homer The pronephros is almost always
Smith.)
functionless in the adult, and usually
disappears altogether. The mesonephros is generally the functional kidney
in the lower (anamniote) (pp. 153, 253) Craniata, in which no metanephros
is developed, and the mesonephric duct, in addition to carrying the seminal
fluid of the male, acts as a urinary duct.
In the higher forms (amniotes) (p. 457) the early developed mesonephros
largely atrophies. In these, the metanephros is the adult functional kidney,
and the metanephric duct becomes the true ureter (Fig. gg).
The kidney (meso- or meta-nephros) of the adult is a massive highly vascular
structure containing vast numbers of convoluted urinary tubules (Fig. roo),
separated from one another by connective-tissue. The tubules are lined by a
 PHYLUM CHORDATA I 55

single layer of glandular epithelial cells and each ends blindly in Bowman's
capsule, a globular dilatation. In many of the lower Craniata, a branch goes
off from the tubule, near the Malpighian body, and, passing to the ventral
surface of the kidney,
ends in a ciliated funnel- proximal distal
tubule tubule
like body (Fig. g8 A, nst.),
resembling the nephridio-
stome of a worm, and, like it,
opening into the ccelom.
At their opposite ends the
tubules join with one an-
other, and finally discharge
into the urinary duct.
The renal arteries branch
extensively in the kidney, so
that each of the countless
Malpighian corpuscles is
supplied by a minute afferent
arteriole (Fig. roo, aff. a.).
This invaginates the wall
of Bowman's capsule and
breaks up in a bunch of
looped capillaries, the
glomerulus, suspended in
the interior of the capsule.
The blood is carried from
the glomerulus by an efferent descending and
ascending loop
vessel (e./f. v.), which joins of Henle
the general capillary system
of the kidneys, forming a
network over the urinary
FIG. xoo.-Excretory system: A renal unit (nephron) of
tubules. Finally the blood a terrestrial mammal. To the encapsulated glomerulus (wit h-
is returned from this net- in Bowman's capsule) comes blood from the arterial cir-
culation. Fluid, containing solutes of small molecular size
work to the renal vein. (e.g. urea, NaCI), is filtered and sent into the convoluted
The components of the and tubule. This is enmeshed in capillaries into which fluid
beneficial substances are reabsorbed for canalisation back
above apparatus differ into the venous system. Discarded material flows down the
collecting tubule into the kidney pelvis (see Fig. 99).
greatly (even within a single (Modified after Homer Smith.)
order) according to the ex-
cretory requirements of the animal and the environment in which it exists.
In general, however, blood pressure forces fluids, salts, and waste metabolites
of small molecular size to filter from the tuft of capillaries into the Bowman's
capsule. The apparatus is impermeable to blood-cells and plasma proteins.
 ZOOLOGY
The stranded plasma proteins, however, exert an osmotic pressure which tends
to retain fluid and solutes within the capillaries. Depending upon the inter-
action of these factors, varying amounts of protein-free filtrate are allowed to
flow into the confluent renal tubule. From here quantities of fluid, together
with glucose and beneficial salts, can be reabsorbed through the tubule walls
into the bloodstream via the adjacent capillary net. The remaining substances
are excreted as urine or its equivalent.
All vertebrates, whether aquatic or terrestrial, are constantly exercised to
maintain the integrity of their internal environment. The historic migrations
between salt and fresh water and on to dry land have been made possible
partly by manifold modifications (slime, exoskeletal structures, sweat-glands,
gill, and kidney adaptations, etc.) in the structures that are involved in con-
servation or elimination of internal fluids and the regulation of the concentra-
tion of their contained salts. An animal is in large degree a system of physio-
logical membranes. When iso-osmotic sea-animals (with watery tissues rich
in the salts of the surrounding sea) explored the rivers they must have developed
or already possessed (Fig. ror) a mechanism that prevented the fatal dis-
ruption of their salt balance by the osmotic inflow of fresh water (seep. r64).
The development of a waterproof exoskeleton by ostracoderms probably
improved such protection, as well as conferring obvious advantages against
predators (p. r66).
The kidney (meso- or meta-nephros) is the primary organ of excretion, al-
though gills, lungs, skin, intestine, and other structures are important in this
respect among different groups. The structure of the primary renal unit, the
nephron, though differing in detail between animals of various habitats,
preserves a basic similarity that can be traced from the hypothetical proto-
vertebrate arrangement throughout the craniate series (Fig. ror). The
kidney is segmental and mesodermal in origin (see below) and there is little
reason to doubt that the vertebrate ancestor bore pairs of bilaterally symmetri-
cal mesodermal nephric tubules (cf. however, Amphioxus, p. 47), which
communicated through crelomic nephrostomes with the external environment
(or with some other structure opening thereto) and which discharged fluid and
metabolites and probably sexual products as well. In such a structure (Fig.
ror A) the crelomic cavity participated in excretion and a 'relatively feeble
stream of crelomic fluid would serve to wash the excretion out of the tubules
... with this meagre equipment the protovertebrate tried to enter the brackish
lagoons or freshwater rivers of the Palreozoic continents' (Homer Smith) (see,
however, p. r64).
Now, however much the external surface might come to be guarded by
slime or exoskeletal modifications, a considerable amount of fresh water would
be absorbed during feeding and respiration. The evolution of efficient homeo-
stasis, therefore, inevitably depended upon the development of a more elaborate
 PHYLUM CHORDATA 157
excretory mechanism in which the newly emergent, or at least increasingly
efficient, force-pump-the heart-could play an important part.
So, perhaps, arose the glomerulus (Fig. IOI B). The blood was now driven
continuously into an arteriole which entered the open nephrostome and which
branched into a knot of semi-permeable capillaries. Through the glomeruli
large quantities of fluid and salts could be eliminated into the renal tubule,
while, at the same time, suspended blood cells and proteins (of larger molecular

A B c D
Frc. IOI. - Excretion and osmoregulation: Probable stages in the evolution of the vertebrate
nephron. A, the hypothetical protovertebrate tubule drained the ccelom through the open
nephrostome (ccelomostome); B, earliest vertebrates developed the glomerulus in loose association
with the ccelomostome as a more efficient excretory device under hydrostatic (blood) pressure;
C, the glomerulus later became sealed in a Bowman's capsule, though with the retention, in
some species, of a C<Elomostome; which has disappeared in advanced vertebrates (D) (see Fig.
wo). (After Homer Smith.)

size) were conserved. The glomerular capillaries were in turn canalised back
into the venous system. Indications of this arrangement occur in the em-
bryonic piscine pronephros and persist in the pericardia! cavity of the adult
Myxine (p. 197).
Probably an added refinement was the closure and invagination of the
nephrostome (characteristic of the adult forms of extant vertebrates), possibly
in association with an unmodified nephrostome (Fig. ror C), which was later
lost (Fig. ror D) upon the perfection of the apparatus of glomerular filtration
and tubule reabsorption (Fig. roo).
 ZOOLOGY

Meanwhile paired internal channels arose to carry urine into the cloaca.
With the emergence of such enhanced excretory efficiency, various alternative
mechanisms for the discharge of gametes arose (Fig. 98). The transitional
animals referred to above had no need to conserve water; and in freshwater
fishes of to-day tubule reabsorption appears to be very limited. Undoubtedly,
the tetrapods inherited the glomerular apparatus through the crossopterygian
line (p. 353) and have themselves come to adopt many interesting excretory
specialisations in relation to later changes in habitat (e.g. see pp. 158, 548, 88g).
Fishes that took to the sea, on the other hand, were compelled to ingest salt
water and developed a remarkable excretory apparatus at the gill-surface
(p. 318) and so succeeded in maintaining the stability of their internal environ-
ment. In general (excluding the ureotelic elasmobranchs, p. 253), marine
fishes have reduced their glomerular equipment and, it follows, urine output.
Some species have become aglomerular (p. 347).
The evolution of the glomerulus was concerned in an interesting way
with advances in the venous system in general. The renal capillaries early
received blood at low pressure from a variety of sources which contributed to
the renal portal system that still persists in fishes, frogs, reptiles and, to some
extent, in birds. It was advantageous that the blood flowing through the
glomeruli should do so at relatively high pressure. The capillaries, after
leaving the Malpighian corpuscles, unite with those carrying blood from the renal
portal system in a peri-tubule network, and thereafter the two groups cannot
be distinguished. They unite as the renal vein and carry 'purified' blood back
to the heart for re-distribution. In the Mammalia, however, the renal portal
system has disappeared in the adult. The blood enters at high pressure through
the renal artery (a branch of the dorsal aorta). The capillary network contains
only post-glomerular blood, which is carried by the renal vein to the post-caval
vein and the heart.
The development of the kidney reveals a resemblance to the ccelomoducts
of annelids (Vol. I) which would hardly be suspected from its adult structure.
The pronephros (Fig. g8, A, p. nph.) originates as a small number of coiled
tubes formed from mesoderm in the body-wall at the anterior end of the
ccelom. They are arranged metamerically, and each opens into the ccelom by
a ciliated funnel (nst.). Obviously such tubes are ccelomoducts. Their chief
peculiarity is that their outer ends do not open directly to the exterior, but
into a longitudinal tube, the pronephric or segmental duct (sg. d.), which passes
backwards, and discharges into the cloaca. It seems probable that this
arrangement is to be explained by supposing that the ccelomoducts originally
opened externally into a longitudinal groove, which, by the apposition of its
edges, was converted into a tube. All the tubules of the pronephros open, by
their ciliated funnels, into the narrow anterior end of the ccelom. Into this
projects a branch of the aorta ending in a single large glomerulus or' glomus'.
 PHYLUM CHORDATA I 59

The pronephros soon degenerates. Its tubules lose their connection with
the pronephric duct (B), but in the meantime fresh tubules appear in the
segments posterior to the pronephros, and together constitute the mesonephros
or Wolffian body (B, ms. nph.). From this structure the permanent kidney is
formed in most of the lower Craniata. The mesonephric tubules open at one
end into the pronephric duct (sg. d.), at the other, by ciliated funnels (nst.), into
the crelom. A short distance from the funnel each gives off a blind pouch,
which dilates at the end and forms a Malpighian capsule (m. c.). A branch
from the aorta that enters it gives rise to a glomerulus.
In elasmobranchs the pronephric duct now becomes divided by a longitudinal
partition into two tubes. One retains its connection with the mesonephros and
is known as the mesonephric or Wolffian duct (C, ms. n. d.). The other has no
connection with the tubules, but opens into the crelom in the region of the
vanishing pronephros, and is called the M iillerian duct (p. n. d.). In most
Craniata the Mullerian appears quite independently of the Wolffian duct.
The latter is then simply the pronephric duct after its union with the meso-
nephric tubules.
In the higher vertebrates, from reptiles to mammals, a diverticulum (D, E,
mt. n. d.) is given off from the posterior end of the Wolffian duct, and this
grows forwards and becomes connected with the hindmost tubules. In this
way a metanephros (mt. nph.) is formed. This forms the permanent kidney
and a metanephric duct (mt. n. d.), which gives rise to the ureter. The Wolffian
body ceases its renal function, and becomes a purely vestigial organ in the
female while its residue in the male provides the epididymis.
In many fishes there is a dilatation of the mesonephric duct, the urinary
bladder, which serves as a receptacle for the urine. In the higher Craniata the
ventral wall of the cloaca sends off a pouch, the allantoic bladder (al. bl.), which
serves the same purpose, although morphologically an entirely different
structure.
The gonads (gon.) are developed as ridges growing from the dorsal wall
of the crelom and covered by crelomic epithelium, from the cells of which, as
in so many of the lower animals, the ova and sperms are derived. The testes
consist of lobules (fishes) or tubules (tetrapods), lined with spermatogenetic
epithelium, and usually discharge their products through delicate vasa efferentia
(D, v. e.) into the Wolffian duct, but in some groups into the coelom (e.g.
Petromyzon, p. 176). The sperms are always small, and motile. Between
the tubules occur Leydig or interstitial cells which collectively form an endocrine
gland which produces the male sex hormone (testosterone). The ovary is formed
basically of connective-tissue or stroma. It is covered by specialised epi-
thelium, certain of the cells of which become enlarged to form ova which
become spread through the stroma. In the majority of cases the ova are dis-
charged from the surface of the ovary into the crelom and thus into the open
r6o ZOOLOGY

ends of the Mullerian ducts (E, p. n. d.). These thus function simply as
oviducts, having no connection in the adult with the urinary system. In
some groups the ova and the sperms are shed into the crelom and escape by the
genital pores, and in many bony fishes the ovary is a hollow organ (as in Arthro-
poda (Vol. I)) discharging its ova into an internal cavity. From there they
are carried by a duct that is continuous with the gonad. The ovary has
specialised cells that produce female sex hormone (a:strogen).
In close topographical relation with the urinogenital organs are found the
endocrine inter-renal glands (p. 276) or, in tetrapods, adrenal (suprarenal) glands
of compound structure and function. The adrenals are developed partly from
ridges of the dorsal wall of the crelom, i.e. from mesoderm, partly from the
sympathetic ganglia. Adrenal tissue may be segmentally arranged in certain
lower groups (p. 380).
Development.-The ova of Craniata are usually telolecithal, but the amount
of food-yolk varies within wide limits. When it is small in quantity, cleavage
is complete but usually unequal, when abundant, incomplete, and discoidal.
In the latter case the embryo proper is formed, as in cephalopods, from a com-
paratively small portion of the zygote, the rest giving rise to a large yolk-sac
and, in the higher forms, to other embryonic membranes.
There is never a typical invaginated gastrula, as in Amphioxus, but in
some of the lower Craniata a gastrula stage is formed by a combination of
inpushing and overgrowth. Details will be given in the sections on the various
groups.
The mode of development of the mesoderm and of the crelom differs strik-
ingly from the process we saw in Amphioxus. At an early stage the mesoderm
is found in the form of paired longitudinal bands (Fig. 102, A, msd.) lying
one on each side of the middle line. Here they are separated from one another
by the notochord (nch.), and completely fill the space between the ectoderm
and the endoderm. Each mesoderm-band becomes differentiated into a dorsal
portion, the vertebral plate, bounding the nervous system and notochord, and
a ventral portion, the lateral plate, surrounding the mesenteron. The vertebral
plate undergoes metameric segmentation, becoming divided into a row of
squarish masses, somites or mesodermal segments (B, pr. v.). The lateral plate
splits into two layers, a somatic (som.), adherent to the ectoderm, a splanchnic
(spl.), to the endoderm. The space between the two is the crelom (ca:l.),
which is thus a schizoca:le, or cavity hollowed out of the mesoderm, and is,
except in the head-region in the lampreys (p. 176), at no stage in communica-
tion with the mesenteron, as are some of the crelomic pouches of Amphioxus.
In Lampetra, approximately the first score of segments are segmentally
separate, sausage-shaped chambers extending from myotomes down to the
bottom of the lateral 'plate' material so that this structure is not, in fact, a
plate. Behind the anterior segments the crelom arises in the lateral plate as a
 PHYLUM CHOI~DATA r6r

continuous cavity (as in the higher Craniata) and there is no such segmentation
of the lateral plate region posteriorly. The above points are of high phylo-
genetic interest.
In most craniates the crelom extends upwards into each somite. This
arrangement is only temporary, and these somitic cavities soon disappear.
From the dorsal portions of the somites are formed the derm atomes (which give
rise to the dermis); and the myotomes (from which myomeres develop) . From
their medial portions arise the sclerotomes.
The development of the principal organs has been described in general
terms along with a general account of the organs themselves. It will be

FIG. !oz .-Development: (A) earlier and (B) later embryos of frog. ccel. coolom ; ccel'.
prolongation of coolom into a somite ; ent. mesenteron ; med. gr. medulla ry groove; msd. meso-
d erm ; nch. notochord ; pr. v . somites; sg. d. segm ent al duct; som. somatic layer of mesoderm;
sp . c. spinal cord ; spl. spla nchnic layer of mesoderm ; y k. yolk -cells. (After A.M. Marshall. )

convenient to defer further consideration of this subject until we come to deal

with the development of the various types of Craniata, and with the embryo-
logical characteristics of the classes and sub-classes.
Metamerism.- A t endency, more or less strongly marked, to a serial repeti-
tion of parts is to be observed in a number of different systems of organs. In-
st ances of this have already been pointed out in the skelet on, and the muscular,
nervous, and excret ory systems. This phenomenon seems to lead to the con-
clusion that the structure of the Craniata can be understood only when they
are regarded as metamerically-segmented animals.
Segmentation in the Craniata, however, is never visible on the ext erior.
Even in the case of organs which present metameric characters, the meta-
merism often appears indefinite and uncertain. Thus the segmentation of the
spinal column, which in the adult is most pronounced of all, does not coincide
with the segmentation of the muscular and nervous system. Yet when we
t ake the phenomena of embryonic development into account, it becomes
VOL. II. L
r6z ZOOLOGY
sufficiently clear that in the Craniata we are dealing with animals possessing
a metameric segmentation of the same general type as that possessed by
Amphioxus and that the apparent anomalies are due to processes of secondary
modification.
It is in the trunk region that the metamerism is most strongly pronounced,
and that more particularly in the lower groups. In the head there is great
specialisation in co-ordination with the presence of the brain, the chief organs
of special sense, and the mouth and jaws. Therefore, though there are in-
dications of metamerism of various parts, it is only by the study of develop-
ment that it is possible to interpret the structure of the head in terms of a
metameric segmentation which becomes so much modified in the adult animal.
When the development is followed out, it becomes evident that (as in the
Arthropoda, Vol. I) the head in Craniata is formed as a result of a process
of fusion between a number of metameres, the individuality of which is evident
in early stages (more particularly among lower forms) and is most pronounced
in the region behind the auditory capsules.
 SUB-PHYLUM CRANIATA (VERTEBRATA)

V
INTRODUCTION
ARIOUS classifications of Vertebrata have been proposed, but agreement
has been reached on only one point-£.e. that this great group is essen-
tially divisible into two unequal sections : (r) The jawless Agnatha
(composed of a small number of primitive but highly specialised fish-like
animals), and (2) the Gnathostomata, which embraces all other vertebrates
from the true fishes to the Mammalia, including Man. In the past each of these
distinctive assemblages has been sometimes accorded the rank of sub-phylum,
but, however radically they differ in their buccal and visceral skeletal anatomy
in the adult, such an arrangement seems untenable when we consider the criteria
on which the three more primitive chordate sub-phyla have been erected and,
equally, when we survey the many fundamental similarities existing between
these two craniate groups. In recent years there has been a tendency to consider
the Agnatha and Gnathostomata as ' branches ', ' groups ', or ' super-classes '
of a single, and final, sub-phylum of the animal kingdom, and that is the arrange-
ment followed here.
We are faced now, as we shall be confronted again, with the perennially
vexed question as to what classificatory status various included groups should
be accorded. For example, different authorities assign the Euphanerida (p.
167) variously to Family, Order, and Class. At the present stage of our know-
ledge, the conventional arrangements of many fossil, and even living, groups
are essentially provisional and arbitrary. We shall see later, for example, that
the <Orders' erected by ornithologists have no validity whatever when
judged by criteria employed by ichthyologists and herpetologists. No
arrangement in any group can hope to be universally agreed at the present
time. As mentioned above, several opinions are available concerning the
most valid form of representation within each branch of the sub-phylum
Vertebrata. 1
1 Goodrich recognised three branches of craniates: the Ostracodermi, including all the fossil
agnathans; the Cyclostomata, including the lampreys and hags; and the Gnathostomata.
Stensio and Jarvik classify the Agnatha into the Sub-classes Cephalaspidomorphi, including the
cephalaspids, anaspids, and lampreys; and the Pteraspidomorphi, including the pteraspids and
hags. Romer includes all except the pteraspids in the Sub-class Cephalaspidomorphi.
 ZOOLOGY
SUPER-CLASS AGNATHA
Classes Euphanerida (Silurian)
Heterostraci (Fteraspida) (Ordovician-Devonian)
Anaspida (Silurian-Devonian)
Osteostraci (Cephalaspida) (Silurian-Devonian)
Petromyzontia (?-Recent)
Myxinoidea (?-Recent)
It is almost certain that we know only a small proportion of the Agnatha
that existed during the Palreozoic. Further, next to nothing is known of their
ancestry. The earliest known vertebrate relics come from the Ordovician,
and the nature of these makes it possible that the first vertebrate animals
appeared in the Cambrian (Table I, p. 3). Opinion is sharply divided as to
whether the first true vertebrates were fresh-water or marine in habitat. It
is possible that the relatively strongly swimming vertebrates may have arisen
in fresh water, where such powers would enable them to withstand river
currents sweeping seawards. The vertebrate kidney, so different from the
excretory apparatus of Amphioxus (p. 47), possibly arose during the slow
colonisation of coastal and inland waters (p. 156). The earliest fresh-water
chordates no doubt took in surrounding fluid by osmosis through the permeable
regions of the body. The water had to be discharged. At the same time it
was necessary for vital solutes to be retained, and waste metabolites to be
discarded, if the integrity of the internal environment were to be maintained.
Pioneer movement into brackish and fresh water would impose the necessity
for considerable elaboration of the simple mesodermal funnels of excretion
which carried fluid from crelom to the exterior. In any case there arose a
device which made it possible for the power of the heart-pump to be used to
eliminate unwanted fluid, and contained substances, through a filter (Fig. roo).
This seems to have been at first a relatively simple knot of capillaries near each
tubule mouth. Upon this basic regulatory mechanism there have been
imposed the many modifications made necessary by the re-colonisation of
sea-water, and the exploitation of land, including deserts and other environ-
mental niches. A good case can be made out that all vertebrates, what-
ever their habitat to-day, are of fresh-water ancestry (see Romer, Homer
Smith).
But even stronger evidence, perhaps, can be adduced for an opposite view
(see Krogh, Watson, Robertson). While it is undeniable that most fossils of
very early vertebrates so far discovered have come from fresh-water sediments
(e.g. see p. 174), it is equally true that the earlt"est known remains (plates and
scales of Middle Ordovician ostracoderms) were unearthed from sandstone
that, judged by an associated, and indubitably marine, invertebrate fauna,
was not of fresh-water origin. It has been suggested, but not established, that
 PHYLUM CHORDATA r6s
these fragments are the remnants of fresh-water animals that were washed
downstream and incorporated in littoral deposits. As indicated above, there
have been suggestions that the powerful lateral body flexure of vertebrates
would help the earliest fresh-water craniates to withstand currents tending to
sweep them seawards. But it is perhaps equally easy to believe that the
development of such enhanced locomotion and stability could occur in the
relative turmoil of the ocean, and that such pre-adapted (p. 385) ancestral
forms could then swim against estuarine currents to colonise the brackish and
fresh-water streams and lakes in which so many have left their bones.
It should be remembered too that all three protochordate groups, the
Adelochordata (p. 5), Tunicata (p. 20), and Acrania (p. 47), are, and perhaps
always have been, restricted to the sea. Again, it is possible that the glomer-
ular kidney (p. ror) existed in marine protovertebrates, thus additionally
pre-adapting them for life in fresh water. Finally, the blood of some extant
agnathans (myxinoids, p. 197) has an ionic constitution strikingly similar to
that of the surrounding sea: ' this may well be a primary feature, derived
directly from marine chordate ancestors' (Robertson).
The first vertebrate group of which numerous specimens are available is
the Agnatha. The first common representatives of these were fresh-water,
bottom-living animals of the Silurian (p. 3). Groups continued to expand
during the Devonian, when they attained their maximum development.
Between that time (some 270 to 300 million years ago), and the present day,
little is known of them. Extant agnathans, unlike most of the early forms,
lack bones, true teeth, and other durable parts, and that is probably why
fossils are unknown.
It has been sometimes claimed that the not uncommonly found conodonts
are the teeth of unknown armoured agnathans. This is unlikely. Conodonts,
however, should perhaps for the time being find a place in a volume of the
present character, and this is probably the best place to discuss them. They
are small (0·2-3 mm. long), cone- or tooth-shaped, widely distributed, fossils
which show a great deal of variation but fall possibly into two groups, laminated
and fibrous. Their chemical composition is essentially bone-like and not very
different from that of the armour of Devonian fishes (p. 285). Laminated
conodonts occur in sedimentary strata from the Ordovician to the Triassic.
Fibrous conodonts occur possibly only in Ordovician deposits and may re-
present a distinct group. A highly suspect binomial classification has been
erected, and subscribed to, in many of the several hundred papers dealing with
conodonts.
Conodonts seem to be formed by the accretion of material around a basal
(but not pulp-) cavity. Radial canals, extending from cavity to surface,
occur. The fibrous specimens often have very bone-like material attached to
them. A few lamellar specimens have a similar attachment. There is some
r66 ZOOLOGY
evidence that conodonts were regenerated when broken, but they show little
evidence of wear.
These enigmatic objects have been variously held to be the branchial filters,
or skeletal supports, of primitive vertebrates or, sometimes, the teeth of
Agnatha or of primitive elasmobranch fishes. They have also been claimed
to be the teeth, or copulatory organs, of worms. Some have believed them to
be part of the armour of crustaceans or to be allied to the radular teeth of
gastropods. Rhodes, the latest reviewer, concludes that : (r) 'if of vertebrate
origin they may represent some group . . . other than fishes, now extinct,
and, apart from the conodonts, entirely unknown', or (2) they may be mastica-
tory organs of annelid affinity. Apart from their anatomy and chemistry,
the one thing that is perfectly clear is that nobody at present has any real idea
of their function or identity.
The Agnatha, apart from their antiquity per se, are of great interest in
possessing tru-e bone in the oldest groups. It was for many years confidently
held that cartilage was the forerunner of bone. To-day, however, it is now
widely agreed, with de Beer, that bone is the more primitive material and that
cartilage in the adult is probably a neotenous phenomenon. On palceonto-
logical evidence Romer has reached the same conclusion concerning its anti-
quity. It has not yet been proved, but there are excellent reasons for suppos-
ing, that cartilage may be essentially an adaptation of embryonic tissue in the
adult form. Although it could be held that Silurian and Devonian dermal
bone (as well as skeletal bone in the Cephalaspida) might be derived from car-
tilage of an even earlier date, there is no doubt that the earliest known verte-
brate fragments (from the Ordovician, about 400 million years ago) were
ossified. Stensi6 believes it possible that bone arose as early as the Lower
Ordovician, or perhaps even in the Upper Cambrian. There is no evidence of
a progressive increase in ossification in any craniate group.
The early agnathans were generally heavily armoured (Figs. 104, ros).
There is no certainty why this was so. Their buccal arrangements reveal that
they did not devour each other. The fresh-water forms were isolated from
attacks by large predacious marine invertebrates. There is evidence, however,
that large water-scorpions (Eurypterida, Vol. I) with formidable claws and
mouth-parts existed in the same aquatic environment. These animals were
allied to to-day's arachnids, and reached a length of about 9 feet-perhaps
the biggest size attained by arthropods. It is an interesting fact that when
faster fishes came to replace the early slow-moving armoured agnathan types
in the succeeding Devonian period, the eurypterids also faded and disappeared.
Homer Smith, on the other hand, believes that the armour of Silurian and
Devonian forms was a defence against ' the osmotic invasion of fresh-water
rather than the claws and tail-spines of eurypterids '. There is little doubt that
dermal ossification would in fact provide such defence; but it would seem that,
 PHYLUM CHORDATA

if protection against osmotic inflow was the primary factor involved, this may
well have been obtained by more economical means. Further, many con-
temporary animals were not protected in this way (e.g. see Fig. 103).

CLASS EUPHANERIDA
Perhaps the most primitive vertebrate yet described was the totally un-
armoured Jamoytius kerwoodi, described by White from the Scottish Silurian
(Fig. 103). First thought to be allied to Lasanius (p. 171), jamoytius was only
about 7 inches long. It was blunt headed, with a rounded, somewhat elon-
gated, body. With the ' possible exception of the supports of the median

Myoc. D.F.

FIG. IOJ.- Ciass Euphanerida, Family Jamoytiidm. Jamoytius ke-rwoodi. Specimen (lacking
tail) shown in three.quarter view, at about !th of natural size. A. F. anal fin supports; D . F .
dorsal fin supports; D. S. displaced skin; E. eye; Int. contents of intestine; L. F. F. lateral fin.
fold; Myoc. septal spaces; Myom. myomere; No. notochord. The above is one of the illustra-
tions used in White's description of the new order, family and species.

fins', there is no indication of internal calcification. It had relatively enormous

and widely separated eye-regions, a brief anal fin, and a dorsal fin continuous
along the posterior two-thirds of the body. The intestine was short and
straight. There was a persistent notochord and myotomes that were simple
in arrangement. No evidence of gill pouches opening to the exterior has been
found.
White believes that the nakedness of J amoytius is a primary character and
that earlier members of this apparently unspecialised group may have con-
stituted the stock from which gnathostomes as well as agnathans were derived.
There is a further suggestion that the Acrania (e.g. Amphioxus) (p. 68) arose
as degenerate forms from animals such as the present one. Gregory has
suggested that ' the boneless J amoytius may well have been on the way towards
Amphioxus, while the bony Cephalaspis (p. 172) may have been on or near the
r68 ZOOLOGY
line leading to the lampreys'. This view, however, implies that jamoytius
was secondarily naked.
The unusual name J amoytius is derived from that of the late J. A. Moy-
Thomas, a distinguished worker in the present field who tragically died at the
height of his powers.

CLASS HETEROSTRACI (PTERASPIDA)

A great deal is now known of the three armoured agnathan groups of the
Pal;eozoic. These, the Heterostraci, Anaspida, and Osteostraci, and probably

FIG. 104.-Class Hetero-

straci, Family Pteraspidre.
Pteraspis. Restoration of
P . rostrata from dorsal (left-
hand figure) lateral and
ventral views. a. l. p.
anterior lateral plate ; b. p.
branchial plate; c. p. corn-
ual plate; d. d. dorsal disc;
d. sp. dorsal spine; o. p. oral
plate; or. p. orbital plate;
pi. pineal plate; p. l . p.
posterior lateral plate; r .
rostrum; v. d. ventral disc.
(After E . I. White.)

the naked lampreys and hags as well, are clearly related. None, however,
appears to have been directly derived from another. Each stock probably
branched successively from the ancestral stem in the order used below.
The Heterostraci, although probably more advanced than the Euphanerida
(see above), are known to have existed in the Middle Ordovician, from which
plates of Astraspis have been described. Such animals (e.g. Pteraspis, Fig.
104) were apparently seldom more than a foot long, although some grew to a
 PHYLUM CHORD/\.TA r6g
length of 5 feet. They became more plentiful in the Upper Silurian and Lower
Devonian and persisted into the Upper Devonian. They possessed a broad,
depressed head protected by a carapace of exoskeletal plates. The eyes were
widely separated, and lateral in position. There was a sub-ventral mouth at
the extreme anterior end of the body which was equipped with probably mobile
plates. White has suggested that this buccal apparatus was protrusible and
able to scoop up bottom-mud and the contained organic material that served
as food.
There were no fins on the body. The tail behind the carapace was laterally
compressed, downwardly turned (hypocercal), and protectively covered with over-
lapping rhomboid scales of the same composition as the plates of the carapace.
The Heterostraci differed widely from the Osteostraci (see below) in several
essential points. There are no traces of a median nostril and there are strong
indications that the olfactory organ was a paired structure opening into the
roof of the mouth. The plates and scales lacked bone-cells. The plates
were formed of three layers, of which the outer was composed of a substance
allied to dentine. The middle layer was formed of vascular 'bone'; the
lower was laminated. The gill-pouches, apparently of the adult cyclo-
stome pattern, united as a single exhalent pore placed rather far back on the
sides of the carapace. There was no sign of the complicated and prominently
innervated sensory organ formerly thought to be one of electrical discharge
(Fig. ro8, p. 173).
The four groups which form the Heterostraci (Paleaspidce, Cyathaspidce,
Pteraspidce (Fig. 104), and Drepanaspidce (Fig. 105)) were distinguished from
one another by the number of plates in the carapace. The Paleaspidce had
an undivided upper shield ; the Cyathaspidce had it divided into four plates ;
the Pteraspidce nine and the Drepanaspidce twelve major and many smaller
plates.
A description of Pteraspis rostrata, a species of the best-known genus, will
serve for the remainder. The carapace was somewhat elongated and rect-
angular in section. The dorsal surface was composed of nine plates, of which
three, the rostral, pineal, and large dorsal, were unpaired. The remaining six
plates formed three pairs. These are the orbital (in which lay the eye-sockets),
the branchial (with the single gill opening at the posterior border), and just
behind these, the cornual plates. In the adult all these plates were bound
together by a fusion of their inner lamince. There was a prominent median
spine inserted at the base of the dorsal disc. On the ventral surface, a large
unpaired plate covered most of the area except for a number of small oral
plates round the lower border of the mouth. Immediately behind these were
three pairs of small plates, the post-oral and the anterior and posterior laterals.
The trunk was covered with thick rectangular scales and there was a series of
thorn-like scales along the upper and lower surface. From impressions of the
IJO ZOOLOGY
internal surface of the upper carapace it seems clear that there were seven
pairs of gill pouches. An X-like impression just behind the pineal opening has
been interpreted as evidence of two semicircular canals (seep. 145) on each side.
There was a prominent lateral line system (seep. 135).

FIG. 105.-Class Heterostraci, Family Drepanaspidm. DTepanaspis. Dorsal and ventral

views. D. gemundenensis. C. cornual plate; D, median dorsal plate; M.-M3 , ventral oral
pla tes; 0 1 -03 , ocular plates; R, rostral plate; V, ventral plate; B , branchial plate. (From
Stensi6, a fter Ki;er.)

CLASS ANASPIDA
Anaspids share some characters with the pteraspids but show a greater
resemblance to the cephalaspids (Osteostraci) although, it should be emphasised
again, they sprang from a common ancestry and were derived from neither.
They were fish-shaped animals, a few inches long, with a downwardly turned
tail (hypocercal) (Fig. 106). The internal, presumably cartilaginous, skeleton
is unknown, but a highly developed dermal skeleton was present. The scales
on the head were small and showed a complicated arrangement which differed
in pattern in different genera. The scales on the trunk were arranged in lateral
and ventral series, which, with a set of tall lateral scales, showed great resem-
blance to the condition found in cephalaspids, as does also the mode of their
articulation. There are no paired appendages, unless a pair of pectoral spines
represented them. There was-an anal spine, and a series of ridge spines along
the dorsal surface. The eyes were lateral, and the pineal and nasa-hypophysial
openings were placed on the top of the head in relatively the same position as
 PHYLUM CHORDATA IJI

in the cephalaspids. A series of gill openings, varying in number in different

genera, were placed in front of the pectoral spine. The mouth had a shape
that resembled that of jawed animals, but there is no evidence that jaws

FIG. 106.-0rder Anaspida, Family Birkeniidre: Rllyncholepis parvulus. (After Ki.er.

existed. Lasanius, from the Upper Silurian, was almost unarmoured except
for dorsal and pectoral spines. There were several groups. Birkenia, Rhyncho-
lepis (Fig. 106), Saarolepis ( = Anaspis) are typical genera.

CLASS OSTEOSTRACI (CEPHALASPIDA)

The cephalaspids (Fig. 107) had developed prominent ' pectoral fins ' and
a heterocercal tail. Much splendidly preserved material has been discovered,
allowing a relatively clear picture of their essential features. They were
bottom-living animals with a single nasa-hypophysial opening placed far back

FIG. I O].- Order Osteostraci, Family Cephalaspidre. Hernicyclaspis. H . murchisoni . D 2

dorsal fin; d. cr. dorsal crest of trunk; sd. cr. do rsal scutes; Ppl. pineal plates ; sclr. sclerotic ring ;
lsf. lateral 'sensory field ' ; P ee. pectoral fin; vhp. caudal fin. (After Stensii:i.)

on the head. The head region was expanded and flattened. It was protected
by a hard, bony carapace or head-shield which was often ornamented with
tubercles, and usually produced on each side into a backwardly directed ' horn'
which left a bay, or pectoral sinus, between itself and the trunk. Beneath the
true dermal bone in at least some cephalaspids there occurred a fragile and
continuously ossified endocranium, equally curious both in shape and constitu-
tion. This was usually cartilaginous in later forms. The body was protected
by rows of scales in which true bone-cells were present. A dorsal fin occurred.
172 ZOOLOGY
Two Orders, Cephalaspidiformes (Fig. 107) and Tremataspidiformes, are
generally recognised.
In Cephalaspis, the most completely known genus, there was, as mentioned
above, a pair of lobed appendages arising from the pectoral sinus on each side.
These were neomorphs, and therefore not homologous with the pectoral fins of
vertebrates. Of the internal anatomy, too, a good deal is known. The brain,
with its ten pairs of cranial nerves, was closely comparable with that of the
living cyclostomes, but in addition there was a well-developed system arising
in the acoustico-lateralis region. From here five pairs of stout vessels ran to
depressions on the upper surface of the head-shield. By analogy with such
modem forms as torpedoes (p. 271), Stensio suggested that these nerves and
depressions formed a kind of electric organ (Fig. 108).
There are good reasons for believing this theory to be invalid. Westall
has reinterpreted the so-called ' electric fields ' as a sensory organ. He be-
lieves them probably to have been ' receptors of vibrationary stimuli ', con-
sisting of invaginated pockets, probably derived from placodes, and richly
supplied by neuromast organs innervated by the acoustico-lateralis system.
In short, they possibly compensated for the relatively reduced lateral line
system (p. 135), which was in them much less extensive than that of the
Heterostraci existing in the same habitat. Watson, too, is of the opinion
that they may constitute a sensory apparatus that might allow 'pressure
waves incident on the head to be conveyed along the fluid-filled canals to the
ear ' .
Cephalaspis was a bottom dweller. The eyes were placed close together on
the top of the cephalic shield, with the pineal aperture between them. It is
possible that the pineal was a photoreceptor which helped the animal gauge the
depth of water and, by means of pituitary hormones, protectively control its
colour in a manner comparable with that of the ammocrete larva of Petromyzon
(p. 196). The nasa-hypophysial opening was a little anterior to the pineal
aperture. (See p. 345.)
The ventral mouth was followed on each side by ten pairs of gill-pouches,
of which the first two (anterior and spiracular gill-cleft) were innervated by the
ophthalmicus profundus and trigeminal nerves respectively. Each such nerve,
then, was in Cephalaspis a typical branchiomeric nerve, i.e. one associated with
a functional gill-slit. We will see that in jawed animals (pp. 205, 210) these
nerves supply apparently quite unrelated structures (i.e. the snout and jaws).
This remarkable change in situation and function has been dictated by the
enlargement of the mouth and buccal cavity in relation to the shift to macro-
phagous habits. The comers of the mouth migrated backwards, obliterating
the first, and absorbing the second, pairs of gills (Fig. 137). For many reasons
it now appears certain that in this way the mouth came into proximity with
the branchial arch (posterior to the second pair of previously functional gills),
 PHYLUM CHORDATA 173

F rG. IoS.-Osteostraci: Cranial anatomy of Kireraspis.

Up per left: top v iew of hea d and anterior pa rt of trunk region: deo. d orsal sensory (supposed
' electric') organ; leo. lateral organ ; n . opening of single dorsal nost ril and h ypoph ysis ; o. orbit ;
p. p ineal opening.
Lower left: diagra mma tic dissect ion of top of h ead, t o show b ra in, nerves, a nd sense organs.
N erves to sensory organ shown on the left ; other major nerves (numbered) on right, a rranged
in a series corresponding to the gilt slits. c. Cerebellum; d. nerve to dorsal sensory field; e.
orbit ; f. forebra in ; m . m idbrain ; mo. medulla oblongat a; n . opening for nostril a nd h ypophysis ;
o. ear region (two canals) . I n this a nd t he next figure Roman numera ls indicat e cra nia l nerves.
Upper right: restoration of ventral surface of head, with the plates covering the t h roat re.
m oved, showing the ten gill sacs a nd t he small, a nteriorly placed m out h. dpr. mou t h; ibs.
par t it ions between gill sacs ; kebr. d uct s from gill pouches t o surface; res. cesophagus ; psg. a
p respiracula r gill pouch lost in higher vertebrates; rpm. rostral region in front of gills ; sg. gill
pouch corresponding to t he spira cle of higher fishes ; vm. muscles of gill pouches.
L ower right: restoration of ventral surface of head, covered by sma ll plates: ebrc. opening of
the gills; 111. mouth. (From R omer , after St ens io .)
174 ZOOLOGY

and the skeleton of this arch and its musculature came to be modified into a
powerful masticating apparatus, i.e. the modern jaw.
In Cephalaspis the whole of the ventral mouth and gill region was supported by
a system of small, tessellated, bony plates, constituting the oro-branchial area.
The considerably expanded gills had nothing in common with those of fishes.
It has been suggested that their structure resembled that found in lampreys of
to-day (p. 175) or, alternatively, perhaps that of the ammocrete larva (p. 196).
It is generally held that ostracoderms strained organic particles from debris
sucked in from the floor of fresh-water lakes and streams : the small ventral
mouth, dorsal eyes, and generally flattened form point to this conclusion.
The Osteostraci ranged from the Upper Silurian to
the Upper Devonian, and a considerable number of
genera and species is known. They showed great
variation in the proportions of the cephalic shield and
in its ornamentation. Representative genera are
Cephalaspis, H emicyclaspis, A teleaspis, Benneviaspis,
Kia:raspis (Fig. ro8), Didymaspis, Thyestes, and others.
The tremataspids differ from other cephalaspids

I
chiefly in the absence of the pectoral sinus, and, in con-
sequence, of the paired appendages. The cephalic shield
was well developed, and extended backwards on to the
trunk. The lateral sensory fields were divided into
anterior and posterior portions. The members of some
genera of the Cephalaspidiformes were transitional in
shape; the two groups were clearly allied.

THE ' C<ELOLEPIDA '

FIG. 109.-Lanarkia
spinosa, possibly related Several genera of small fish-like animals which
to the Heterostraci and usually appear as flattened impressions with a broad
formerly often placed with
head-region, a somewhat fusiform body, and a hetero-
'Order' Ccelolepida. R e·
stored outline in the posi-
cercal or hypocercal (e.g. Ccelolepis) tail have been de-
tion in which it occurs as
a fossil, the head beingscribed. They occurred from the Upper Ordovician to
flattened and the t ail
the Devonian. Their armour appeared to be restricted
twisted round so as to a p-
pear in profile. On eachto minute denticles (Fig. rog) that produced, in some
side a much-enlarged der-
mal denticle is shown. forms at least, a shagreen surface perhaps not unlike
(From the Cambridge Natu-
that of selachians (p. 267). It has often been considered
ral History, afterTraquair.)
that the ' Crelolepida ' forms a single well-defined
assembly. Westoll, however, has made it very probable that two or more
structurally distinct groups exist. One such is represented by Thelodus and
Lanarkia (Fig. rog) and another by Cephalopterus.
The first group were related to the Heterostraci and the second to the
 PHYLUM CHORDATA IJ5
Osteostraci. Other forms (e.g. Pldebolepis, Co::lolepis) showed certain resem-
blances to the Anaspida. It is still uncertain whether the ccelolepids represent,
or include (r) forms that belonged to each of the above groups and never
became ossified, (2) secondarily unossified types that descended from armoured
ancestors, and (3) the ' larval ' stages of animals that might have developed
dermal ossifications at a later stage of their development.

EXTANT AGNATHA (CYCLOSTOMATA)

Lampreys and hags, the only surviving agnathans, are often grouped to-
gether in the Class or Order Cyclostomata, distinguished from all other living
craniates by the possession of pouched gills and a single nostril, and the
primitive absence of jaws or paired fins. However, such features, which are
shared with one or other of the fossil groups, do not provide evidence that hags
and lampreys are more closely related to each other than they are to the
earliest agnathans. It seems probable that the lampreys are related to a
cephalaspid-anaspid stock, but the affinities of the hags are very doubtful. In
view of this it is advisable to regard the Cyclostomata as a 'group of con-
venience' and to treat the lampreys and hags as separate Classes of the Agnatha.

CLASS PETROMYZONTIA
This group comprises the lampreys, Agnatha with a round sucking mouth,
a pharynx which ends blindly, and (like the cephalaspids) a nasa-hypophysial
sac opening by a single nostril on the dorsal surface of the head. There are
about half a dozen genera, differing from one another only in minor anatomical
fea tures. Petromyzon (Fig. rro) occurs on both coasts of the North Atlantic,
and Entosphenus on both coasts of the North Pacific. Geotria occurs along the
southern coast of Australia, and in New Zealand, Chile, and Argentina.

d. f.,
g.
1.1.

·-e~~_.{l~~~
.- l ·~·-~.,- _:.:....:,............:.....,.~~~~
b. f.
FIG. 1 10 .- Class Petromyzontia, Family Petromyzonidre. Petromyzon. Adult Lamprey
(P. marin us). b. f. buccal funnel; c. f. cauda l fin; cl. cloaca; d. f. 1 , 2 1st and znd dorsal fins ; g. ex-
terna l gill aperture; /.I. la tera l line organs; n. nasal aperture. (Drawn by R. Strahan.)

M ordacia is restricted to the Southern Pacific. The remaining genera, all

Holarctic, appear to be degenerate descendants of the Northern genera already
mentioned.
 ZOOLOGY

EXAMPLE OF THE CLASS:-

THE LAMPREY (PETROMYZON)
Three species of lamprey are common in European river systems. The
Sea-lamprey (P. marinus) attains a length of a metre, the Lampem, or common
fresh-water lamprey (Lampetra fluviatilis), reaches a length of about go em.,
and the Sandpride, or lesser Fresh-water Lamprey (L. planeri), does not exceed
45 em. in length.
Two quite different phases occur in the life-cycle of Petromyzon. The larval
phase or ammoccete feeds on microscopic particles by means of an endostyle
not unlike that of Amphioxus. The adult has a suctorial mouth, and a rasping
apparatus that enables it to cling to fishes and feed on their tissues. Com-
paratively little is known about the habits and life-cycle of even the common
species. The adult lives in the sea for most of its life. It attaches itself to a
fish, rasps away some of the host tissue, and then feeds on its blood. It is often
found in this situation when the host is caught. At the onset of its single
breeding season it migrates coastwards and ascends a river into fresh water.
In Europe this occurs in the autumn, when great numbers of L. fluviatilis
invade favoured streams. They are not infrequently used as food, though
more particularly in past eras, as the sad fate of Henry I of England will recall.
In some instances they ascend rapids in their urge to reach the breeding-
ground, and cling to the rocks with their suckers between leaps forward and
upward. During this migratory phase the gut atrophies and they cannot feed.
They are sustained by fat reserves accumulated sub-cutaneously and in the
body-muscles. Towards the end of winter the gonads become mature. Secon-
dary sexual characters develop-a prominent anal fin in the female, and a
penial-tube and a thickening at the base of the dorsal fin in the male.
In the spring, lampreys aggregate in pairs in shallow spawning-grounds
and build their nests. They move stones from the stream-bed with their
suckers and construct a sandy floor with a crescentic ridge on the downstream
border. Although copulation occurs (the male clinging to the female by means
of its buccal funnel), fertilisation is external. Both eggs and spermatozoa
(which are ejected through the penial tube and are very short-lived) are ex-
truded into the surrounding water, whence they are carried by the current into
the interstices of the stony rim of the nest. After each of the numerous short
matings, sand is stirred up from the floor of the nest by vigorous tail move-
ments, and this serves to embed the eggs more firmly among the stones. It is
said that the seasonally developed anal fin of the female assists in this operation.
When spawning is completed, the adults drop away from the nest and die within
several days.
It is of great interest that lampreys that inhabit lakes in North America
(where their ancestors were isolated) have successfully adapted themselves to
 PHYLUM CHORDATA 1 77
live their whole lives in fresh water, but nevertheless move a few miles up
lake-side streams at the onset of the breeding season.
External Characters.-The head and trunk (Figs. no, rn) are nearly
cylindrical, with the tail-region compressed or flattened from side to side. At
the anterior end, and directed downwards, is a large, basin-like depression, the
buccal funnel (b. f.), surrounded by a marginal membrane (m. m.) outside of
which arise the overlapping oral fimbrice (o. f.) and the longer, sensory cirri (c.).
The inside of the buccal funnel is beset with radiating rows of yellow horny
teeth (i. o., l. t., s. o.). The teeth rest on cartilaginous pads and are principally
mesodermal in origin, and therefore not homologous with true vertebrate
teeth, which first appear in fishes.
From the bottom of the buccal o. f. s.o.
funnel there projects a prominence,
the so-called ' tongue ', bearing even
larger horny teeth (t. t.), and having t.
immediately above it the narrow
mouth (m.). On the dorsal surface of
the head is a single median nostril
(n.), and immediately behind it a
transparent area of skin indicates the
position of the pineal organ. The
paired eyes have no eyelids, but are
covered by a transparent area of skin.
The gill-slits (g.) are seven pairs of
small apertures on the sides of the FIG. I I I . -Petromyz on: Buccal funnel.
cirrus; i. o. infra-oral tooth p lates; l . t.
head, the first a little behind the eyes. c. lateral bicuspid tooth; m . mouth; m . rn.
On the ventral surface, marking the marginal membrane ; o. f. oral fimbri;e ; s. o.
supra-oral tooth; t. t. tongue tooth. (Redrawn
junction between trunk and tail, is the after Bigelow and Schroeder.)
very small anus lying in a slight de-
pression. Immediately behind it is a small papilla pierced at its extremity by
the urinogenital aperture (cl.). There is no trace of paired appendages. Two
dorsal fins of approximately equal dimensions (separated by a notch) and a
caudal fin are present, the second dorsal being continuous with the caudal. All
these fins are median and unpaired. As previously mentioned, the sexes during
the period of gonad maturation can be distinguished by the number and structure
of the fins and, in addition, by the appearance of a penial tube in the male.
This is formed by the junction of the cloacal margins. At this period, too, the
cloacal margins of the female become distended and sometimes reddened.
The skin is soft and slimy and, as in Myxine, lacking in exoskeletal structures
(Fig. II2). Among extant forms scales, like true teeth, do not appear until
we ascend further the chordate stem. Keratin (p. 85) does not occur in the
epidermis although it possibly forms the caps on the 'teeth' (p. 184). As the
VOL. II. M
 ZOOLOGY
outermost layers of the epidermis become worn away by abrasion, new cells are
added by division of those at the base of the layer. Thus, these forms differ
from Amphioxus and resemble the higher vertebrates in their possession of a
several-layered, more durable, epidermis. Underlying the epidermis is a layer
of collagen and elastic fibres which forms the cutis. This layer contains also
pigment-cells or chromatophores. The pigment in these star-shaped cells changes

FIG. II 2.-Myxine: Integument. Vertical section showing structure and cell-types in M .

glutinosa. a. t. adipose tissue; b. v. blood vessels; c. t. f. connective t issue fibres; d. dermis; e.
epidermis ; m. c. mucous cell ; p. c. pigment cell; t. c. thread cell. (Thread cells are peculiar to t he
Myxinid<e. Their mucous cells are atypical in that t he nucleus lies in the centre of the cell
(cf. Fig. 113, r) . (Dra wn by G. H. 0. Burgess. )

its position under various conditions of illumination. Thus the ' colour' of the
lamprey changes from pallid to dusky and vice versa. This faculty is particularly
well shown in the larva (p. 196).
The epidermis is equipped also with numerous unicellular glands which
secrete a slime which makes Petromyzon difficult, or impossible to hold with the
bare hand. In addition, two other types of secretory cell are formed (Fig. II3).
One, the granular cell, is of yet unknown function. The second, the club cell
(Kolbenzelle) , is an elongated cell with hyaline cytoplasm. These are found also
in certain t eleosts, notably in Anguillid<e (eels). There have been suggestions
 PHYLUM CHORDATA 179
that club cells are neural (but fibres have not been demonstrated to end in
them), or scab-forming (but at least in Anguilla and Tinea such is not the
case). On the other hand, there is evidence that club cells contribute to the

t.
n.

.
_

n.

Frc. 1 13.-Integument: Epidermal cells of agnathans and fishes. 1. Mucous cell of Salmo lrttlta;
2. club cell of Conger conger; 3- thread cell of Myxine glutinosa; 4- granular cell of Petro-
myzon marinus.
c. cytoplasm; g. granules; n. nucleus; m. mucus cell; I. ' thread' in thread cell. (Drawn by
G. H . 0 . Burgess.)

sliminess of eels in which mucous cells are present only in relatively small
numbers.
The segmental sense-organs take the form of a lateral line (p. 136). These
organs are restricted to fishes and fish-like vertebrates (including tadpoles of
Amphibia) . In lampreys, lateral line organs are not enclosed in a canal, but
are exposed to the exterior.
r8o ZOOLOGY
Endoskeleton.-The axial skeleton of the trunk is simple. There is a per-
sistent notochord (Fig. II4, nc.) with a tough sheath composed of an inner
fibrous and an outer elastic layer. This longitudinal central rod probably pre-
vents the body from shortening when the muscle segments contract and drive
the animal forward. Attached to the sides of the notochord are small, vertical,
rods of segmentally arranged
cartilage, bounding the spinal
canal on each side, and which
correspond to the rudimentary
neural and interneural arches.
In the caudal region these fuse
into a single plate perforated
by foramina for the spinal
nerves and send off processes
to the base of the fin. For
the rest of its length the spinal
canal is enclosed only by
nc.
tough, pigmented connective-
c.t. tissue. Slender rods of carti-
lage support the median fins.
cd. d. ao. The cranium also is primi-
tive in structure. Its floor is
formed by a basal plate (Fig.
ns, b. pl.) , made by the union
S·C. S.
mes.

of the parachordals and trabe-

culre, surrounding posteriorly
the anterior end of the noto-
chord. Immediately anterior
to the notochord is a large
l.v. aperture, the basi-cranial fon-
F IG. 114.- Petromyzon: Trunk region. Transverse
section. cd. posterior cardina l vein; c. t. connective
tanelle (b. cr. f.), due to the
tissue ; c. r. cartilaginous rods ('fin rays'); d. ao. dorsal non-union of the posterior ends
aorta ; f. c. fat column; int. intestine ; i. v. intra-intestinal
vein; mes. mesonephros ; mes. d. mesonephric duct ; my. of the trabecul<e. Through
myotome; n . ca. spina l canal; nc. notochord ; s-c. s. sub- this passes the nasa-hypophy-
cardinal venous sinus; sp. c. spinal cord; sp. v. spira l
valve ; ts. test is. (Drawn by R. Strahan .) sial sac (Fig. n 8, na. hy p.;
Fig. 121 , pty. p.) extending
from the olfactory sac t o the ventral surface of the notochord. Lateral
walls extend upwards from each side of the basal plate, but the roof of the
cranium is formed by membrane except at one point. H ere a narrow transverse
bar (cr. r.) extends across between the side-walls and furnishes a rudimentary
roof. United with the posterior end of t he basal plate and forming the end
of the neurocranium are the otic capsules (Fig. n s. au. c). The side-walls are
 PHYLUM CHORDATA r8r

pierced by apertures for the cranial nerves. So far the skull is typical, though
extremely simple; its remaining parts, however, are in many cases very
difficult of interpretation.

cn.c
FIG. IIj.- Petromyzon : Skull. Dorsal (A). ventral (B). and sectional (C) views of P. marinus.
The cartilaginous parts are d otted. a. d. c. anterior dorsal cartilage; an. c. annular cartilage;
au. c. auditory capsule; b. cr. f. basi-cranial fontanelle; b. pl. basal plate; en. c. cornual cartilage ;
cr. r. cranial roof; n. a. neural arch; na. ap. nasal aperture; nclt. notochord; Nv. I, olfactory
nerve; Nu. z, 5, and 8, foramina for the optic, trigeminal, and auditory nerves; Nv s'. trigeminal
nerve; olf. c. olfactory capsule; p. d. c. posterior dorsal cartilage; p. lat. c. posterior lateral
cartilage; sb. oc. a. sub-ocular arch; st. p. styloid process. (After \'V. K. Parker.)

The olfactory capsule (olf. c.) is an unpaired concavo-convex plate which

supports the posterior wall of the olfactory sac and is pierced by paired apertures
for the olfactory nerves. It is unique in being united to the cranium by fibrous
tissue only. Extending outwards and downwards from each side of the basal
182 ZOOLOGY
plate is an inverted arch of cartilage, called the subocular arch (Figs. n6 and
IIJ, s. o. a.) because it supports the eye. From its posterior end a slender
styloid process (st. p.) passes directly downwards and is connected at its lower end

ling . c. corn. c.

FrG. n6.-Peh·omyzon: Skull and branchial basket in adult. P . maJ•inus. a. d . p. anterior

dorsal plate; a.l. c. anterior lateral cartilage; an. c. annular cartilage; br. 1 rst branchial bar ; corn. c.
cornual cartilage; cr. w. cranial wall; ep. b. epitrematic bar; ex. hy. extrahyal bar; g . ap. gill
aperture; hy. b. hypotrematic bar; ling. c. lingual cartilage; m. v. b. m edian ventral bar; m. v. c.
median ventral cartilage; n. a. neural arch; nc. notochord; olf. olfactory capsule; at. otic capsule;
p. d. p . posterior dorsal plate; per. c. pericardia! cartilage; p. l. c. posterior lateral cartilage;
s. c. b. subchordal bar; s. o. a. subocular arch ; st. c. styloid cartilage; sty. c. stylet cartilage ; trab.
trabecula; v. p. vertical pillar of subocular arch. (Modified after Parker.)

cr. w.
o/1. st. c.

corn. c.
ex. hy.
FrG. II].-Lampetra: Skull during metamorphosis. (Tongue skeleton omitted.) L. plane~·i.
a. d. p. anterior dorsal plate; an. c. annular cartilage; br. 1 rst branchial bar; corn. c. cornual cartilage;
cr. w. cranial wall; ex. hy. extrahyal bar; nc. notochord; olf. olfactory capsule; ot. otic capsule;
pch. parachordal; p. d. p . posterior dorsal plate; p . l. c. posterior lateral cartilage; s. o. a. sub-
ocular arch; st. c. styloid cartilage; sty. c. stylet cartilage ; trab. trabecula; trab. c. trabecular com.
missure; v. p. vertical pillar of subocular arch. (Drawn by R . Strahan, reconstructed after Damas.)

with a small cornual cartilage (corn. c.). These two cartilages arise in a similar
manner to the skeleton of the gills and probably represent the reduced arch
of the mandibular somite. The styloid process has been identified with the
extra-mandibular cartilage of gnathostomes (p. 203), and the cornual cartilage
 PHYLUM CHORDATA

may possibly represent the mandibular cartilage. In close relation with the
angle of the subocular arch is an upwardly directed plate, the posterior lateral
cartilage (p. l. c.).
Connected with the anterior end of the basal plate is the large bilobed
posterior dorsal plate (p. d. p.). Below and projecting in front of it is the
anterior dorsal plate (a. d. p.). Also supporting the upper lip are the paired
anterior lateral cartilages (a. l. c.) which arise in the premandibular somite and
possibly represent the internal visceral arch of that segment. The great ring-
shaped annular cartilage (an. c.) supports the edge of the buccal funnel and
carries at its sides two backwardly directed rods, the stylet cartilages (sty. c.).

ap.

vent. atr. per. c.

FIG. 1 r8.-Petromyzon: Head region of adult. (Diagrammatic half-section.) a. d. p. anterior
dorsal plate; an. annular cartilage; ap. apical cartilage; atr. atrium; b. f . .buccal funnel; b1·.
brain; ccel. ccelom; gon. gonad; g. op. opening to gill; hs . hydrosinus; int. intestine;
l. liver; ling. c. lingual cartilage; m. v. b. median ventral bar of branchial skeleton;
m. v. c. median ventral cartilage (copula); na. nostril; na. hyp. nasa-hypophysial duct; nc. noto-
chord; ces. cesophagus; o. f. oral fimbri;e; olf. olfactory capsule; p. d . p. posterior dorsal plate;
per. c. pericardia! cartilage; ph. pharynx; s.c. spinal cord; t. t. tongue teeth; vel. velum; vel. t.
velar tentacles; vent. ventricle. (Drawn by R. Strahan.)

The' tongue' is supported by a long unpaired lingual cartilage (Fig. n8, ling.
c.). This is tipped in front by a small median, and a pair of still smaller lateral,
cartilages. Below it occurs a slender T-shaped median ventral cartilage (m. v. c.).
The visceral skeleton differs remarkably from the ordinary craniate type and
has many elements which cannot be homologised with it. It is most probably
represented in the extra-branchial cartilages of the lower gnathostomes, the
inner series, which forms the bulk of the branchial skeleton, being absent from
the post-mandibular segments in the lamprey.
The visceral skeleton consists of a branchial basket formed, on each side,
of nine irregularly curved vertical bars of cartilage (Fig. n6, br. 1 ). The
first is placed almost immediately posterior to the styloid cartilage and is
homologous with the extra-hyal cartilage of elasmobranchs (p. 219). The
second lies immediately in front of the first gill-cleft, and the remaining seven
just behind the seven gill-clefts. These bars are united together by four
 ZOOLOGY
longitudinal rods. One, the subchordal bar (s. c. b.), lies alongside the noto-
chord and is connected in front with the cranium. Two others, the epitre-
matic bar (ep. b.) and hypotrematic bar (hy. b.), are placed respectively above and
below the gill-clefts. The fourth, the median ventral bar (m. v. b.), is situated
close to the middle ventral line and is partly fused with its fellow of the opposite
side. The posterior vertical bar is connected with a cup-like pericardial
cartilage (per. c.), which supports the posterior and lateral walls of the peri-
cardium. The whole branchial basket lies external to the gill-pouches and
branchial arteries, not, like typical visceral arches, in the walls of the pharynx.
The median fins are supported by delicate cartilaginous rods (Fig. II4, c. r.,
Fig. 125, cr.). These are more numerous than the myomeres, and lie parallel to
one another in the substance of the fin, extending downwards to the fibrous
neural tube. The structure of the cartilage is peculiar and varies in different
parts. It has very little matrix. Such rods are probably not homologous
with the fin-rays of true fishes (p. 307).
The muscles of the trunk and tail are arranged in myomeres which take a
zigzag course. In the branchial region they are divided into dorsal and
ventral bands, which pass respectively above and below the gill-slits; but in
the trunk there is no division into dorsal and ventral parts. A great mass
of radiating muscle is inserted into the buccal funnel, and the 'tongue' has
an extremely complex musculature which derives its nerve-supply from the
trigeminal nerve.
Alimentary Canal and Associated Structures.-The teeth are laminated horny
cones. Beneath them lie mesodermal papillre (covered with ectoderm) which
bear a superficial resemblance to the germs of true calcified teeth. When
worn out they are succeeded by others developed at their bases. Opening
into the mouth below the tongue are paired ducts which lead from the two
buccal glands. These are embedded in the basilaris muscles and appear to
secrete a substance which prevents the coagulation of the blood of the host.
The mouth leads into a buccal cavity (Fig. n8, b. f.) formed from the stomo-
dreum of the embryo, and communicating behind with two tubes placed one
above the other. The dorsal of these is the cesophagus (as.); the ventral canal
is the respiratory tube (ph.), or pharynx. Guarding the entrance to the latter
is a curtain-like fold, the velum (vel.). The <:esophagus extends above the
pericardium and enters the intestine (int.) through a valvular aperture. The
intestine is unconvoluted. Its anterior end is slightly dilated, and is the only
representative of a stomach. Its posterior end is widened to form the rectum
(Figs. no, 125), which ends at a cloaca. As Barrington has shown, true gastric
digestion involving the action of pepsin does not seem to occur until we reach
the true fishes (as far as the vertebrates of to-day are concerned). The develop-
ment of the capacious stomach is correlated with the ancient change from
micro- to macro-phagy (p. 206). Although a stomach is lacking in some of the
 PHYLUM CHORDATA r85

higher fishes (p. 336), it is probable that in the Agnatha its absence is a primitive,
rather than a secondary, feature.
The whole of the intestine of Petromyzon is formed from the mesenteron of
the embryo, and the blastopore becomes the anus, there being no proctod<eum.
The lumen of the intestine is crescentic owing to the presence of a typhlosole
in which lie the anterior mesenteric artery and hepatic portal vein. Between
these vessels and the wall of the typhlosole is a reticular tissue whose cavities
communicate with capillaries. Blood cells of all types are formed in this spleen-
like tissue and passed from there to the blood. The typhlosole takes a slightly
spiral course along the intestine and is hence known as the spiral valve (Fig. II4,
sp. v.). There is no continuous mesentery, but only a number of narrow
supporting bands.
The liver (Fig. u8, l.) is a large bilobed organ. Gall-bladder and bile-duct
occur in the larva, but are at least sometimes absent in the adult. There is
no pancreas as such. In both larval and adult lampreys, however, there occur
in the gut epithelium aggregations of seemingly secretory cells of two different
kinds. There is suggestive experimental evidence that one kind (zymogen
cells) produce a proteolytic enzyme, and that the other sort is involved in
carbohydrate metabolism. This second, probably endocrine, group has no
communication with the lumen of the intestine. It arises in the larva as' cords
of cells' of one type (Barrington), and forms what have been termed follicles of
Langerhans. These may be homologous with the islets of Langerhans (p. 153)
of higher forms. Both secretory elements-hypothetical endocrine and zymo-
gen-occur in the sub-mucosa at the anterior end of the broader mid-gut.
The follicles of Langerhans are aggregated near the entrance of the bile-duct.
It must be emphasised, however, that the hvo elements, though occurring in
the same region, are not as intimately associated as are the acinous cells and
insulin-producing elements in the compound pancreas of the Gnathostomata
(p. 203).
Respiratory Organs.-Adult lampreys differ from all other Vertebrata in
that the pharynx ends blindly. By virtue of this difference it is referred to,
perhaps unnecessarily, as the ' respiratory tube ' (Fig. u8, ph.). In the
ammoccete (p. 196) the pharynx communicates posteriorly with a short,
narrow <:esophagus but, during metamorphosis, this connection is lost and the
cesophagus becomes extended forward dorsal to the respiratory tube and opens
separately into the buccal cavity.
The respiratory organs are typical gill-pouches. They have the form of
biconvex lenses, with numerous gill-lamell<e developed on the inner surfaces,
and are separated from one another by wide interbranchial septa. The gills
are much less pouch-like in the ammoccete, where they open widely into the
pharynx (Fig. ug). In the embryo an additional gill pouch rudiment occurs
in front of the first of the adult series.
r86 ZOOLOGY
Circulatory System.-The blood-vascular system shows a radical advance-
ment from the primitive Amphioxus plan in the possession of a distinct heart.
The atrium (Fig. II8, atr.) is situated to the left of the ventricle (v.) and receives
blood from a small sinus venostts (s. v.). There is no conus arteriosus, but the
proximal end of the ventral aorta presents a slight dilatation or bulbus arteriosus.
Both afferent and efferent branchial arteries supply each the posterior hemi-
branch of one gill-pouch and the anterior hemibranch of the next. They are
thus related to the gills, not to the gill-pouches. In addition to the paired
jugulars there is a median ventral inferior jugttlar vein returning blood from
the lower parts of the head. There is no renal portal system (p. 244), the two
trab. c. nc. sp.c.
br.

u.l.

1.1. an. r. ling. r. vel. g. end.

FIG. I 19.- Geot-ria: Ammoc<ete head region. (Diagrammatic half-section of G. australis.) an. r.
rudiment of annular cartilage (mucocartilage); ap. r. rudiment of apical cartilage (mucocartilage) ;
br. brain; b. tent. buccal tentacles ; end. endostyle; g. gill; ling. r. rudiment of lingual cart ilage
(mucocartilage); l. l. lowe r lip; na. nasal aperture; nc. notochord; olf. c. r . rudiment of olfactory
capsule (cartilage); r. d. p. rostra-dorsal plate (mucocartilage); sp. c. spinal cord; lrab. c. trabe-
cular commissure (cartilage); u. l. upper lip; vel. velum. (Drawn by R. Strahan.)

branches of the caudal vein being continued directly into the cardinals.
Blood from the kidneys drains into two sub-cardinal sinuses (Fig. II4, s.-c.s.)
which connect by numerous small passages with the cardinal veins above.
The left precaval disappears in the adult, so that the jugulars and cardinals
of both sides open into the right precaval. On the sides of the segmental
arteries and veins occurs diffuse chromaffin tissue (p. 152). There is evidence
that an extract of this tissue has properties strongly resembling that of mam-
malian adrenalin. Such tissue may represent a diffuse adrenal medulla. Islets
of cells around the lateral and ventral walls of the cardinal veins have been
thought possibly to represent adrenal cortical tissue (p. 152).
The erythrocytes of the blood contain a respiratory pigment, hcemoglobin,
which is intermediate in many of its properties between the hcemoglobins of
invertebrates and those of gnathostomes.
 PHYLUM CHORDATA

As previously mentioned, the formation of blood cells, hcematopoiesis,

occurs primarily in the spiral valve. In the adult, this function is shared
with the reticular tissue of the kidney. The erythrocytes, in common with
those of true fishes, amphibians, reptiles, and birds, retain their nucleus through-
out the life of the corpuscle. The presence of hcemoglobin and a force-pump,
the heart, enormously increases the capacity of the circulatory system to trans-
port speedily the oxygen (taken in at the gills) to the tissues throughout the
body. An extensive system of lymphatic sinuses (p. n6) also occurs.
Nervous System.-This, too, shows a great advance on that of Amphioxus.
An elaborate brain is developed along with a system of cranial nerves. It is
of great interest, however, that the dorsal and ventral roots of the spinal nerves
are unjoined, and therefore retain the Amphioxus plan. Similar to those of
Amphioxus, and differing from those of other vertebrates, the nerves of the
Lamprey and other cyclostomes are unmyelinated. The spinal cord is wholly
composed of transparent grey matter. It contains no blood-vessels, but is
dorso-ventrally flattened in a manner that may possibly allow metabolic
exchanges to take place without them. As in the fishes and other vertebrates,
the nerve cell-bodies are arranged around the narrow central canal, but synaptic
contacts occur at the periphery of the cord in what would be the 'white matter'
in other vertebrates. This arrangement gives rise to a neuropil (nerve felt-
work). In this, afferent sensory fibres terminate in close proximity to the
dendrites of motor-cells whose axons become associated in the ventral nerve-
roots. It is thus probable that the motor-fibres can be stimulated directly as
a result of sensory impulses initiated by contact with environmental stimuli.
The connection between brain and motor-fibres appears to be by means of the
large and prominent Muller's fibres which originate from giant cells in the
mid- and hind-brain and probably stimulate motor-cell dendrites in the ventral
region of the cord.
The brain (Figs. 120, 121) exhibits the typical pattern that is found through-
out the vertebrate series. Some of the structures that are well-developed in
the higher forms occur in a relatively primitive condition. This of course is
essentially a reflection of the life of the animal. The optic lobes (Figs. 120, 121,
opt. l.) are very imperfectly differentiated in the tectum of the mid-brain.
In the hind-brain the small size of the cerebellum is remarkable. This organ
occurs as a mere transverse band roofing over the anterior end of the 4th
ventricle, and its lack of complexity is not surprising when we observe the
Lamprey's mode of locomotion even when it is not being transported by its
host. The rest of the hind-brain is highly developed ; among other functions
it controls the powerful sucking structures by means of the trigeminal nerve.
The central region of the tectum of the mid-brain is epithelial and is related to
a vascular thickening of the overlying pia-mater which forms a choroid plexus,
so that when the membranes are removed an aperture is left. No choroid
r88 ZOOLOGY

plexus is found in the mid-brain of higher vertebrates, where such structures

are limited to the fore- and hind-brain.
On the dorsal border of the lateral wall of the diencephalon are two ganglia
habenulce, the right (r. gn. hb.) being much larger than the left (l. gn. hb.).
These constitute an epithalamus and are connected to the pineal apparatus.

Nut
.Nv.f
o(/l - .._
Pn- - -- -+-

opt.l - ~ --. - cr. c rh

I ~ ~ - Nu .S
,•
'
- Nu.S
m.ed.ohl
~- m.ed . ohl

FIG. I2o. -Petromy~on: Brain. Dorsal (left) and ventral (right) views of P . marinus. ch. pl. I ,
anterior choroid plexus forming roof of prosencephalon and diencephalon; ch. pl. 2, aperture in
roof of mid-brain exposed by removal of middle choroid plexus ; ch. pl. 3, m etacrele exposed by
removal of posterior choroid plexus; crb. cerebellum; cr. crb. crura cerebri ; crb. h. olfactory
lobes ; dien. diencephalon; inf. infundibulum ; l. gn. hb. left habenular ganglion; med. obl. medulla
oblongata ; Nv. I, olfactory, Nv. 2 , optic, Nv. 3, oculo-motor, Nv. 5, trigeminal, and Nv . 8, auditory
n erves; olf. l. olfactory bulbs; opt. l. optic lobes; pn. pineal eye; r. gn. hb. right ganglion
habenul<e. (After Ahlborn.)

This consists of the pineal eye which lies against a transparent area of the cranial
roof and the smaller parapineal body lying below and somewhat behind it. A
stalk containing nerve-fibres leads from the pineal eye to the right habenular
ganglion, and a similar one from the parapineal body leads to the left habenular
ganglion. Since the habenular ganglia are paired structures, it would seem
that pineal and parapineal eyes were originally paired and have only secon-
darily come to lie one on top of the other. The pineal eye contains a pigmented
 PHYLUM CHORDATA 189
retina with sensory cells, and a flattened imperfect lens. In the parapineal
body these structures are extremely reduced and hardly recognisable.
The pineal apparatus is better developed than in any of the higher forms
now living, and Young has shown that it is of vital importance in the everyday
life of the Lamprey. As light conditions in the environment change, so does
the colour of the larval form. The pineal apparatus is photosensitive, and it
seems that impulses are conveyed via the well-developed hypothalamus (in the
ventral region of the diencephalon) to the endocrine pituitary gland. The
pituitary or hypophysis is closely applied to the hypothalamus : there is no
hypophysial stalk, such as is developed in the higher Vertebrata. The gland
is a compound one, consisting of a comparatively large pars anterior and

FIG. J2z.-Petromyzm1: Brain with olfactory and nasa-hypophysial ducts. Cblm. cerebellum ;
crb. h. olfactory lobe; dien . diencephalon; f. fold in nasal duct; gl. accessory olfactory organ ;
inf. infundibulum; I. gn. hb. left habenular ganglion ; m ed. obl. m edu lla oblongata; na. ap . nostr il ;
nch. notochord; Nv. I, olfactory ner ve; Nv. :2, optic n. ; Nv. 3, oculomotor n.; Nv. 4, trochlea r
n.; Nv. 5, trigeminal n.; Nv. 6, adbucens n. ; Nu. 7, facial n. ; Nv . S, auditory n .; Nv. IO, vagus
n.; Nv. 1 2, hypoglossal n. ; olf. cp. olfactory capsule ; olf. I. olfactory bulb; olf. m . rn. ol·
factory mucosa; opt. l. optic lobe; pn. parapineal organ; pn. e. pineal eye; pty. b. pituitar y
gland; pty. p. hypophysial sac; sp. median septum of olfactor y capsule; sp. I, dorsal root of
Jlrst spinal nerve. (Combined from figures by Ahlborn and Ka:nsche.)

a flattened pars intermedia. These are separated from the third ventricle
by a thin tract of epithelial tissue which may represent the pars nervosa (of
the posterior lobe) of higher forms. There is experimental evidence that the
expansion of melanophores, causing the Lamprey's skin to darken, is caused
by a secretion from the intermedia when this is stimulated by nerve impulses
originating from the pineal.
In front of the diencephalon are paired bean-like masses, the cerebral
hemispheres, each consisting of a small posterior portion, the olfactory lobe (crb.
h.) and a larger olfactory bulb (olf. l.).
The cavity of the fore-brain extends laterally into these hemispheres
to form rudimentary lateral ventricles. The part of the lateral ventricle
which is contained in the olfactory bulb is sometimes referred to as the
rln'noca:le.
rgo ZOOLOGY
Organs of Special Sense.-The external nostril (Fig. no, n.; Fig. uS, na.)
leads by a short passage into a rounded olfactory sac (Fig. II8, olf.) placed
just in front of the brain and having its posterior wall raised into
ridges covered by the olfactory mucous membrane (Fig. r2r, olf. m. m.).
Here are located specialised olfactory receptor-cells, and from these travel axons
which become aggregated in the short olfactory nerve (Fig. r2o, nv. I). This
joins the highly organised olfactory bulb (olf. l.) close by. From the bottom of
hyp. ' ·

v.
FIG. 122.-Petrmnyzon: Development. Five stages in the development of the cavities of the
anterior part of the body. A , B, C- successive embryonic stages; D-ammoccete; E-adult.
b. t. buccal tentacles; end. endoderm; g. cesophagus ; h. m. hind-mouth ; hs. hydrosinus ; hyp. r.
hypophysial rudiment ; hyp. s. hypophysial sac ; inf. infundibulum ; I. I. lower lip; na. nasal
a perture ; na. r. nasal rudiment ; nc. notochord; olj. c. olfactory capsule ; p . ant. pars anterior of
pituita ry; ph. pharynx ; pin . pinea l eye : pin. r. pineal rudiment : p . int. pars intermedia of
pituitary; pit. p ituitary gla nd; st. stomod<eum; 11 . l. upper lip; v. v elum; v. t. velar tentacles.
(Drawn by R . Strahan, after various authors.)

the olfactory sac is given off a large nasa-hypophysial sac (Fig. u8, n. hyp., Fig.
121, pty. p.) which extends downwards and backwards between the brain and the
skull-floor, passes through the basicranial fontanelle, and ends blindly below the
anterior end of the notochord. This elongated, compressible pouch seems to
act in a manner analogous to the rubber bulb of a pipette. The expanded and
closed end of the pouch lies directly above the dorsal wall of the pharynx and
between the first pair of gill-pouches. Directly above the pouch is the un-
yielding anterior end of the notochord (Fig. 121, nch.). Respiratory movements
squeeze the water in the pouch in a forward direction out through the nostril
 PHYLUM CHORDATA 191

(na. ap.) situated on the dorsal surface of the head. When the pressure relaxes,
a fresh stream of water flows into the pouch, and thus comes in contact with
the olfactory receptor-organs. If the aperture is experimentally blocked, the
Lamprey no longer swims away from noxious substances liberated in its vicinity.
The relations between the olfactory sac, nasa-hypophysial sac, and the
pituitary body are extraordinary. In the embryo, before the stomodreum
(Fig. 122, A, st.) communicates with the gut, two unpaired ectodermal invagina-
tions appear in front of the mouth. The foremost of these is the nasal rudiment
(na. r.). The other (which is situated between the olfactory sac and the

FIG. 123.- Lampetra: Eye and adjacent structures. (L. jluviatilis, anterior end to the left .)
Accommodation is accomplished by a lteration of the shape of the eyeball by the extrinsic cornea its
muscle. The tendon of this muscle is inserted on the rim of the spectacle, a specialised window-
like area of transparent skin. Muscle contraction draws the spectacle taut and fl attens
the subjacent cornea. This in turn is in close contact with the lens which is thus pushed
closer to the retina. The elasticity of sclera a nd vitreous b ody, and the equality of the intra -
ocular pressure, returns the eyeball to the resting shape. av. anterior surface of vitreous; c.
cornea; cr. external rectus; io. inferior oblique; ir. internal rectus; s. spectacle; sk. skin; sp.
space between spectacle and cornea; sr. superior rectus; t. t endon of comealis; v. venous sinuses
(cushioning the eyeball). (After Walls.)

mouth) is the hypophysial rudiment (hyp . r.), which in this case opens just
outside the stomodreum instead of within it as in other Craniata. Its inner
or blind end extends to the ventral surface of the fore-brain and terminates just
below the infundibulum (inf.). As development proceeds, the olfactory and
hypophysial invaginations become sunk in a common pit (B), which, by the
growth of the immense upper lip (up. l.), is gradually shifted to the top of the
head (C, D). In the ammoccete (p. rg6), this common nasa-hypophysial cavity
extends only a short distance under the forebrain. The hypophysial rudiment
grows out as a thin plate of tissue. This buds off (r) the intermedia, which
interdigitates with the nervous tissue from the floor of the third ventricle, and
(z) the anterior body of the pituitary gland. Later, all connection with the
rgz ZOOLOGY
naso-hypophysial cavity is lost. At metamorphosis the last-named grows out
into its adult condition independently of the pituitary gland. The peculiar
monorhinal condition of the lamprey is due to the hypertrophy of the upper
lip. This, in turn, is related to the function of the upper lip, which is the bur-
rowing organ of the ammocrete. We may therefore assume, with Watson, that
the cephalaspids, anaspids, and hags similarly possessed ancestors in which the
upper lip performed this function.
The paired eyes of Petromyzon are essentially similar to those of other
vertebrates. Each is moved by six external muscles, as in higher forms, but
the arrangement of these is peculiar. The accommodation of the eye is brought
about by an extraordinary arrangement (Fig. 123). A remarkable cornealis
muscle, which arises from the special-
i.e.
a.c.
isation of two anterior myotomes, pulls
on the cornea from one side and flattens
it. This corneal movement thrusts the
lens in towards the retina. Thus, the
.. ::<..
?:iJtfi"'
·:;,~ , whole lens moves in a manner roughly
similar to the lens of a camera, being
cr. w. omp moved back and forth in relation to the
end. d. ~........_ sacc. photographic plate. It is said that the
mac.
cornea of the lamprey does not move
when the position of the rest of the eye
Frc. 124.-Petromyzon: Membranous
labyrinth (left). a. c. anterior semicircular is altered by the extrinsic muscles.
canal; amp. ampulla; cr. crista; end. d.
endolymphatic duct; i . c. invagination canal; The membranous labyrinth (Fig.
mac. m acula; p. c. posterior semicircular 124) is remarkable in having only two
canal; sacc. sacculus; utr. utriculus. (Re-
drawn from Neal and Rand, after Hesse.) semi-circular canals. These, the an-
terior (a. c.) and posterior semi-circular
canals (p. c.), curve at right angles to each other and are equipped with a
sacculus (sacc.) and utriculus (utr.). Little or nothing is known concerning the
actual hearing capacity of Petromyzon. Sensory apparatus within the above
compartments appreciates changes in the animal's orientation, and resultant
nerve-impulses lead to appropriate reflex movements and keep it on an even
keel.
In eye ostomes the lateral line sense-organs (p. 136) are relatively simple and
are exposed to the surrounding water. They occur in the head region and in
a single line extending along each side of the body. They are innervated by
cranial nerves; those on the body and tail are connected to the central nervous
system by a branch of the vagus.
There is experimental evidence that skin photoreceptor organs occur in the
tail of the ammocrete larva at least ; the work of Young suggests that the caudal
lateral line organs are involved. Finally, organs of taste occur in the wall of
the pharynx between the gill-sacs.
 PHYLUM CHORDATA 193
Urinogenital System.-This shows a great advance on that of Amphioxus.
Complicated organs of excretion, the kidneys (Figs. II4, 125, mes.), have
appeared. These enable the animal to eliminate excess fluid and waste
nitrogenous and other substances, and at the same time maintain an appro-
priate osmotic balance whether it is living in a fresh- or a salt-water medium.
The kidneys are long, strap-shaped bodies developed from the mesonephros
of the embryo. The essential renal units within the adult kidney are numerous
minute Malphigian corpuscles. Each contains a 'knot' of microscopic blood-

in!. mes. mes. d. g .d. cl. u. g .p. my.

FrG. I 25.-J>efrotnyzon: Cloacal region. P. marinus. cd. posterior cardinal vein; cl. cloaca
c. r. cartilaginous rods(' fin rays'); d. ao. dorsal aorta; d. j. 2 second dorsal fin; g. d. genital pore
int. intestine; m. muscles of fin; m es . mesonephros; mes. d. mesonephric duct; my. myotome
n. a. neural arch; nc. notochord; u. g. p. urinogenital papilla. (Redrawn after Goodrich .)

vessels, the glomerulus, and opens into a winding tubule (p. 155). The heart-
beat pumps blood through the glomeruli. Excess fluids and excretory products
are shed down the tubules towards the exterior.
Each kidney is attached along one edge to the dorsal wall of the body-
cavity by part of the peritoneum. Along the other (free) edge runs the meso-
nephric duct (Fig. II4, mes. d.), which opens posteriorly into a small urino-
genital sinus placed just behind the rectum. This opens by a urino-genital
papilla into a pit, the cloaca, in which the alimentary canal also ends. The
side-walls of the sinus are pierced by a pair of small apertures, the genital pores
(Fig. 125, g. d.), which place its cavity in communication with the crelom.
VOL. II. N
194 ZOOLOGY
The kidneys contain lymphoid tissue where, as has been mentioned, blood
cells may be formed. This tissue also performs a phagocytic function by
destroying old blood cells and removing suspended material from the circulation.
The sexes are separate in the adult. However, sexual differentiation occurs
at a comparatively late stage, and so both oocytes and spermatocytes may be
found in a single gonad in the young Lamprey. The gonad (Fig. n8, gon.,
Fig. II4, ts.) is unpaired. The ovary in the female ruptures and extrudes
eggs directly into the ccelom, as in higher vertebrates. The testis, however,
is unique among vertebrates in the manner in which it discharges its sperm-
atozoa. The sperm-containing follicles of the testis rupture and the sperm-
atozoa, like the ova, are shed into the ccelom. The gametes escape through
the pair of genital pores mentioned above. These, alike in each sex, lead
as short canals to the urinogenital sinus from which eggs or sperms escape
to the exterior where fertilisation takes place. In adult males there occur
prominent aggregations of interstitial Leydig-eells in the typical vertebrate
arrangement (Fig. 97, p. 152).
Development-The egg is telolecithal, having a considerable accumulation
of yolk in the v"egetal hemisphere. In correspondence with this, cleavage is
complete but unequal, a condition comparable with that in the frog (p. 418).
Early segmentation divides the animal hemisphere into a number of small,
relatively yolk-less micromeres (Fig. 126, A, micr.), and the vegetal hemisphere
into a smaller number of yolky macromeres (A, y. end.). A cavity, the blastocccle
(blc.), develops between these two cell Ia yers. The faster -dividing micro-
meres grow around to enclose the macromeres and also invaginate through a
crescentic groove, the dorsal lip (B, d. l.) of the blastopore (B, blp.) just below
the equator of the egg. As development proceeds, the blastopore moves to-
wards the vegetal pole of the embryo (C) and later passes it (D). In contrast
to the condition in Rana (p. 418), the position of the blastopore bears no relation
to the plane of the first cleavage. The process of invagination through the
blastopore gives rise to the cavity of the archenteron (C, arch.). The ventral
wall of this cavity is formed from the macromeres, while the dorsal and lateral
walls are formed from invaginated chorda-mesoderm (C, ch. m.) which later
develops into the notochord (D, nch.) and somites (F, sam.). Somewhat later,
a groove appears in the endodermal floor and the sides of the groove come
together to form an endodermal roof (F, end. r.) to the gut. Apart from this
contribution, the macromeres take no part in organ-building. At the time of
hatching they still retain considerable food stores. Owing to the mass of this
tissue, the lumen of the archenteron is much narrower than in Amphioxus.
By the completion of gastrulation, the blastopore has taken up a position
at the postero-ventral end. The development of the central nervous system
differs widely from the corresponding process in Amphioxus, and is only
approached among the Craniata by the bony fishes. The dorsal surface be-
 PHYLUM CHORDATA rgs
comes flattened along a narrow longitudinal area, and along this a groove
appears. This terminates immediately in front of the blastopore. The area
along which the groove runs soon becomes raised up above the general surface
and forms a narrow longitudinal elevation. Sections of this stage show that
the ectoderm has developed a thickening along the course of the longitudinal
groove, and this comes to grow downwards towards the archenteron as a

blc.

Frc. rzG.-Peh·mnyzon: Development. Successive stages in gastrulation shown in recon-

structions from which parts of the embryo have been removed to reveal internal structures. The
centre of gravity shifts during gastrulation and this affects the orientation so that the region
marked by the arrow eventually comes to lie dorsally. The shift in orientation is a gradual one,
but for convenience is shown in two stages. an. anus; anh. archenteron; blc. blastocrele; blp.
blastopore; ch. nz. chorda-mesoderm; d. l. dorsal lip of blastopore; ect. ectoderm; end. r. endo-
dermal roof of archenteron; mes. mesoderm; micr. micromere layer; n. c. nerve cord; nch. noto-
chord; n. k. neural keel; sam. somite; slam. stomod<eal invagination; y. end. yolky endoderm.
(Drawn by R. Strahan, reconstructed from Brachet after Weissenberg and de Sely-Longchamps.)

solid longitudinal neural keel. This is the rudiment of the central nervous
system. Subsequently the keel becomes separated off from the surface ecto-
derm, and lies below it as a solid cord. It is only at a considerably later period
that a lumen appears in this cord, and gives rise to the ventricles of the brain
and the central canal of the spinal cord. In contrast to the condition in
Amphioxus, somite formation is limited to the dorsal mesoderm on either side
of the mid-line, a feature which is also characteristic of the remainder of the
Craniata. The blastopore does not close; it is converted into an anal aper-
ture, so that there is no proctodceum. The dorsal lip of the blastopore, very
rg6 ZOOLOGY
prominent from the first , is produced into the rudiment of the tail region. The
mouth is developed later than the anal aperture by stomodreal invagination.
The membrane previously separating the fore-gut from the mid-gut becomes the
velum (Fig. 122, v.) of the larva.
The young is hatched as an ammoca:te larva (Fig. 127) some 10 mm. long.
This differs from the adult in several important respects. The median fin is
continuous. There is a semi-circular, hood-shaped ztpper lip (u. l.) instead of
the suctorial buccal funnel of the adult. Teeth are absent. A set of branched
buccal tentacles (Fig. 119, b. tent.) guards the entrance to the alimentary canal.
Behind the tentacles lies the velum (vel.), a pair of cup-shaped structures
attached to the anterior wall of the pharynx.
A B These act as paddles and maintain a stream
-na .ap of water through the pharynx. In this
e-Ye they are assisted by pumping movements
of the pharynx. Although the ammoccete
l .l / burrows in mud, it feeds by taking in
fairly clean water from the oral end of
its burrow. The endostyle (end.), homo-
logous with that of Amphioxus, passes a
stream of mucus forward a.long a groove
in the floor of the pharynx to a pair of
peripharyngeal grooves behind the velum.
Movements of the velum dislodge strands
of mucus which form a sort of net across
t the entrance to the pharynx. Further
FIG . 127.- Lcnnpetra: Development. down the pharynx, the strands come to-
Head of larval L. fluviatilis. A, from be- gether to form a cord which passes to the
n eath; B, from the side. br. 1, first
branchial aperture; eye, eye; l. l. lower cesophagus. Propulsion is now achieved
lip; na. ap. n ostril; u. l. upper lip.
(After W. K Parker.) by the action of a:sophageal cilia. Food is
in this manner filtered from the water
current and passed to the digestive region of the alimentary canal without being
diluted with a great amount of water. In Amphioxus the feeding current is
maintained by cilia; in the ammoccet e by muscles.
This evolution of a muscular, in place of a ciliary, ingestive apparatus was
an immense step forward in evolution. It enabled a far swifter and greater
intake and, consequently, the development and sustenance of far bigger
animals.
During its early life the larva spends most of its time buried in a U-shaped
burrow in the mud, emerging only at night to change its feeding ground. It
swims with its head downwards and then burrows again with great rapidity.
Two major instinctive responses combine to maintain the ammoccete in its
proper environment: photokinesis and thigmotaxis. The photokinetic response
 PHYLUM CHORDATA 197
is shown when ammoccetes are illuminated. Their reaction is to swim in a
random fashion until reaching a shaded area, when locomotion becomes much
more limited. The thigmotactic response is shown when ammoccetes are given
the alternative of a smooth bottom or tubes into which to swim. Spontaneous
movements are much fewer when touch receptors are stimulated over a large
part of the body-surface than when only a limited number are, so that animals
tend to stay in a tube after reaching it, even if the illumination is fairly high.
Together these responses lead the animals to seek crevices or burrow in dark
places. A batch of photoreceptors occur in the skin of the tail, as Young has
shown. These presumably act as an extra warning device to prevent the animal
from leaving its tail exposed above the burrow.
We have seen that larval lampreys exhibit a muscular feeding device, yet
retain an endostyle, a characteristic part of the feeding mechanism of ancestral
forms. At the time of metamorphosis, the endostyle becomes modified and
develops into an entirely different structure that occurs throughout the rest
of the vertebrate series-the endocrine thyroid gland. The endostyle loses
its mucus-secreting capacity and develops a series of iodine-containing
vesicles that are clearly of the same nature as the thyroid follicles of higher
animals.
At the same time the buccal region becomes elongated to accommodate the
tongue ; and its margin becomes circular. The tentacles recede and are re-
placed by teeth on the buccal funnel and tongue. The velum becomes reduced,
and skeletal rods develop from it to form a filter guarding the respiratory
pharynx: food material is thus diverted to the cesophagus, which has already
been cut off from the pharynx proper. The gills are modified and open into
the pharynx. The nasa-hypophysial sac becomes enlarged posteriorly. The
larval cranium, which consists of little more than the parachordals, trabeculre,
and olfactory and otic capsules, becomes filled out into the adult condition.
The skeleton of the upper lip and tongue, which is absent in the larva, also
develops during metamorphosis. The eyes complete their development and
move to the surface. The continuous dorsal fin is excavated to form the hvo
dorsal, and single caudal fins; and the colour of the skin changes from muddy
brown to metallic on the dorsal surface. In some species, brilliant-coloured
stripes also appear at this stage. The Lamprey now leaves its river habitat
for the open sea and begins the carnivorous phase of its life-history.

CLASS MYXINOIDEA
The exclusively marine hags or slime-eels are exemplified by the widespread
genera Myxine, Paramyxine, and Eptatretus (= Bdellostoma). These inhabit
muddy bottoms, where they rest in temporary burrows. They feed on a wide
range of dead animals and may perhaps prey upon living polychaetes and
rg8 ZOOLOGY
priapuloids. They eat dead or dying fish, but the old view that they are
parasitic upon fishes is incorrect. Their method of feeding is unique among
vertebrates in that they bite with protrusible horny teeth (Fig. 130, d. pl.)
which move against each other from side to side. Other unique features
include the direct communication between the nostril and the pharynx, the
existence of auxiliary hearts in the caudal and visceral circulation, and the
fusion of the two semicircular canals form a ring-shaped structure.

COMPARISON OF THE MYXINOIDEA WITH THE LAMPREY

Myxinoids show a much greater range of variation than lampreys. Here
we shall only indicate the points in which they differ from lampreys and from
each other. The body of myxinoids is
n.r.
cylindrical and from 40 to So em. long.
n n

They are remarkable for the immense

amount of slime they can produce when
disturbed. The slime comes from thread
cells, peculiar to m yxinoids, which occur
singly in the epidermis (Fig. II3) or
massed in the segmental slime-glands
(Fig. rz8). During cell development a
.
. .. . . continuous thread of fibrous protein is
. ·:. !~ laid down within the membrane. When
. .. ·.::·: the hag is stimulated by agitation or local
: :· ~ . .:.. :
---:--~ . br. op. pressure, large numbers of these cells
...
I
-·~ .
burst, releasing their threads, which are
·:.:: -. -. ;.:~.
-':~.:-. : ·. from r to 3fL in diameter and several em.
in length. The threads absorb water
t':. . .
0 : • • .:

,. • • 0 • ~

·. };~ c. op almost instantaneously and form around

the body a tenacious slimy gel as much as
ro em. thick. One m edium-sized Myxine
can gelatinise as much as 500 cc. of sea
·::·~; water in less than a minute.
A B The buccal cavity is even longer than
FIG. I28. -Class Myxinoidea: Myxine in lampreys, but the mouth is not ex-
glutinosa (A } a nd Eptatretus (= Bdello-
stoma) stoutii (B) from below. br. ap ., , panded into a funnel. The mouth and
first branchia l aperture; c. ap . common h ·1 h d
branchial aperture; mt!t. mouth; n . nostril; t e nostn are eac surroun ed by
n. t.,, 2 nasal tentacles; o. t .1. 2 oral t entacles; tentacles (Fig. 128) supported by carti-
ph. cut. aperture of pharyngo-cutaneous
duct; st. gl. slime g lands. (Drawn by R. !ages (Fig. 130, tent. 1- 4). There is a
Strahan.) single median palatine tooth (Fig. 129,
pal. t.) above the oral aperture, and two rows of smaller teeth arising from the
dental plate (Fig. 130) on the 'tongue'. The papill~ beneath the cone-like horny
 PHYLUM CHORDATA rgg
teeth bear a closer superficial resem-
blance to rudiments (or vestiges) of true
calcified teeth than is the case in
lampreys; but it appears that no odon-
toblasts and no calcified substance of any
kind are formed in connection with them.
The terminal nostril opens into a tubular
nasal duct (Fig. rzg, na. d.) leading to
the olfactory organ and is continuous
with the naso-hypophysial duct (na.
hyp.) which opens into the buccal cavity
just anterior to the velum (vel.). This
is homologous with the velum of the
ammoccete and, although having a
much more complex structure, performs
the same function of maintaining the
respiratory current. The continuous
flow of \Vater over the olfactory mucosa
permitted by this arrangement is pro-
bably valuable to a blind scavenger.
The only fin is a narrow caudal fin
surrounding the end of the tail. The
respiratory organs present striking
differences in the major genera. In
Eptatretus there are in different species
six to fourteen very small external
branchial apertures (br. ap . I) on each
side. Each of these communicates by a
short efferent duct with one of the gill-
pouches which is again connected with
the pharynx by another tube. Behind
and close to the last gill-slit, on the left
side, is an aperture leading into a tube,
the pharyngo-cutaneous duct (ms. eztl.),
which opens directly into the pharynx.
In A1yxine the efferent ducts (Fig. rzg,
ef. d.) all unite together before opening
on the exterior, so that there is only a
single external branchial aperture (br. ap.)
on each side. The pharyngo-cutaneous
duct (ms . cut.) opens into the left ex-
ternal branchial aperture (br. ap.) and
200 ZOOLOGY
serves for the expulsion of inhaled particles too large to enter the afferent
ducts.
The spinal cord (sp. c.) is covered only by fibrous tissue. There is no
trace of neural arches in the trunk, but in the posterior part of the caudal
region both neural canal and notochord are enclosed in a continuous cartila-
ginous plate. Similarly, the roof of the skull is entirely membranous (Fig. 130) .
The skull has a fundamental similarity to that of lampreys, but this is obvious

a.v.n.b.
pal. b.

ex. br. 1

vel. sk.

corn. c.

,.....~ ·3

FIG. IJo.-1Yfy:rine: Skull of adult. il!I. glutinosa. Portion of the palatine bar is removed on
left side. a. v. n. b. anterior vertical nasal bar; bas. basal cartilage; br. 1 Jst (internal) bra nchial arch ;
corn. c. cornual cartilage; d. l . b. dorsal longitudinal bar ; d. pl. dental plate; ex. br. 1 , 2 1st and
2nd extrabra nchial arches; ex. pq. extrapalatoquadrate; hy. hyoid arch; hyp. c. hypophysial
cartilage ; n c. notochord ; ot. otic capsule ; pal. b. p a latine bar; pelt. parachordal; pq. palata.
quadrate; p. v. n. b. posterior vertical nasal bar; s. n . c. subnasal cartilage; tent.,, 2 , 3 , 4 tentacular
cartilages; trab. trabecula; v. br. b. ventral branchial bar ; vel. sk. velar skeleton. (Drawn by R.
Strahan, reconstructed from Cole.)

only in the embryo (Fig. 131). Later developments, associated with the growth
of the nasa-hypophysial sac, the oral tentacles, and the backward migration of
the gills, bring about a considerable distortion. The nasal duct (Fig. 129, na. d.)
is supported by rings of cartilage (na. c.), and the oral tentacles by
flexible rods of the same tissue. The cornual cartilage (Fig. 131, corn. c.) and
the subnasal cartilage (s. n. c.) represent the skeleton of the upper lip of lampreys
and the basal plates (Figs. 1 29 and 131, bas.) are, in part, the homologue of the
lamprey tongue skeleton. The sub-ocular arch (Fig. 130, ex. pq., pq., hy.) re-
presents anteriorly the visceral skeleton of the mandibular arch and posteriorly,
 PHYLUM CHORDATA 201

the hyoid arch. Of the remaining branchial arches, only the first two are
represented, these being the first extra-branchial cartilage (ex. br. I.), and the
first branchial cartilage (br. I), and the second extra-branchial cartilage (ex. br. 2).
In the region of the common branchial apertures a pair of asymmetrical
cartilages (Fig. 129, p. br. sl?.) may represent a posterior remnant of the
branchial skeleton.
The myotomes of one side alternate with those of the other.

tent.3

FIG. I3L-My:rine: Skull of embryo. 111. glutinosa. a. v. n. b. anterior vertical nasal bar; bas,
basal skeleton; b·r. 1 rst (internal) br anchia l arch; corn. c. corn ua l cartilage; d. l. b. dorsal longi-
tudinal bar; ex. br. 1 , 2 rst and 2nd extrabranchial arches; PX. hy. extrahyal arch; ex. pq. extra-
palatoquadrate; hyp. com. hypophysial commissure; lab. c. labial cartilage; n c. notochord; ot.
otic capsule ; pal. b. palatine b a r ; pch. p a rac hordal; pq. palatoquadrate ; p . v. n . b. posterior
vertical nasal bar; s. n . c. subnasal cartilage ; tent.1 , 2 . , tentacu lar cartilages ; trab. trabecula ;
trab. com. trabec ular commissure; v. br. b. ventral branchial bar. (Drawn by R. Strahan, recon-
structed from Holmgren.)

The intestine is wide. The liver consists of separate anterior and posterior
portions, the ducts of which open separately into the gall-bladder. A pancreas-
like gland occurs in both Myxine and Eptatretus in a position comparable with
that of the 'insular organ' of lampreys (p. 185). There is no definite spleen,
but blood-forming lymphoid tissue is found scattered through the submucosa
of the gut. Blood cells may also be formed in the pronephros.
The brain differs considerably from that of Petromyzon, especially in the
larger olfactory lobes, the reduced ventricles, and the smaller mid-brain. The
dorsal and ventral roots of the spinal nerves unite instead of remaining separate.
202 ZOOLOGY
It is important to realise, however, that the junction is different from that
typical of the Gnathostomata.
The eyes are vestigial and sunk beneath the skin, which, as in the ammo-
crete (p. rg6), is light-sensitive. When stimulated these provoke a photo-
kinetic response. The membranous labyrinth (Fig. 132) has only a single semi-
circular canal, which, having an ampulla at each end, probably represents
both anterior and posterior canals (p. 145). Hags lack a pineal organ and a
lateral line system. In comparison with lampreys, their sensory system is
extremely limited.
The pronephros is retained in adult myxinoids as a pair of small bodies
lying dorsal to the pericardia! cavity, into which the nephrostomes open. In
Eptatretus, the tubules open into an incomplete
a.p.c. pronephric duct which, however, has no open-
ing to the exterior. Myxine lacks a pronephric
duct and the tubules end blindly. The func-
tional kidney is the mesonephros, and is speci-
end. d. ally interesting from the fact that in myxinoids
it retains in the adult its primitive segmental
arrangement. The 'ureter' sends off in each
segment a coiled tubule with a single Bowman's
capsule, into which a branch from the aorta
utr. sacc. enters and forms a glomerulus.
FIG. IJZ.-My;rine: Membranous Despite many statements to the contrary,
labyrinth (left). a. p. c. fused anter-
ior and posterior canals ; cr. crista; the sexes are probably separate in Myxine but,
end . d . endolymphatic duct; utr . sacc. as in Lampreys, differentiation of the immature
urticulo-sacculus. (Redrawn from
Neal and Rand, after Hesse.) gonad does not occur until late in life. The
female may thus retain an undeveloped testis
and the male a rudimentary ovary. The large, cylindrical eggs are enclosed
in a horny shell bearing terminal hooked processes.
The early development of Myxine is unknown, but there is no reason to
assume that it differs markedly from Eptatretus, in which segmentation is
markedly meroblastic, being confined to one pole of the elongate egg. The
blastoderm thus formed extends gradually over the surface of the yolk, which
it completely encloses only at a late stage, when the gill-clefts are all formed.
Eptatretus differs from Petromyzon and resembles the majority of the Craniata
in the mode of development of the central nervous system. This is formed not
from a solid ectodermal keel, but from an open medullary groove. The lips
of the groove bend inwards and unite to form a medullary canal. There is no
metamorphosis in the life-history.
 SUPER-CLASS GNATHOSTOMATA

T
INTRODUCTION

HE Gnathostomata includes all vertebrates with upper and lower jaws.

It comprises a wide range of animals, from fish to the various tetrapod
classes, which have in turn been derived from a fish or fish-like ancestor.
An entirely satisfactory classification of fishes is not easy to construct, because of
the incompleteness of several parts of the fossil record, and the consequent lack
of connecting links. Complete agreement as to their arrangement, therefore, has
not been reached, and the student will meet with a considerable choice of classi-
fications. It is at least certain that within the old group of Pisces (fishes
in the widest sense of the expression) there are fundamental divisions which
are of great antiquity. Animals of these divisions appear to have little to do
with one another beyond the possession of a possible, but unknown common
ancestor at a very remote period.
The old classification of the Gnathostomata into five classes-Pisces,
Amphibia, Reptilia, Aves, and Mammalia-has therefore to be modified by
the division of the fishes. Here these will be divided into three classes, as
follows:
CLASS PLACODERMI (APHETOHYOIDEA)
Sub-classes Acanthodii (Silurian-Permian)
Arthrodira (Silurian-Devonian)
Petalichthyida (Devonian)
Antiarchi (Pterichthyomorphi) (Devonian)
Rhenanida (Devonian)
Palreospondylia (Devonian)
CLASS ELASMOBRANCHII (CHONDRICHTHYES, SELACHII)
Sub-class Selachii
Orders Cladoselachii (Devonian-Permian)
Pleuracanthodii (Devonian-Triassic)
Protoselachii (Devonian-Recent)
Euselachii (Jurassic-Recent)
204 ZOOLOGY

Sub-class Bradyodonti
Orders Eubradyodonti (Devonian-Recent)
Holocephali (Jurassic-Recent)
CLASS OSTEICHTHYES
Sub-class Actinopterygii
Super-orders Chondrostei (Jurassic-Recent)
Holostei (Triassic-Recent)
Teleostei (Jurassic-Recent)
Sub-class Crossopterygii
Order Rhipidistia
Sub-orders Osteolepidoti (Devonian-Permian)
Cmlacanthini (Devonian-Recent)
Order Dipnoi (Devonian-Recent)

Compared with the tetrapods (p. 381) the three classes mentioned above
have certain characters in common. They have organs of respiration and
locomotion related to a permanently aquatic life. The chief, and generally the
only, respiratory organs are the gills. These are in the form of a series of
vascular processes attached to the septa of the branchial arches and persistent
throughout life.
The organs of locomotion are the paired pectoral and pelvic fins and the
unpaired dorsal, anal, and caudal fins (p. 79). These fins are all supported
by fin-rays of dermal origin, in addition to the endoskeletal supports. The
fin-rays are of diagnostic significance. A dermal exoskeleton is typically
present, but is occasionally secondarily lost. In the endoskeleton the notochord
is usually partly replaced by vertebrre, either of bone or cartilage. There is a
well-developed skull and a system of visceral arches, of which the first pair form
the upper and lower jaws, the latter movably articulated with the skull. Both
normally bear teeth (p. 103). The first pair of visceral arches mentioned above
are actually the second pair, because primitively there was a premandibular
arch that is represented in the Agnatha (p. 164; and see Petromyzon, p.
176). There is no middle ear (p. II4) and no allantoic bladder, the latter
structure arising for the first time in the Amphibia (p. 381). These two nega-
tive characters are therefore also diagnostic.
There are other characters found in one or other of the three classes which,
though highly characteristic when they occur, cannot be called diagnostic.
It is the unequal distribution of these characters that makes the division of
fishes into four classes desirable. A swim-bladder (p. 339), for instance, is
normally present (though it may be secondarily lost) in all fishes except elasmo-
branchs, which never at any time possessed one. In the placoderms and other
extinct groups the presence of the swim-bladder can only be surmised. The nasal
 PHYLUM CHORDATA 205

capsules open by inhalent and exhalent apertures (p. 138). These may be
only partly separate, as in the elasmobranchs (p. 230), or completely separate,
as in the Actinopterygii, where they are both dorsal in position (p. 298). In
the Crossopterygii (p. 353) one pair is external, the other internal in the mouth,
although in some very ancient forms the internal nares have been lost. The
condition of the circulatory system, the structure of the brain, the urinogenital
organs, fins, and scales all yield characters which help to diagnose the various
groups.
The kidney is a mesonephros with an occasional persistence of a few pro-
nephric tubules (p. 153). As in all gnathostomes, there are three semicircular
canals (p. 145).

CLASS PLACODERMI (APHETOHYOIDEA)

It seems certain that these animals were the earliest jawed vertebrates,
They appeared in the Silurian, flourished in the Devonian and Carboniferous,
and became extinct in the Permian (p. 3). As mentioned previously, it was
once believed that the cartilaginous fishes represented the most primitive
type. It now appears evident, however, that the present group of bony ani-
mals were the forerunners of some of the modern fishes, and perhaps even of
the elasmobranchs. This appearance of bone (accompanied by the develop-
ment of mobile jaws allied to the advantages conferred by an elaborate nervous
system, a heart, and highly developed blood-vascular system) early provided
a basis for the later development of animals of extraordinary complexity and
adaptive efficiency.
The Placodermi are so called because of their heavy, defensive armour of
bony plates (Figs. 133, 135). Members of this polymorphic class show a wide
degree of adaptive radiation, and in some instances an appearance so unfishlike
that their zoological position has been one of uncertainty. Some have been
thought to be Agnatha (p. 164), some as being allied to the Dipnoi (p. 361), and
some to the elasmobranchs (p. 219).
All placoderms possess jaws. Hyoid gill-slits persisted, and consequently
spiracles had not arisen, and, further, the hyoid arch remained unmodified and
did not support the jaws-the aphetohyoidean condition was retained (Watson).
There thus persisted a type of jaw suspension (p. 94) more primitive than
that in any other class of jawed fishes and one, moreover, which probably
existed in ancestral gnathostomes. We shall see later that in modern fishes
the hyomandibular bone (of the hyoid arch) helps support and brace the jaw
against the cranium. It seems probable that the jaws have been derived
from paired anterior gill-bars (seep. 93). This evolution of relatively massive
jaws appears to have been influential in reducing the first pair of gill-slits and
enabling the development of quite separate spiracles which became small
apertures of specialised function in the former first gill-slit region.
zo6 ZOOLOGY
For many years it was believed that at some early stage of craniate evolu-
tion primitive jaws had probably appeared long before the emergence of the
hyoid arch as observed today. It is now known that this anticipated condition
did actually occur in the Placodermi. The hyoid arch was in no sense a jaw
support, and the cleft was probably a typical gill-cleft (p. 94). The placo-
derm jaw is autostylic, i.e. it is articulated by its own processes to the cranium.
We see in the buccal arrangements of these Palceozoic placoderms an enormous
advance on the Agnatha, but, at the same time, a mechanical and masticatory

FrG. IJJ.- Sub-class and Order Arthrodira, Sub-order Coccosteidre. Dinic lithys. This carni-
vorous p lacoderm, seen h ere hunting the contemporary shark Cladoselaclte (p. 222 ) , probably
reached a length of 26 feet. The head and thorax were armoured but the posterior parts were
unprotect ed. (After Heintz.)

efficiency very inferior in comparison with later types of articulation that we

shall subsequently examine.
Most placoderms possess paired fins, but these are essentially adaptive
organs, variable in the extreme, and no generalised' ancestral pattern' is shown.
At the same time there are a number of other characters which suggest an ulti-
mate common ancestor for all placoderm orders. The hypothetical condition
required for such ancestry is probably most nearly approached by the Acan-
thodii, which flourished between zoo and 300 million years ago. It has been
suggested, too, that the acanthodians (or their close allies) may have been
ancestral to the elasmobranchs, and perhaps other gnathostomes as well.
A possible objection to this, however, may be in the acanthodian operculum,
which extends over the spiracular region. At the same time, this operculum
was variable, and little is known of its arrangement in the earliest forms.
Although in Devonian times the Acanthodii were many and varied, the sup-
 PHYLUM CHORDATA 207

posed acanthodians from Silurian deposits are known chiefly from seemingly
characteristic scales and dermal fin-bones. It is, perhaps, possible that one of
these may have given rise to some, if not all modern gnathostomes.
SUB-CLASS ACANTHODII
These-the so-called 'needle-finned sharks '-are the oldest and least
specialised known gnathostomes. Acanthodians were not fish. (See, partic-
ularly, Watson on their myotome and lateral line structure.) Appearing

Hy.Op.
Mand.Op. Bc.Ac.l. Br: 11.

L.L.

Pect. 5p.
FIG. 134. -Sub-class and Order Acanthodii. FamilyBrachyacanthidre. Climatius: Head of C.
reticulatus. Ant. Lat. antero-lateral dermal boneoftheshoulder-girdle; Br. Ar. I- IV.dermalelements
of the branchia l arch es and their opercula; D. Endo.Lym. foramen ductus endolymphaticus; Eye,
orbit; Hy. Op. hyoid operculum; L. L. lateral line ; Mand. mandible; Mand. Op. dorsal part of
mandibular operculum; M and. Ray. lower part of mandibular operculum; F eet. Sp. pectoral spine;
Sc. scapula ; Sup. Or. C. supra-orbita l cana l. (After Watson.)

first in the Upper Silurian, they reached their maximum development during the
Lower Devonian at a time when other classes of fishes were only just beginning
to evolve, and persisted until its extinction in the Lower Permian (p. 3).
Westoll separates the Acanthodii from the other Placodermi (see below) to
form two separate classes of a ' Grade Aphetohyoidea ', concluding that,
although the two groups show many resemblances, neither seems to have been
derived from the other. They may have diverged from a not very remote
common ancestry.
Acanthodians are fresh-water, superficially shark-like organisms. Usually
they have elongated fusiform bodies, which, in the final stages of evolution,
became almost eel-like. It is probable that later forms were sea-dwellers.
They were generally only a few inches long. The snout was blunt, the mouth
terminal, and the eyes large and forwardly placed (Fig. 134). The tail was
zo8 ZOOLOGY
heterocercal, and the relatively immobile fins were supported each by a strong
jacket of dermal bone. There were primitively two dorsal fins (reduced to
one in the later forms) and one anal fin. Between the pairs of pectoral and
pelvic fins was an intermediate series of pairs of fins increasing in size posteriorly.
Primitively this series was more numerous, consisting of as many as five pairs.
This suggests a derivation from an originally continuous fin-fold. Later the
series became reduced to one small pair, and finally to none.
A pectoral girdle was present (Fig. 135), consisting of a scapular and coracoid
part, to which the spine of the fin was affixed. The pectoral fin showed traces of
basal elements, as did also the dorsal fin. The peripheral part of the fin was
supported by ceratotrichia.
Externally the body was covered with scales of highly characteristic appear-
ance. These had nothing in common with the isolated placoid scales which
push through the skin of elasmobranchs (p. 103). They formed a continuous
Co....

FrG. 135.-Diplacantlm.~: Pec-

toral girdle. A dm . Peel. Sp. admed -
ian pectoral spine; Cor. coracoid ;
S c. scapula ; Derm. Pl. dermal
p late ; Lat. P ect.Sp.latera l pectoral
spine. (After ·watson.)

"'-Adm P.c:t. Sp.

dermal armour of true scales which, rhomboid in shape, were formed essentially
of a bone-like material. These scales were ornamented with stri<e, and were
small enough to appear almost granular. The dermal armour extended on to
the head (Fig. 134), where the scales were larger, and arranged in a definite
pattern. In section they can be seen to have been made up of layers of bone
substance in a manner comparable to a ganoid scale (p. 84). Teeth were
usually absent, but, when present, seemed to be derived from the modified
scales. Further, they appeared to be replaceable as in the elasmobranchs.
This, however, was more probably a primitive feature than evidence of any
close connection between the two classes.
In the primitive Lower Devonian acanthodian Climatius (Fig. 134) there
occurred a series of long gill-slits, each of which was overhung by an opercular
fold. The largest gill-slit, which was functional as such, was the homologue of
the spiracle (Fig. 168, p. 248), its operculum being borne by the jaws. Preserva-
tion of some specimens of the much later, Lower Permian Acanthodes is suffic-
iently good to allow a description of the neurocranium, visceral arches, and
adjacent structures. Although more specialised than the earlier species, this
 PHYLUM CHORDATA 209

toothless form nevertheless showed the essential features of the sub-class and
class.
The neurocranium (Figs. 136, 137) jaws and branchial arches (Fig. 137)
were largely cartilaginous, but ossified in parts by a layer of external peri-
chondrial bone, a primitive type of ossification such as occurs also in certain
agnathans. The ossifications of the neurocranium resembled in general those
of other primitive vertebrates. They consisted of paired lateral and dorsal
plates which may fuse. There were anterior, middle, and posterior ventral
plates. In front there were five small ossicles, one unpaired, supporting the
rostrum. A pair of lateral occipital ossicles, one lying on each side of the

AntOs

FIG. 136.- Acanthodes:

Skull. Ventral aspect. Ant.
Bas. anterior basal; Ant. for
Os. anterior ossicles; ? A o.
groove for dorsal aorta;
Dor. Os. dorsa l bone; Hyp.
For. hypophysial foram en;
jug. C. jul'(u lar canal ; Lat. Ot.Pr.Art.
Oc. lateral occipital ; M.
Bas. middle basal; Oc. N.
foramina for occipital
nerves; Ot. Pr. Art. articu-
lation for the otic process;
Pal. Ba.s. Art. articulation
for the palatobasal process ;
Post. Bas. posterior basal;
Post. Orb. Col. post-orbital
column; X . notch for vagus
nerve. (After Wa t son.)

posterior ventral plate, but at a higher level, completes the list. The jaws
consisted of a stout palato-quadrate bar for the upper, and the mandible for
the lower, portion. The interesting and important point about these elements
is that each ossified from two centres. Two centres ossi!y the upper and two
the lower elements. Thus the jaws were formed from four pieces, exactly as
are the hyoid and subsequent gill-arches, and so provide further evidence that
the jaws were themselves modified gill-arches. Later in life a special ossifica-
tion occurred in the otic process. The hyoid arch is complete, and took no
part in the jaw suspension. It was supplied throughout its length with gill-
rakers. This is sufficient proof that there was a complete hyoid gill-slit. In
earlier forms a small operculum was present on each gill-arch, and a larger one
on the jaws. In the later forms the mandibular operculum gradually extended
VOL. I~ 0
2!0 ZOOLOGY
back at first to cover the lower part of the gills, and fmally to take over the
whole function of covering them. This produces an operculum comparable in
extent and external appearance with that of the later bony fishes.
The acanthodians may be classified into six families. Of these, four
(Brachyacanthidre, Diplacanthidre, Ischnacanthid.e, and Gyracanthidre) possess

B•lld,

FIG. I37.-Acantlwdes: Head region. Reconstructions: A. \Vith the mandible and palata-
quadrate removed. B. Complete except for the remo,·al of the scales. Ant. Bas. anterior basal;
Ant. Os. anterior ossification in the basis cranii; Ba. Br. I. basi branchial; Cer. Br. I.-I I. cerato-
branchials; Cer. Hy . ceratohyal; Circ. Or. circum-orbital bones; Cor. coracoid; Dar. Os. dorsal
bone of neurocranium; Ep. Br. epibranchials 1-4; F. R. Ao. foramen for aorta; Hyp. Br. & Hy.
hypobranchial and hypohyal; Lat. Oc. lateral ossification in neurocranium; M. Gill. Cit. margin
of gill chamber; ;VIand. Ant. anterior ossification of Meckel's cartilage; 1'>fand. Post. posterior
ossification of Meckel's cartilage; l'iiand. Ray. ray of the mandibular operculum; Mand. Spl.
mandibular splint; Oc. N. foramina for occipital nerve; 01. Pr. Art. articular facet for otic
process ; Pal. Bas. Art. palato-basal articulation; Pal. Qu. Ant. anterior bone of palato-quadrate
cartilage; Pal. Qu . Post. posterior bone of palato-quadrate cartilage; Peel. Sp. spine of pectoral
fin; Sc. scapula; X, vagus foramen. (After ·watson.)

two dorsal spines. The two remaining families (Mesacanthidre and Acantho-
didre) have only one. The Brachyacanthidre (e.g. Climatius, Fig. 134), which
occurred from the Upper Silurian to the Lower Devonian, is probably the most
primitive group (Moy-Thomas).
SUB-CLASS ARTHRODIRA
This group flourished in the Devonian. The Arthrodira were swift pre-
dators (Fig. 133, p. 206) which seem, at first sight, to have been very different
 PHYLUM CHORDATA 2II

from acanthodians. In the past some have claimed them to be allied to the
lung-fishes (p. 361) or, subsequently, to the Gll-;:i
elasmobranchs (p. 219). Watson, however, has ~ ........
s ~ .5
shown them to be closer to the Acanthodii than t ~:I:
was believed. The head was always protected "'"'
8 ~E
by bony plates which are united to form a -a_~~
strong cranial roof. This was articulated by :2 ~- ·
means of sockets to a pair of condyles on an f-< ~ E
equally strong bony cuirass covering the anterior ~ -~ ~
~ s ro
~~~
parts of the body (Fig. 138). These plates are Q..c::.
usually ornamented by tubercles. The remain- ~:; 1l
Q..c:: .....
ing free part of the body (where known) tapered t;.J _:; .!!l

to a whip-like tail, and was either naked or, in aif§

some, covered with scales. A dorsal fin was
commonly present, and a pair of fins on the
] --g ~
_ g~ --
.... ~
: a 0 cu
ventral si d e must be regar d ed as representing f 8:.::: ~
the pelvics. Free pectoral fins did not occur, ~ ,q;~
but were represented by an immovable bone- • Scd ~> §s
sheathed structure on each side, which gradually · ~ ~
u

s
became reduced almost to extinction. The · .!:: "' "'
"" cd..c::
presence of an operculum which covered the · _a ~ :.
branchial arches can be inferred. The vertebral -~ "5::s -~""gdl·~
column, where known, had well-developed · ~~ ~
neural and hremal arches but no centra or ribs. · ~ ro :S
Jaws and t eeth, the latter rather soon worn .o"'
9o .~
::~- >
.,
down, have been seen in some forms, a nd may r.n -~ .8
..s.;;; cd
be presumed to have normally been present ;8 li --g
throughout the group. Arthrodires have been 8 ~ -:::1
,d p."
divided into three sub-orders, the Euarthrodira, 1: ~ ~
< .,a.
Ptyctodontida, and Phyllolepida. ~~ ~
The Euarthrodira contains the most primi- .... <+::
0 bo .,
tive family, the Arctolepida (e.g. Arctolepis), -g]~
which extended from the Lower to the Upper : ~ ....:..
Devonian. The Coccosteidre (e.g. Coccosteus, ] ';:; ~
Fig. 138), Dinichthys (Fig. 133, p. zo6) ..6 g~
~~~
ranged from the Middle to U ppcr Devonian. 1 ~ g
.
In this family belonged also the remarkable ~ ~ .:
ro~

.... ~<+::
Titanichthys, which may have reached a length . ~-;;
~-~ ~
of some 30 feet. Its jaws, about 2 feet long, ~ -~tl
were produced into a beak equipped with ~ "'
~ :::.

shearing blades. The Mylostomidre (e.g. Mylostoma) possessed a plate-like,

crushing dentition and are known only from the Upper Devonian. The
212 ZOOLOGY

Homostiidre (e.g. Homostius), characterised by a dorsoventral compression,

existed from the Middle to Upper Devonian.
~ .S The sub-order Ptyctodontida was perhaps restricted to
5~ the Middle and Upper Devonian and is characterised by a
""-o reduction of both cephalic and pectoral armour (e.g.
:::lo:i
ro·~

~~ Rhamphodopsis). The Phyllolepida are known only from

£;.+->g the Upper Devonian. Phyllolepis was once considered to
·~ ~ be allied to the heterostracan Drepanaspis (p. 170), which
'E.,...,_i it superficially resembles .
:.2o
..........
"::l
SUB-CLASS PETALI CHTHYIDA
·a.>
~~ These were Devonian forms (Fig. 139). They somewhat
~:3' resembled the Arthrodira, but the ball-and-socket joint
"; s
·~"'

~8
between the head and body did not exist. Again, the
§- 8 pattern of the head plating was somewhat different, and
the pectoral and pelvic fins were constructed on a very
~-·~
~ »
.;;:o
~ _g different plan. On the other hand, the facts that the
0
ai P.. neurocranium (Fig. 140) so much resembled that of an
l~ arctolepid, and that the hyoid arch took no part in the
:£;.:::c::if suspension of the jaws, are evidence that the order is rightly
~~ placed with the placoderms. Certain characters have caused
CD-
~ C:
.. 0
some authorities to look upon these fishes as allied to the
~0 elasmobranchs. The paired fins in particular have been
:s1S
»'0
:-:::Q) cited as evidence. The structure of the shoulder-girdle, the
§~ tribasal pectoral fin (Fig. 141), and the Cladoselache-like
~~
•o pelvic fin are undoubtedly points of resemblance. The
~~ "'"" shoulder-girdle, however, was equally like the primary girdle
:£ -~ of such acanthodians as Climatius or Diplacanthus. Further,
3 § the tribasal pectoral fin is paralleled, except for the absence
ct .t. of a spine, by that of Acanthodes. Moreover, a tribasal fin
~~ at this period is too advanced in structure to lead to the
o"'0~.-..8 more simple but later fin of the earliest known elasmo-
~] ;3 branchs, such as Cladoselache. The pelvic fin certainly re-
E "' <.> 0

u
-[P:i sembles that of Cladoselache, but is of a type that is found
....
;, "g B equally in the early sturgeons and palreoniscids, and is too
1 j"=""' tl ::5. generalised and primitive to be of much value as evidence
\ 1 ~~ .,; of any particular affinity. Other elasmobranch characters,
:, '0
:: ~ ~]' such as the ventral position of the mouth and the flattening
:,1 ~.!:: B of the neurocranium, may be accounted for as the result of
~ ~
' ;. "' adaptation to a bottom-living life. The different arrange-
ment of the head-plates from the general arthrodire pattern is not so great as
appears at first sight. The chief difference is along the mid-dorsal line. This
 PHYLUM CHORDATA 213

might be put down as the result of a secondary fusion. Both this group and
the Rhenanida include primitive members that possessed fin-structures pro-
RP

FIG. 14o.-Petalichthyida: Skull

and pectoral shield. A . Lunaspis
heroldi. in dorsal view. ADL. antero-
dorsal lateral plate; A L. antero-
lateral plate; M. marginal plate ; MD.
median dorsal plate; N. nuchal plate;
OP. opercular? plate; PA. paranuchal
plate; PDL. postero-dorsal lateral
plate; PN. postnuchal plate; PO. pre-
orbital plate; PT. postorbital plate;
RP. rostra-pineal plate; SO. sub-
orbital plate; SP. spinate.
B. Macropetalichthys rapheidolabis
in lateral, C. in dorsa l and D. in ventral
view. ce. external carotid foramen;
cs. cranio-spinal process; de. ductus
endolymphaticus; hv. head vein fora-
men; ju 1-2. anterior and posterior
openings of the jugular canal; no.
position of nasal openings; oc. occipital
part of skull; op. optic foramen; or.
orbit; pi. pineal foramen; ra. radix
aort<e; VII. facial nerve foramen;
Vllp. foramen for palatine branch of
facial nerve; IX. foramen for glosso-
pharyngeal nerve, X. foramen for vagus
nerve. (From May-Thomas after Gross,
Stensio.)

FIG. J4I.-Macropetaliclltl1ys: Pec-

toral girdle. Co. coracoid; Sc. scapula; Po.,
A1o. , l'vlt. pro-, m eso-, ana meta-pterygia;
R. radials. (After Broili.)

tected by a jacket of dermal bone and may be perhaps more properly combined
in a single group.
In this sub-class are placed the genera M acropetalichthys (Fig. 139) a Middle
Devonian form and Lunaspis (Fig. 140) of the Lower Devonian. The affinities
of certain other supposed members are in doubt.
214 ZOOLOGY

SUB-CLASS ANTIARCHI (PTERICHTHYOMORPHI)

This is a compact sub-class which was confined to the Devonian period.
It contained small fresh-water animals that were common, but apparently left
no descendants. Fossils sufficiently well preserved to reveal certain internal
details have been found. A spiral valve (p. 237), such as occurs in elasmo-
branchs, lung-fishes, and coclacanths, has probably been detected. Further,

FIG. 142.-Sub-class and Order Antiarchi, Family Asterolepidre. Pterichthyodcs. This

group contained small placoderms less than a foot long. Like other aphetohyoideans they had
a well-developed lateral line system. Upper figure is P. milleri in side view. Left-hand figure
in dorsal, right-hand figure in ventral view. ADL. antero-dorsolateral; A V L. antero-ventrolateral;
MD. median dorsal; NIV. median ventral; PD. posterior dorsal; PDL. posterior dorsolateral;
PVL. posterior ventrolateral; S. semilunar plate. (After Traquair.)

a pair of large backwardly directed sacs communicated with the pharynx

and appear to have been lungs (see p. 108).
Like the Arthrodira, the Antiarchi were provided with a strong armour
over the head and shoulders, but with a somewhat different arrangement of the
plates. There was also a different method of articulation of the head to the
body. The sockets in this case were on the cuirass and on the articulating
processes on the head. The pectoral appendages were of a much more elaborate
structure.
 PHYLUM CHORDATA zrs
Pterichthyodes ( =Pterichthys) (Fig. 142) may be taken as the type of the
single family (Asterolepidre). The genera of this group differ only in relatively
unimportant details. The eyes were placed close together on the top of the
head, a modification which was no doubt responsible for a rearrangement of the
neighbouring plates. The cuirass was formed by a large median dorsal and a
posterior dorsal plate, both unpaired. Along the sides were the anterior and
posterior dorso-laterals. The floor was made up anteriorly by a pair of anterior
ventrolateral plates, which enclosed between them a pair of small semilunar
plates. Posteriorly it was made up by a pair of posterior ventral plates. Be-
tween these pairs lay an unpaired median ventral. The anterior ventrolateral
plates had at their sides an exca \'ation which held the condylar head of the' arms'
or pectoral appendages. These were highly developed, and were the leading
characteristic of the order. They were formed of a number of plates, closely

FIG. 143.-Sub-class and Order Antiarchi, Family Asterolepidre. Bothriolepis. This small
placoderm was heavily armoured in front and unprotected behind. It possessed weakly developed
jaws and ate minute animals or plants (cf. Dinichthys, Fig. 133. p. 206). Its pectoral appendages
were freely movable and probably sculled the animal along. (From Romer, after Patten.)

united except for a flexible joint dividing them into a proximal and distal
section. The fossilised arms are hollow, but were in life provided internally
with muscles, nerves, and arteries. The jaw apparatus consisted of a pair of
large superognathals, each bearing a notch at the side. Into this was fitted a
small elongated element, and this in turn articulated with a third. The system
therefore corresponded to some degree with that of Coccosteus. The lower ja\vs
were more weakly developed.
The trunk was protected by well-developed scales and terminated in a
heterocercal tail with a ventral web. A single dorsal fin was present supported
by an anterior spine. A series of fulcra! scales ran along the upper border of
the body from the fin to the tail. In this genus there were no pelvic fins.
Another well-known genus is Bothriolepis (Fig. 143), which had a naked
body, an extra plate on the head, and a pair of fleshy protuberances at the
ventrolateral border of the cuirass which are supposed to represent a pair of
pelvic fins. Remigolepis lacked the median joint to the pectoral fins. Micro-
braclzius and Asterolepis, Ceraspis, and Ceratolepis are other recorded genera.
216 ZOOLOGY

SUB-CLASS RHENANIDA
In shape these peculiar, probably placoderm, Devonian animals resembled
the modern rays. Their skin, too, was denticulate. However, the denticles
were attached to a body-armour. The skull was ossified, and the gills were
protected by an operculum.
Spiracles have not been found,
nor is there any suggestion of
hyoid support for the lower jaw.
It seems obvious that these ani-
mals were bottom gliders, and
that the various similarities be-
tween them and the later-evolved
skates are examples of conver-
gent adaptation. In the Rhena-
nida the bones were not greatly
developed, and became pro-
gressively less so in later forms.
Gemuendina (Fig. 144) is the
best-known form. It had very
much the shape of a skate. The
head had on the top a central
plate of bone, and ranged on
each side were three pairs of
plates which did not articulate,
but were surrounded by areas
of skin bearing tubercles. The
large pectoral fins were supported
by a secondary elongation of the
radials, and behind them are t he
smaller, flattened pelvic fins.
The body then tapered to a
FIG. 144.- Sub-class a nd Order Rhenanida, Family
Gemuendinidre. Gemuendina. Although superficially pointed tail. A dorsal fin was
skate-like, this 'stegoselac hia n ' (G . sturtzi) belonged t o present and supported by an
an operculate group that preceded, and was probably not
r elated to, the elasmobranchs. Note the position of the anterior spine much as in Acan-
eyes, the pectoral and pelvic fins, a nd the dorsal spine.
(After Broili.) thodes or Pterichthyodes. Be-
hind this fin was a row of dorsal
spines, and there was a row of similar spines on each side which started from a
point just behind the pelvic fins and ran to join the dorsal row at the end of
the tail. The pectoral girdle (Fig. 145) was comparable with that of a ptycto-
dont (p. zn), but lacked the lateral spine.
 PHYLUM CHORDATA 2!7

The presence of gills supported on branchial arches and covered by an

operculum can be demonstrated in some instances, and from them the presence
of gills of a similar type can be reasonably inferred in the others. The jaws,
when known, showed no signs of any hyoid suspension. From this arrangement,
the placoderm condition with its complete hyoid gill-slit naturally follows.
The general impression given by these forms is that they were placoderms which
were losing their armour, and had developed special fins that enabled them to

A In.Lat. B ln.lat.

FIG. 145.-Gemuendina: Anterior part of the body with head removed. Restoration of G,
sturtzi. A. D . L. antero-dorsolateral ; A . L. antero-lateral; In . Lat. inter-lateral; !vi. D. median
dorsal plate; Peel. pectoral fin; Peel. Ant. Mar. anterior margin of pectoral fin; Pelv. pelvic fin;
Vert. vertebral column. (After \Vatson.)

exploit the lake- and sea-floor. Only three genera have so far been described:
Gemuendina (Fig. 144) (Lower Devonian), Asterostetts (Middle Devonian), and
]agorina (Upper Devonian).

SUB-CLASS PALJEOSPONDYLIA
Palceospondylus (Fig. 146), the sole representative of this group, occurs only
in two restricted areas of the Mid-Devonian in Scotland. From the time of its
discovery it has been the subject of much speculation, and has in turn been
considered either as the larva of a dipnoan fish, of Coccosteus, or of an amphi-
bian; as a fish of chondrichthyan affinity, and even as a t eleost. For many
years the general consensus of opinion was that its nearest affinity was with
the Agnatha, and certain of its features led some to see in it a relationship,
possibly ancestral, with the Myxinidre. Moy-Thomas however showed with
new as well as old material that Palceospondyhts was not a member of the
218 ZOOLOGY
Agnatha, but instead a true gnathostome. Despite its lack of dermal armour it
should probably be placed in a separate sub-class of the placoderms.
Palf2ospondylus gunni reached a maximum length of about two inches, and
occurs in great numbers in certain layers of the flagstones of a small quarry at
Achanarras in Scotland. Unfortunately the preservation of only a very few
specimens is sufficiently good to yield details of the structure. It had a

FIG. q6.-Sub-class and Order Palreospondylia,

Family Palreospondylidre. Palreospondutus gunrJi.
This tiny (ca. 5 em.) enigmatic anima l is t he sole
described species of its group. It lacks any apparent
exoskeleton but is of the placoderm 'level'. A bove:
Complete animal. The head and anterior vertebr<e in
dorsal view, the posterior vertcbr<e and tail in lateral
view. CE. centrum; HA. h<emal a rch; NA . neural
arch ; PC?. pectoral girdle; PV. pelvic girdle ;
RA. radials.
Head in ventral view. AO. probably a ventral
brain-case structure; APQ?. possibly anterior part
of palata-quadrate; BA . branchial arch; BB. basi-
branchial; C£. ceratohyal ; HY. hyomandibular;
MA. possibly mandible; PPQ. posterior part of
palatoquadrate. (After l\loy-Thomas.)

depressed skull, a well-calcified vertebral column, and a heterocercal tail. The

lower radials were two-jointed, and the second row of radials was bifurcated
(Fig. 146). Paired fins were present, and traces of pectoral and pelvic girdles
have been observed. In the vertebral column the neural arches were low in
front, but gradually lengthened into neural spines towards the caudal region.
Ventrally there were short hremal ribs which posteriorly became hremal spines.
The vertebral centra were stout and ring-shaped. The neurocranium (Fig.
 PHYLUM CHORDATA 2!9

146), with its ring of tentacles in front, was complicated, and the interpretation
of some of its structures presents difficulties. At first sight it gives the im-
pression of a larval condition, but against this view is the fact that much of it
was calcified. Further, the vertebral column was fully calcified even in the
smallest specimens. The neurocranium is dorso-ventrally compressed, com-
plete and fully calcified on its ventral surface and sides but it lacked a roof,
being uncalcified in this region. The auditory capsules were large, and formed
the end of the neurocranium. This character is probably not found in any
other placoderms, but occurs in the Agnatha (p. 164). The upper jaws (Fig.
146, APQ?, PPQ) were formed by a pair of elements, an anterior and posterior
palatoquadrate, on each side. The lower jaws were represented by a single
pair which ended bluntly (MA). If, as seems possible, there was an anterior
uncalcified piece on each side, the jaw apparatus would be comparable to that
found in the Acanthodii (Fig. 137). The hyoid arch (HY, CE) was complete
(and took no part in the support of the jaws), but, unlike that of other placo.
derms, was very different from the succeeding branchial arches. A pair of
ventral elements lay posterior to the auditory capsules and represented a
branchial arch, and a small element that lay outside the second of these may
represent part of the pectoral girdle.
It would seem probable that Palmospondylus was an animal of the apheto-
hyoidean grade. At the same time there is no evidence of its relationship to
any known placoderm.

CLASS ELASMOBRANCHII (CHONDRICHTHYES)

INTRODUCTION
This class comprises the living sharks, rays, and chimreras, together with a
number of extinct orders. Elasmobranch fishes are generally almost ex-
clusively marine in habit, although a few have successfully colonised brackish
and even completely fresh water. Most of them are predacious. They include
the largest fishes that ever lived. The biggest known was probably a giant
Tertiary species of Carcharodon, which may have been about So feet in length
(Fig. 147). Teeth of this monster measure about 6 inches long, and in shape are
very like those of to-day's representatives. Other smaller teeth, but at the
same time still larger than those of any living form, have been dredged from
the Pacific floor and appear to be of very recent origin. The largest living
shark is the oceanic, tropical oviparous whale shark (Rhinodon), which reaches
nearly 70 feet. It lives on plankton, as does also the Basking Shark (Cetorhinus),
which grows to a length of 40 feet. Both of these species are harmless to Man,
as are, in fact, most elasmobranchs.
The man-eating Great White Shark or White Pointer (Carcharodon car-
clzarias) attains a length of 36 feet. It has flat, triangular, serrated teeth, wide
220 ZOOLOGY

at the base. and about 3 inches long. Fundamentalists who have been troubled
by the fact of the narrow resophagus of the large baleen whales, and who are
emotionally and spiritually incapable of accepting certain biblical narratives as
picturesque allegory, have sometimes held that it was the Great White Shark
and not a mammal, that swallowed Jonah. In any case, sperm whales,
sufficiently wide of gape and throat, sometimes enter the Mediterranean
Sea.
In the South-west Pacific Ocean, particularly, several man-eating species
occur, and this has led to the establishment of elaborate protective measures,
including aerial patrols, on eastern Australian beaches. Unquestionable man-
eating sharks are the Black (Common) Whaler (Galeolamna) (reaching a length
of 12 feet and weighing about goo lbs.), and the Tiger Shark (Galeocerdo) (16
feet and more than 1,400 lbs.). In addition, the Grey Nurse (Carcharias) (9ft.
6 ins.), and probably the Blue
Pointer or Mako (I suropsis) (13 feet)
also kill Man. During this present
century more than 120 people
were attacked in Australian waters,
of whom more than fifty died of
h::emorrhage and shock after re-
ceivmg fearful injuries. One
aboriginal trochus-diver lived after
injuries which required 238 stitches
(North Queensland, 1939). During
the same period at least ten boats
were attacked by sharks, and in-
numerable dogs, and even horses,
have been mauled. Although at
least one shark fatality occurs every
summer, this mortality is quite in-
considerable when it is considered
FIG. 147--Carcharodon megalodon: Teeth that surfing is the principal Austra-
and jaws. The triangular fang of this Pleistocene lian sport, and more than 10o,ooo
shark are up to 6 inches long. The jaws, restored Dn
the pattern of a modern mat.-eater, may have had a people expose themselves to some
6· or 7-foot gape. (Redrawn from an exhibit in the
Am. Mus. Nat. Hist.) danger each week-end in the surfing
season. It is a remarkable fact that
shark attacks are rare in Western Australia and North America. During the
early phases of the war with Japan the United States air force authorities
impressed on pilots destined for Pacific service that they had nothing to fear
from sharks. Soon both nations were busily testing repellents (p. 273).
The elasmobranchs are a well-defined group which fall into two clear-cut
divisions, the Selachii and the Bradyodonti (see below). There have been
 PHYLUM CHORDATA 22!

several attempts to ally elasmobranchs with the somewhat shark-like poly-

morphic Placodermi (p. 205), but no convincing evidence of anything but very
remote relationship has been adduced. It is possible that the elasmobranch
ancestor was of placoderm affinity, but the known aphetohyoidean groups were
much more closely allied to each other, and of a much lower degree of organisa-
tion, than even the most primitive known elasmobranchs. The primary
elasmobranch division occurred very early in the history of the class. Sharks
with a typically bradyodont dentition (p. 256), and also selachian clado-
donts both appear in Upper Devonian deposits. Suggestions that various
fragments found in Mid-Devonian and even earlier strata were those of elasmo-
branchs have been rejected by most recent authorities.
The elasmobranch skeleton is entirely cartilaginous, but in it there may
be occasionally heavy subsidiary mineralisation, although never true bone.
This persistent lack of bone led early workers to believe that the sharks and
their allies antedated the bony fishes, but this view is now generally abandoned.
We know now that bone (p. 74) is an exceedingly primitive material, and
in any case the earliest known truly cartilaginous fishes do not seem to have
appeared before mid-Devonian. And, in fact, even bony fishes (Actino-
pterygii) (p. 285), as distinct from placoderms (p. 205), have been found in
Lower Devonian deposits. It is true that cartilaginous structures do not
preserve as well as bony ones, but if elasmobranchs are as ancient as was once
widely believed it would be expected that teeth of hypothetical, very ancient,
species would be preserved and some at least probably found. All in all, it is
likely that the elasmobranchs arose from a bony ancestor, and perhaps by
neoteny (p. 3). We will see later that the extant Axolotl (p. 456) has arisen
because of its capacity to become sexually mature and to reproduce in an
essentially larval condition. Cartilage is embryonic tissue. It is by no means
improbable that the elasmobranch ancestors became sexually mature while
their skeletons remained relatively immature. It is possible that the un-
armoured, later placoderms (p. 217) may have reached the soft-bodied con-
dition by means of an essentially similar, if not so hypoth~tically simple,
manner as has been discussed above (p. 3). Neoteny, incidentally, has been
demonstrated to occur in teleosts (e.g. Clariallabes, p. 338).
There is little agreement concerning the most appropriate classification of
the cartilaginous fishes. We will use the following :

CLASS ELASMOBRANCHII (CHONDRICHTHYES)

Sub-class Selachii
Orders Cladoselachii (Devonian-Permian)
Pleuracanthodii (Ichthyotomi) (Devonian-Triassic)
Protoselachii (Devonian-Cretaceous)
Euselachii (Jurassic-Recent)
222 ZOOLOGY
Sub-order Pleurotremata (Jurassic-Recent) 1
Super-families Heterodontoidea (Jurassic-Recent)
Notidanoidea (Jurassic-Recent)
Galeoidea (Jurassic-Recent)
Squaloidea (J urassic-Reccnt)
Sub-order Hypotremata (Jurassic-Recent)
Sub-class Bradyodonti (Devonian-Recent)
Orders Eubradyodonti (Devonian-Permian)
Holocephali (Jurassic-Recent)
An operculum is absent (even in Chlamydoselachus, frilled sharks), but is
present in Holocephali (p. 256). The gill-slits open directly to the exterior and
the gills are laminar (p. 106). A cloaca is present. Its aperture serves as
a common outlet for the rectum and the renal and reproductive ducts. A
uniquely high concentration of blood urea occurs (p. 253). A rhythmically
contractile conus arteriosus with several transverse rows of valves is always
well developed. The venous system is expanded in places into larger sinuses,
which do not occur in other gnathostomes. Other characters are pelvic claspers
in the male (absent only in one extinct group, the Cladoselachii) and large egg-
cases. Fertilisation is always internal. Many species are viviparous (p. 278).
The large spiral valve in the short intestine, and the abdominal pores, are
characteristic, but occur in some other fishes. The placoid scales are diagnostic.

SUB-CLASS SELACHII
ORDER CLADOSELACHII (PLEUROPTERYGII)
These were extinct, shark-like Chondrichthyes with a fusiform body, a
heterocercal tail with a large lower lobe, and a horizontal keel-like fin on each
side of the caudal peduncle (Fig. 148). There were two dorsal fins, sometimes
with a spine in front of the first. There were very prominent pectoral fins
and pelvic fins were also present. Each kind had a primitive, very wide, basal
attachment to the body and was without the anterior and posterior con-
striction from the body-wall that is usual in the paired fins of almost all fishes.
The skeleton of the fins consisted of parallel cartilages of simple structure.
The large sub-terminal mouth (without pronounced rostrum), long sharp teeth,
relatively enormous forwardly directed eyes, and generally streamlined form
suggesting a hunter of great power and voracity. The body was covered with
small denticles. The typically selachian teeth each consisted of a slender,
pointed main cusp with smaller lateral cusps fixed on a broad base. The jaw
1 There is much to be said for abandoning the classical Pleurotremata{H ypotremata dichotomy
and including the rays as a Super-family Batoidea (Upper Jurassic-Recent) of the Order Euselachii
(true sharks). There exist squaloidean forms (e.g. monk-fishes, Squatina, banjo-fishes, Rhinobatis)
that seem morphologically intermediate between 'sharks' and 'rays'. The other super-families
of the Pleurotremata were distinct in the Lower and Middle Jurassic, whereas the rays first
appeared at the top of the Jurassic.
 PHYLUM CHORDATA 223

suspension was amphistylic. A well-developed postorbital process engaged the

upper jaws and a further, more anterior articulation with the floor of the
cranium probably existed. Such a suspension was probably a primitive
feature, as was also the absence, unique among elasmobranchs, of claspers
(p. 255) in the male. The large notochord was unconstricted. There were
paired nostrils, and large scale-like denticles surrounded the eyes. A hyoid
arch and five pairs of branchial arches lay behind the jaws. Cladoselache
jyleri (Fig. 148) from the Upper Devonian (p. 3) is the best-known form , and

FIG. 148.-Sub-class Selachii, Order Cladoselachii, Family Cladoselachidm. Cladoselache.

General structure. C. fyleri grew to a length of about 3 feet. In some species, perhaps only in
males, there was a small, laterally -compressed spine in front of the anterior dorsal fin. (After Harris.)

Cladod-us and Symmori-um were related forms ranging from the Upper Devonian
to the Upper Carboniferous.
It is possible that the Cladoselachii were a group closely allied to the
ancestral stock from which the other Chondrichthyes have radiated. Exclud-
ing the somewhat specialised Pleuracanthodii mentioned below (p. 224), there
seem, broadly speaking, to have been two main radiations. One of these, the
Bradyodonti (p. 256), eventually gave rise to the Holocephali (p. 256). The

FIG. 149.- 0rder Cladoselachii, Family ctenacanthidm. CtefUJCanthus. Some ctenacanths

(e.g. Goodrichia) were more than twice as long as C. costellatus, shown above. (After Moy-Thomas.)

other produced the Ctenacanthidre and Hybodontidre (p. 224), which are on the
line of the modern sharks and rays. A third group known only from the
Devonian is the Coronodontidre (e.g. Diademon-us, Coronodus). These possessed
peculiar t eeth with conical cusps. Diademonus was completely covered with
224 ZOOLOGY
minute, ridged scales which strikingly resembled its curious teeth (see Harris).
It had pelvic claspers.
The Ctenacanthidre (Fig. 149) are found from the late Devonian to the
Lower Permian. They were ancient dorsal-spined sharks with teeth like those
of Cladoselache. The jaw suspension was
amphistylic. Claspers were absent. The
radial elements of the fins were segmented
as in modern forms, and so showed a great
advance.

ORDER PLEURACANTHODII
(ICHTHYOTOMI)

The Pleuracanthodii were slender, fresh-

water, shark-like fishes that appear to have
arisen as an early offshoot from the central
elasmobranch stem. They extended from
the Upper Devonian until the early Triassic.
In them the pectoral fin had acquired the
form of an 'archipterygium '-an elongated
axis extending out from the body and bearing
pre- and post, axial rays. The postaxial rays
were fewer in number than the pre-axial.
This may be a primitive feature. A well-
known species, Pleuracanthus decheni (Fig.
150), occurred in the Carboniferous and
Permian. It was clearly in many respects a
specialised form. The long body, the slender
diphycercal tail (unusual in early elasmo-
branchs), the continuous fin-fold along the
back, and the anal fin (with its unusual fore
and aft duplication and direct suspension from
the hremal arches), and the long, movable
spine on the head, all point to a considerable
degree of specialisation. Scales have not
been observed in any pleuracanth, and are
thought to have been absent. The cartilage
of the skull, the amphistylic jaws, and the
branchial arches were sometimes well calcified.
The teeth were bipronged and had a central cusp. The presence of claspers
in the male is a typically chondrichthian feature. Like other Chondrichthyes,
the Pleuracanthodii are presumed to have branched off from the early
Cladoselachian stock, but to have given rise to no successors.
 PHYLUM CHORDATA 225

ORDER PROTOSELACHII

The most primitive protoselachian group was probably the Hybodontidre

(e.g. Hybodus), the members of which were almost certainly of ctenacanthian
ancestry. The order became relatively common in the late Palreozoic and
Mesozoic. It was probably composed of numerous sub-groups, but only the
Hybodontidre are well known. A second family (Tristychiidre) has been

Frc. 151.-0rder Euselachii, Sub-order Pleurotremata, Super- family Heterodontoidea.

Heterodontus (Crested P ort Jac kson Shark H . galea/us.) This harmless eastern Australian species
grows t o a length of about 4 feet. It may be trawled a t a depth of 50 fa thom s, but is common
in harbours , where its skin and t eeth are ofte n stained reddish, p erhaps through feeding on
large Purple Sea-urchins in the sha llows. It is ovipa rous. T h e egg-cases arc usually found among
weed a t a depth of about 50 feet. They arc about 4 inches long with a fla nge o f seven o r eight
spirals a nd terminal anchoring t endrils that may reach a length of seven feet . (Modified a fter
McCulloch.)

erected on material from the lower Carboniferous. The protoselachians

differed from the Cladoselachii in two principal ways. Firstly, the pectoral
fins were no longer fin-folds but became ' notched at their posterior margins as
in modern elasmobranchs' (Moy-Thomas) and secondly, pelvic claspers had
arisen in the males- a feature to be ret ained by all ' modern ' sharks and their
derivatives. The hybodonts retained the amphist ylic jaw suspension, but
had already evolved narrow-based, flexible fins similar to those of extant forms.
In addition to sharp anterior fangs, they developed flattened, crushing posterior
VOL. II. p
226 ZOOLOGY
teeth, with which they could break the thick shells of molluscs. It was from the
Protoselachii that the Euselachii (p. 226), embracing all modern selachians (but
not bradyodonts), arose in the Mesozoic. Although the hybodonts disappeared
in the late Mesozoic, there still exist to-day a few hybodont-like sharks which
exhibit many of the ancient characters. These belong to the genus Hetero-
donttts (e.g. the harmless Port Jackson
Shark, Figs. 151, 152), which occurs also in
Jurassic strata (p. 3).

ORDER EUSELACHII

This group is composed of the sharks

and rays. Although these do not occur in
vast numbers, as do many species of bony
fishes (e.g. herring, cod), they are neverthe-
less abundant in equatorial and temperate
seas.
The living and extinct orders which
FIG. 152.-Jieterodonflls: Dentition.
The hybodont condition is retained. The occur from the Jurassic period onward are
anterior fangs are pointed. The posterior distinguished, in addition to the general
elements are rounded and constitute a
mechanism adapted to crushing echino- characters of the class, by having numerous
derms and molluscs (cf. Fig. 147. p. 220). teeth developed in continual succession,
(After Deary.)
and by the pectoral fins having three basal
pieces (the pro-, meso-, and meta-pterygium), from which a number of pre-axial
radials spread out. 1
The order is divided into two sub-orders, the Pleurotremata (sharks)
and the Hypotremata (rays), chiefly on the position of the gill-slits. These are
lateral in the first group and ventral in the second. The division is perhaps
essentially one of convenience (see page 222), but other notable differences
exist. In the Pleurotremata the anterior margin of the pectoral fin is free from
the body. The pectoral radials are simple and of few segments, and, as a rule,
only the anterior ones reach the free edge of the fin. The two halves of the
pectoral arch are well separated above. In the Hypotremata, on the other
hand, the pectoral fin is joined to the side of the body or to the head. The
pectoral radials are numerous, multi-segmented and bifurcated at the ends. All
of them reach the free edge of the fin. The halves of the pectoral girdle either fuse
with one another or else both fuse to the vertebral column. There is a further
difference : the skull of the Pleurotremata is without cartilages attached to
the olfactory capsules, and the pterygo-quadrate has a process articulating
with, or attached by ligament to, the cranium.
In the skull of the Hypotremata there are paired preorbital cartilages
1 In the hybodonts and in early stages of some Euselachii there are traces of reduced postaxial
radials.
 PHYLUM CHORDATA 227

attached to the olfactory capsules, which are often very well developed. The
pterygo-quadrate has no articulating process to the skull. Finally, in the
Pleurotremata the hyomandibular and ceratohyal both bear cartilaginous
rays, and both take part in supporting the first gill. The ceratohyal is a single
cartilage, and is attached to the lower end of the hyomandibular. In the
Hypotremata the hyomandibular lacks rays, and so takes no part in the support
of the gill, and the ceratohyal is segmented and attached to the hyomandi-
bular either high up or not at all.
The Pleurotremata can be divided into four super-families : the Hetero-
dontoidea, Notidanoidea, Galeoidea, and Squaloidea.
The Heterodontoidea are accorded ordinal rank by some authorities
and relegated to family rank within the Squaloidea by others. One genus,
Heterodontus, (Fig. 151) survives as an obvious though somewhat modified
derivative of Hybodus stock (p. 224). The genus occurred in the Jurassic
(p. 3)-

FIG. 153·-0rder Euselachii, Sub-order Pleurotrem.ata, Super-family Notidanoidea. Clllamy-

doselaclms. C. anguineus (Frilled-gillcd Shark). This swift and slender shark is found prin-
cipally, but only relatively rarely, in J apanese waters. Teeth occur in Tertiary deposits. An ally,
Hexanchus, occurred in the Jurassic. The frills are elongations of inter-branchial septa (p. 106).
(After Gunther.)

When the Port Jackson Shark (H. phillippi) was discovered in the
days of Australian colonial settlement in the late eighteenth century it im-
mediately aroused great attention. It still retains the curiously diversified
dentition and a sub-terminal mouth. Although the palato-quadrate post-
orbital articulation has been abandoned, it remains essentially amphistylic.
The animal lives chiefly on molluscs, crustacea, and echinoderms. Several
species occur in Pacific waters.
The Notidanoidea, too, are archaic survivors. The group includes Chlamy -
doselachus ( = Notidanus) (the frill-gilled sharks) (Fig. 153); also H eptranchias
and Hexanchus. These animals are characterised by a single dorsal fin (unique
to the group), a simple vertebral column, a sub-terminal mouth, and the
presence of more than five gill-slits. In the Galeoidea there are two dorsal
fins without spines, an anal fin, and five gill-slits. This group includes (r) the
Odontaspidre (Grey Nurse and Goblin Shark), (2) the lsuridre with numerous
genera such as Lamna (the mackerel sharks, e.g. Porbeagle, L. nasus) , Carcharo-
don, Cetorhinus (Basking Shark), and Alopias (Thresher); (3) the Orectolobidre
228 ZOOLOGY
(carpet and tiger sharks), Orectolobus and Stegostoma; the Scyliorhinidre,
Scyliorhinus (dogfishes); and lastly (4) the Carcharinidre, with Carcharinus
(blue sharks), Galeus (topes), Mustelus (nurse hounds), and Sphyrna (hammer-
heads).

FrG. 154.- 0rder Euselachii, Sub-order Hypotremata: Radiation and adaptation in selachians.
Saw-fishes (Pristis, Family Pristid<e) are survivors of a group that was common in the Cretaceous.
They are not t o be confused with the squaloid saw-sharks (Pristiophorid<e), with which they share
convergent, but not diagnostic characters. Saw-fishes grow to a length o f over 24 feet. They
hack and disable their prey before swallowing. Eagle-rays (Aetobati s, Family Myliobatid<e)
weigh as much as 400 lbs. A etobatis eats molluscs, sometimes uncovering t h em by a gitating the
sandy sea-floor with its wing-like pectoral fins. Both the above groups are strikingly adapted to
bottom-living. (From photographs and specimens.)

The Squaloidea have two dorsal fins, with or without an anterior spine.
The anal fin is usually absent. In this order are placed the Squatinidre,
Squatina (angel- or monk-fish), the Squalidre, Squalus (spiny dogfishes), the
Pristiophoridre (saw sharks), and others.
The Hypotremata are sometimes divided into two groups: the Narco-
batoidea or electric rays, and the Batoidea, which include all other skates
and rays such as the Rhinobatidre (guitar fishes), the Pristidre (sawfishes),
Myliobatidre (eagle rays), Raiidre (true skates and rays) (Fig. 154), etc.
The Sub-class Bradyodonti will be dealt with on p. 256.
 PHYLUM CHORDATA 229

EXAMPLE OF THE SUB-CLASS.-A DOGFISH (SCYLIO-

RHINUS ( = SCYLLIUM), SQUALUS, BRACHJELURUS, ETC.)
General External Features.-The flesh of these fishes is sold in British fish-
shops under the names of ' Rock Salmon' or ' Flake'. The general shape of
the body (Fig. ISS) is roughly fusiform. At the anterior end it is broader and
depressed. Posteriorly it tapers gradually and is compressed from side to side.
The head terminates anteriorly in a short, blunt snout. The tail is narrow and
bent upwards towards the extremity. The colour is grey with brown markings,
or dark brown above, lighter underneath. The entire surface is covered closely
with minute hard denticles-placoid scales or dermal teeth, (p. 103)-which
are rather larger on the upper surface than on the lower. These are pointed,
and directed somewhat backwards, so that the surface appears rougher when

FIG. ISS--Order Euselachii, Sub-order Pleurotremata, Super-family Squaloidea, Family

Hemiscyllidre. Brachrelurus waddi (Bloch and Schneider, I 801). The Sydney(' Blind' or' Dusky')
Dogfish lives around rocky shores and feeds on small fishes, squid, sea-anemones, crustaceans, and
seaweed. It reaches a length of about 3 feet and is ovo-viviparous (\Vhitley). Above is a drawing
of a fcetus about 6 inches long from which the yolk sac has been erased and a suggestion of the
lateral line system put in. The dark bands become less prominent with maturity (as occurs in
many animals). ' Blind ' is a fisherman's misnomer arising from the fact that the comparatively
small eyes are closed upon removal of the fish from water.

the hand is passed over it forwards than when it is passed in the opposite
direction. When examined closely each scale is found to be a minute spine
situated on a broader base. The spine consists of dentine covered with a
layer of enamel. The base is composed of bone-like substance and there is a
vascular pulp cavity. The whole scale has thus the same essential structure
as a tooth, but, at the same time, the enamel is of different origin from that
of mammals, in that it arises in the mesodermal dentine and not as a speci-
fically ectodermal secretion.
Along each side of the head and body runs a faint depressed longitudinal
line or slight narrow groove-the lateral line, marking the position of the lateral
line canal. This contains integumentary sense-organs (pp. 136-137).
As in fishes in general, two sets of fins occur-the unpaired or median fins,
and the paired or lateral. These are all flap-like outgrowths which run vertically
and longitudinally in the case of the median fins and nearly horizontally
in the case of the lateral. They are flexible , but stiffish (particularly towards
the base) owing to the presence of a supporting framework of cartilage. Of the
 ZOOLOGY
median fins two-the dorsal-are situated as the name indicates and are of
triangular shape. The anterior, which is the larger, is situated at about the
middle of the length of the body, the other a little further back. The caudal
fin fringes the tail. It consists of a narrower dorsal portion and a broader
ventral, continuous with one another round the extremity of the tail, the latter
divided by a notch into a larger anterior, and a smaller posterior lobe. The
tail is heterocercal, i.e. the posterior extremity of the spinal column is bent
upwards and lies in the dorsal portion of the caudal fin so that the lobes of the
fin have come to be unequal. The so-called anal fin is situated on the ventral
surface. In Scyliorhinus it is to be found opposite the interval between the
anterior and posterior dorsals. In Hemiscyllium it is situated behind the pos-
terior dorsals, which it resembles in size and shape.
Of the lateral fins there are two pairs, the pectoral and the pelvic. The
pectorals are situated at the sides of the body, just behind the head. The
pelvics, which are the smaller, are placed on the ventral surface, close together,
about the middle of the body. In the males the bases of the pelvic fins are
united together in the middle line, and each has connected with it a clasper or
copulatory organ. The latter is a stiff rod, on the inner and dorsal aspect of
which is a groove leading forwards into a pouch-like depression in the base of
the fin.
The mouth-a transverse, somewhat crescentic opening-is situated on the
ventral surface of the head, near its anterior end. In front and behind it is
bounded by the upper and lower jaws, each bearing several rows of teeth with
sharp points directed backwards, and adapted for holding and wounding.
The teeth are replaced as they become severely worn or broken. The teeth
of elasmobranchs were formerly confidently homologised with those of other
vertebrates, but this is now called into question. In some elasmobranchs
accessory teeth are borne on the branchial cartilages. The nostrils are situated
one in front of each angle of the mouth, with which each is connected by a wide
groove-the naso-buccal groove. In Squalus the outer edge of the groove is pro-
longed into a narrow subcylindrical appendage-the barbel. A small rounded
aperture, the spiracle-placed just behind the eye-leads into the large pharynx.
In the spiracle is located a leaf-shaped gill-like organ, the pseudobranch (p. 205).
This may be an organ of special sense. It is not essentially respiratory in
function, since before the blood reaches the pseudobranch it is already oxy-
genated. There are five branchial slits on each side of the head behind the
spiracle (Fig. 155). They lead internally into the pharynx. A large median
cloacal aperture opens on the ventral surface at the root of the tail between the
pelvic fins. The cloaca is a chamber forming the common outlet for the intes-
tine and the renal and reproductive organs. A pair of small depressions, the
enigmatic abdominal pores, are situated behind the cloacal opening and lead
into narrow passages opening into the abdominal cavity.
 PHYLUM CHORDATA 231

The skeleton is composed entirely of cartilage (p. 73), with, in certain

places, depositions of calcareous salts. As in vertebrates in general we dis-
tinguish two sets of elements in the skeleton-the axial set and the appendicular.
The former comprises the skull and spinal column ; the latter the limbs and
their arches.
The spinal column is clearly divisible into two regions-those of the trunk
and tail. In the trunk-region each vertebra (Fig. 156, A and B) consists of a
centrum (c), neural arch (n. a.), and transverse processes (tr. pr.). In the
caudal region there are no transverse processes, but inferior or hcemal arches
(C., D., h. a.) take their place. The centra of all the vertebrre are deeply
biconcave or amphiccelous, having deep conical concavities on their anterior

FIG. 156.- Scyliorhintts:

Vertebral column. A and H,
from t h e trunk ; C and D , from
the middle of the t a il; A a nd
C, two vertebr;e in longitudinal
section ; Band D, single verte -
brre viewed from one end. b,
calcified portion of centrum;
c. centrum; for. foramen for
d orsal, and for' . for ventral root
of spinal nerve; h. a. h <emal
arch (basi-ventral) ; h. c. h<em al
canal ; h. sp . h;em alspine ; i.n.p. C/ er.;:a·•~'nPn..p
intercalary piece (int erdorsal,
or interneural plat e); n. a. / J, "·"/'
neural arch; n. c. neural canal ; n.a -,r,.· ' -, '> n .c
n. p. neural pla te (basi-dorsal) ; - nlc
n. sp. neural spine; ntc. inter-
v ertebral substance (remains of h
notoch ord); r. proximal portion
of rib ; tr. pr. tra nsverse process - h.c
(basal stump ). (After T. J. h.op
P arker.)

and posterior surfaces. Through the series of centra are found remnants of
the notochord (ntc.). These are constricted in the centrum itself. In the large
spaces formed by the apposition of the amphiccelous centra of adjoining
vertebrre, a residual pulpy mass occurs. The concave anterior and posterior
surfaces of the centra are covered by a dense calcified layer, and in Squalus
eight radiating lamellre of calcified tissue run longitudinally through the sub-
stance of the centrum itself. The centra, unlike those of the higher forms, are
developed as chondrifications of the sheath of the notochord into which cells of
the skeletogenous layer have migrated. On the dorsal side of the row of centra
the spinal column is represented by the series of neural arches which support
the walls of the spinal canal (n. c.). Owing to the presence of a series of
intercalary cartilages the neural arches appear to be twice as numerous as the
centra. Each neural arch consists on each side of a process, the neural process
(given off from the centrum), and of a small cartilage, the neural plate (basi-
dorsal), which becomes completely fused with the neural process in the adult.
232 ZOOLOGY
Between successive neural plates (the width of each of which is only about half
the length of the centrum) is interposed a series of plates of very similar shape,
the interdorsal or interneural plates (inp.) . Small median cartilages, the neural
spines, fit in between both neural and interneural plates of opposite sides and
form keystones completing the arches.
The transverse processes are very short. Connected with each of them is a
rudimentary cartilaginous rib (r.) about half an inch in length.
The cranium (Fig. 157) is a cartilaginous case, the wall of which is con-
tinuous throughout, and not composed, like the skulls of higher vertebrates, of

b.h_y
FIG. 157--Brachrelurus: Skull, visceral arches, and anterior part of spinal column. The bran.
chial rays are not represented. The skull and hyoid arch are somewhat drawn downwards, so
that the hyoid and first branchial arch are not exactly in their natural relations . cer. hy. cerato.
hyal; ep . br. I. ep. br. s. first and fifth epibranchials; gl. aperture for glossopharyngeal nerve ;
b. hy. basihyal; hy. mn. hyomandibular; interc. intercalary (interdorsal) plates; m ck. Meckel's
cartilage; neur. neura l processes ; olf. olfa ctory capsule; oc. foram en for oculomotor; oph. I,
foramen for ophthalmic division of facial nerve ; oph. 2, foramen for ophthalmic division of
trigeminal; opt. optic foramen ; pal. q. palatoquadrate; path. foramen for 4th nerve; ph. br. I and
ph. br. s. first and fifth pharyngobranchial; sp. neural spines; tr. transverse processes and ribs;
tri. foramen for trigeminal nerve.

a number of distinct bony elements fitting in together. At the anterior end is

a rostrum, consisting in Scyliorhinus of three cartilaginous rods converging as
they extend forwards and lateral ones meeting anteriorly. At the sides of the
base of this are the olfactory capsules (olf.). These are thin rounded cartilaginous
sacs opening widely below, the cavities of the two capsules being separated
from one another by a thin septum. The part of the roof of the cranial cavity
behind and between the olfactory capsules is formed not of cartilage, but of a
tough fibrous membrane, and the space thus filled in is termed the anterior
fontanelle . In contact with the lower surface of the membrane is the pineal
body (p. 128). Each side-wall of this part of the skull presents a deep concavity
-the orbit-over which is a ridge-like prominence, the supraorbital crest.
This terminates anteriorly and posteriorly in obscure processes termed re-
 PHYLUM CHORDATA 233
spectively the preorbital and postorbital processes. Below the orbit is a longi-
tudinal infraorbital ridge.
Behind the orbit is the otic region in which the parts of the membranous
labyrinth of the internal ear are embedded. On the upper surface of this
posterior portion of the skull are two small apertures situated in a mesial
depression. These are the openings of the endolymphatic ducts, leading into
the vestibule of the membranous labyrinth. Behind this again is the oc-
cipital region, forming the posterior boundary of the cranial cavity, and
having in the middle a large rounded aperture-the foramen magnum. Through
this the spinal cord, contained in the neural canal and protected by the neural
arches of the vertebrre, becomes continuous with the brain. Below this, on
either side, is an articular surface-the occtpital condyle-for articulation with
the spinal column. Between the two condyles is a concavity, like that of the
vertebral centra, containing notochordal tissue.
A number of smaller apertures, or foramina, chiefly for the passage of nerves,
perforate the wall of the skull. Behind and to the outer side of the anterior
fontanelle are apertures (oph. 2, I) through which the ophthalmic branches of
the fifth and seventh nerves leave the skull. Piercing the inner wall of the
orbit are foramina through which the optic or second pair of cerebral nerves
(opt.), the oculomotor or third, (oc.), the trochlear, or fourth (path.), the trige-
minal, or fifth (tri.), the abducens, or sixth, and the facial, or seventh, leave the
interior of the cranial cavity. Just behind the auditory region is the foramen
for the glossopharyngeal (gl.). In the posterior wall of the skull, near the
foramen magnum, is the foramen for the vagus.
In close connection with the cranium are a number of cartilages composing
the visceral arches (Figs. 157 and 158). These are incomplete hoops of cartilage,
mostly segmented, which lie in the sides and floor of the mouth-cavity or
pharynx. The first of these forms the upper and lower jaws. The upper jaw,
or palatoqttadrate (pal. q.), consists of two stout rods of cartilage firmly bound
together in the middle line and bearing the upper series of teeth. The lower
jaw, or Meckel's cartilage (mck.), likewise consists of two stout tooth-bearing
cartilaginous rods firmly united together in the middle line. This kind of
union is a symphysis. At their outer ends the upper and lower jaws articulate
with one another by a movable joint. In front the upper jaw is connected by a
ligament with the base of the skull.
Immediately behind the lower jaw is the hyoid arch. This consists of two
cartilages on each side, and a mesial one below. The uppermost cartilage is
the hyomandibttlar (hy. mn.). This articulates by its proximal end with a
distinct articular facet on the auditory region of the skull. Distally it is con-
nected by ligamentous fibres with the outer ends of the palatoquadrate and
Meckel's cartilage. The lower lateral cartilage is the ceratohyal (cer. hy.). Both
the hyomandibular and ceratohyal bear a number of slender cartilaginous rods
234 ZOOLOGY
-the branchial rays of the hyoid arch. The mesial element, or basihyal (b. hy.),
lies in the floor of the pharynx. Behind the hyoid arch follow the branchial
arches, which are five in number. Each branchial arch, with exceptions to
be presently noted, consists of four cartilages. The uppermost of these-
pharyngobranchial (ph. br. I-ph. br. 5)-lie in the dorsal wall of the pharynx,
not far from the spinal column. The pharyngobranchials of the last two arches
are fused together. The next in order-the epibranchials (ep. br.)-with the
exception of those of the last arch, bear a number of slender cartilaginous rods-

FIG. I58.-Bracllrelurus: Visceral arches. Ventral view. Letters as in preceding figure.

In addition-b. br. basibranchial plate; cer. br. ceratobranchials; hyp. br. hypobranchials.

the branchial rays-which support the walls of the gill-sacs. The next-the
ceratobranchials (cer. br.)-are, with the same exception, similarly provided.
The hypobranchials (hyp. br.), which succeed these, are absent in the case of the
first and fifth arches. In the middle line on the floor of the pharyngeal cavity
is a mesial cartilage-the basibranchial (Fig. 158, b.br.)-which is connected
with the ventral ends of the third, fourth, and fifth arches. A series of slender
curved rods-the extrabranchials-lie superficial to the branchial arches, along
the borders of the corresponding external branchial clefts. The branchial arches
not only support the gills; their jointed flexible structure also allows muscular
pharyngeal movements which maintain the respiratory current of oxygenated
 PHYLUM CHORDATA 235
sea-water. The pharyngeal walls expand, and the floor of the mouth is de-
pressed. This causes water to flow into the buccal cavity. The mouth is now
closed and the floor raised. This drives the imprisoned water out through the
gill-slits. As it passes over the extensive capillary surface carbon dioxide
is liberated and oxygen is taken up by the hamtoglobin carried in the red blood
cells (pp. 70, 343).
The above movements are carried out by the lateral plate and myotomal
hypoglossal muscles, the first innervated by vagus and glossopharyngeal nerves
and the second by the hypoglossal (p. 130).
Two pairs of delicate labial
cartilages are present at the sides
of the mouth, and a couple at the
margins of the openings of the
olfactory capsules.
The skeleton of all the fins-
paired and unpaired-presents a
considerable degree of uniformity.
The main part of the expanse of
the fin is supported by a series of
flattened segmented rods, the
pterygiophores or cartilaginous fin-
rays, which lie in close apposition.
In the case of the dorsal fins these
may be partly calcified. At the
outer ends of these are one or more
rows of polygonal plates of carti-
lage. On each side of the rays
FJ G . 159--- BJ"achrelurus: Pectoral arch and fin.
and polygonal cartilages are a d. r. dermal horny rays; meso. mesopterygium; meta.
number of slender 'horny' rays or metapterygium; pect. pectoral arch; pro. proptery-
gium.
ceratotrichia of dermal origin. On
account of their appearance and horn-like consistency, these structures are
commonly referred to as horny. They do not, however, consist of true horn,
which is always epidermal in origin. They are composed of elastin, charac-
t eristic of elastic connective-tissue fibres.
In the smaller median fins there may be an elongated rod of cartilage con-
stituting the skeleton, or cartilage may be entirely absent. In the pectoral
fin (Fig. 159) the fin-rays are supported on three basal cartilages articulating
with the pectoral arch. The latter (pect.) is a strong hoop of cartilage incom-
plete dorsally, situated immediately behind the last of the branchial arches.
It consists of a dorsal, or scapular, and a ventral, or coracoid portion, the cora-
coid portions of opposite sides being completely continuous across the middle
line, while the scapulas are separated by a wide gap in which the spinal column
 ZOOLOGY
lies. Between the two portions are the three articular surfaces for the three
basal cartilages. The coracoid portions are produced forwards in the middle
line into a flattened process supporting the floor of the pericardia! cavity in
which the heart is lodged. The three basal cartilages of the fin are named,
respectively, the anterior, propterygium (pro.), the middle, mesopterygium
(meso.), and the posterior, metapterygium (meta.). Of these the first is the
smallest and the last the largest. The first bears only one large ray; the other
two bear twelve or more rays, differently arranged in the two genera.
The pelvic fin (Fig. 160) has only a single basal cartilage (meta.) articulating
with the pelvic arch, with which also one or two of the fin-rays articulate
directly. The pelvic arch (pelv.) is a nearly straight bar of cartilage which
runs transversely across the ventral surface of
pdv
the body, just in front of the cloacal opening.
Brief mention may be made of locomotion
in elasmobranch fishes. The dogfish swims
largely by means of the contraction of the
muscle-fibres of its myotomes. Forward
motion is carried out by alternate rhythmic
waves of myotomal contraction passing
posteriorly from the head and transverse
thrusts of the tail. The first process depends
partly on the presence of the vertebral column,
which forms a resistant central axis. As Gray
has shown, the muscle contractions undulate
FIG. r6o.-Brachrelurus: Pelvic the streamlined body and thrust it ahead
arch and pelvic fin. m eta. meta-
pterygium ; p elu. p elvic a rch. by transmitting a backward momentum to the
surrounding water and pushing this back in a
manner not essentially different from the operation of the man-made propeller
(see also p. 86) . The dogfish cannot move backwards. Lampreys (p. 176)
and eels (p. 296) are able to do so because they can send contraction waves
anteriorly. Most bony fishes can swim smoothly in reverse by fin-motion. In
these, the evolution of the swim-bladder (p. 339), a hydrostatic organ, opened
up all kinds of locomotory possibilities.
In the dogfish and in sharks generally the fins (including the caudal) are
essentially stabilising and steering organs. Lacking a swim-bladder and lungs,
cartilaginous fishes are heavier than sea-water. The specific gravity of a dogfish
is about 1-06 compared with the 1·03 of sea-water, and so it must exert a lift
with pectoral fins and the lower lobe of its tail in order to stay afloat. It
is of great interest that all fishes that have developed an air-bladder or lungs
seem to have at earlier stages of evolution possessed a shark-like tail, having
only later developed the outwardly symmetrical caudal fin of familiar 'fish-
tail' form (Figs. 204, 302).
 PHYLUM CHORDATA 237
Alimentary Canal and Associated Structures (Fig. 161). The mouth leads
into a very wide cavity, the pharynx, into which open the lateral aspects of
the internal apertures of the branchial clefts and the spiracle. From this runs
backwards a short, wide, muscular-walled tube-the resophagus (~s.)-which
passes behind into the stomach. It will be recalled that no true stomach occurs
in the Agnatha (p. 164). Among the Gnathostomata it is of almost universal
occurrence. It exhibits great variation, however, and has been secondarily
lost, in so far as histological structure is concerned, in certain highly specialised
teleosts (p. 293), the Dipnoi (p. 361) and the Holocephali (p. 262). In the dog-
fish it is of a peculiar U-shape with a long left limb continuous with the reso-
phagus, and a short right one passing into the intestine.
The stomach is a capacious, muscular-walled, epithelium-lined bag in which
the remains of fish, crabs, and other prey may be found. The acid secretion of
the stomach prevents bacterial decomposition. The action of the powerful
stomach-muscles helps to disintegrate the solid particles, thus providing a
greater area for attack by proteolytic pepsin or a similar enzyme.
At the pylort~s (pyl.)-the point where the stomach joins the intestine-
is a slight constriction, followed by a thickening, the pyloric sphincter. This
circular muscle ensures the retention of food in the stomach until stimuli
cause it to relax and allow the particles to move through into the small intestine.
The small intestine is exceedingly short, measuring only an inch or two
long. There next follows the large intestine, which is longer, much wider, and
divisible into an anterior colon (col.) and a narrow rectttm which terminates at the
cloaca. Two prominent glands pour their secretions into the small intestine
-the liver (liv.) and the pancreas (pancr.). The liver is a large bi-]obed gland.
At the anterior end of the left lobe is embedded a rounded sac, the gall-bladder
(g. bl.). Ducts connect each lobe of the liver with the gall-bladder and carry
to it the bile or gall, which is subsequently passed down the bile-duct (b. dct.) into
the small intestine near the beginning of the colon.
The pancreas is a pale, compressed gland consisting of two main lobes
with a broad connecting isthmus. It lies in the angle between the right-hand
limb of the stomach and the small intestine. Its duct, the pancreatic duct,
carries digestive enzymes elaborated by the gland and enters the wall of the
intestine, runs in it for about half an inch, and eventually opens at a point near
the beginning of the colon.
Little is known of the digestive processes of the elasmobranch fishes. No
doubt the bile acts primarily as an emulsifying agent upon ingested fats,
separating them into minute droplets more readily accessible to the action of
lipases. From the pancreas, and possibly also from the gut-wall, come other
enzymes which hydrolyse carbohydrates and proteins. Absorption takes place
largely in the colonic region, which is equipped with an anterior spiral valve.
This begins at the end of the duodenum ; it is a richly glandular fold of tissue
 !-.)
w
(XI

N
0
0
1:""
0
C"l
><!

lpl

FIG. I6I.-Braclw~lurus: Visceralstmcture. The left oviduct, kidney, and the stomach have been removed; the cloaca has been
opened. The head is represented in median vertical section. The chambers of t he heart have been opened. an. opening of rectum
into cloaca; aur. atrium; barb. barbel; b. br. basi-branchial plate; b. del. bile.duct; br. cl. internal branchial clefts; clo. cloaca;
ccel. art. creliac artery (cut short); col. colon; con. conus arteriosus; con. v. connecting vein between the genital (ovarian) sinus and
the hepatic portal system; c. ovd. common crelomic aperture of oviducts; g. bl. gall-bladder ; gen. sin. genital sinus; hy . basi-
branchial; inter. intervertebral substance; liv. liver; mck . Meckel's cartilage; nas. nasal cartilage; ces. resophagus; ov. ovary;
pal. palato-quadrate; pancr. pancreas; pect. pectoral arch; pnl. stalk of pineal body; port. v. hepatic portal vein ; pyl. py lorus;
pyl. st. pyloric portion of stomach; rect. rectum; 1·ect. gl. rectal gland; r. meson., r. metan. anterior and posterior portions o f kidney;
r. ov. ap. aperture of right oviduct into cloaca; r. sh. gl. right shell-gland; r. ur. sin. right urinary sinus; sin . sinus v enosus; sp. co.
spinal cord; sp. co' . anterior part of spinal cord in media n vertical section; spir. spiracle ; spl. spleen; sp. v. spira l valve; thyr.
thyroid; ur. p. urinary papilla; vent. ao. ventral aorta; vert. c entra of vertebra-.
 PHYLUM CHORDATA 239

which runs spirally around the intestine and both retards the too rapid passage
of food and affords a more extensive area for absorption of digested sub-
stances. The intestinal wall is invested with a ramification of capillaries which
unite to form larger vessels which communicate with others from elsewhere
(p. II2} as the hepatic portal vein and carry absorbed substances to the liver.
There is little doubt that this gland is the site of manifold activities as in higher
craniates. Apart from its role in the utilisation and transformation of absorbed
substances, it probably also destroys effete red blood cells since Kupffer cells
have been demonstrated in the liver of dogfish.
In addition to its exocrine activity, the pancreas has a second and endocrine
function. In the pancreatic tissue (cf. Petromyzon, p. 185) islets of Langerhans
occur in close association with blood capillaries, which carry away a hormone
which probably has many of the same actions as insulin in higher vertebrates.
There is some evidence that the islet-tissue is derived from intra-lobular ducts.
Thus we may have another (see also p. 148) example of an older structure
being converted to endocrine function.
The third gland opening into the alimentary tract, the rectal or digitiform
gland, is small yet prominent. It is only about three-quarters of an inch long,
and is of enigmatic function. It would seem, however, that it is by no means
inactive. The gland has a central space lined with layers of cuboidal cells.
Other cells of apparently secretory activity have what may be collecting ducts
in their vicinity, and these ducts unite to form the short principal duct that
empties into the rectum. At the same time the gland is highly vascular and
conspicuously lymphoid. It is absent in actinopterygians, but appears to be
present in the ccelacanth Latimeria (p. 360).
Morphologically associated with the alimentary canal
is the spleen (spl.), a dark red body attached to the con-
vexity of the U-shaped stomach. It sends a narrow lobe
along the right-hand limb and is attached by means of a
gastrosplenic omentum (see p. 74). The spleen is part of
the blood-vascular system and in higher vertebrates has
blood storage and other less obvious functions (p. 273).
Respiration.-The organs of respiration are the gills,
situated in five gill-sacs. Each branchial sac (Fig. r62) is
FIG. !62 .-Braclrw-
an antero-posteriorly compressed cavity opening intern- lurus: Branchial sac.
(Exposed from the ex-
ally into the ph<3;rynx and externally by the correspond- terior.)
ing gill-slit. The walls of the pouches are supported by
the branchial and hyoid arches with their rays, the first pouch being situated
between the hyoid and first branchial arches, the last between the fourth and
fifth branchial arches. On the anterior and posterior walls of the pouches are
the gills, each hemibranch (p. ro6) consisting of a series of close-set parallel
folds of highly vascular membrane. Through this exceedingly thin membrane
240 ZOOLOGY
occur gaseous exchanges between the animal and the sea-water that is forced
by buccal and pharyngeal musculature into the mouth and out through the
gills (Fig. 73, p. ro6). Carbon dioxide is given off by the blood and oxygen
is taken up by its hcemoglobin. There-oxygenated blood meanwhile continues
its journey towards the dorsal aorta for re-distribution throughout the body
(see below).
Separating adjoining gill-pouches, and supporting the gills, is a series of
broad interbranchial septa, each containing the corresponding branchial arch
with its connected branchial rays. The most anterior hemibranch is borne
on the posterior surface of the hyoid arch. The last gill-pouch differs from the
rest in having gill-folds on its anterior wall only. On the anterior wall of the
spiracle is the pseudobranch (p. 320).
Blood-vascular System.-This follows the general vertebrate plan already
observed in the Agnatha. The heart is situated in the pericardia! cavity on
the ventral aspect of the body, in front of the pectoral arch, and between the

efferent
pseudobranchial

hypobranchial
plexus
FIG. 163.-lUustelus: Heart and branchial circulation. (Redrawn after various authors.)

two series of branchial pouches. The dorsal wall of the pericardia! cavity is
supported by the basibranchial cartilage. Placing it in communication with
the abdominal cavity is a canal-the pericardia-peritoneal canal. The heart (Fig.
r6 r) consists of four chambers-sinus venosus (sin.), atrium (aur.), ventricle (vent.),
and conus arteriosus (con.)-through which the blood passes in the order given.
The sinus venosus is a thin-walled, transverse, tubular chamber, into the end of
which the great veins open. It is here that the heart-beat originates (p. rog),
and in it is an extensive nerve plexus connected with a branch of the vagus.
Vagal stimulation slows the heart-rate. There is no 'antagonistic' accelerator
branch from the sympathetic system such as occurs in higher forms (seep. rrg).
The contractile sinus venosus communicates with the atrium by an
aperture, the sinu-atrial aperture. The atrium is a large, three-cornered,
 PHYLUM CHORDATA

thin-walled chamber. It is situated in front of the sinus venosus and dorsal

to the ventricle. Its apex is directed forwards, and its lateral angles project
at the sides of the ventricle; it communicates with the ventricle by a slit-like
aperture guarded by a two-lipped valve. Contraction of the atrium drives
blood into the ventricle. This is a thick-walled, globular chamber, forming
the most conspicuous part of the heart when looked at from the ventral surface.
The muscles of the ventricle contract and raise the blood pressure to fill the
conus arteriosus, which runs forwards as a stout median tube to the anterior
end of the pericardia! cavity, where it gives off the ventral aorta. It contains
two transverse rows of valves, anterior and posterior, the former consisting of
three, the latter of three or four. The ventral aorta (Fig. 163) gives origin to a
series of paired afferent branchial arteries, which carry deoxygenated blood
to the branchial pouches. In Scyliorhinus the two most posterior arise close
together near the beginning of the ventral aorta, the third pair a little further
forwards. The ventral aorta then runs forwards and bifurcates to form the left
and right common stem of the first and second afferent arteries, each of which in
turn bifurcates to form the first and second afferent vessels of its side. In
Squalus (Fig. 164 A, B) the arrangement is somewhat more primitive.
From the gills oxygenated blood passes by means of the efferent branchial
arteries. These efferent vessels (Fig. 163) form a series of loops, one running
around the margin of each of the first four internal branchial clefts. A
single vessel runs along the anterior border of the fifth branchial cleft and
opens into the fourth loop. The four main efferent branchial vessels (epi-
branchials) run inwards and backwards from the loops above the mucous
membrane investing the roof of the pharynx to unite in a large median
trunk-the dorsal aorta (d. ao.). From this thick-walled artery the blood is
redistributed to tissues all over the body. A dorsal carotid artery is given off
from the first efferent branchial. A branch (hyoidean) given off from the
same efferent vessel supplies the pseudobranch (p. 230) and is then taken up by
the ventral carotid. Both carotids run forwards to supply the head.
The large dorsal aorta (Fig. 163) runs backwards throughout the length of
the body-cavity, giving off numerous branches, and is continued as the caudal
artery, which runs in the canal enclosed by the inferior arches of the caudal
vertebrre. The first pair of branches are the subclavians, for the supply of the
pectoral fins. These are given off from between the third and fourth pairs of
epibranchial arteries. The next branch is the unpaired cceliac (Fig. r6r, ccel.).
This runs in the mesentery and divides into branches for the supply of the
stomach and liver, the first part of the intestine, and the pancreas. The
anterior mesenteric artery, also median, supplies the rest of the intestine and
gives off branches to the reproductive organs. The lienogastric supplies part
of the stomach, the spleen, and part of the pancreas. The posterior mesenteric
is a small vessel mainly supplying the rectal gland. Paired renal arteries carry
VOL. II. Q
 v.f.

la.v.

'
'
' c.m.
I.e."
V. I.

fem.v
cl.v.

lt.l!.
cd.v. m.an~.

A. l.c.v_ v.t.a .

FIG. 164.-Squalus: Circulation. A .

ventral; B , lateral view of a relatively
primitive genus with persistent larval
characters. a. ca. anterior cardinal
sinus; a. c. e. external carotid artery;
af, hyoidean afferent branchial artery ;
af 4, fourth afferent branchial artery;
a. h. s. superficial hyoid a rtery; a. m. v.
ventral mandibular artery ; a. s. sta-
pedial artery; a. t. anterior thyroid artery
(last three b ranch from the external
carotid); Atr. atrium; b. a . b rachial B.
artery ; b. v. brachial vein ; c. a. conus
arteriosus; cd. a. caudal a rtery ; cd. v. caudal vein; ce. a. anterior cerebral artery; cl. fifth gill
slit; cl. v. cloacal vein; c. m. cauda l mesonephros; cw. a. creliac artery; d. a. paired dorsal aorta;
e. a. anastomosis between efferent collector loops; e. c. efferent collector loop; ep. r- 4. first and
fourth epibranchial arteries; e. s. efferent pseudobranchial artery; fem. v. femoral ve·n; f. v.
anterior facial vein ; g. a. ga:>tric a rter y ; h. a. hepatic artery; h. e. hyoidean epibranchial artery:
h. p. hepat ic portal vein; h. s. hepatic sinus; hy. s. hyoidean sinus; i . c. internal carotid artery;
i.e. c. intracranial branch of the spiracular epibranchial artery; i.f inferior jugular sinus ; il. a.
iliac artery ; in. a. innominate artery ; i. p. intestine-pyloric artery ; i. v. interorbital vein; l. a.
lat era l artery ; Ia. v. latera l abdominal vein; l. c. v. superior lat era l cuta neous vein; li. a. lieno-
gastric artery; m. ant. ant erior mesenteric artery; m . post. post erior mesenteric a rtery ; o. m .
great ophthalmic artery; or. p. post-orbital sinus; or. s. orbital sinus ; p. c. posterior cardina l
sinus; p. ce. posterior cerebral vein; Pel. pelvic cartilage; p. s. afferent pseudobranchial artery ;
r . p. renal portal vein; reel. a. rectal artery; s. a. subclavia n artery; s. c. subclavian vein; s. m.
submental sinus; s. s. subscapular sinus; s. v. sinus venosus; t. a. ventral aorta; v. a. vertebral
artery; Vent. ventricle; v. f. thyroid vein; v. i . inter-renal portion of posterior cardina l sinus;
v. i. a. anterior intestinal vein; v. i . p. posterior intestinal vein; v. j. base of jugular vein; v. l. a.
ventro-lateral artery. (After O'Donoghue a nd Abbot.)
 PHYLUM CHORDATA 243
a small quantity of arterial blood to the kidneys, and a pair of iliac arteries,
likewise of small size, pass to the pelvic fins. In addition to these, a number
of small parietal arteries, which supply the wall of the body, are given off
throughout the length of the aorta.
The gills, as we have seen, stand between the heart and the dorsal aorta.
Because of their resistance, the blood pressure in the dorsal aorta is much
lower than in the ventral aorta. This is in great contrast to animals having a
double circulation where the pressure lost in the passage through the respira-
tory capillaries is made up again when the blood returns to the left ventricle.
In such animals distribution from the dorsal aorta is always at high pressure.
Venous return is probably partly assisted by the large and widespread blood
sinuses that characterise the venous system of elasmobranchs. The veins
have very thin walls, but the larger trunks are remarkable for sinusoidal
dilatations, which are, of course, true vessels, and not to be confused with the
blood sinuses found in invertebrates. Little knowledge concerning their
precise mode of action is available but, as resistance is lowered by the widening
of a vessel, it would seem that the sinuses may be adaptations that, along with
the thin-walled sinus venosus, help to ensure that a supply of blood is constantly
available to the muscular chambers of the heart. Little is known about
the possible auxiliary action of somatic muscles of the kind that help squeeze
the blood towards the heart in higher groups (see p. 78). There is evidence
that a suction mechanism, made possible by the box-like cartilaginous frame-
work (lined by the pericardium), considerably assists venous return. This
structure is made up above by the basibranchial plate and below by the
pectoral girdle. A pericardia-peritoneal canal penetrates from pericardium
into the crelom of the abdomen, where it narrows markedly. This canal may
act as a valve that helps maintain a negative venous pressure by means of a
one-way flow of fluid from pericardium to crelom. Certainly when the atrium
and ventricle powerfully contract, the pressure in the pericardium falls sharply
and a compensatory flow of venous blood pours into the thin-walled sinus
venosus in much the same way as the mammalian lung is filled with air.
An outline of the veins in Scyliorhinus is as follows. The blood comes
back from the head region by a pair of jugular or anterior cardinal sinuses,
and from the trunk by a pair of posterior cardinal sinuses. At the level
of the sinus venosus the anterior and posterior cardinals of each side unite
to form a short, nearly transverse sinus, the precaval sinus or ductus Cttvieri,
which is continued into the lateral extremity of the sinus venosus. Into the
precaval sinus, about its middle, opens an inferior jugular sinus which
brings back the blood from the floor of the mouth and from the ventral
surface of the branchial region. The twQ posterior cardinal sinuses extend
backwards throughout the length of the body-cavity. In front these are
enormously dilated ; behind they lie between the kidneys. Anteriorly each
VOL. II. Q*
244 ZOOLOGY

receives the corresponding subclavian vein bringing the blood from the pectoral
fin and adjacent parts of the body-wall. The lateral vein (la. v.), instead of
joining with the subclavian, opens separately into the precaval. The genital
sinus discharges into the posterior cardinal sinus.
A B
VH .};.ol
~
....

TF•D

Cp

IY

.llH

p,..Jto
mi
.NlT

Sv
FIG. I6j .- Scylior·llinus: Brain. A, dorsa l view ; B, ventral view ; C, lateral view. F. rlto.
fossa rhomboida lis (fourth vent ricle): Gp, epiphysis; H H, cerebellum ; HS. H, hypophysis ;
L. o/. olfactory b ulb ; AIH, mid-bra in; NH, m edulla oblongata; S v, saccus vasculosus; T ro,
olfactory p eduncle; Ul., lobi inferiores; VH, prosencepha lon ; Z H , d iencephalon ; II, optic
nerves; Ill, oculomotor; IV, trochlea r; V , trigemina l ; VI, abducens; VII, facia l; VI II,
au d itory; I X , glossopharyngea l ; X, vagus. (After \Viedersheim.)

There are two portal systems of veins, t he renal and the hepatic portal,
by which the kidneys and liver, respectively, are supplied with venous
blood. The caudal vein, which brings back the blood from the tail, runs,
along with the caudal artery, through the inferior arches of the vertebra:.
 PHYLUM CHORDATA 245
It divides on entering the abdominal cavity into right and left renal portal
veins. These end in a number of afferent renal veins supplying the kidneys.
The hepatic portal vein (h. p.) is formed by the confluence of veins derived
from the intestine, stomach, pancreas, and spleen, and carries blood and
digested food-substances forwards to enter the liver to the right of the mid-line.
The above does not apply to all species of dogfish (which fall into two sub-
orders). Differences in Squalus, a less advanced form retaining certain larval
characters in its blood vascular system, can be seen in Fig. 164.
Nervous System.-In the elasmobranch fishes the brain is large and highly
organised and shows a considerable advance on that of the Cyclostomata.
Anteriorly it consists of a rounded smooth telencephalon: both roof and floor
are considerably thickened and meet internally in the mid-line, dividing the
original median ventricle of the end brain into paired ventricles. It is probable
that the end-brain in these animals serves principally as an apparatus for ana-
lysing olfactory impulses. From its antero-lateral region each hemisphere gives
off a thick cord, which dilates into large masses, the paired olfactory bulbs (L. ol.),
each closely applied to the posterior surface of the corresponding olfactory
organ. This structural arrangement is not inconsistent with the numerous
accounts of naturalists and fishermen claiming high olfactory powers for
selachians (p. 275).
The diencephalon (ZH) has a very thin roof. Its lateral walls are composed
of two thickish masses, the thalami. Attached to the roof is a slender tube, the
epiphysis cerebri or pineal organ (Gp.), which runs forwards and terminates in a
slightly dilated extremity fixed to the membranous part of the roof of the skull.
The pineal body does not appear to be secretory, nor does it retain any structure
suggesting visual function (cf. Petromyzon, p. r8g; Minnow, p. 345). The
hypothalamus in the floor of the diencephalon is well-developed and may, as in
'higher ' vertebrates, be concerned with the regulation of various uncondi-
tioned reflexes and visceral functions. Two rounded bodies-the lobi inferiores
-are prominent. These are dilated portions of the infundibulum. Behind,
these give off a thin-walled, pigmented, vascular outgrowth-the saccus
vasculosus (Sv.). We will see that this peculiar characteristic organ is highly
developed in the bony fishes as well. It may be a pressure receptor centre.
Attached to the infundibulum and extending backwards from it is a thin-
walled sac-the pituitary body or hypophysis (HS.H). In the pituitary, anterior,
intermediate, tuberal, and neural (or posterior) 'lobes' occur. Probably a
chromatophore-expanding hormone and gonadotrophic hormones occur. The
thyroid gland (p. rsr) is said not to be markedly influenced by hypophysectomy
(cf. teleost fishes, p. 346).
In front of the infundibulum, and also on the lower surface of the dien-
cephalon, is the optic chiasma, formed by the decussation of the fibres of the
two optic tracts.
 ZOOLOGY
The mid-brain (MH) is large. It consists dorsally of a pair of oval optic
lobes which form the tectum. These receive the optic tracts and, in addition,
fibres which transmit a variety of afferent impulses unconnected with optical
stimulation. There is, too, experimental evidence that efferent fibres from the
mid-brain influence spinal centres. The floor of the mid-brain is almost con-
tinuous with the ventral surfaces of hind-brain and spinal cord.
The cerebellum (HH) is large and elongated in the antero-posterior direction.
Its anterior portion overlaps the optic lobes, its posterior, the medulla ob-
longata. Its surface is marked with a few fine
grooves. Into it lead sensory fibres from the ear
r"- and others from the facial, glossopharyngeal, and
vagus nerves which carry afferent impulses from the
lateral line system. The main part of the cerebellum
(the median b01ily) is disproportionately large in the
biggest sharks and rays. The medulla oblongata
(NH), broad in front, narrows posteriorly to pass
into the spinal cord. The fourth ventricle (F. rho.) is
a shallow cavity in the medulla oblongata covered
dorsally only by a thin vascular membrane, the
choroid plexus: it is wide in front and gradually
narrows posteriorly. At the sides of the anterior
part of the fourth ventricle are a pair of folded ear-
FIG. I 66.-Hemiseyl•
shaped lobes, the auricular lobes of the cerebellum.
lium: Relationship of brain These correspond with the flocculi of more advanced
ventricles. cer. dilatation from
which the epiccele is given off; vertebrates. The medulla gives off a series of cranial
dia. diacrele (third ventricle) nerves (p. 133). Of particular interest are those
pointing to the opening leading
into the infundibulum; iter. which are concerned with the control of branchial
iter or mesoccele; meta. meta-
ccele (fourth ventricle); opt. respiration and cardio-vascular activities. We have
optoccele; para. paracrele; seen that in air-breathing vertebrates an important
pros. prosocrele ; rh. rhinocrele.
part of the mechanism for respiratory control is still
located in the medulla in spite of the fact that the mode of respiration is
radically different.
The fourth ventricle (Fig. 166, meta.) is continuous behind with the central
canal of the spinal cord. It extends into the cerebellum dorsally, and in front
is continuous with a narrow passage, the aqueduct of the mid-brain, which
opens anteriorly into a wider space, third ventricle (dia.) occupying the interior
of the diencephalon. The third ventricle extends into the telencephalon and
is prolonged into the cerebral hemispheres as the lateral ventricles on each
side.
Cranial Nerves. From the anterior enlargements of the olfactory bulbs
already mentioned spring numerous fibres which constitute the first pair of
cranial nerves (olfactorius) and enter the olfactory capsules. Between the
 PHYLUM CHORDATA 24'7

two olfactory lobes two small nerves, the terminal or pre-olfactory, arise from
the telencephalon. These connect with the interior of the olfactory sacs.
From the optic chiasma two tracts (see p. 130) (Figs. 92, p. 143; and 167)
run outwards through the optic foramina into the orbits, each perforating the
sclerotic of the corresponding eye and terminating in the retina. The third
(oculomotorius), fourth (trochlear), and sixth (abducens) pairs of nerves have
the general origin and distribution which have already been described as uni-
versal in the Craniata (p. 130).
The fifth nerve (trigeminal) (p. 131, Figs. 165, 167, 168) arises in close
relation to the seventh (JaciaUs). As it passes into the orbit it swells into a
ganglion-the Gasserian. It has three chief branches. The first given off is the

FIG. 167.-Scyliorhinus: Brain

and spinal nerves. From the dorsal
surface with the right eye removed.
The cut surfaces of the cartilaginous
skull and spinal column are dotted.
cl.r-cl.s, branchial clefts; ep.
epiphysis; ext. rect. posterior rectus
muscle of the eye·ball; gl. ph. glos-
sopharyngeal; hor. can. horizontal
semicircular canal; hy. mnd. V II .
hyomandibular portion of the fa cial ;
inf. obl. inferior oblique muscle; in!.
rect. a nterior rectus muscle; lat. vag.
lateral branch of vagus; mnd. V,
mx. V. mandibular and maxillary
divisions of the trigeminal; olf. cps.
olfactory capsule; olf. s. olfactory
sac; oph. V. Vll. superficial ophthal-
mic branches of trigeminal and facial ;
path. fourth nerve; pl. V ll. palatine
branch of facial ; sp. co. spinal cord;
sp. and spir. spiracle; s. reel. superior
rectus muscle; s. obl. superior ob-
lique; vag. vagus; vest. vestibule.
(After Marshall and Hurst.)

superficial ophthalmic (Fig. 167, oph. V; Fig. 168), a nerve which runs forwards
through the orbit above the origin of the recti muscles, and in very close relation
with the ophthalmic branch of the facial. Anteriorly it breaks up into branches
distributed to the integument of the dorsal surface of the snout.l The main
trunk of the nerve then runs forwards and outwards across the floor of the orbit,
and divides into two branches, the maxillary and mandibular, or second and third
divisions of the trigeminal. The former supplies the skin of the ventral surface
of the snout, the latter the skin and muscles of the lower jaw.
1 In most Chondrichthyes a nerve of considerable size- the ophthalmicus profundus (p. 131)
- arises from the dorsal and anterior part of the Gasserian ganglion, and is usually regarded as a
branch of the trigeminal. It runs forwards over the posterior rectus muscle and under the
superior rectus, and perforates the pre-orbital process to end in the integument of the snout.
Among other branches it gives off ciliary branches to the eyeball; these are joined by the ciliary
branches of the oculomotor. The ophthalmicus profundus is not present in Scyliorhinus in the
adult condition.
 ZOOLOGY
Of the branches of the seventh (facial), the ophthalm-ic runs through the
orbit in close relation to the superficial ophthalmic branch of the trigeminal,
and is distributed to the lateralline and ampullary canals of the snout (see p. 25 r).
The buccal runs forwards in intimate relation with the maxillary division of the
trigeminal, and breaks up into branches which are distributed to the sensory
canals and ampullre of the region of the snout. The palatine passes to the roof
of the mouth. The main body of the nerve-hyomand-ibular-then runs out-
wards close to the edge of the hyomandibular cartilage and behind the spiracle,
eventually becoming distributed to the muscles between the spiracle and the
first branchial cleft. A small external mandibular branch comes off from it

rhm
roL_~a~f~f__r~s_l~~------~a_l_-7----~~---

imd emd pr pt ph
FIG. 168.-Selachii: Components of dorsal root cranial nerves. The ventral root nerves
(somatic motor component) to eye muscles and hypoglossal muscles are omitted. The post-
trematic branches of branchial n erves (as well as the hyomandibular and mandibular branches of
the Vllth and Vth nerves respectively) include a visceral motor component absent from pre-
trematic branches. These nerves supply visceral muscles which constrict t he arch in question.
The jaws and other arches are opened or expanded by the contraction of somatic muscles inner-
vated by ventral branches of the anterior spinal nerves which are collected together to form a
hypoglossal nerve emanating from the ventral aspect of the medulla. (Sec also Fig. 8o, p. 129.)
al. accessory line: an. auditory; emd. external mandibular; imd. internal mandibular; md. mandi-
bular; mx. maxillary; osf. superior ophthalmic o f facial ; ost. superior ophthalmic of trigeminal;
ph. pharyngeal; pl. palatine; pr. pretrematic; pt. post-trematic; ra. dorsal ramus; rhm. hyo-
mandibular; ro. ra mus oticus; rst. ramus supra-temporalis; spn. dorsal root of spinal nerve; vsd.
vestigal dorsal root. Compo nents : black line = lateral line; broken line = gen eral cutaneous;
beaded line= visceral motor; cross-hatched line = visceral sensory. (Redrawn after Goodrich.)

and goes to the lateral line and ampullary canals of the lower jaw. The eighth
(acousticus or aud-itory) nerve passes directly into the internal ear, and breaks
up into branches for the supply of its various parts. The ninth (glossopharyn-
geal) perforates the posterior part of the auditory region of the skull, and, after
it emerges, passes directly to the first branchial cleft, where it bifurcates. One
branch now goes to the anterior, and the other to the posterior, wall of the cleft.
The tenth and last nerve of the series-the vagus or pneumogastr-ic-is a large
nerve which emerges from the skull by an aperture situated between the
auditory region and the foramen magnum. It first gives off a series of four
branchial branches, each of which bifurcates to supply the anterior and posterior
 PHYLUM CHORDATA 249
borders of the last four branchial clefts. The lateralis nerve is frequently
referred to as a branch of the vagus, but it has a distinct origin in the medulla.
After becoming separated from the vagus trunk it runs in the horizontal
septum opposite the lateral line, which it supplies, to the posterior end of
the body. The rest of the vagus runs backwards to divide into cardiac branches
for the heart and gastric branches for the stomach. It will be observed that the
system of neuromast organs (lateral line and ampullary organs) is supplied by
nerve-fibres which pass out in various branches of the facial and in the lateralis
branch of the vagus. All these fibres originate in a centre in the medulla, the
acousticolateral centre, common to them and the fibres of the auditory nerve.
Spinal Cord. This is a cylindrical cord which extends from the foramen
magnum, where it is continuous with the medulla oblongata, backwards
throughout the length of the neural canal, enclosed by the neural arches of the
vertebrre. As in the Craniata in general (seep. 124), it has dorsal and ventral
longitudinal fissures and a narrow central canal, and gives origin to a large
number of paired spinal nerves, each arising from it by two roots.
Autonomic Nervous System. The sympathetic chain of selachians, unlike
that of the teleosts, and in fact uniquely among vertebrates, does not extend
into the head (cf. Fig. 82, p. ng). Otherwise it occurs in the typical gnatho-
stome pattern as a series of paired lateral ganglia approximately segmentally
arranged, extending from about the level of the sinus venosus posteriorly over
the kidneys to a little past the cloaca (Fig. 16g). The prominent sympathetic
ganglia, closely associated with adrenalin-chromaffin tissue (p. 152), contain
motor nerve-cells (postganglionic) which send fibres to plain muscle (p. 76)
variously investing the alimentary canal and other visceral organs and arterial
walls. These motor-cells are in turn controlled via preganglionic fibres which
originate in cells in the cord and emerge in the ventral spinal roots and pass to
the rami communicantes.
Many of the fibres in the sympathetic chain are sensory. Postganglionic
fibres, such as run in higher vertebrates (e.g. p. ng) from the sympathetic
ganglia back to the spinal nerves to terminate in the skin, do not occur. Young
found no evidence of the sympathetic control of chromatophores or other
skin functions.
Although the vfl.gus (Fig. 168) is massive and branched extensively to the
heart, alimentary canal, and elsewhere there has been as yet no clear recognition
of an antagonistic parasympathetic system (p. 121).
Organs of Special Sense.-The olfactory organs (Figs. 167; 8g, p. 138) are
rounded chambers enclosed by the cartilage of the olfactory capsules of the
skull, and opening to the exterior by the nostrils on the ventral surface of
the head. Water flows inwards and passes over chemoreceptors which lie in the
epithelium covering a series of close-set ridges running out from a median
septum.
zso ZOOLOGY
Taste-buds, too, occur in the pharyngeal region.
The eye (Fig. 184, p. 273) has the general structure already described as
characterising the Craniata in general (Figs. go, 276, pp. 139, 413). The retina
(except in Mustelus and the eagle-ray Myliobatis aquila) lacks cones: most
selachians are apparently colour-blind. The lens is supported primarily by a
dorsal thickening of a membrane that covers the ciliary body. Ventrally
there is situated a papilla containing smooth muscle-fibres, the contraction of
which moves the lens towards the cornea, achieving an accommodation (for
nearer vision) which may be enhanced by the action of the protractor lentis
muscle on the cornea itself. The primary apparatus of photo-adaptation in all
species is, of course, the iris, which can contract into a narrow oblique slit in
bright conditions.
The sclerotic is cartilaginous. The choroid has a silvery layer, the tapetum
lucidum, beneath the photosensitive rods of the retina. This, an adaptation to
life in dim light, reflects additional light on to the retinal cells (see alsop. 140).
The pigment epithelium is devoid of pigment cells in most species, but a few
specialised forms which lack the tapetum have them (e.g. the Basking Shark
Cetorhinus and the abyssal shark Lcemargus). There are the usual eye-muscles,
the two obliques situated anteriorly, the four recti posteriorly, not embracing
the optic nerve (Fig. 92, p. 143). The eyelids are represented by stiff folds.
The structures that make up the ear in vertebrates subserve three senses :
r. orientation (in regard to gravity) ; 2. acceleration (changes in speed and direc-
tion) ; and 3· hearing (the appreciation of air-, earth-, or water-borne
sound waves). In the dogfish the apparatus consists only of the membranous
labyrinth (Fig. 93, p. 144), equivalent to the internal ear of higher Craniata.
The middle and outer ear are absent. The membranous labyrinth consists of
a utricle and saccule (incompletely separated and forming a vestibule), and three
semicircular canals. The vestibule communicates by a narrow passage-the
endolymphatic duct-with the exterior, in the position already mentioned. Of
the three semicircular canals, the anterior and posterior are vertical and the
external horizontal, as in Craniata in general.
Each of the endolymph-containing semi-circular canals expands at one end
to form an ampulla. At the other end each opens into the vestibule into which
chamber the ampullre also open, establishing a continuity of the fluid system.
The actual receptors occur as sensory cristre in the ampullre with a neuro-
epithelium made up of neuromast cells which are equipped with hair-like
processes which are ensheathed in a gelatinous cupola terminalis. The de-
tailed working of the six canals of this system in the maintenance of dynamic
equilibrium is well discussed by Lowenstein (see references), but in general it
can be said here that the fluid-filled apparatus rapidly perceives rotational
changes and that each semi-circular canal is a bi-directional receptor.
In the endolymph of the utriculus and sacculus (and in the lagena of at least
 PHYLUM CHORDATA 251

some elasmobranchs) are suspended, in a mass of gelatinous tissue, numerous

minute calcareous otoliths, which give it a' milky' character. These are gravity
receptors of broadly the same function as those found in many invertebrate
phyla. In the elasmobranchs studied the widest range of reception is found
in the utriculus. It seems established, too, that all otolith organs respond not
only to gravitational stimuli but to all other linear accelerations as well. The
old functional concept which distinguished between dynamic receptors (canals)
and the static receptors (otolith organs) must be abandoned.
It will be seen above that no cochlea (p. 145) occurs. The apparatus seems
essentially concerned with the appraisal of position and the maintenance
of equilibrium. There is, however, some evidence that selachians, with the
nerve supply to skin and lateral line (see below) cut, can respond to low-
frequency vibrations. Section of the auditory nerve drastically reduces the
response.
Situated in the head are deep mucus-filled innervated canals called the
ampullce of Lorenzini (see alsop. 359). There is some evidence that these are
sense organs concerned with the appreciation of changes in water temperature.
They appear to be part of the lateral line system which extends down each side
of the body. This apparatus appreciates vibrations in the surrounding water.
Specialised hair-like processes, tipped with a concretion of relatively heavy
material, are sunk in minute pits and are connected with the lateral line nerves
(see also p. 136). Thus, it is not easy to touch a fish, even if it cannot see
the approaching objects. Blind species (e.g. the teleost Anoptichthys) catch
minute animals that they never see. Pumphrey has shown that lateral line
receptors act to some degree like the aerial of a direction-finder. Thus is the
animal aware of vibrations set up in its neighbourhood and, then, apprised of
the presence of predators or prey, it is able to take appropriate action (see
below). At the same time it must be remembered that most fishes depend a
good deal on their eyes (p. 273).
Endocrine Organs.-In addition to the pituitary (p. 150), pancreas (p.
152), and gonads (see below, p. 255), a thyroid gland (Fig. 161) and adrenal
tissues (Fig. 169) also occur. We saw in Petromyzon (p. 176) the development
of a simple, but seemingly typically craniate thyroid from the endostyle
(pp. 196, 197) in the metamorphosis from ammoccete larva to adult, together
with the secretion of an iodine-containing thyroxin, or a thyroxin-like sub-
stance, that will experimentally hasten anuran metamorphosis. In the dog-
fishes we see the thyroid as a pharyngeal downgrowth sometimes still associated
with a minute and an apparently functionless ciliated cavity that emphasises
its endostylic origin. This is easily located just below the ventral aorta,
where the vessel finally turns upward to branch and supply the first pair of
branchial pouches. Evidence concerning thyroid function in selachians is
confusing.
 ZOOLOGY
The thymus has often been considered, without proof, to be an endocrine
organ. In dogfishes it can be found inconspicuously placed on each side of
the thyroid and below the upper angles of the branchial clefts.
An adrenal gland (p. 152) does not occur as such, but
there exist separately the homologues of both medullary and
cortical tissues. The adrenalin-producing chromaffin tissue
is distributed in a long segmental series of small glands
(Fig. r6g), some of them penetrating the roof of the
posterior cardinal sinus. Anteriorly they are conjoined
into elongated aggregations on each side of the resophagus.
Posteriorly, as Young has shown, they lie in kidney tissue.
They are closely associated with the sympathetic ganglia
(see also p. 249), and probably have broadly the same
functions (p. rrg) as in higher groups in which they, and
the ganglia, share a common neural crest origin. The
posterior elements are bigger in the male.
Inter-renal glands occur near the kidneys. Their position
and form vary in different species-they may be single or
diffuse. Essential to the life of the fish, they represent the
adrenal cortex of mammals and the corresponding inter-
digitating lipoidal cells found in the adrenals of reptiles,
birds, and Amphibia (p. 152).
Excretion and Osmoregulation.-Each kidney consists
of two parts, anterior and posterior. The former (Fig.
r6r, r. meson., Fig. IJO, k1) is a long, narrow ribbon of soft
reddish tissue, which runs along throughout a great part of
the body-cavity at the side of the vertebral column. It
is covered by peritoneum. The posterior portion is a
compact, lobulated, dark red body, lying at the side of
FIG. I6g.-Scyl- the cloaca, continuous with the anterior portion. It, too,
liun~: Medullary and
cortical homologues is covered by peritoneum. Both portions possess ducts.
of adrenal. Aorta Those of the anterior are narrow tubes, which run over its
and kidneys with
associated diffuse ventral surface and become dilated behind to form a pair
chromaffin tissue of elongated chambers, the ttrinary sinuses (Fig. r7r, ur.
(black) and inter-
renal gland (stippled) sin.). These unite behind into a median sinus (med. ur.
from ventral aspect.
(Redrawn from sin.), which opens into the cloaca by a median aperture
Chester Jones, after situated on a papilla, the urinary papilla. The ducts of
Vincent.)
the posterior portion, which are usually from four to six
in number, open into the small urinary sinuses. These have the function
of a bladder, but are derived from mesodermal tissue, and are therefore not
strictly homologous with the urinary bladder in tetrapods. The kidneys
of the male differ from those of the female. The posterior part of the male
 PHYLUM CHORDATA 253
kidney has the same diameter as in the female. Its ducts, usually five in
number on each side, open into the urinogenital sinus, some of the most
anterior first uniting to form a common tube. The sinus has a median aperture
into the general cavity of the cloaca situated on the summit of a prominent

FrG. I?o.-Scyliorllinus:
Urinogenital system. From
ventral side. A, male and B ,
female. Only the anterior end
of the gonad is represented in
each figure, and except that in
B both kidneys are shown, the
organs of the right side only are
drawn. In A the seminal
vesicle and sperm-sac are dis-
sected away from the kidneys
and displaced outwards, and
the ureters inwards. ab. p.
depression into which the ab-
dominal pore opens; ct. cloaca;
cls. clasper; cf. d. efferent
ducts of spermary; k. kidney;
k', k" . anterior non-renal por-
tion of the kidney, forming in
the male the so-called' Leydig's
gland,' which, together with
the coiled spermiduct, consti-
tutes the epididymis; lr. an-
terior portion of liver; m. d.
vestigial Mullerian duct in the
male; res. gullet; ov. ovary ;
ovd. oviduct ; ovd'. its ccelomic
aperture; ovd". the common
apertureoftheoviducts into the
cloaca ; r . rectum; sh. gl. shell -
gla nd; spd. spermiduct; sp. s.
sperm-sac; s. v. seminal v esicle ;
s. v' . its aperture into the urino-
genital sinus; Is. testis; u. g. s.
urinogenital sinus; '"'·posterior
mesonephric ducts ; ur'. their
apertures into the urinogenital
sinus; u . s. urinary sinus.
(After T. ] . Parker.)

urinogenital papilla. The anterior non-renal part of the male kidney (Leydig's
gland) is mentioned below (p. 255). The anterior part of the female kidney
(Fig. 171) is degenerate and possibly functionless.
The urea concentration in the circulation of elasmobranchs- both sharks
and rays-is uniquely high. Smith has shown that the blood of one of the
dogfishes is more than 2 per cent. urea. The relative impermeability of the
254 ZOOLOGY
gills to urea ensures an osmotic concentration a little above that of the surround-
ing sea. A complex renal mechanism ensures the reabsorption of urea through
part of the urinary tubules of the hind kidney. These
fishes tolerate a concentration of urea that is toxic
in other animals. The twenty-odd species of elasmo-
branchs living in fresh water to-day are probably re-
colonisers: they (e.g. Trygon, Pristis) still maintain
an exceedingly high urea level.
Elasmobranchs still convert some ammonia to
trimethylamine oxide. It has been suggested that
they solved both excretory and osmotic problems by
the development of the ureotelic mechanism that
allowed ancestral forms to colonise salt water. When
a habitually freshwater animal is put in salt-water
the osmotic pressure of its blood is far less than that
of the new medium. The gradual evolution of a
mechanism that secreted and retained a high level of
urea could counterbalance the
dangerously high (for a fresh-
rmed.u.r. sin. water form) osmotic effects
of salt-water (pp. 156, 164).
Thus the urea-laden elasmo-
FIG. I7I.-BTachreluTus: branch blood does not lose fluid
Kidney and urinary sinus of to the surrounding sea but, in
female (Right). m ed. ur.
sin, median urinary sinus ; fact, achieves a modest intake
neph. kidney; ur. sin, right
urinary sinus. which is dealt with by the kid-
neys. Teleosts, as we shall see,
have solved the same problem in another way. Their
total salt concentration is much the same as in elasmo-
branchs. They carry no great amount of urea, but
maintain a balance by drinking sea-water and excreting
chloride at the gill-surfaces (p. 318).
All elasmobranch tissues except brain and blood can
synthesise urea. The animals are so perfectly adapted
to the high urea concentration that the heart will not beat
without it. Even the dogfish egg-case (the 'mermaid's
purse' (Fig. 172) contains a urea reserve. The embryo, FIG. 172.- Dogfl.sh:

too, synthesises urea and stores it·in the yolk. Egg-case with anchoring
elements. (Afte r D ean.)
Reproduction.-ln the female there is a single large
elongated, lobulated, ovary (Figs. 161, 170, ov.) lying a little to the right of
the middle line of the abdominal cavity and attached by a fold of peritoneum,
the mesovarium. On its surface are rounded elevations or follicles of various
 PHYLUM CHORDATA 255
sizes, each containing an ovum coloured bright yellow. As well as eggs, the
ovary produces female sex hormones (restrogen). There are two oviducts
(Mullerian ducts) which are unconnected with the ovaries. These are repre-
sented in the male by vestiges at their anterior aspect (Fig. 170, m.d.). Each
oviduct (Figs. 161 and 170, ovd.) is an elongated, thick-walled, muscular tube
extending throughout the entire length of the abdominal cavity. Anteriorly,
the two unite behind the pericardium to open into the abdominal cavity by
a wide oviducal aperture (ovd'.). At about the point of junction of the middle
and anterior thirds is a swelling marking the position of the shell-gland (sh. gl.).
The posterior part dilates to form a wide uterine chamber. In Scyliorhinus the
two unite to open into the cloaca by a common aperture situated immediately
behind the opening of the rectum, while in Squalus they remain distinct and
have separate cloacal openings. Each duct is guarded in the immature fish
by a delicate membrane, the hymen (see alsop. 894).
After the bursting of the follicles, and their extrusion from the ovary, the
liberated ova-which are more than one centimetre in diameter-are propelled,
often for comparatively long distances, by epithelial cilia into the oviducal
aperture. There is evidence that the shell gland begins to secrete even before
the eggs enter the oviduct. The eggs are driven by ciliary action down the
oviducts, fertilised by spermatozoa stored in folds in the wall of the shell
gland, and invested with a protective covering before entry into the uterus.
In the male (Fig. 170, A) the paired, soft, lobulated testes are each attached
to the abdominal wall by a fold of peritoneum, the mesorchium. Each is
composed essentially of convoluted seminiferous tubules which produce sperm-
atozoa, and between these, interstitial or Leydig cells which secrete the male sex
hormone (testosterone or its homologue) (Fig. 97, p. 152). Anteriorly, fro~n
each testis a small number of efferent ducts (ef. d.) pass to the anterior end
of a long, narrow, strap-shaped body, which corresponds to the vestigial
anterior portion of the kidney in the female. This is the epididymis. Its duct,
the vas deferens, runs along the entire length of the non-renal part of the kidney
(Leydig's gland). This structure, not to be confused with the hormone-produc-
ing Leydig (interstitial) cells of the testis, possibly adds a secretion that nourishes
the spermatozoa (see alsop. 88g). Leaving the kidney, the vas deferens widens
and opens into the urinogenital sinus (u. g. s.). This is a median chamber
projecting into the cloaca. Posteriorly each spermiduct dilates to form a wide
thin-walled sac, the vesicula seminalis. Closely applied to the latter is a thin-
walled elongated sperm-sac, of uncertain function. Anteriorly the sperm-sac
narrows to a blind extremity. Posteriorly the right and left sperm-sacs
combine to form the urinogenital sinus. The posterior part of the kidney has
the same character as in the female.
As mentioned previously, spermatozoa are transferred to the female
through the agency of paired, grooved, and erectile claspers which are modified
 ZOOLOGY
pelvic fins. These can be experimentally erected by means of adrenalin
and electrical stimulation of appropriate spinal nerves. The claspers may be
smooth, in others (e.g. Squalus, Scyliorhinus) they may be equipped with
exoskeletal structures. These, analogous to the copulatory grapples of certain
reptiles (p. sso) and mammals (p. 8go). serve to maintain contact when inserted
into the female cloaca, while the semen, with its cargo of spermatozoa, is
probably forced along the grooves by the action of sea-water ejected from the
capacious sac-like siphon. This lies beneath the skin of the ventral surface of
the body and communicates with the claspers by means of paired ducts. In
some species only one clasper is inserted at a time.

SUB-CLASS BRADYODONTI
These remarkable fishes, represented today only by Chimcera and its allies,
emerged in the Upper Devonian. They may be divided into two orders. The
first is composed of the more primitive Eubradyodonti, which, after a period of
successful expansion, disappeared at about the end of the Palceozoic (p. 3).
Extinct forms are known chiefly from their dentition. The principal teeth
were essentially flattened crushing plates. A characteristic of the dentition was
that there were available at any one time relatively few teeth which were
successively replaced (e.g. Helodus). The second group is as follows:

ORDER HoLocEPHALI 1
These arose in the Jurassic, or possibly Triassic and replaced the Eubradyo-
donti (see above). Today a few survive as the remarkable mollusc-eating,
generally deep-sea chimceras, ghost-sharks or rabbit-fishes (Fig. 173). They are
seemingly elasmobranchian, as evidenced by the male claspers, the large
egg-cases (and, when present, by the placoid scales) and by their general
anatomical similarity to the Euselachii. In modern species the skin is naked
except for a few such denticles restricted to the head-claspers.
Nevertheless, the group has a number of well-defined characters which
some consider entitle its representatives to be placed in a separate sub-class.
In the first place, the holostylic jaw suspension is characteristic (see p. 94) in
that the upper jaws are immovably united with the cranium. Thus, support
from the hyomandibular is not required and, in fact, neither jaws nor skull are
attached to the hyoid arch. Characteristic, too, are the extra claspers on the
head and in front of the ordinary pair on the abdomen. The presence of an
operculum is a character which differentiates the Holocephali from the Euse-
lachii. The dentition is also highly peculiar in the composition of the tooth-
plates, as well as in their shape. There is no enamel, but, instead, a layer of
vitrodentine, and the pulp cavity is much reduced. The teeth are in the form
1 0rvig (rg6o) suggests that the Holocephali are derived from coccosteomorphs (p. 211),
implying that the elasmobranchs are at least diphyletic.
 PHYLUM CHORDATA 257

I.

lf.

]II.

FIG. 173--Sub-class Bradyodonti, Order Holocephali: Representatives of constituent families.

I. Cltimmra monstrosa, male. A, ventral, B, front view of head. II. Callorhynchus antarctiws.
Dorsal, lateral, and ventral yiews. III. H arriotta raleighana .
 ZOOLOGY
of tritural plates formed, probably, by a fusion of originally separate denticles.
Of these plates there are two pairs in the upper jaws, a small anterior and a
larger posterior pair, and one large pair on the lower jaws. The arrangement
of these plates, producing the 'parrot-like' beak, has contributed largely to the
peculiar modification of the whole skull. The microscopic structure of the
teeth is much like that of the Bradyodonti. An operculum covers the gills and
there is no spiracle.
The earliest known representatives of the Holocephali differ very little in
essential characters from the living forms. Squaloraja and Myriacanthus are
representative and fairly completely known examples.
The relatively few existing representatives of the Holocephali form three
families, the Callorhynchidre (e.g. Callorhynchus), Chimreridre (e.g. Chimcera),
and Rhinochimreridre (e.g. Rhinochimcera, Harriotta). Chimcera, the so-called
' King of the Herrings' (Fig. 173, I) (a quaint name shared also with the bony
fishes Trachypterus and Regalecus, p. 297), is found on the coasts of Europe,
Japan, Australia and New Zealand, the west coast of North America, and off the
Africa coast. Callorhynchus (Fig. 173, II) is tolerably abundant in the south
temperate seas. Harriotta (Fig. 173, III) is a deep-sea form. These fishes
are sometimes known as spook-fishes or ghost-sharks in Australian waters
because of the pale ' ghostly ' iridescence that some of them emit.
External Characters.-The general form of the body is shark-like, but the
large, compressed head and small mouth are strikingly different from the
depressed, shovel-shaped head and wide mouth of most elasmobranchs. The
mouth is bounded by lip-like folds, two of which, placed laterally and sup-
ported by labial cartilages, resemble the folds in which the premaxillre and
maxillre of many bony fishes are enclosed. A third fold, external to and
concentric with the mandible, is also supported by labial cartilages and has
the appearance of a second or external lower jaw. In Chimcera the snout is
blunt, in Harriotta long and pointed. In Callorhynchus, the so-called' Elephant
Shark', it is produced into a rostrum, from the end of which depends a large
cutaneous flap abundantly supplied with nerves and evidently serving as an
important tactile organ.
A still more important difference from sharks and rays is the possession
of only a single external branchial aperture, owing to the fact that a fold of
skin, the operculum, extends backwards from the region of the hyoid arch.
This covers the true gill-slits, which thus come to open into a common chamber
situated beneath the operculum and communicate with the exterior by a
single secondary branchial aperture placed just anterior to the shoulder-
girdle : there is no spiracle. Equally characteristic is the fact that the urino-
genital aperture is distinct from and behind the anus-there is no cloaca.
There are two large dorsal, and a small ventral fin. The caudal fin is of the
ordinary heterocercal type in the adult Callorhynchus, but in the young (Fig.
 PHYLUM CHORDATA 259
179) the extremity of the tail proper is not upturned, and the fin-rays are
arranged symmetrically above and below it, producing the diphycercal form.
In Chimcera the tail may be produced into a long, whip-like filament. The
pectoral and pelvic fins are both large, especially the former.
In the male there is a horizontal slit situated a little in front of the pelvic
fins. This leads into a shallow glandular pouch, from which can be protruded
a peculiar, and indeed unique, apparatus, the anterior clasper. This consists
of a plate covered with recurved dermal teeth, to which is added, inC allorhynchus,
a plate rolled upon itself to form an incom-
plete tube. The precise use of this apparatus
A
is not known. A rudiment of the pouch
occurs in the female, although the clasper
itself is absent. The male possesses, in
addition, a pair of the ordinary myxopterygia
or posterior claspers, and is further dis-
tinguished by the presence of a little
knocker-like structure, the frontal clasper, on
the dorsal surface of the head. In Harriotta
l1..r
the paired claspers are poorly developed,
and the frontal clasper is absent.
8
The lateral line is an open groove in
Chimcera, a closed tube in Callorhynchus,
and there are numerous sensory pits,
arranged in curved lines, on the head. The :n.a
skin is smooth and silvery, sometimes of an c.r
iridescent, but probably not luminescent,
character. For the most part there are no
exoskeletal structures. There are, however, column. FIG. 174.- Cllimrera: Vertebra
A, transverse section, B
delicate, recurved dermal teeth on the lateral view. c. r. calcified ring; h. r
h;emal ridge; int. intercalary piece; n. a.
anterior and frontal claspers, and the first neural arch; nch. position of notochordal
dorsal fin is supported by an immense bony n. tissue; nch. sh. sheath of notochord
sp. neural spine. (After Hasse.)
spine or dermal defence. In the young,
moreover, there is a double row of small dermal teeth along the back.
Endoskeleton.-The vertebral column consists of a persistent notochord with
cartilaginous arches. In Chimcera, but not in Callorhynchus, there are calcified
rings (Fig. 174, c. r.) embedded in the sheath of the notochord. The anterior
neural arches are fused to form a high, compressed, vertical plate, to which
the first dorsal fin is articulated. The cranium (Figs. 175 and 176) has a
characteristic form, largely owing to the compression of the region between
and in front of the large orbits, which are separated from the cranial cavity
only by membrane in Callorhynchus (Fig. 176, or.). In Chimcera they lie above
the level of the cranial cavity and are separated from one another by a median
z6o ZOOLOGY
vertical partition of fibrous tissue (Fig. 175, i. o. s.). At first sight the palata-
quadrate, or primary upper jaw, appears to be absent, but it is, in fact, repre-
sented by a triangular plate (pal. qu.) which extends downwards and outwards
from each side of the cranium and presents at its apex a facet for the articula-
tion of the mandible. The palatoquadrate is therefore fused with the cranium
and furnishes the sole support for the lower jaw; the skull is holostylic. The
pituitary fossa (Fig. 176, s. t.) is very deep and inclined backwards. On the
ventral surface of the basis cranii
of'.>-.cl
r I is a pit (pt.) for the extra-cranial
portion of the pituitary body.
The posterior portion of the
cranial cavity is very high. The
anterior part-containing most
-p .s .c of the fore-brain-is low and
tunnel-like, and has above it a
cavity of almost equal size (Nv.
5o'.) for the ophthalmic branches
of the fifth nerves. The greater
part of the membranous labyrinth
is lodged in a series of pits on
the side-walls of the cranium
FIG. 175.-Chimrera: Skull. Lateral view of C. (a. s. c., p. s. c.), and is separated
monstrosa. a. s. c. position of anterior semicircular
canal; c. hy. ceratohyal; e. hy. epihyal; Jr. cl. from the brain by membrane
frontal clasper; h. s. c. position of horizontal semi- only. The occipital region arti-
circular canal; i. o. s. interorbital septum; lb. I ,
lb. 2, lb. 3, labial cartilages; Mck . C. mandible; culates with the vertebral column
Nv. 2, optic foramen; Nv. 10, vagus foramen; olf.
cp. olfactory capsule; op. r. operacular rays; pal.qu. by a single saddle-shaped surface
palatoquadrate; ph. hy. pharyngohyal; p. s. c. or condyle (oc. en.). There is a
position of posterior semicircular canal; qu.
quadrate region; r. rostrum. (After Hubrecht.) great development of labial car-
tilages. Particularly noticeable
is a large plate which, in Callorhynchus, lies just externally to the mandible,
nearly equalling it in size and having the appearance of a secondary or external
jaw. In Callorhynchus the snout is supported by three cartilaginous rods
growing forward from the cranium. The medial and dorsal rod represents the
rostrum (r). These rods, as well as the great lower labial, are represented by
comparatively small structures in Ckimcera (Fig. 175, lb. 3).
The hyoid resembles the branchial arches in form and is little superior to
them in size. Above the epihyal (Fig. 175, e. hy) is a small cartilage (ph. hy.),
evidently serially homologous with the pharyngobranchials, and therefore to be
considered as a phary ngohyal. It represents the hyomandibular of selachians,
but, having no function to perform in the support of the jaws, it is no larger
than the corresponding segments in the succeeding arches. Long cartilaginous
rays (op. r.) for the support of the operculum are attached to the ceratohyal.
 PHYLUM CHORDATA 261

The first dorsal fin is remarkable for having all its pterygiophores fused
in a single plate, which articulates with the coalesced neural arches already
referred to. The remaining fins are formed quite on the selachian type, as is
also the shoulder girdle. The right and left halves of the pelvic arch are
separate from one another. They are united in the middle ventral line by
ligament only. Each presents a narrow iliac region and a broad flat pubo-
ischial region perforated by two apertures or fenestrce closed by membrane,

Frc. 176 .- Callorhyn.clms: Skull. C. antarcliws. Sagit tal sectionwithlabialcartilages removed.

a . s. c. apertures through which the anterior semicircular canal passes from the cranial cavity into
the aud itory capsule; e. l . d. aperture for endoly mphatic duct; m ck. c. Meckel's cartilage; mnd. t.
m a ndibular t ooth; nch. notoch ord; Nv. s. trigeminal foramen; Nv . 5· o. foramen for exit of
ophtha lmic nerves; Nv. 5.'o', canal for ophthalmic nerves with apertures of entrance and exit;
Nv. 10, vagus foramen; oc. en. occipital condyle; or. fenestra separating cranial cavity from orbit;
pal. gn . palatoquadrate; pal. I. palat ine tooth; pn. position of pineal body ; pt. pit for extra-
cra nial portion of pituitary body; p. s. c. apertures through which the posterior semicircula r canal
passes into the auditory capsule; qu. quadrate region of palatoquadrate; r . rostrum; sac. depres-
sion for sacculus ; s. t. sella turcica ; tr. tritor; vo. t. vomerine teeth.

one of them of great size in Callorhynchus. The skeleton of the anterior clasper
articulates with the pubic region.
Alimentary Canal.-The teeth (Fig. 173, 176) are very characteristic, having
the form of strong plates with an irregular surface and a sharp cutting edge. In
the upper jaw there is a pair of small vomerine teeth (vo. t.) in front. Imme-
diately behind them occurs a pair of large palatine teeth (pal. t.), and in the lower
jaw a single pair of large mandibular teeth (mnd. t.). They are composed of
vasodentine, and each palatine and mandibular tooth has its surface slightly
raised into a rounded elevation of a specially hard substance, of whiter colour
than the rest of the tooth, and known as a tritor (tr.). The stomach is absent as a
VOL. II. R
 ZOOLOGY
histological entity. This probably represents a secondary loss as in the case
of numerous highly specialised teleosts (p. 336). In the Holocephali the alimen-
tary canal passes in a straight line from resophagus to anus. There is a well-
developed spiral valve in the intestine.
Respiratory Organs.-There are three pairs of holobranchs or complete gills
borne on the first three branchial arches, and two hemibranchs or half-gills,
one on the posterior face of the hyoid, the other on the anterior face of the
fourth branchial arch. The fifth branchial arch is, as usual, gill-less, and
there is no cleft between it and its predecessor.
Blood-vascular System.-This resembles that of the dog-fishes in all essen-
tial respects. The heart is formed of sinus venosus, atrium, ventricle, and
conus arteriosus, the last with three rows of valves.
Brain.-This (Fig. 177), on the other hand, is very unlike that of Scyliorhinus,
but presents a fairly close resemblance to that of Scymnorhinus. The medulla

FrG. I7]. -Callorllyt~cllus: Brain. C. an/arcticus. cblm. cerebellum ; ch . pl:r r , choroid plexus of
fore-brain, and ell. pix. 2, of hind-brain; cp. rst. corpus restiforme; crb. h. cerebral hemisphere ;
dien. diencephalon; lb. inf. lobus inferior ; med. obi. m edulla oblongata; olf. l. olfactory bulb;
olf. p. o!Iactory peduncle; opt. l. optic lobe ; pn. b. pineal body; pn. s. pineal stalk ; pty. pituitary
body.

oblongata (med. obl.) is produced laterally into large frill-like restiform bodies
(cp. rst.), which bound the hinder half of the cerebellum (cblm.). The dien-
cephalon (dien.) is long and trough-shaped. The chief difference from the brain
of the dogfish is that the end-brain is greatly compressed and elongated by the
large orbits. The combined diacrele and prosocrele are widely open above in a
brain from which the membranes have been removed, but in the entire organ
(Fig. 177) are roofed over by a conical, tent-like choroid plexus (ch. plx. 1).
The cavities of the small, spindle-shaped hemispheres (crb. h.). sometimes
regarded as corresponding to olfactory lobes only, communicate with the third
ventricle by wide foramina of Monro, partly blocked by up hemispherical
corpora striata. Each hemisphere is continued in front into a slender, thin-
walled tube, the olfactory peduncle (olj. p.), bearing at its extremity a com-
pressed olfactory bulb (olj. l.).
The optic nerves form a chiasma. The pineal body (pn. b.) is a small
rounded vesicle borne on a hollow stalk (pn. s.) which runs just outside the
 PHYLUM CHORDATA 263
posterior wall of the tent-like choroid plexus. The pituitary body (pty.)
consists of intra- and extra-cranial portions. In
advanced embryos the two are united by a delicate
strand of tissue.
Urinogenital Organs and Reproduction.-The A
kidneys (Fig. 178, kd.) are lobed, deep-red bodies,
like those of dogfishes, but shorter and stouter.
They are much longer in the male than in the female.
The anterior portion is massive, and consists mainly
of a mass of true renal tubules. It is indistinctly
divided into segments. The posterior portion is
narrower and also indistinctly segmented. From
both parts arise a number of ducts (meso-nephric
ducts) the majority of which open into the vas
deferens, while the last six open into the urino-
genital sinus. In the female the ducts all open into
a rounded median urinary bladder or urinary sinus,
situated between the two oviducal apertures. The
female reproductive organs are also constructed on
the general elasmobranch pattern, and are chiefly
noticeable for the immense size of the shell-glands
and uteri.
The male organs present certain unique charac- B
ters. The testes (ts.) are large ovoid bodies the
tubules of which apparently do not contain fully-
developed sperms, but only immature sperm-cells.
These latter are probably passed through vasa
efferentia into the vas deferens, which is coiled in a
highly complicated manner to form a body of con-
siderable size, commonly termed the epididymis,
closely applied to the surface of the anterior part
of the kidney. In this the sperms become aggre- FIG. 17 s.-callorhynchus:
gated into spermatophores in the form of small Urinogenital System. C. ant-
arcticus . A, male urinogenital
ovoidal capsules surrounded by a resistant mem- organs of left side, ventral
brane and full of a gelatinous substance in which aspect; B. anterior part of
vesicula seminalis in section.
bundles of sperms are imbedded. The lower end of cl. cloaca; epid. epididymis;
kd. kidney; mul. d. Mullerian
t he vas def erens (v. d:t .) lS dilated to form a large duct; sph. spermatophores; ts.
lf ·

cylindrical vesicula seminalis (vs. sem.) imperfectly testis; u. g. s. opening of urino-

genital sinus; v. dj. vas de-
divided into compartments by transverse partitions ferens; vs. sem. vesicula semi-
(B) and filled with a greenish jelly. The spermato- nalis. (A after Redeke.)
phores (sph.) are passed into these compartments and finally make their way
through the central passage into the urinogenital sinus (u. g. s.). The vestigial
VOL. II. R *
 N
~

N
0
0
t""'
0
C'l
><

FIG. IJ9.-Callorhynchus: Reproduction. Egg-shell of C. antarcticus with embryonic chamber cut open to show the contained
embryo. br. f. branchial filaments; v. valve through which young fish escapes; yk. s. yolk·sac.
 PHYLUM CHORDATA z6s

Mullerian ducts (mul. d.) are much more fully developed than in dog-fishes.
They are complete, though narrow, tubes which open in front by a large com-
mon aperture into the crelom. Behind they are connected with the urinogenital
sinus.
As in selachians, fertilisation is internal. The egg becomes surrounded, as
in dog-fishes, by a horny egg-shell secreted by the shell-glands. The egg-shell
of Callorhynchus (Fig. 179) is of extraordinary size-about 25 em. in length, or
fully five-sixths as long as the abdominal cavity-and the elongated chamber
for the embryo is surrounded by a broad, flat expansion covered on one side
with yellow hair-like processes. This gives the shell a close resemblance,
doubtless protective, to a piece of kelp. The early development resembles that
of selachians ; but the yolk becomes divided into nucleated masses which
divide into smaller segments, and the smallest break away and become dis-
solved in a milky nutrient liquid which fills the spaces of the shell. The
advanced embryo has elongated gill-filaments (br. f.) projecting through the
branchial aperture (and probably serving to absorb the nutriment derived from
the yolk), a diphycercal tail, and a curiously lobed and nearly sessile yolk-sac
(yk. s.). (For selachian development, seep. 279.)

GENERAL ORGAN/SA TION OF ELASMOBRANCHII

External Characters.-Sharks (sub-order Pleurotremata) are usually some-
what fusiform and slightly compressed laterally. Rays occupy a separate
sub-order (Hypotremata), but despite their radical dorso-ventral compression,
the migration of the eyes to the dorsal surface, and their apparent dissimilarity
from the sharks in other ways, the two groups have many features in common.
The flattened form of the rays is an adaptation to a bottom-dwelling existence.
They can commonly be seen gliding along the bottom of all relatively warm
seas, and they extend into circumpolar regions as well. Although they are
popularly supposed to be confined to shallow waters, some inhabit great depths
and are blind. Their peculiar form and, in particular, their highly unpleasant
and efficient weapons of defence have brought sting-rays and electric-rays to
the attention of mankind from earliest historical times. That small withered
curiosity, the 'Jenny Haniver', frequently sold on the water-fronts, is made
from a skate (Family Rajidce) that has been dried, and then cut and twisted
into shape. Skates are fashioned into grotesque shapes and sold also as
' mermaids', ' sea-eagles ', ' dragons ', 'monkey-fishes ', and 'basilisks ' (see also
p. 645). Rays appear on Etruscan mosaics and vases. Some attain great
bulk. Among the true sting-rays, Captain Cook's Stingaree or Whai Repo
(a Maori name) (Bathytoshia) of the Pacific reaches a length of 14 feet, a width
of 6 to 7 feet, and a weight of 770 lbs. It is generally caught at from zo to 6o
fathoms, although sometimes speared in a few feet of water. Devil-rays
('Devil- or Diamond-fish') (Family Mobulidce) may grow even bigger and are
266 ZOOLOGY
wider than the body is long (Fig. 154, p. 228). Manta (which uniquely has
cephalic fins) reaches a width of 22 feet across its diamond-shaped body and
'wings' (pectoral fins). It may weigh more than 3,000 lbs.
The elasmobranch head is in many cases produced forwards into a long
rostrum. This is of great length and bordered with triangular teeth in the
sawsharks (Pristiophorus) and sawfishes (Pristis) (Fig. 154, p. 228). In the
Hammerhead Shark (Sphyrna = Zygcena) the anterior part of the head is
extended transversely and the eyes are carried at the lateral extremities.
There are well-developed median and paired fins. The caudal fin is large
and, as a rule, strongly heterocercal in the sharks and shark-like rays. It is
reduced in most of the latter group. The dorsal and anal fins are large in the
sharks, the former completely divided into two. In the rays the dorsal fin is
usually small, and the anal fin absent. The paired fins differ widely in the two
groups. In the sharks both pairs are well developed, the pectoral being the
larger. In the rays the pectoral fins are extremely large and much bigger than
the pelvic fins. The pectoral fins fringe the greater part of the length of the
flattened body, and are prolonged forwards on either side and even in front of
the head. Thus, the animal presents the appearance of a broad, fleshy leaf.
In all recent Chondrichthyes the male has, connected with the pelvic fins, a
pair of grooved appendages-the claspers or myxopterygia-which subserve
copulation (p. 255). It is of great interest that such organs, which make internal
fertilisation possible, did not, except in the Devonian Diademonus, appear
before the hybodont sharks (p. 224). The latter were equipped also with
hook-like cephalic spines, which probably helped anchor the male during
coupling, and some species of Cladoselache possessed a large head spine of
perhaps similar function (Harris).
The mouth is situated on the ventral surface of the head, usually a consider-
able distance from the anterior extremity (cf., however, Fig. rsr, p. 225). In
front of each angle of the mouth is the opening of one of the olfactory sacs.
Each of these is frequently connected by a groove-the nasa-buccal groove-
with the mouth-cavity. Behind the mouth, on the dorsal surface in the rays,
and laterally in the sharks, is the spiracle. We saw (p. ro6) that in sharks
oxygenated water is taken in through the mouth, which is then closed, the
buccal cavity contracted, and the water forced through the gill-slits. In the
bottom-living rays the water is taken in through the spiracle. The water is
driven across the respiratory surface by the closure of a special valve. Along
each side of the neck in the sharks, and on the ventral surface in the rays, there
is a row of slit-like apertures-the branchial slits or clefts. These are usually
five in number on each side; but in Pliotrema, Hexanchus, and Chlamydo-
selachus there are six, and in Heptranchias seven. In Chlamydoselachus (Fig.
153) a fold comparable to a rudimentary operculum extends back over the
first branchial cleft, and is continuous across the middle line ventrally. In the
 PHYLUM CHORDATA

remainder of the sub-class no such structure is represented. A large cloacal

opening is situated just in front of the root of the tail, and in most members of
the sub-class a pair of small openings placed close to it-the abdominal pores-
lead into the abdominal cavity.
When the integument develops any hard parts as is the case in the majority
of the Chondrichthyes, they take the form, not of regular scales, as in most
other fishes, but of numerous rough, hard placoid scales. These vary greatly
in shape, are usually extremely minute, but are in some cases developed, in
certain parts of the surface, into prominent tubercles or spines. vVhen these
hard bodies are small, and set closely together in the skin, they give the surface
very much the character of a fine file. Such skin, known as 'shagreen ', was
formerly used for various polishing purposes in the arts, for armour, sword-
hilts, and even as striking surfaces affixed to the sides of lucifer match-boxes.
Each of the hard bodies has the same basic structure as a tooth (p. 103).
Such denticles differ in superficial
structure and arrangement in
different groups and are of diag-
nostic significance. A highly
specialised and dangerous integu-
mentary structure-the so-called
'sting'- characterises the sting-
arees, or dasyatid sting-rays i
(Family Dasyatid~). One or WK
FIG. r8o.-Scymnorhinwi:
several deciduous spines, capable I c. intercalary Spinal column.
cartilages ; Ob. neural arches; W K,
of independent movement, are centra. (After Weidersheim.)
attached some distance up the
tail. Grooves in the spine carry toxic secretions from contiguous glands.
Sting-rays produce agonising wounds and hideous scarring. A large ray may
slash right down to the bone, and septica:mia often results. In 1938 a New
Zealand girl aged eighteen was killed by stab-wounds from a fish that was
almost certainly a sting-ray. In addition to inflicting three thigh wounds,
the spine penetrated the thorax and both ventricles of the heart.
The endoskeleton is composed of cartilage, often with a deposition of
calcareous matter in special places- notably in the jaws and the vertebral
column. The entire spinal column may be almost completely cartilaginous
(Hexanchus and Heptranchias). Usually the centra are strengthened by
radiating or concentric lamell~ of calcified tissue; or they may be completely
calcified. They are deeply amphicrelous and the remains of the notochord
persist in the large inter-central spaces. Intercalary pieces (Fig. r8o, Ic.)
are interposed between both superior and inferior arches. In the rays (Fig.
181) the anterior part of the spinal column becomes converted into a con-
tinuous solid cartilaginous and calcified mass-the anterior vertebral plate
z68 ZOOLOGY

(a. v. p.). As in fishes in general, two regions are distinguishable in the

spinal column-the precaudal and the caudal, the latter being characterised
by the possession of inferior or hremal arches. In the precaudal region short
ribs may be developed, but these are sometimes rudimentary or entirely
absent. In the sharks pterygiophores, sometimes jointed, fused at their bases
with the hremal spines, support the ventral lobe of the caudal fin. The
dorsal lobe of the same fin is supported by a series of pterygiophores resembling
produced neural spines, but only secondarily related to the spinal column,

FIG. 181.-Vrolophus:
Endoskeleton of sting-ray.
Ventral view. a. v. p.
anterior vertebral plate;
bas. br. basibranchial plate;
br. I-br.s branchial arches.
(The branchial rays are not
represented, the round dots
indicating their articula-
tions with the arches.) cl.
skeleton of clasper; h. m.
hyomandibular; hy. hyoid
arch; lab. labial cartilage;
lig. ligament connecting the
hyomandibular with the
palatoquadrate and Meck-
el's cartilage; mck. Meckel's
cartilage; ms. pt. meso-
pterygium, and mt. pt.
metapterygium of pectoral
fin; mt. pt' . metaptery.
gium of pelvic fin; nas.
nasal cartilage; pal. palata-
quadrate; pect. pectoral
arch; pl. pelvic arch; pro.
pt. propterygium; sp.
spiracular cartilage.

and sometimes also divided by joints. The dorsal and ventral fins are
sometimes supported by similar pterygiophores; but in many cases the
cartilaginous supports of these fins consist, in whole or in part, of expanded
plates of cartilage. The marginal portions of the unpaired fins beyond the
limits of the endoskeleton are supported by dermal fibre-like structures
(ceratotrichia) composed of elastin.
The skull is an undivided mass of cartilage, hardened, in many cases, by
deposition of calcareous matter, but not containing any true bony tissue. It
consists of a cartilaginous case which protects the brain and the organs of
special sense. The structure of this cartilaginous brain-case as it occurs in
 PHYLUM CHORDATA 269
dogfishes has already been described. The main differences observable in
the different families are connected with the size and form of the rostrum.
In the rays the lower lip of the foramen magnum is deeply excavated for the
reception of a short process, the so-called odontoid process, which projects
forwards from the anterior vertebral plate, and on either side of this is an
articular surface-the occipital condyle-for articulation with corresponding
surfaces on that plate. In the sharks the skull is not so definitely marked
off from the spinal column. The apertures of the endolymphatic ducts in the
rays are not situated in a median depression, such as is observable in dog-
fishes and in all the sharks. The articular surface in the auditory region for
the hyomandibular is sometimes borne on a projecting process, sometimes
on the general level of the lateral surface. Sometimes in the rays there is a
smaller articulation behind for the first branchial arch.

I
FIG.
Skull.
182 .-Heplranchias:
Lateral view. mck.
/
Meckel's cartilage ; pal. qu. pal-
atoquadrate; pt. orb . post-
orbital process of the cranium,
with which the palatoquadrate
articulates. (After Gegenbaur.) I
I
?at.yu
. rnc/~

The upper and lower jaws-the palatoquadrate and Meckel's cartilage-are

connected with the skull through the intermediation of a hyomandibular
cartilage (Fig. 157, hy. mn. ; Fig. r8r, h. m.). The skull is thus of the hyostylic
type as regards the mode of suspension of the jaws. In sharks the palata-
quadrate has a process (absent in rays) for articulation with the base of the
skull in the pre-orbital region. In Hexanchus and Heptranchias (Fig. r82)
there is, in addition to this, a prominent postorbital process of the palata-
quadrate for articulation with the postorbital region of the skull (amphistylic
arrangement). Cestracion is also in a sense amphistylic; the palatoquadrate
is firmly united with the skull, articulating with a groove on the base, and the
hyomandibular takes only a small share in the suspension of the jaws. At the
sides of the mouth in all elasmobranchs is a series of labial cartilages, usually
two pairs above and one pair below. Attached to the hyomandibular is a thin
plate of the cartilage-the spiracular (Fig. r8r, sp.)-which supports the
anterior wall of the spiracle.
 270 ZOOLOGY
The hyoid arch proper is in most elasmobranchs connected at its dorsal
end with the hyomandibular-sometimes at its distal extremity, sometimes
near its articulation with the skull. In some rays, however, it is not so re-
lated, but articulates separately and independently with the skull behind the
hyomandibular. In the genera Hypnarce and Trygonorhina it articulates
with the dorsal portion of the first branchial arch. In sharks the hyoid is
usually relatively massive. In rays it is smaller, and in most cases closely
resembles the branchial arches, and bears similar cartilaginous rays. A larger
or smaller median element, or basihyal, is present in all cases.
There are always five pairs of branchial arches except in Hexanchus and
Chlamydoselachus, which have six, and Heptranchias, in which there are seven.
Their dorsal ends are free in sharks, but in most rays they are articulated with
the anterior vertebral plate of the spinal column. Externally they bear a
series of slender cartilaginous branchial rays. The median ventral elements
of the branchial arches are usually more or less reduced, and in some cases are
represented by a single basibranchial plate (Fig. r58, b. br.). In the rays the
fifth branchial arch articulates with the pectoral arch, a connection which is
absent in sharks. A series of slender cartilages, the extra-branchial cartilages,
support the branchial apertures. These are probably modified branchial
rays.
The pectoral arch (Figs. 159, r8r, pect.) consists of a single cartilage. In
most sharks, a mesial flexible portion occurs by which it is divided into right
and left halves. Each lateral half consists of a dorsal scapular and a ventral
coracoid part, the two being separated by the surfaces on which articulate
the basal cartilages of the fin. In rays, but not in sharks, the dorsal
ends of the pectoral arch are connected with the spinal column (anterior
vertebral plate) by a distinct articulation. The portion of the arch on which
the articular surface is situated sometimes forms an independent supra-
scapula cartilage.
The basal pterygiophores of the pectoral fin are typically three, pro-, meso-,
and meta-pterygium (Figs. 159 and r8r), but there are sometimes four, and
the number may be reduced to two. The pro- and meta-pterygia are divided
in the rays (Fig. r8r) into several segments. The former articulates, through
the intermediation of a cartilage termed the antorbital, with the olfactory
region of the skull.
The pelvic arch (pl.), like the pectoral arch, is usually a single cartilage, but
in some exceptional cases it consists of two lateral portions. In some cases a
median epipubic process projects forwards from the pelvic arch, and frequently
there is on each side a prepubic process. A lateral iliac process, which becomes
highly developed in the Holocephali, is sometimes represented, and may attain
considerable dimensions. The pelvic fin has usually two basal cartilages,
representing the pro- and meta-pterygia, but the former is often absent. In
 PHYLUM CHORDATA 2JI

the male special cartilages attached to the metapterygia support the claspers.
With the basal cartilages of both pectoral and pelvic fins are connected a
number of jointed cartilaginous fin-rays supporting the expanse of the fin.
The arrangement of the muscles is simple. The trunk-muscles are divided
into a pair of dorsal and a pair of ventral divisions each, composed of many
myomeres with intercalated myo-
commata (Fig. 62, p. 86), following
a metameric arrangement. The
ventral part, where it forms the
muscles of the wall of the ab-
dominal cavity, is composed ex-
ternally of obliquely running
fibres, and represents one of the
two oblique muscles of the ab-
domen of higher forms. Mesially
this passes into a median band of
longitudinally running fibres cor-
responding to a primitive rectus.
The muscles of the limbs are dis-
tinguishable into two main sets-
(r) those inserted into the limb-
arch and (z) those inserted into the
free part of the appendage. The
latter, according to their insertion,
act as elevators, depressors, or
adductors. A series of circular
muscles passes between the carti-
lages of the visceral arches and,
when they contract, have the FIG. 183. -Tm·pedo: Electric organs. On the
right the surface only of the electric organ (O.E.) is
effect of contracting the pharynx shown, on t he left the nerves passing t o the organ
and constricting the apertures. A are shown. The roof of the skull is removed to
bring the brain into view. br. gills; f. spiracle; o.
set of muscles pass between the eyes; tr. trigeminal; tr'. its electric branch; v.
vagus; I, fore -brain; II, mid-brain; III, cere.
various arches and act so as to bellum; IV, electric lobe. (After Gegenbaur.)
approximate them. A broad sheet
of longitudinal fibres divided into myomeres extends forwards from the shoulder-
girdle to the visceral arches.
Electric organs occur in several elasmobranchs. They are best developed
in the electric-rays, numb-fishes, or torpedoes (Fig. 183), in which they form a
pair of large masses running through the entire thickness of the body, between
the head and the margin of the pectoral fin. A network of strands of fibrous
tissue forms the support for a number of vertical prisms, each divided by
transverse partitions into a large number of compartments or cells. Numerous
2]2 ZOOLOGY
nerve-fibres pass to the various parts of the organ. These are derived mainly
from four nerves, which originate from an electric lobe of the medulla oblongata,
with a branch from the trigeminal. Rays of this family often conceal them-
selves in the sand rather than retreat; they are sometimes trodden upon.
Torpedo generates a greater potential than even the South American electric
'eel' (p. 334). Fishermen get electric shocks from captured torpedoes when
hauling in nets or while standing with bare feet on a large catch of bony
fishes under which a Numb-fish lies hidden. It will be seen that the electric
organ can be a highly successful weapon of offence as well as defence. Electric-
rays often eat bony fishes as large as is possible for them to swallow. In
certain other rays electric organs are comparatively small and situated at the
sides of the root of the tail. In all cases the cells are formed from specialised
muscle-fibres.
Although no extant marine fish is known to use electrical discharges in
direction finding (see p. 335), it is perhaps not impossible that the device
originated for this purpose in shallow off-shore gulf-waters constantly flooded
by muddy rivers. There is, of course, no direct comparison, but it is not with-
out interest that extant teleosts of several families have suffered visual degen-
eracy in such situations.
Luminous organs occur on the surface of a few oceanic elasmobranchs.
Spinax molleri, a small (2o-inch) shark of Australian waters, gleams vivid
greenish from its ventral surface. According to one early nineteenth-century
author it imparts to itself 'a truly ghostly and terrific appearance'.
Alimentary Canal and Associated Structures.-Teeth are developed on the
palatoquadrate and on Meckel's cartilage. They are arranged in several
parallel rows, and are developed from a groove within the margin of the jaw.
Successive rows come to the front, and, as they wear out, fall off and are replaced
by others. In the sharks the teeth are usually large, and may be long, narrow,
and pointed, or triangular with serrated edges (Fig. 147, p. 220), or made up of
several sharp cusps. In the rays the teeth are more or less obtuse. Some-
times, as in the eagle-rays, they form a continuous pavement of smooth plates
covered with enamel. Thus they are adapted to crushing food such as molluscs,
crabs, and other animals. An exceptional shark is the large (up to 40 feet)
Basking Shark (Cetorhinus), which lives on plankton. Water and krill are
taken in through the mouth. The latter are strained and prevented from
passing through the branchial clefts by elongated gill-rakers. There is recent
evidence that during the winter these are lost and that Cetorhinus undergoes a
quiescent demersal phase until the seasonal improvement of food supplies.
Sharks generally have a prominent tongue supported by the median basihyal;
this is entirely or almost entirely absent in the rays. The various divisions of
the alimentary canal are similar in all the members of the class to those already
described in dogfishes. A spiral valve is always present in the large intestine,
 PHYLUM CHORDATA 273
though its arrangement varies considerably in the different families. In some
cases (e.g. Carcharimts) the fold is not a spiral one, but, attached by one edge
in a nearly longitudinal line to the intestinal wall, is rolled up in the shape of a
scroll. A pair of pyloric creca occur in Somniosus. Connected dorsally to the
rectum is a median glandular ccecum, the rectal gland. The rectum always
terminates in a cloaca, into which the urinary and genital ducts also lead.
There is always a large liver and a prominent pancreas.
A thyroid can be found in the middle line behind the lower jaw. A repre-
sentative of the thymus lies on either side, a little below the upper angles of the
branchial clefts.
The respiratory and associated organs of the Chondrichthyes always have
the general structure and arrangement already described in the case of the

c
i
I cf
ch
r
ext
sc
sf
0

FrG. 184.-Carcharodon: Eyes and mechanism ol accommodation. The left figure shows the
peculiar prop-like optic pedicel (op) which is, however, absent (probably lost), in some elasmo-
bra nchs (e .g. Dogfish, Scyliorhinus and certain deep-sea forms) and not fully a rticulated
in others. The eye is above represented in full accommodation. W ere the protractor lentis
muscle relaxed, the lens would be withdrawn from the cornea. a, horizontal section; b, vertical
section (cf. teleost eye, p. 316); c. cornea; ch. chorioid; cj. ciliary folds, from which gelatinous
zonule passes to lens equator; ext. external rectus; i. iris; in{. inferior rectus; int. internal rectus;
l. lens; op. optic pedicel; p. papilla (bearing protractor lentis muscle); r. retina; s. suspensory
'ligament' (a thickening in the zonule); sc. scleral cartilage; sf. fibrous portion of sclera; so.
superior oblique ; sup. superior rectus. (After Walls.)

Dogfish. In the rays the water for respiration is taken in mainly through the
spiracles; in the sharks through the mouth.
In addition to the gills supported on the hyoid and branchial arches there
is also in the Notidanidre a gill on the anterior side of the spiracular cleft-the
spiracular gill-represented in many others by a rete mirabile or network of
blood-vessels (pseudobranch, p. 230).
Blood-vascular System.-The heart has, in all essential respects, the same
structure throughout the group. The conus arteriosus is always contractile,
and contains several rows of valves. The general course of the circulation is the
same in all (see pp. 240-245), with some variation in the precise arrangement
of the vessels. In some of the rays the ventral aorta and the roots of the
afferent vessels are partly enclosed in the cartilage of the basi-branchial plate.
Hcematopoietic tissue occurs in the spleen. In addition, elasmobranchs of
274 ZOOLOGY

both sexes possess lymphoid epigonal organs (Fig. 185) that are concerned with
the manufacture of both red and white cells. In Cetorhinus, unlike most
selachians, the testis is almost completely surrounded by the epigonal organ
(Matthews). In the female the organ is bilateral even though the left ovary is
absent or rudimentary. The organ of Leydig, too, is said to produce gran-
ulocytes. As in the Agnatha, the lymphatic system has not been shown to be
clearly distinct from the venous: blood is commonly found in lymph vessels
(Mott).
Brain.-The fore-brain greatly exceeds the other divisions in size. In
Scymnorhinus there are two widely-separated cerebral hemispheres containing
large lateral ventricles. In other genera there is at most, as in dogfishes, a
median depression of greater or less depth, indicating a division of the anterior
end of the fore-brain into two lateral portions. In Scyliorhinus, as already
pointed out, there is a median ventricle which gives rise anteriorly to two
lateral ventricles, and the same holds good for Squatina and Squalus. In most
rays there is only a very small median ventricle without anterior prolongations;
in M yliobatis this is absent. The olfactory bulbs are of great size, in some cases
with short and thick, in others longer and narrower, stalks. In Scyliorhinus,
Squatina, and Squalus, as well as in Scymnorhinus, they contain ventricles
(rhinoca:les) forming part of the lateral ventricles. In the rays they are solid.
The diencephalon is of moderate extent. On its lower aspect are a pair of
rounded lobi inferiores, which are of the nature of dilatations of the infundi-
bulum, a part of the hypothalamus, and a saccus vasculosus, which is a diverti-
culum of the infundibulum. Directly below the saccus vasculosus lies the
hypophysis. The epiphysis is long and narrow.
In the hind-brain the cerebellum is relatively greatly elongated and overlaps
the tectum of the mid-brain and sometimes also the diencephalon in front, while
behind it extends over the posterior part of the medulla oblongata. It usually
contains a cerebellar ventricle. The medulla is elongated in the sharks,
shorter and more triangular in the rays. The electric rays possess electric lobes
which are rounded elevations of the floor of the fourth ventricle.
Organs of Special Sense.-Integumentary sense-organs (neuromasts, p. 137)
are highly developed in the Chondrichthyes. They are supplied, as already
mentioned, by branches of the nerves of the lateral system, comprising, in addi-
tion to the lateralis, nerves in relation with the facial and sometimes the
glossopharyngeal. These integumentary sense-organs occur in the interior
of a continuous system of closed tubes, the sensory tubes, more rarely of open
grooves. The chief canals of this system are a lateral-line canal, running along
the middle of each side of the body, which is continuous with certain canals in
the head. These communicate with the exterior at intervals by small pores.
In addition to the canals of the lateral-line system there are a number of
isolated canals, the ampullary canals, with neuromasts contained in terminal
 PHYLUM CHORDATA 275
enlargements or ampulla. These, almost peculiar to the Chondrichthyes, are
most numerous about the snout region. Of similar essential character are
the vesicles of Savi which occur in the electric rays. (The teleost catfish
Plotosus also possesses ampulla:.)
The olfactory organs are a pair of sacs opening on the lower surface of
the head, a little distance in front of the mouth, and enclosed by the cartilagin-
ous olfactory capsules of the skull. Their inner surface is raised up into a
number of ridges on which the fibres of the olfactory nerves are distributed.
There is ample evidence that at least some sharks hunt primarily by scent.
Collecting parties wading through shallows on the Great Barrier Reef have
found that small sharks prefer to follow people whose legs have been gashed
by coral. Commercial shark fishermen, who hunt Australian sharks for their
hide and liver oils, sometimes throw overboard buckets of bullocks' blood ob-
tained from abbatoirs, and find this a successful lure. Researches during the
Second World War revealed that copper acetate repelled sharks to a certain
extent.
The eye (Fig. r84) is usually big. It has a cartilaginous sclerotic, and is
in most, but not all, cases attached to the inner wall of the orbit by means of a
cartilaginous stalk or optic pedicel 'running prop-like from cranium to eyeball'
(Walls). At the eyeball this is often expanded and cupped, and forms a socket
joint for the rotation of the eye. In a few sharks and rays this pedicel is slender
and sufficiently bent and elastic that its own tendency to straighten itself can
proptose (protrude) the eyeball when the extra-ocular muscles act together.
This action, with broadly the effect of a levator bulbi muscle (p. 413), may have
been the original function of the apparatus. In the elasmobranchs studied,
accommodation is brought about by smooth muscle-fibres that swing the whole
lens towards the cornea. Thus the action is towards near vision, as in Man,
and not towards distant vision, as in lampreys and bony fishes. The protractor
lentis muscle may, in addition, cause the lens to bulge the cornea to some
degree. A fold of the conjunctiva, resembling the nictitating membrane of
higher vertebrates, occurs in some sharks (e.g. Carcharinidce). One deep-sea
electric ray, Benthobatis moresbyi, possesses degenerate eyes. Most elasmo-
branchs lack cones in the retina, but a few-e.g. Jlr1ustelus and particularly
the eagle-ray (Myliobatis)-possess them, probably derived independently, in
these animals, from rods (p. 140). The tapetum (p. 141) has been lost in at
least two groups of unusual habit-the Basking Shark (Cetorhinus) and the
abyssal shark Lcemargus.
Audio-equilibration.-The membranous labyrinth consists of a utricle and
saccule, incompletely separated, and together forming a memlfranous vestibule
from which arise the three semicircular canals with their ampullce, and also
the endolymphatic duct. This retains its opening to the exterior on the dorsal
surface of the head and represents the stalk of the embryonic otic vesicle. In
 ZOOLOGY
the rays the semicircular canals form almost complete circles and open separately
into the vestibule by narrow ducts.
Endocrine Organs.-Relatively little work has been done in this field in
selachians. There is evidence that the pituitary exerts central governing
influences (p. 149), but it would be unwise in the extreme to assume that
elasmobranch endocrines exactly parallel functionally those of higher creatures.
In some cases there is evidence that they do not.
The inter-renals are homologous with the adrenal cortex of higher animals,
but differ in their position and, apparently, some of their functions. The
selachian inter-renal can be single, median, and elongated (as in Scyliorhinus),
paired, and diffuse (as in Raja), or single, asymmetrical, and condensed (as in
Torpedo). Deprivation leads to death, but cortical extracts from a mammal
(rat) will prolong life in Torpedo after removal of its inter-renal gland. Ex-
tracts from the skate inter-renal will likewise maintain life in adrenalectomised
rats.
The medullary homologue has been dealt with above (p. 252). Little is
known of the thyroid functions in selachians. Corpora lutea of a supposedly
mammalian structure have been described, but these have not yet been proved
to be true post-ovulatory, rather than pre-ovulatory, bodies. There is some
evidence that extracts of dogfish pituitary will cause lactation in mammals,
but nothing is known of the possible endocrine control of the so-called 'uterine-
milk' of certain viviparous selachians (p. 278). Interstitial aggregations
(Leydig cells) of the typical vertebrate kind (e.g. Petromyzon, Man) occur in the
testes (see p. 152). There is evidence that testosterone increases clasper-
length in young males (see p. 256 in regard to erection), and that restrogens
accelerate growth in the reproductive tract in young females.
The pancreas contains islets.
Urinogenital Organs and Reproduction.-The kidneys differ somewhat in the
two sexes (p. 252). In the male the anterior portion persists in the epididymis,
and its duct becomes the spermiduct, while the posterior portion, which is the
functional kidney, has a duct or ducts of its own. In the female selachian there
is no direct connection between the reproductive and renal organs ; the
anterior portion of the kidney may be functional, and its duct persists, opening
along with those of the posterior portion. In the male the urinary ducts open
into a median chamber-the urinogenital sinus-which extends into the cloaca,
and receives also the spermiducts. It communicates with the general cavity of
the cloaca by a median opening situated on a papilla-the urinogenital papilla.
In the female there is a median urinary sinus, into which the urinary ducts
open, or the latter may open separately into the cloaca (see Fig. IJO, p. 253).
Save in certain exceptional cases (e.g. Scyliorhinus), there are two ovaries.
These vary considerably in form, and towards the breeding are characterised
by the great size of the follicles enclosing the mature ova. The oviducts
 PHYLUM CHORDATA 277

(Mullerian ducts) are separate from the ovaries. The right and left oviducts
come into close relationship anteriorly, being united in the middle on the
ventral surface of the resophagus. Here each opens by a wide orifice into the
abdominal cavity, or both open by a single median aperture. The following
part of the oviduct is very narrow. At one point, however, it exhibits a
thickening, due to the presence in its walls of the follicles of the nidamentary
gland. Posteriorly, the oviduct gives rise to a dilated uterus, and this communi-
cates with the cloaca through a wide vagina. In ovoviviparous elasmobranchs
(p. 278) the shell-gland is small or vestigial.
The ova are very large, consisting of a mass of yolk-spherules held together
by a network of protoplasmic threads, with, on one side, a disc of protoplasm-

L. gt.

FIG. 185.- Cetorhinus: Urinogenital and associated organs. Lefi: offemale (with resophagus);
Right: of male. amp. ampulla ductus deferentis; epi. epididymis; EP. 0. epigonal organ (seep.
274); isth. isthmus ; kid. kidney; L. gl. Leydig's gland; nid. gl. nidamentary (' shell ') gland; ov.
ovary; ov. d. oviduct; res. resophagus; test. testis; ut. uterus; ut. (f.) uterus (lined with folds); ut. (t.)
uterus (lined with trophonemata). (Redrawn after Harrison Matthews .)

the germinal disc. The process of maturation is similar to that observable in

holoblastic ova. One polar body is thrown off in the ovary, the other apparently
at impregnation. The ripe ovum ruptures the wall of the enclosing follicle and
so passes into the abdominal cavity to enter one of the oviducts through the
wide abdominal opening. Impregnation takes place in the oviduct. The
impregnated ovum in the oviparous forms becomes surrounded by a layer of
semi-fluid albumen and enclosed in a shell of keratin secreted by the shell-
gland. The shell varies in shape somewhat in the different groups. Most
commonly, as in many dogfishes (Fig. 172), it is four-cornered, with twisted
filamentous appendages at the angles, by means of which it becomes attached
to sea-weeds and the like. In the skates the filaments are absent. In Hetero-
dontus it is an ovoid body the wall of which presents a broad, spiral flange.
In Rhinobatus and Trygonorhina, which are both viviparous, each shell encloses
not one egg, but three or four.
 ZOOLOGY
Somniosus, formerly thought to differ from other Chondrichthyes in having
the ova externally fertilised, produces remarkably small ova. The eggs of the
Basking Shark, too, are relatively small.
Placentation.-It is among the elasmobranchs that we see the first develop-
ment of a true placentation as distinct from the primitive arrangements that
occur among arthropods (e.g. certain insects and Peripatus) and in Salpa among
the Tunicata (p. 20). We will see that vivipary or internal gestation with the
formation of a placenta (as apart from mere ovovivipary) has been developed
independently also in teleosts (p. 351), and reptiles (p. 549), among poikilo-
thermous animals. A placenta has been defined as 'any intimate apposition
or fusion of the fcetal organs of the maternal (or paternal) tissues for physi-
ological exchange' (Mossman), although, as Amoroso points out, people
commonly restrict the expression to mean the chorio-allantoic placenta found in
some reptiles and many mammals (see pp. 550, 904).
In a few sharks, snakes, lizards, and in most marsupials the chorion is vas-
cularised from the allantois, and thus unites with the wall of the uterus or its
equivalent structure. Selachians commonly nourish their internally developing
young with material from the yolk-sac (ovovivipary), but in many of these,
even though the egg-case is intact, the maternal epithelium secretes products
that assist in the nutrition of the embryo. In some species there is a con-
siderable development of new epithelium (Acanthias), or even villi (Torpedo,
Trygon), but in others a so-called uterine-milk is produced without gross alter-
ation of the maternal mucosa.
In the ovoviviparous elasmobranch Lamna the yolk is soon absorbed, but
immature eggs or degenerating ovarian tissue pass into, and down, the oviducts
to the uterine chamber, where the material is eaten by the embryos, which
are said to remain within the parent for more than a year.
Among the truly viviparous selachians, placentation has been extensively
investigated in Mustelus, Scoliodon, and other genera. The embryos of
M ustelus each develop in separate compartments in which villi develop and
fuse with grooves in the yolk-sac. This gives rise to a simple yolk-sac placenta,
involving modification of both parental and embryonic tissues and the in-
vasion of maternal capillaries to a degree that brings both circulations into an
intimacy that allows efficient functional, including gaseous, exchange.
In several species of Scoliodon an almost complete union is formed between
the yolk-sac of the embryos (again in compartments) and the maternal epithel-
ium. The mucosa is raised, and then invaginated, into a series of small,
highly vascular cups, and to each one of these a yolk-sac is joined. A placental
cord is formed on which thread-like appendicula assist in the absorption of
maternal secretions. The arrangement of the placenta differs in various
species in the genus. In 5. sarrakowah the placental cord is formed so that (as
in higher mammals) a wholly vascular connection is obtained.
 PHYLUM CHORDATA 279
Development-Except in one species of H eterodontus, cleavage is meroblastic.
It is confined to the germinal disc. A segmentation cavity appears early
betweeh the blastoderm cells and the yolk; the floor of the blastoccele becomes

FIG. 186.-Selachii: Development.

Embryo in longitudinal section. A, sec-
tion of the young blastoderm with seg-
mentation-cavity enclosed in the lower
layer cells; B, older blastoderm with
embryo in which endoderm and meso-
derm are distinctly formed, and in which
the alimentary slit has appeared. The
segmentation-cavity is still represented
as being present, though by this stage it
has in reality disappeared. C, older
blastoderm with embryo in which the
neural canal has become formed and is
continuous posteriorly with the alimen-
tary canal. Ectoderm without shading;
mesoderm and also notochord black with
clear outlines to the cells; endoderm and
lower layer cells with simple shading.
al. alimentary cavity; ch. notochord;
ep. ectoderm; m. mesoderm; n. nuclei
of yolk; nc. neuroccele; sg. segmen-
tation-cavity; x. point where ectoderm
and endoderm become continuous at the
posterior end of the embryo. (After
Balfour.)

covered with a syncytium which probably plays a part in yolk digestion (Fig.
r86, n). When cleavage is complete the blastoderm is a lens-shaped disc
thicker at one end-the future caudal
extremity.
7td Mass migrations of cells now take
place round the dorsal lip, which is at
the posterior edge of the blastodisc.
An archenteron is formed (Fig. r86,
' I al.). (An archenteron is not formed in
the teleosts, but in most other respects
~- -:
.................. . gastrulation is similar in both groups;
see p. 319.) The invaginated cells
~l.f!­ blp cd
migrate forward forming mesodermal
and endodermal structures and elimi-
FIG. •87.- Selachii: Development. Embryo nating the blastoccele at the same time.
of Scyliorhinus caniculus with the tail-swellings
well marked and the medullary groove just It is possible that in addition to this
beginning. bl. e. edge of blastoderm; bl. p. forward migration some endodermal
blastopore; cd. caudal swellings ; hd. head.
(After Sedgwick.) cells are derived in selachians by delam-
ination from the early gastrula.
At the end of gastrulation the medullary groove begins to form in the same
way as in other vertebrates. Similarly, somites, lateral plate, notochord,
 280 ZOOLOGY
and intermediate cell mass become laid down. The archenteron becomes
the alimentary canal.
As the blastoderm extends over the yolk, its posterior edge assumes the
form of two prominent caudal swellings
(Fig. 187, cd.). The medullary groove
meanwhile deepens, and its edges grow
over so as to form a canal (Fig. r86, C;
Fig. r88). The union takes place first
in the middle, the anterior and posterior
parts (Fig. r88, neur.) remaining open
for a time. When the posterior part
closes, it does so in such a way that it
encloses the blastopore, and there is ~
thus formed a temporary passage of
communication between the medullary
canal and the archenteron-the neu-
renteric passage.
By degrees the body of the young
fish becomes moulded on the blasto-
derm. This is effected by the formation
of a system of anterior, posterior, and

11
FrG. r8g.- Selachii: Development.
Three views of the developing egg, showing
the embryo, the blastoderm, and the vessels
of the yolk-sac. The shaded part (bl.) is the
blastoderm, the white part the uncovered
yolk. A, young stage with the embryo still
neur attached at the edge of the blastoderm; B,
FrG. r88.-Selachii: Development. Em- older stage with the yolk not quite enclosed
bryo of a Ray with the medullary groove by the blastoderm; C, stage after the com-
closed except at the hind end. The notched plete closure of the yolk. a. arterial trunks
embryonic part of the blastoderm has grown of yolk-sac; bl. blastoderm; v. venous
faster than the rest and come to project over trunks of yolk-sac; y. point of closure of the
the surface of the yolk. bl. e. edge of blasto- yolk-blastopore; x. portion of the blasto-
derm; hd. head; nettr. unenclosed part of the derm outside the arterial sinus terrninalis.
neuroccele. (After Sedgwick.) (After Balfour.)

lateral folds. These grow inwards and separate the body of the embryo from
the rest of the blastoderm enclosing the yolk. As the folds approach one
another in the middle, underneath the embryo, they form a constriction
connecting the body of the embryo with the yolk enclosed in the extra-
 PHYLUM CHORDATA z8r

embryonic part of the blastoderm. The process may be illustrated by pinching

off a portion of a ball of clay, leaving only a narrow neck connecting the pinched-
off portion with the rest. The body of the embryo is thus gradually folded off
from the yolk-sac and comes to be connected with it only by a narrow neck
or yolk-stalk (Fig. 189). The head and tail of the young fish soon undergo
differentiation, and a series of perforations at the sides of the neck form the
branchial clefts and spiracle. A number of very delicate filaments (Fig. 190)
grow out from these apertures and become greatly elongated. These are the
provisional gills, which atrophy
as development approaches .... _ ,.,..,.brn
completion, their bases alone
persisting to give rise to the
permanent gills. The great
development of these gill-fila-
ments in the embryos of some
viviparous forms suggests that,
in addition to their respiratory
functions, they may also serve
as organs for the absorption of
nutrient fluids secreted by the FIG. I go.-Selachii: Development. Left: Head of
embryo of Scyliorhinus caniculus, with the rudiments of
villi of the uterine wall. the gills on the first and second branchial arches. Right:
The fins, both paired and at a somewhat later stage. The gill-filaments have in·
creased in number and are present on the mandibular
unpaired, appear as longitud- arch. ang. angle of the jaw; hy . hyoid; m. brn. mid· brain;
mnd. mandible; nas. nasal sac; spir. spiracle. (After
inal ridges of the ectoderm en- Sedgwick.)
closing mesoderm. In some
Chondrichthyes the paired fins are at first represented on each side by a con-
tinuous ridge or fold, which only subsequently becomes divided into anterior
and posterior portions-the rudiments respectively of the pectoral and pelvic
fins. Into these folds penetrates a series of buds from the somites. These,
the muscle-buds, give rise to the fin-muscles. At first , from their mode of
origin,. these present a met americ arrangement, but this is in great measure
lost during development.
While half the embryo is still in the yolk sac, and the body as yet 3 mm.
long and possessed of only fifteen somites, its body-muscles contract rhythm-
ically and cause a lateral bending of the head region. There is experimental
evidence that such rhythmicity seems to be independent of neural control. Its
function may be to keep the surrounding fluid moving and to assist oxygenation.
It is of interest that the ovoviviparous Squalus acanthias undergoes one of
the longest gestation periods known. The 2 to 12 embryos may be held from
20 to 22 months (see alsop. 907). During this period each is probably entirely
dependant for nourishment on its extremely large yolk sac (Gilbert).

VOL. II. s
 CLASS OSTEICHTHYES

Sub-Class Actinopterygii
INTRODUCTION
The ray-finned 'bony fishes' include all the commonest marine species of
to-day (e.g. cod, sole, herring, eels) as well as the numerous forms that occur in
freshwater lakes and streams of almost every country. They include also
some of those peculiar and archaic survivals which used to be called 'ganoids'
(pp. 287, 326)-e.g. the sturgeons, Garpike (Lepisosteus) 1 and the Bowfin (Amia)
(p. 292). Other survivors of ancient lines, Polypterus and Erpetoicltthys
(p. 290), which were once placed with the crossopterygians-lung-fishes (p. 361)
and crelacanths (p. 356)-are now believed to be actinopterygians.
The earliest known actinopterygians came (as did the crossopterygians)
from freshwaters of the mid-Devonian. The groups are undoubtedly older
than the strata from which they were described. It is a curious fact that
while only very few osteichthyian fragments of dubious early Devonian age
have been discovered, great numbers of diverse species had appeared by the
mid-Devonian and they actually dominated the freshwaters of the time.
Whether the Crossopterygii and the actinopterygians are of immediate
common ancestry is uncertain. It is likely that some common progenitor
did in fact exist in Upper Silurian or in very early Devonian waters. Each
group had already diversified widely and characteristically by mid-Devonian,
from which the earliest known indisputable examples come. The actinoptery-
gians were at first much rarer than the crossopterygians, but to-day they are
among the commonest vertebrates. The crossopterygians, on the other hand,
have dwindled drastically, until to-day only four genera, including a mere
handful of species, survive.
The Actinopterygii get their name from the structure of the paired fins, as
seen in the more modem representatives of the group. The ancestral animals
were more heavily scaled. Further, they were clumsier, and probably slower,
than the more advanced and streamlined bony fishes of to-day. Unlike the
paired fins of the Chondrichthyes (or the archipterygial fins of certain other
1 This animal is usually called 'Lepidosteus ', but the original orthography is Lepisosteus and
this is here used, with strong misgivings.
 PHYLUM CHORDATA

fishes which have cartilaginous or bony basal supports projecting outside the
body-wall), the modern actinopterygian fin has these supports reduced almost
to vanishing point. Further, the fin-web lying outside the body-wall is sup-
ported by the fin rays alone. The Polypterini and Chondrostei (sturgeons)
(p. 288), and some of the early palreoniscoids (p. 285) which are somewhat
more primitive as regards their fin structure, are exceptions.
The structure of the scales, being ganoid as opposed to cosmoid of early
Crossopterygii (see p. 353), is an important diagnostic feature. The scales of
most of the group, however, especially in the more recent forms, have gradually
become modified by the loss of many elements, and may be reduced to thin
horny structures, or may even be absent. The tail is at first heterocercal,
i.e. the lower fin-lobe is disproportionately developed, and is supported only
by the hypurals. It next becomes semi-heterocercal, and finally homocercal
(Fig. 204, p. 302) or, more rarely, diphycercal (Fig. 251, p 365). The investing
bones of the skull and jaws can be compared with those of the Crossopterygii,
but show a certain amount of numerical reduction and modification in shape.
The earlier forms, judged by what we can see in the persisting species of
the more primitive groups (such as the sturgeons, Polypterini, Amia, and Lepi-
sosteus), possessed spiracles, abdominal pores, and a spiral valve in the gut.
More advanced species have lost these structures. The respiratory arrange-
ments have changed from a gill-condition near the laminar type to the fully
filamentar (Fig. 73, p. ro6). In exchange for a large valvular conus in the heart,
a prominent elastic bulbus arteriosus has been evolved, the conus becoming
reduced to one or, at most, two rows of valves. There is a gradual alteration
of the genital ducts towards the specialised type of the teleosts (p. 293). The
brain structure is characteristic in the large corpora striata, cerebellum, and
medulla, presence of the valvula cerebelli, and in the absence of cerebral
lobes.
The actinopterygians in general show no trace of internal nostrils. In the
few forms that possess them (p. 339) such structures are not homologous with
those of lung-fishes and tetrapods (p. 381). The more primitive actinoptery-
gians, again unlike the lung-fishes, have only one dorsal fin (instead of two).
Although no bony fish reaches a size rivalling that of the biggest sharks (p. 219),
the chondrostean Huso huso (a sturgeon) grows to a length of about 24
feet and may weigh as much as 2,ooo lbs. A few bony fishes are as ferocious
as the most dangerous sharks. At least two kinds-the marine barracudas
and the fresh-water Piranha (Caribe or Characin-fish) of South America-
will attack Man without fear, or provocation. The barracudas (Sphyrama)
are swift tropical and sub-tropical forms with dagger-like teeth. Some grow
to a length of about eight feet and are much feared in West Indian and Central
American waters. The river-dwelling Piranha (Serrasalmus) rarely exceeds two
feet in length but hunts in large shoals. It possesses powerful jaws with
 ZOOLOGY
cutting teeth that can inflict grievous injury. Like sharks (p. 275) the Piranha
is said to be acutely sensitive to the smell of blood.
A pineal aperture, common in early crossopterygians, rarely occurs. Little
muscle or other soft tissue extends into the paired fins. These depend essen-
tially on flexible dermal rays for their support. The differences in scale
<;tructure have been mentioned above and elsewhere (p. 83).
As stated previously, the actinopterygians seem to have arisen in fresh
water, as did other bony fishes. There was some invasion by them of the sea
as early as Palreozoic times, and in the Triassic a great marine radiation oc-
curred, a trend that has been continued ever since. The numerous freshwater
teleosts of to-day have arisen as the result of former reinvasions of rivers and
lakes.
As in most vertebrates, stories relating to the longevity of fishes are generally
grossly exaggerated. Pike and carp are traditionally claimed to live for as
long as rso to 200 years, but no reliable records are available. Regan has
suggested that pike of 6o or 70 lb. may be 'at least as many years old' (cited by
Norman).
As usual, there is a variety of opinions concerning the most revealing
classification within the group. In the arrangement preferred below, the
assembly is divided into three super-orders, for which are retained the old and
familiar names Chondrostei, Holostei, and Teleostei. At the same time we
must remember that these names in no way truly reflect the history of actino-
pterygian-and osseous-development as was believed when they were first
assigned.

SUB-CLASS ACTINOPTERYGII
Super-order Chondrostei
Orders Palreoniscoidea (Devonian-Recent)
Acipenseroidei (] urassic-Recent)
Polypterini (Eocene-Recent)
Super-order Holostei
Orders Semionotoidea (Upper Permian-Recent)
Pycnodontoldea (Upper Triassic-Eocene)
Aspidorhynchoidea (Upper ] urassic-Cretaceous)
Amioidea (Triassic-Recent)
Pholidophoroidea (Triassic-Cretaceous)
Super-order Teleostei 1 (Lower Jurassic-Recent)
Orders Isospondyli ( = order Clupeiformes)
Haplomi ( = sub-order Esocoidei)
Iniomi ( = order Scopeliformes)
1 This classification is essentially that of Regan. The more modern arrangement of Berg is
given in brackets.
 PHYLUM CHORDATA 285
Orders Miripinnati
Giganturoidea ( = order Giganturiformes)
Lyomeri ( = order Saccopharyngiformes)
Ostariophysi (=order Cypriniformes)
Apodes ( = order Anguilliformes)
The above group is composed of' soft-rayed' orders (e.g. Fig. 201).
Orders Heteromi ( = orders Halosauriformes and N otocanthiformes)
Synentognathi (=.order Beloniformes)
Salmopercre (=order Percopsiformes)
Microcyprini ( = order Cyprinodontiformes)
Solenichthyes ( = order Syngnathiformes)
Anacanthini (=orders Gadiformes and Macruriformes)
Allotriognathi ( = order Lampridiformes)
The above group is composed of' intermediate' orders.
Orders Berycomorphi ( = order Beryciformes)
Zeomorphi ( = order Zeiformes)
Percomorphi ( = order Perciformes)
Gobiomorphi ( = sub-order Gobioidei)
Scleroparei ( = sub-order Cottoidei)
Thoracostei ( = order Gasterosteiformes)
Hypostomides ( = order Pegasiformes)
Heterosomata (=order Pleuronectiformes)
Discocephali ( = order Echeniformes)
Plectognathi ( = order Tetraodontiformes)
Malacichthyes ( = order Icosteiformes)
Xenopterygii (= sub-order Gobiesocoidei)
Haplodoci (= sub-order Batrachoidei)
Pediculati ( = order Lophiiformes)
Opisthomi (=order Mastacembeliformes)
Synbranchii ( = order Synbranchiformes)
The above group is composed of' spiny-rayed' (e.g. Fig. 202) and allied orders.

SUPER-ORDER CHONDROSTEI
ORDER p AL/EONISCOIDEA
Palreoniscids and crossopterygians occur in the same Devonian deposits, but
the former, although less numerous at first, quickly came to outnumber the
crelacanths and lung-fishes, and by Carboniferous times (p. 3) were already
the dominant fresh-water vertebrates. The earliest known form is Cheirolepis,
which, as with most of its late Palreozoic allies, was only 6 or 8 inches long.
The primitive ray-finned Palreoniscoidea probably arose from some ancestor
common to themselves and the Crossopterygii. Palreoniscoids, however, had
286 ZOOLOGY
some points of difference from the osteolepids, ccelacanths, and Dipnoi on the
one hand, and others, perhaps less deep-seated, distinguish them from later
actinopterygians.
The main differences between the early actinopterygians and the crosso-
pterygians can be seen by comparing two such forms as PalaJoniscus and
Osteolepis. The former had large eyes with no more than four sclerotic plates,
as against the large eyes and many sclerotic plates of the latter. In PalaJoniscus

A.

B.
FIG. 191.-Super-order Chondrostei, Order Palreoniscoidea, Family Palreoniscidm. Palalonis-
cus. Upper: Reconstruction (after Nicholson and Lydekker). Lower : A, lateral, and B, dorsal,
aspect of h ead. Ant. antorbital; Br. branchiostegals; Clei. cleithrum; D. dentary; D. Pt.
dermo-pterotic; D. Sph. derma-sphenotic; E. Sc. extrascapulas; Fr. frontal; I. 0. infraorbitals;
Mx. maxilla ; Na. nostril ; Op. opercular; PA. parietal; P. op. preopercular; P.o. postorbital;
Pt. Ros. rostral; S. 0. supraorbitals ; Sub. 0. suborbitals; S. Clei. supracleithrum; S. op. sub.
opercular; S. Sc. suprascapular. (After Westoll.)

(Fig. 191) the maxilla had a wide postorbital extension which ran far backwards.
This was firmly supported by a large preopercular, which functionally replaced
the squamosal of Osteolepis. The cheek extended dorsally as far as the upper
surface of the skull and helped to obliterate the spiracular cleft. In Osteolepis
(Fig. 244, p. 355) the squamosal is a large bone. The preopercular was very
small, and the maxilla itself was smaller. The osteolepid skull was further
characterised by the internal nostril, the transverse meso-cranial joint (p. 358),
and by peculiarities of dentition. All of these are entirely unrepresented in the
actinopterygian skull. In the fins of palreoniscoids, while some of them had
 PHYLUM CHORDATA

a series of divergent radials which formed a short, rounded lobe, there was no
defined central axis. They were in no way archipterygial. The structure of
the scales was ganoid, not cosmoid (p. 83). Their arrangement on the tail
was characteristic : there was an apparent break of pattern in the lines of
scales on the upper tail-lobe; these appear to have to run in a different direc-
tion from those on the body. There was one dorsal fin only, compared with
two in osteolepids.
The palreoniscoids were essentially, though not exclusively, Palreozoic fishes.
They reached the peak of their development in the Carboniferous, and faded in
the Triassic when they were being replaced by Mesozoic offshoots which

FIG. 192.-Super.order Chondrostei,

Order Palreoniscoidea, Family Perleididre.
Helichthys. Lower Triassic. Upper :
Restoration of H. elegans. Lower:
Head. Clav. clavicle; Clei. cleithrum;
Man. lower jaw; Max. maxilla; Op.
opercular; Pre. Op. preopercular; Pt.
Clei. postcleithrum; Pt. Fr. postfrontal;
Pt. Orb. postorbital; Pt. tem. post-
temporal; Sup. Clei. supracleithrum;
Sub. Op. subopercular; Sup. tem .
supratemporal; Tab. tabular; X. un-
certain homology. (After Brough.)

specialised in various directions from the more primitive stock. Of these

offshoots only a bare remnant has persisted in the four lines to-day represented
by the sturgeons, Polyptertts, Lepisosteus, and Amia (see pp. 290-293). There
were, however, many other families evolved during the late Palreozoic
and Mesozoic, all pursuing their own evolutionary lines, only to become extinct.
Some primitive forms persisted until the Jurassic and early Cretaceous. All
these may be placed in a rather loosely defined group, the Holostei (p. 291).
However, such a group cannot be clearly defined from the earlier palreoniscoids
on the one hand, nor their later representatives from the t eleosts on the other.
It is largely a matter of various trends of evolution, but all the Holostei show a
general trend which is quite separable from that of the sturgeons (Chondrostei),
in that there is no considerable loss of bone nor reduction of jaws. The fin
structure and gradual loss of many primitive characters also differentiate
288 ZOOLOGY
them from Polypterus. The general trend of evolution away from their earlier
palreoniscoid ancestors is shown by the forward movement of the point of
suspension of the lower jaw and the greater freedom of movement of the
maxilla so as to allow a wider gape. The fins become more definitely actino-
pterygian, and the tail more and more homocercal. The whole skeleton
becomes more mechanically efficient.
The Palreoniscoidea therefore, for the present, must be regarded as a collec-
tion of families of separate lines of evolution whose actual relations one to
another are still uncertain. Several lines had already become differentiated by
the Devonian. These are so diverse that it is not possible to select one as
absolutely typical of the whole group. Cheirolepis, although probably the
earliest known species, was already specialised in several respects. The small
scales were atypical in shape, the eyes unusually small, the skull had a very
large posterior extension, and its dorsal surface was fiat instead of rounded.
Families that may be placed in this order include the following: I. Palreonis-
cidre (e.g. Palceoniscus (Fig. 191), Cheirolepis, Birgeria). 2. Platysomidre,
deep-bodied fishes (e.g. Cheirodus, Platysomus). 3· Saurichthyiidc.e, long-
bodied fishes (e.g. Saurichthys, Belonorhynchus). 4· Catopteridre (e.g. Cato-
pterus). 5· Perleididre (e.g. Helichthys (Fig. 192), Perleidus).
ORDER AciPENSEROIDEI
This order is represented at the present time by the sturgeons-viz.,
Acipenser (Fig. 193), a genus of about twenty species living in the rivers of
Europe, North America and Asia; Scaphirhynchus, the 'Shovel-nosed Sturgeon'

p cf:t"
FIG. 193.-Super-order Chondrostei, Order Acipenseroidei, Family Acipenseridre. Acipcnser.
One of the smaller sturgeons, the Sterlet (A. ruthenus) inhabits the Black and Caspian Seas. Stur.
geons are mostly marine. They ascend rivers to breed and a few spend all their lives in fresh
water. The tactile chin-barbels are a pointer towards the ground-feeding habits of the genus.
Theoretically all sturgeons caught in English waters are the property of the Monarch (decree of
Edward II, 1284- 1327). b. barbels ; c. f. caudal fin ; d. f. dorsal fin; pet. f. pectoral fin; pv. f.
pelvic fin; sc. scutes; v.f. ventral fin. (After Cuvier.)

of North America and Central Asia; Polyodon, the Spoonbill Sturgeon or

Paddlefish of the Mississippi, and Psephums, a spoonbill sturgeon from China.
The roe of various species of Acipenser is sieved, salted, and preserved for
the market as caviar. Sturgeons have colonised both salt and fresh water.
They stir up mud with the snout, and by means of variously modified buccal
and pharyngeal arrangements are able to scoop up, or suck in and filter, various
bottom-dwelling invertebrates.
 PHYLUM CHORDATA

The group as a whole shows a mixture of primitive and secondarily modi-

fied characters. In the first category are such features as the unconstricted
notochord, the persistence of basi- and inter-
dorsals and of basi- and inter-ventrals as
separate elements in the vertebrre (Fig. 194),
the retention of a clavicle in the shoulder-
girdle, the primitive structure of the fins with
their broad bases, the fulcra! scales on the
dorsal and caudal fin, the heterocercal tail,
the open spiracle and spiracular pseudo-
branch, the spiral valve and the arrangement
of the urinogenital organs. In the second
category are certain characters which are
more developed in the earlier than in the
later forms, and which are therefore known
to have become degenerate or lost. These FIG. 194.-Acipenser: Pre-caudal

include the reduction of the jaws (owing to vertebne. a. c. aortic canal formed by
the union of ingrowths from the basi-
the suctorial method of feeding which has and interventrals of opposite sides;
b. d. basidorsals; b. v. basiventrals;
been acquired), the loss of ganoine on the i. d. interdorsals; i. v. interventrals;
scales and head bones, the reduction of the n. notochord; n . c. neural canal; n. sp.
neural spine; nt. s. sheath of notochord ;
scales to a few rows on the sides of the body, P . parapophysis; r. rib ; s. n . foramen
for root of spinal nerve. (From Cam-
and the irregularity and loss of some of the bridge Natural History.)
head bones and secondary multiplication of
centres of ossification of others. There is also a general loss of ossification in
the whole skeleton.
FIG. I95· -Chon-
drosteus: Skull and
pectoral girdle. C. aci -
penseroides. Ant. Br .an-
terior branchiostegal;
Ant. Orb. antorbital; C.
Hy . ceratohyal; Clav.
clavicles; Clei. clei-
thrum; Fr. frontal; I.
T em. intertemporal;
]u. jugal; Lac. lachry-
mal; Mx. maxilla;
Op. opercular; P. 0 .
postorbital; P. Tem.
posttemporal ; Pt. Fr.
postfrontal ; S. Clei.
supracleithrum; s. op.
subopercular; S. T em.
supratemporal; Tab.
tabular. (After Wat-
son.)

A Jurassic species, Chondrosteus acipenseroides (Fig. 195), which reached a

length of about 3 feet, was the earliest known true sturgeon. It was transitional
in structure, and although degeneration had already set in, was probably of
290 ZOOLOGY
palreoniscoid origin through some such form as Coccocephalus. Fishes of Poly-
odon affinity occurred in the Cretaceous. Their ancestry, however, is obscure.

ORDER POLYPTERINI (CLADISTIA)

Few fossils are known, but the Cladistia are obviously survivals from an
ancient type. Polypterus (Fig. rg6) and Erpetoichthys, the only known
members of the order, were once classed as crossopterygians largely be-
cause of the appearance of the paired fins and of the arrangement of the paired
ventral lung-like air bladder and its outlet into the pharynx. This structure
may have helped the survival of Polyptems in a periodically drought-stricken
environment. Spiracles persist and serve to expel excess water gulped in
through the mouth. In Erpetoichthys a pharyngeohypophysial duct remains
open throughout life; in Polypterus it is closed in the adult. The balance of

FIG. 196.-Super-order Chondrostei, Order Polypterini, Family Poly-

pteridre. Polypterus bichi-r. The Central African Polypterus (a nd t he B
closely related West African Erpetoichthys) , together with the paddle-
fishes (Poly odon) are aberrant freshwater 'chondrostean' survivors. See
also sturgeons (Fig. 193). A, entire animal; B, ventral view of throat.
an. anus; br. m. branchiostegal membrane; c. f. caudal fin; d. f. dorsal
finlets; j'ug. pl. gular plates; na. nostril; pet. f . pectoral fin; pv.f. pelvic fin; u. f. ventral fin .
(After Cuvier.)

evidence favours an origin from some palreoniscoid stock. The thick scales, for
instance, are of typically ganoid form. The skull lacks the division into the
orbito-ethmoidal and occipita-otic regions so characteristic of the crossoptery-
gian skull (p. 368). The position of the nostrils on the dorso-lateral surface of
the snout is typically actinopterygian, as is the anatomy of the soft parts such
as a pyloric crecum in the gut and the teleostean-like character of the urino-
genital organs. The teeth are simple in structure, and are without the peculiar
structure and method of replacement found in the Crossopterygii. The
structure of the paired fin is peculiar, with its long posterior axis and preaxial
radials (Fig. 252, p. 367) and muscular lobe lying outside the body-wall. The
resemblance to the fin of the Chondrichthyes is superficial ; and it is, however,
quite unlike the archipterygial type of the Crossopterygii. I t can be interpreted
as a modification of the basic actinopterygian pattern. On the other hand,
less modified form of the scales, the fin structure just mentioned, and various
characters of the skull prevent any close association of these two fishes with
other Actinopterygii and, for the present, they are best considered as represent-
ing a persistent line of palreoniscoid affinity.
 PHYLUM CHORDATA 29I

There are nine species of the genus Polypterus from the Congo, Nile, Niger,
and Senegal river systems. The West African Erpetoichthys ( = Calamoichthys)
calabaricus is small and more elongated, but otherwise very similar. The most
noticeable features are the rhomboid scales, the presence of a pair of gular plates
between the rami of the lower jaws, the lobed pectoral fins, the absence of
ventral fins, the abbreviated heterocercal tail, and the division of the dorsal fin
into a series of repeated finlets ('sails'), each supported by an anterior spine.

SUPER-ORDER HOLOSTEI
As mentioned above this is an extensive, extremely varied and possibly very
artificial group of ray-finned fishes that present features that seem to indicate
a position between the Chondrostei (p. 285) and the teleosts (p. 293). Thus,
the Holostei represent a fairly broad grade of development rather than a
demonstrably related, however polymorphic, group. Such fishes possess an

FIG. 197.- Super-

order Bolostei, Order
Semionotoidea, Family
Semionotidm. Dapedius
politus. This family,
containing also Lepidotus,
were common holosteans
in Mesozoic seas. (After
Smith Woodward.)

abbreviated heterocercal tail which sometimes superficially resembles the homo-

cereal (Fig. 204, p. 302) appendage of later types; but the internal tail-
structure remains unmistakably heterocercal. The scales are greatly modified,
in that they have lost the middle layer of cosmine, yet the outer ganoine layer
is retained in varying degree. There is a considerable loss of bony fin-rays.
The spiracle has disappeared, and in most no trace remains of a clavicle. There
is also a single dorsal swim-bladder (p. 339) principally of hydrostatic signi-
ficance, but which is said to possess some slight respiratory function in modern
representatives. The Holostei show a considerable degree of uniformity in
the reduction, modification, and freedom of the maxilla, together with striking
changes in the preopercular and orbital regions. These signalled the develop-
ment of the more advanced teleost architecture with which we will become
familiar later on (p. 304) and which achieved dominance in the Cretaceous
(Table I, p. 3).
The Holostei probably appeared first in the late Upper Permian and
 ZOOLOGY
Triassic (e.g. Semionotus) and became very plentiful in the Jurassic (e.g.
Lepidotus, Dapedius) (Fig. 197). As far as is known, they ranged from a few
inches to a few feet in length. Two restricted fresh-water genera, Lepisosteus
and Amia, linger on to-day.
ORDER SEMIONOTOIDEA

This group arose (Family Semionotidre) in the late Palreozoic and faded
in the Cretaceous but still persists in a sole surviving genus of gar-pikes (Lepi-
sosteus) of the fresh water of North and Central America. The second family
(Lepisosteidre) arose in the later Cretaceous or early Tertiary when the former
was dying out.
Lepisosteus (Fig. 198) shows a number of primitive features, such as a long
valvular conus to the heart and a corresponding absence of a bulbus arteriosus,
a tail still hemi-heterocercal, an air bladder with cellular walls, and thick

P'd'
FIG. 198.-Super-order Bolostei, Order Semionotoidea, Family Lepisosteidm. Lepisosteus.
The 'Bony Pike' or Garpike (L. platystomus) and the bowfins (Amia, Fig. 199); these swift, pre-
daceous freshwater fishes are the only surviving holostean genera. c. f. caudal fin; d. f. dorsal
fin; fl. fulcra ; l. l. lateral line; pet. f. pectoral fin; pv. f. pelvic fin; v. f . ventral fin. (After
Curvier.)

rhombic scales with a complete covering of ganoine. Fulcra! scales are still
retained, and traces of a clavicle are identifiable. On the other hand, the
central jugal plate has been lost, and the spiracle is closed. The presence of
pyloric creca and the structure of the generative organs are teleostean features.
Some specialised characters appear to be peculiar to the genus : the reduced
maxilla and its functional replacement by a number of tooth-bearing infra-
orbital elements ; the numerous small cheek plates and, above all, the opis-
thocrelous vertebrre as opposed to the much more common amphicrelous
vertebrre of most actinopterygian fishes.
There are several living species of Lepisosteus, of which L. tristachus is said
to attain a length of 10 feet.

ORDER AMIOIDEA

This may be the typical holostean group. Mesozoic marine representatives

of at least three families were extremely plentiful and a Tertiary fresh-water
family (Amiidre) persists to-day as a single North American species, the Bow-fin
(Amia, Fig. 199). Like the gar-pikes, Amia shows a mixture of primitive and
advanced characters. Of the former the lung is dorsal, but bilobed and cellular,
 PHYLUM CHORDATA 293
so that a certain amount of air-breathing is still possible. A vestigial spiral
valve is present in the intestine. A small valvular conus occurs in the heart, but
there is also a considerable bulbus arteriosus. There is one large jugal plate
between the rami of the lower jaws and well-developed branchiostegal rays as in
teleosts. Unlike the teleost con-
B
dition, there are no pyloric creca;
and the lower jaws still retain the
full number of elements. Of pro-
gressive characters, the scales have
become thin, cycloid, and have lost
the ganoine layer; the fins have
lost all fulcra ; the tail is practi-
cally homocercal ; and the ver-
FIG. 199.-Super-order Holostei, Order Amoidea.
tebrre solid and amphicrelous ex- Family Amiida!. Arnia. Bowfin (A. calva). The
cept in the tail region where single surviving species inhabits North American
freshwater lakes and rivers. The scales have lost
separate basi-dorsals and basi- theirganoine (p. 84). A, Entire animal; B, ventral
view of throat. br. m. branchiostegal membrane;
ventrals still occur. c. f. caudal fin; d. f. dorsal fin; jug. pl. jugular plate;
In addition to the two orders, pet. f. pectoral fin; pv. f. pelvic fin; v. f. ventral fin.
(After Giinther.)
above mentioned, there were also
the extinct Pycnodontoidea, Aspidorhynchoidea, and Pholidophoroidea. The
last-named is perhaps the most interesting in that it gave rise to the Leptolepidre
(see below) which may have contained the ancestors of the teleosts. Although
this family is generally considered to be teleostean, some authorities believe
its members (e.g. Leptolepis) were no more than advanced examples of the
Holostei.

SUPER-ORDER TELEOSTEI
The teleosts, of more than 25,000 living species, are an immensely successful
group. They range through nearly all ocean depths and in the coldest polar
seas and have colonised practically every freshwater habitat (including sub-
terranean waters inhabited by relatively unpigmented, blind representatives)
except those that freeze or are supplied by very hot thermal springs. The
economies of whole tribes of mankind depend on them, and great industries
have been established to exploit them. Teleosts present an almost bewildering
-sometimes grotesque--diversity of form ; and an astonishing-and again
sometimes grotesque-range of psycho-physiological adaptations (cf. Fig. 202,
p. 297) that make those of most birds seem relatively unremarkable. Some
teleosts regularly migrate between extremes of salt and fresh water. Some
occur in inestimable numbers. Teleosts are of outstanding practical importance
to mankind.
True teleosts probably appeared first in the lower Jurassic (e.g. Leptolepis,
Fig. 200), and were certainly very abundant in the salt waters of the Cretaceous,
294 ZOOLOGY
in which they sometimes attained a length of more than IS feet (e.g.
Portheus). It is generally considered, but it has not been proved, that they
were marine in origin. Certain it is that they are an offshoot of the holostean

Par> St.it Exsc Sse

\ I
\ I ~_,_.....,.....;.,.._
Na
Enpt
\
'Rostr>' \
\
Pmx \

I
I I I \
1 I I 1 lop
I I; I
Dent Smx Pop
FIG. 2oo.-Super-order Teleostei, Order Isospondyli, Family Leptolepidre. Leptolepis. Lepto-
lepis bronni, was a small Jurassic fish that seems to be near the a ncestral stem of the modern
teleosts. Upper: restoration. In some genera hypurals were absent (see Berg) . Lower: skull.
Ang. art. angulo-articular; Cl. cleithrum; Co. circumorbital; Dent. dentary; Dsph. derma-
sphenotic; Eept. ectopterygoid; En pt. entopterygoid; Exse. extrascapular; Fr. frontal; I op.
interopercular; Mpt . m etapterygoid; Mx. maxilla ; Na. nasal; Op. opercular; Par. parietal;
Pel. postcleithrum; Pmx. premaxilla ; Pop . preopercular; Psph. parasphenoid; Q. quadrate;
Rostr. rostral bone; Sel. supraclcithrum; Smx. supramaxilla; So. supraorbital; Sop. subopercular;
Sse. suprascapular; St. it. supratemporo-intertemporal; Subo. suborbital; Sym. symplectic.
(After Rayner.)

phase of evolution, but the process of separation was gradual and a definite
line of demarcation is not easy to make.
During the very early Tertiary the teleosts expanded enormously, and
replaced the older phase of evolution so that at the present time they are, with
 PHYLUM CHORDATA 295

the few exceptions already noted (p. 283), the only bony fishes in existence.
The full tide of evolution has not yet been reached ; new species are still in pro-
cess of formation. Teleosts provide striking examples of adaptive radiation
(p. 321).
The marks by which a teleost, whatever its shape and appearance, can be
recognised are partly in the loss of certain characters that are still preserved by
the more primitive actinopterygians, and partly the presence of a number of
structures which are new or, at least, definite modifications of old ones. The
diagnostic features of the teleosts essentially represent the 'further expression
of evolutionary tendencies seen at work in the lower ray-finned fishes' (Romer).
The characters that are lost are : spiracle, spiral valve, conus arteriosus
(reduced to one, or at most two, rows of valves; for this reduction the great
enlargement of the bulbus arteriosus compensates). The swim-bladder (where
retained) is essentially a hydrostatic organ but may be adapted to respiratory,
or other functions such as sound production (p. 346). Ganoine disappears
from the exoskeleton and scales. There are no longer fulcra! scales on the fins.
Gular bones, with very few exceptions where they are much reduced, have dis-
appeared or, rather, have been replaced by branchiostegal rays (Fig. 304).
Dermal bones tend more and more to sink beneath the skin, and to come into
closer association with the endocranium than is the case with the more primitive
palreoniscids and crossopterygians. The bones of the lower jaw are reduced
to three elements only: the dentary, angular, and articular. The pectoral
girdle has lost the clavicle.
Of new or highly modified characters, the gills are now fully filamentar
(p. 299), the tail is externally (and in many cases internally) homocercal, or
even gephyrocercal; but in a slight internal uptilting it still retains a hint of
its heterocercal origin (p. 283). (In development too, it shows distinctly its
origins from fishes with a heterocercal tail.) The endoskeleton has become
almost wholly ossified. The skull, which shows an extreme range of variation
in shape, nearly always has a single prevomer, and at the posterior dorsal
surface a single supraoccipital bone. This element, sometimes stated to
be formed by the fusion of the neural arches of some of the anterior vertebrre,
is insignificant or absent in more primitive groups. Further drastic modifica-
tions appear in the shift of upper crushing teeth (when present) to the para-
sphenoid; the corresponding lower ones are attached to elements of the
pharyngeal floor. The vertebrre are amphiccelous. There are additional 'ribs'
known as the intermuscular bones. In the paired fins the radials are reduced
to a few brachial ossicles, and the fin is completely fan-like, and without any
trace of an axis. The urinogenital organs have become highly specialised in
that both ovaries and testes have acquired new ducts. In the male there is no
longer any connection between the testis and mesonephros. In the female in
some cases there is still an open-mouthed oviduct leading from the ccelom,
296 ZOOLOGY
but usually the oviduct surrounds the ovary, so that the ova pass directly
down to be discharged without being shed into the ccdom.
The brain is highly specialised. The upper walls of the cerebrum are very
thin. (The enlarged corpora striata on the ventral surface show through and
give a false impression of cerebral hemispheres.) The mid- and hind-brains,
however, are often very large. There is a great development of the valvula
cerebelli (Fig. 212) , and the optic nerves cross over outside the brain without
mixing (the absence of the so-called optic chiasma) .

FrG. zor.-Super.order Teleostei, Order Isospondyli, Family Salmonidre. Salmo. Example

from a' soft-rayed' order. Brook Trout (S. t r utta). a . I . adipose lobe o f p elvic fin ; an. anus; c. f.
caudal fin ; d. f. I , first dorsal fin ; d. f. 2, second dorsal or a d ipose fin ; 1. 1. lateral line; op.
operculum ; pet. f. pectora l fin ; pv. j . pelv ic fin; v. f . a nal tin. (After J ardine.)

Teleosts fall essentially into three groups, (r) generalised soft-rayed orders,
(2) spiny-rayed and related forms, and (3) a loose assembly of intermediate
orders.

Soft-ray ed orders:
These include the Isospondyli (e.g. Salmo , trout (Fig. 201); Clupea, her-
rings); Haplomi (freshwater fishes, e.g. E sox, pikes); Ostariophysi ( = Cyprini-
formes) (mostly freshwater fishes, e.g. Cyprinus, carps; Serrasalmus, Piranha ;
Electrophoms, 'electric-eels,' Clarias, catfish; Rhodeus, bitterlings) ; and
Apodes ( = Anguilliformes) (A nguilla, eels) are the best known. Additionally
there are the marine orders Iniomi (e.g. Aulopus, Australian Sergeant Baker),
Miripinnati (a newly erected order, e.g. Miripinna), Giganturoidea (single
genus Gigantua), and Lyomeri (of two families only, e.g. Saccopharynx) .

Intermediate orders:
A group of orders, certain of which are sometimes placed in an 'Order'
Mesichthyes, contains fishes that are distinct from the more primitive soft-
rayed teleosts on the one hand, but have not attained the specialisation of the
spiny-finned fishes on the other. These comprise the following: Heteromi
(deep-sea oceanic fishes, e.g. Lipogenys); Synentognathi (e.g. E xoccetus,
flying-fishes, or Belone, garfishes) ; Salmopercre (three genera of North American
freshwater fishes, e.g. Percopsis); Microcyprinre (freshwater and marine
 PHYLUM CHORDATA 297
fishes, e.g. cyprinodont Fundulus of North America), Solenichthyes (e.g.
Hippocampus, sea-horses), Anacanthini (e.g. Gadus, cods; Merluccius, Hake),
and Allotriognathi (e.g. Regalecus, Oar-fish).

Spiny-rayed and allied orders:

These comprise the following: marine Berycomorphi (e.g. Traichthodes,
Australian Nanygai) and Zeomorphi (e.g. Zeus, John Dory), marine and fresh-
water Percomorphi (e.g. Perea, perch; Cichlidre, e.g. Tilapia (Fig. 202)) ; Scom-
bridre, including tunny, mackerel, bonito; Sphyrrenidre, barracudas; Australian

FIG. 202.-Super-order Teleostei, Order Percomorphi, Family Cichlidlle. Tilopia: ExAmple

from a 'spiny-rayed' order. Most members of the genus Tilapia are mouth-brooders. For some
days after hatching the young ofT. mossambica still take refuge in the female buccal cavity in times
of danger (p. 352). Redrawn from an F.A.O. photograph. (Some young have been eliminated for
the sake of clarity.)

Murray Cod, Moccullochella; marine and sometimes freshwater Gobiomorphi

(e.g. Gobius, gobies); Scleroparei (e.g. Triglidre, gurnards; Synanceja, stone-
fishes); Thoracostei (e.g. Gasterosteus, sticklebacks); the marine Hypostomides
(toothless, bony-armoured Indo-Pacific fishes, e.g. Pegasus, Acanthopegasus);
Heterosomata (e.g. Psetta, turbot and brill; Solea, soles and other flat-fishes);
the marine Discocephali (e.g. Echeneis and Remora, sucker-fishes); the marine
and fluviatile Plectognathi (e.g. Balistes, trigger-fish); the marine Malaci-
chthyes (single family (Costeidre)), Xenopterygii (e.g. Gobiesocidre, cling-
fishes), Haplodoci (e.g. Porichthys, toad-fishes) and Pediculati (angler-fishes);
the freshwater Opisthomi (of Africa, e.g. Mastacembelus, and southern Asia);
VOL. II. T
 ZOOLOGY
and the superficially eel-like marine and freshwater Synbranchii (e.g. land-
going Amphipnous).
It should be remembered that not all systematists approve of the re-
presentation set out above.

EXAMPLE OF A TELEOST FISH.- THE BROOK OR BROWN TROUT

(SALMO TRUTTA)
The Brook Trout is common in the rivers and streams of Europe, and has
been acclimatised in other parts of the world. It varies greatly in size
according to its environment, but it may attain sexual maturity, and
therefore be considered adult at a length of r8-2o em. In large lakes it
may grow to nearly a metre in length. Other allied species such as the Salmon
(S. salar), the Char (Salvelinus alpimts), the American Brook Trout (S.
fontinalis), are common in the Northern He"misphere and of course differ from S.
trutta only in detail.
External Characters.-The body (Fig. 201) is elongated, compressed,
thickest in the middle and tapering both to the head and tail. The mouth
is terminal and very large. The upper jaw is supported by two freely movable
bones, the premaxilla (Fig. 205, p. 304) in front and the maxilla (max.)
behind. Both bear sharp curved teeth arranged in a single row. When the
mouth is opened a row of palatine teeth is seen internal and parallel to those of
the maxilla, and in the mid-line of the roof of the mouth is a double row
of vomerine teeth. The lower jaw is mainly supported by a dentary and
bears a row of teeth. On the throat each ramus of the mandible is bounded
mesially by a deep groove. The floor of the mouth is produced into a promi-
nent tongue bearing a double row of teeth. In old males the apex of the
lower jaw becomes curved upwards like a hook.
The large eyes have no eyelids. The flat cornea is covered by a transparent
layer of skin. A short distance in front of each of the eyes is the double
nostril. Each olfactory sac has two external apertures, the anterior one
provided with a flap-like valve. There is no external indication of the ear.
On each side of the posterior region of the head is the opercttlum (Fig. 205,
op.) or gill-cover, a large flap which, when raised, displays the gills. Between
it and the flank is the large crescentic gill-opening from which the respiratory
current emerges. The operculum is not a mere fold of skin, as in Holocephali,
but is supported by three thin bones. The outlines of these can be seen
through the skin. They are the opercular (Fig. 205, op.), sttb-opercular, and
inter-opercular (inter. op.); the last is attached to the angle of the mandible.
The pre-opercular (pre. op.) is not an opercular support but is one of the cheek-
bones following the outline of the hyomandibular. The ventral portion of the
operculum is produced into a thin membranous extension, the branchiostegal
membrane, which is supported by twelve flat, overlapping bones, the branchi-
 PHYLUM CHORDATA 299
ostegal rays. The narrow area on the ventral surface of the throat which
separates the two gill-openings from one another is called the isthmus. The
gills, seen by lifting up the operculum, are four red comb-like organs, each
having a double row of free gill-filaments. Alternating with the gills are the
five vertically elongated gill-slits which open into the pharynx.
The Trout breathes by drawing water in through the mouth and passing it
outwards over the gills. Inspiration is effected by moving the gill-covers out-
ward and depressing the floor of the mouth. The internal volume is thereby
increased and the pressure decreased. With the buccal cavity thus widened,
water flows in through the open mouth, compensating for the changed pressure.
The branchiostegal membrane then extends and closes the posterior opening into
the branchial chamber by filling the space between the free margin of the
operculum and the body. This prevents any inflow of water from behind the
head. This membrane is so arranged that water under pressure can neverthe-
less pass from the branchial chamber to the exterior. Expiration occurs when
the gill-covers move inwards and the floor of the mouth is raised to force the
water out of the branchial cavity and over the gills to the exterior. The
action of a pair of transversely directed membranous folds, the respiratory
valves, controls the direction of the respiratory current. One of these valves is
attached to the roof, the other to the floor of the mouth. They can become
expanded to block the passage when water presses on them from behind.
Thus the water must pass over the gills (Fig. 73, p. 106).
On the ventral surface of the body, at about two-thirds of the distance
from the snout to the end of the tail, is the anus (Fig. 201, an.). Behind it is
the urinogenital aperture leading into the urinogenital sinus. Into this both
urinary and genital products are discharged.
The region from the snout to the posterior edge of the operculum is the
head. The trunk extends from the operculum to the anus. The post-anal
region is the tail.
There are two dorsal fins. The anterior dorsal (Fig. 201, d. f. I) is large
and triangular, and is supported by thirteen bony fin-rays. The adipose
dorsal (d. f. 2) is small and thick, and is devoid of bony supports : it is dis-
tinguished as an adipose fin, a structure that occurs in a very few families of
teleosts. The caudal fin (c. f.) differs markedly from that of most Chondri-
chthyes in being, as far as its external appearance is concerned, quite symmetri-
cal. It is supported by fin-rays which radiate regularly from the rounded end
of the tail proper. Such outwardly symmetrical tail-fins are called homocercal.
There is a single large anal fin (v. f.) supported by eleven rays. The pectoral
fin (pet. f.) has fourteen rays and is situated, in the normal position, close
behind the gill-opening, but the pelvic fin (pv.f.) has shifted its position and lies
some distance in front of the vent. It is supported by ten rays, and has a
small process or adipose lobe (a. l.) springing from its outer edge near the base
300 ZOOLOGY
The body is covered by a soft, slimy skin through which, in the trunk and
tail, the outlines of the scales can be seen. On the head and fins the skin is
smooth and devoid of scales. A well-marked lateral line (l. l.) (p. 136) extends
along each side from head to tail, and is continued into branching lines on the
head. These run above and below each eye, along the lower jaw, and there is a
well-marked line across the posterior aspect of the skull. The skin is grey
above, shading into yellowish below. It is covered with minute black melano-
phores which, on the sides and back, are aggregated to form round spots two or
three millimetres in diameter. In young specimens orange-coloured spots also
occur.
Skin and Exoskeleton.-The epidermis is a squamous stratified epithelium
and contains mucous glands and pigment cells (p. 82). Beneath the epidermis,
the scales (Fig. 224, p. 329) are lodged in pouches of the dermis. They have
the form of fiat, nearly circular plates of bone. There are no Haversian canals,
lacunre or canaliculi (p. 73). This bone is laid down in concentric rings which
represent the annual growth stages. Such rings represent differential seasonal
growth-rates. Growth is faster in spring and summer and almost ceases during
winter. An examination of the scale-rings, or similar ones in the otoliths (p.
317), reveals the age of a fish. The scales have an imbricating arrangement,
overlapping one another from before backwards, like the tiles of a house, in such
a way that a small three-sided portion of each scale comes to lie immediately
beneath the epidermis, while the rest is hidden beneath the scales immediately
anterior to it. It will be seen that such teleost scales are very distinct from
the placoid scales of elasmobranch fishes which penetrate the epidermis and
give rise to a' shagreen' surface (p. 103). Besides the scales, the fin-rays belong
to the exoskeleton, but will be considered below (pp. 307, 329).
Endoskeleton.-The vertebral column shows a great advance in being
thoroughly differentiated into distinct bony vertebrce. It is divisible into an
anterior or trunk region and a posterior or caudal region, each containing about
twenty-eight vertebrre.
A typical trunk vertebra consists of a centrum (Fig. 203) (CN.) with deeply
concave anterior and posterior faces, and perforated in the centre by a small
hole. The edges of the centra are united by ligaments and the biconvex
spaces between them are filled by the gelatinous remains of the notochord.
Articulation between the arches is made by means of small bony processes,
the zygapophyses (N. ZYG., H. ZYG.). To the dorsal surface of the centrum
is attached, by ligaments in the anterior vertebrre, by ankylosis or actual
bony union in the posterior, a low neural arch (N. A.). This consists in
the anterior vertebrre of distinct right and left moieties, and is continued
above into a long, slender, double neural spine (N. SP.), directed upwards
and backwards. To the ventro-lateral region of the vertebra are attached by
ligament a pair of long, slender pleural ribs (R.) with dilated heads. These
 PHYLUM CHORDATA JOI

curve downwards and backwards between the muscles and the peritoneum,
thus encircling the abdominal cavity. In the first two vertebrre they are
attached directly to the centrum. In the rest they are attached to short down-
wardly directed bones, the parapophyses (PA. PH.), immovably articulated by
broad surfaces to the centrum. At the junction of the neural arch and centrum
are attached, also by fibrous union, a pair of delicate inter-muscular bones
(IM. B.). These extend outwards and back-
wards in the fibrous septa between the myo-
meres (p. 86). The first and second abdominal
vertebrre bear no ribs. In the last three the
neural spines (B, N. SP.) are single.
~s:•
B ~ HA
• H

In the caudal vertebrre the outgrowths r ...."

corresponding to the parapophyses are fused
with the centrum and unite in the middle
ventral line, forming a hcemal arch (C, H. A.),
Through this run the caudal artery and vein. HA
H.IYG

In the first six caudals each hremal arch bears 0

a pair of ribs (R.) ; in the rest the arch is pro-
duced downwards and backwards into a hcemal
spine (D, H. SP.).
The centra as well as the arches of the
vertebrre are formed entirely from the skeleto-
I
FIG. 203.-Sahno: Vertebra!. A,
genous layer, and not from the sheath of the one of the anterior, and B, one of
the posterior trunk vertebr<e; C, one
notochord as in elasmobranchs (pp. 231, of the anterior, and D, one of the pos-
and go). terior caudal vertebr<e. CN. cen-
trum; IM. B. intermuscular bone;
The posterior end of the caudal region is H. A. h<emal arch; H. SP. h<emal
spine; H. ZYG. h<emal zygapo-
curiously modified for the support of the tail- physis; N. A . neural arch; N. SP.
fin. The hindmost centra have their axes not neural spine; N. ZYG. neural
zygapophysis; PA. PH. parapo-
horizontal, but deflected upwards and follow- physis; R. pleural rib.
ing the last undoubted centrum is a rod-like
structure, the urostyle (Fig. 204). This terminal structure consists of the
partly ossified end of the notochord, which has thus precisely the same up-
ward flexure as in dogfishes. The neural and hremal spines of the last five
vertebrre are very broad and closely connected with one another. They are
more numerous than the centra ; and three or four hremal arches are attached
to the urostyle. In this way a firm vertical plate of bone is formed, to the edge
of which the caudal fin-rays are attached fanwise in a symmetrical manner.
It will be obvious, however, that this homocercal tail-fin is really quite as
asymmetric as the heterocercal fin of dogfishes, since, as its morphological
axis is constituted by the notochord, nearly the whole of its rays are, m
strictness, ventral.
The skull (Fig. 205) is an extremely complex structure, composed of
302 ZOOLOGY
mingled bone and cartilage. The cartilage has no superficial mosaic of lime-
salts such as we find in many Chondrichthyes, but certain portions of it are
replaced by cartilage (endochondral) bones, and there are in addition numerous
investing membrane or dermal bones developed in the surrounding connective-
tissue. As in dogfishes, the skull is divided into cranium, upper and lower
jaws, with their suspensory apparatus, and hyoid and branchial arches.

specialized
neural spine neural process epurals uroneurals
and process

dorsal rays

ventral rays

haema/ spine urosty/e hypurals f-3

and process
FrG. 204.-Teleostei: Caudal skeleton. The homocercal tail and its supports in a typical
isospondylid. This form of tail is supported by uroneurals as well as hypurals (cf. p. 283). The
number and arrangement of the skeletal elements are of great taxonomic significance. (Modified
after Hollister.)

The cranium (Fig. 205) is a somewhat wedge-shaped structure, its apex

being directed forwards. At first sight the distinction between replacing and
investing bones is not obvious. After maceration or boiling, however, certain
flat bones (the paired parietals, par., frontals, fron., and nasals, nas., and the
unpaired dermethmoid, d. eth.) can be easily removed from the dorsal
surface as well as two unpaired bones (the parasphenoid, par. sph., and
vomer) from the ventral surface. The above are investing bones. They
are simply attached to the cranium by fibrous tissue, and can readily be prised
off when the latter is sufficiently softened by artificial means. We thus get
a clear distinction between the cranium as a whole, or secondary cranium
(complicated by the presence· of investing bones), and the primary cranium,
 PHYLUM CHORDATA 303
neurocranium or chondrocranium (left by the removal of these bones) which
corresponds with the cranium of elasmobranchs.
The chondocranium (Fig. 206, p. 307) contains the same regions as that of
Scyliorhinus. Posteriorly the occipital region surround the foramen magnum.
Below that aperture is a single concave occipital condyle (for the first vertebra)
produced above into an occipital crest. The auditory capsules project outwards
from the occipital region. Between them, on the dorsal surface of the skull,
are paired oval fontanelles (Fig. 206, Fa.). In the entire skull these are closed
the frontal bones. The posterior region of the cranial floor is produced down-
wards into paired longitudinal ridges, enclosing between them a groove which is
converted into a canal by the apposition of the parasphenoid bone and serves
for the origin of the eye-muscles. In front of the auditory region the cranium
is excavated on each side by a large orbit. A vertical plate or interorbital
septum separates the two cavities from one another. In front of the orbital
region the cranium broadens out to form the olfactory capsules, each excavated
by a deep pit for the olfactory sac. Anterior to these is a blunt snout or
rostrum. The occipital region is formed as usual from the parachordals of the
embryonic skull, the auditory region from the auditory capsules, and the rest
of the cranium from the trabeculre.
The replacing or endochondral bones, formed as ossifications in the chondro-
cranium, correspond in essentials with the typical arrangement already de-
scribed (pp. 95-97). In the occipital region are four bones. These are the
basi-occipital, forming the greater part of the occipital condyle and the hinder
region of the basis cranii or skull-floor; the paired ex-occipitals (ex. oc.), placed
one on each side of the foramen magnum and meeting both above and below it ;
and the supra-occipital (s. oc.), forming the occipital crest already mentioned.
Each auditory capsule is ossified by five bones-i.e. two more than the typical
number (p. g6). These are I. the pro-otic (pr. at.), in the anterior region of the
capsule, uniting with its fellow of the opposite side in the floor of the brain-
case, just in front of the basi-occipital; 2. the opisthotic, in the posterior part of
the capsule, external to the ex-occipital; 3. the sphenotic (sph. at.) (partly
an investing bone), above the pro-otic and forming part of the boundary of
the orbit; 4- the pterotic (pt. at.) (also partly an investing bone), above the ex-
occipital and opisthotic, forming a distinct lateral ridge and produced behind
into a prominent pterotic process; and 5· the epiotic (epi. at.), a small bone,
wedged in between the supra- and ex-occipitals and pterotic, and produced
into a short epiotic process. On the external face of the auditory capsule,
at the junction of the pro-, sphen-, and pter-otics, is an elongated facet (Fig. 205,
hyo. m.). This is covered with cartilage and serves for the articulation of the
hyomandibular.
The trabecular region of the cranium contains six bones. Immediately in
front of the conjoined pro-otics, forming the anterior end of the basis cranii,
 hyo.m.
s. oc.
w
0
4>-

pal.

N
0
0
t""'
0
Gi
....:::

ang . met. ptg. inter op.

Ftc. 205.-Teleostei: Skull showing relationship with pectoral girdle. This skull combines characters which would not be found
togeth er in any order. For example, the median gular is prest>nt only in the most primitive isospondylid fishes. In these the pelvic and
pectoral girdles are no t in contact; the anterior position of the pelvic girdle (as sh own) being c ha ract eristic of more highly organised
orders. It is unlikely that the bones of the circumorbital series arc homologous with similarly named clements in higher vertebrates.
Heading clockwise from s nout : pre. max. premaxilla; pal. palatine ; nasal; d.e/11 . de rmethmoid; p . etll . parethmoid ; pre. Jro 11. prefrontal ;
par. sph. parasphenoid ; supr. orb. supraorbitals; orb. sph . orbitosphenoid; cir. orb. ser. circumorbital series (sec below); d. sph. dermos-
(Continued on bollo:n ofnexl page. )
 PHYLUM CHORDATA

is a small unpaired Y-shaped bone, the basisphenoid. Above it, and forming
the anterior parts of the side-walls of the brain-case, are the large paired
pterosphenoids (ali. sph.). In the interorbital septum is a median vertical
bone, representing fused orbitosphenoids (orb. sph.). Lastly, in the posterior
region of each olfactory capsule, and forming part of the boundary of the
orbit, is the lateral-ethmoid (ec. etlz.), (not shown in Fig. 205).
The investing bones already referred to are closely applied to the roof and
floor of the chondrocranium. They modify its form considerably by projecting
beyond the cartilaginous part, and by concealing apertures and cavities. The
great frontals (!ron.) cover the greater part of the roof of the skull, concealing
the fontanelles, and furnishing roofs to the orbits. Immediately behind the
frontals is a pair of very small parietals (par.). In front of them is an unpaired
dermethmoid (d. eth.). To the sides of this are attached a pair of small
nasals. On the ventral surface is the large parasphenoid (par. sph.), which
forms a kind of clamp to the whole cartilaginous skull-floor. In front of and
below the parasphenoid is the toothed vomer. Encircling the orbit is a ring
of scale-like bones, the circwmorbitals (c. orb. ser.).
In the jaws, as in the cranium, we can see both primary and secondary
structures. The primary upper jaw or palatoquadrate is homologous with the
upper jaw of the dogfishes. Hmvever, instead of remaining cartilaginous, it is
ossified by three replacing bones. The toothed palatine (pal.) is situated in
front, articulating with the olfactory capsule. In the 'pterygoid' region the
only replacing bone is the metapterygoid (met. ptg.) projecting upwards from
the quadrate, but between the quadrate and the palatine are two investing
bones, the ectopterygoid (ect. pt.) on the ventral, and the entopterygoid (en. ptg.)
on the dorsal edge of the original cartilaginous bar. The quadrate (quad.), at the
posterior end of the latter, furnishes a converse condyle for the articulation of
the lower jaw.
These bones, do not, however, enter into the gape, and so do not constitute
the actual upper jaw of the adult fish; external to them are two large investing
bones, the premaxilla (pre. max.) and the maxilla (max.), which together form
the actual or secondary upper jaw. Both these bones bear many teeth. A
phenotic; fran. frontal; spit. at. sphenotic; ali. sp. pterosphenoid ('alisphenoid'); par. parietal;
hyo. m. hyomandibular; s. oc. supraoccipital; epi. at. epiotic; sc. b. scale bone (supratemporal);
pt. at. pterotic (supratemporal in tetrapods); p. temp. post-temporal; ex. oc. exoccipital; s. clei.
supracleithrum; pre. op. preopercular; op. opercular; clei. cleithrum; post. cl. postcleithrum;
scapula; peel. pectoral fin; ptryg. pterygials; ventral or pelvic fin; pelvic girdle; cor. coracoid;
sub. op. subopercular; inter. op. interopercular; inter. hy. interhyal; brnstg. branchiostegals;
met. ptg. metapterygoid; sympl. symplectic; en. ptg. entopterygoid (true pterygoid); epi. hy.
epihyal; ttro. hy. urohyal; quad. quadrate; ang. angular; cer. hy. ceratohyal; ect. pt.
ectopterygoid ('pterygoid'); art. articular; sur. ang. surangular; basi. hy. basihyal; max. maxilla;
med. gul. median gular; sup. max. supramaxilla; dent. dentary.
The circumorbital series consists oflachrymal ( r), jugal (2), su barbitals (3-.5), dermosphenotic (6),
postfrontal, supraorbital, and prefrontal. The opercular series consists of opercular, subopercular,
interopercular, and branchiostegals. The vomer is not visible and the basioccipital is hidden by
the opercular series. The alisphenoid is often called the pterosphenoid and the surangular is often
called the supra-angular. (Modified after Gregory.)
 ZOOLOGY
small scale-like bone, the supramaxilla, is sometimes attached to the posterior
end of the maxilla.
The lower jaw is similarly modified. Articulating with the quadrate is a
large bone, the articular (art.). This continued forwards by a narrow pointed
rod of cartilage which is the unossified distal end of the primary lower jaw
or Meckel's cartilage. The articular bone is its ossified proximal end. There-
fore this bone is a replacing bone. Ensheathing Meckel's cartilage and form-
ing the main part of the secondary lower jaw is a large toothed investing bone,
the dentary (dent.). A small investing bone, the surangular (sur. ang.), is
attached to the lower and hinder end of the articular.
The connection of the upper jaw with the cranium is effected partly by
the articulation of the palatine with the olfactory region, partly by means of
a suspensorium formed of two bones separated by a cartilaginous interval.
The larger, usually called the hyomandibular (hyo. m.), articulates with the
auditory capsule by the facet already noticed. The small, pointed symplectic
(sympl.) fits into a groove in the quadrate. Both bones are attached by fibrous
tissue to the quadrate and metapterygoid (met. ptg.). In this way the suspen-
sorium and palatoquadrate together form an inverted arch. This is freely
articulated in front with the olfactory, and behind with the auditory capsule.
The total result is an extremely mobile upper jaw. As its name implies, the
hyomandibular (together with the symplectic) is commonly held to be the upper
end of the hyoid arch and the homologue of the hyomandibular of Chon-
drichthyes. However, there is some reason for thinking that it really belongs
to the mandibular arch, and corresponds with the dorsal and posterior part of
the triangular palatoquadrate of Holocephali. A perforation in the latter
would convert it into an inverted arch having the same general relations as the
upper jaw plus suspensorium of the Trout, but fused, instead of articulated,
with the cranium at either extremity.
The hyoid cornu is articulated to the cartilaginous interval between the
hyomandibular and symplectic through the intermediation of a small rod-like
bone, the interhyal (int. hy.), which is an ossification in a ligament. It is
ossified by three bones: an epihyal (epi. hy.) above, then a large ceratohyal
(cer. hy.), and below a small double hypohyal. The right and left hyoid bars
are connected by a keystone-piece, the unpaired basihyal (basi-hy.), which
supports the tongue. This bone is toothed in Salmo trutta.
Connected with the hyomandibular and hyoid cornu are certain investing
bones serving for the support of the operculum. The operwlar (Fig. 205, op.) is
articulated with a backward process of the hyomandibular. The sub-opercular
(sub. op.) lies below and internal to the opercular. The inter-opercular (inter. op.)
fits between the lower portions of the three preceding bones, and is attached
by ligament to the angle of the mandible. The sabre-shaped branchiostegal
rays are attached along the posterior border of the epi- and cerato-hyal.
 PHYLUM CHORDATA 307
Twelve such rays occur inS. trutta. Below the basihyal is an unpaired bone,
the urohyal (uro. hy.).
There are five branchial arches, diminishing in size from before backwards.
The first three present the same segments as in dogfishes : pharyngobranchial
above, then epibranchial, then a large ceratobranchial, and a small hypobranchial
below (Fig. zo6; and Fig. 158, p. 234). The right and left hypobranchials of
each arch are connected by an unpaired basibranchial. All these segments are
ossified by replacing bones. The basibranchials are connected with one
another and with the basihyal by cartilage so as to form a median ventral
bar in the floor of the pharynx. In the fourth arch the pharyngobranchial is

IJER

FIG. 2o6.-Salmo: Skull of juvenile. Second week

after hatching with investing bones removed. Au.
auditory capsule; Br. I, first branchial arch; Ch.
notochord; C. Hy. hyoid cornu; Fo. fontanelle ;
G. Hy. basihyal; H. Hy. hypohyal; H. M . hyo- FIG. 207.-Salmo: Dermal
mandibular; r. Hy. interhyal ; 11 , l 2 , labial carti- fin-ray and its supports. D. F. R.
lages; M ck. Meckel's cartilage; M. Pt. metapterygoid Proximal part of dermal fin-ray
region of primary upper jaw; Pa. ch. parachordal ; minus distal tip; PTG. I, proxi-
Pl. Pt. palatopterygoid region ; Q~t. quadrate region; mal pterygiophore (interspinous
S.Or. supraorbital region of cranium; Sy. symplectic bone} embedded in body muscle;
region of suspensorium; T. Cr. cranial roof; Tr. PTG. 2, middle pterygiophore;
trabecula ; I I, optic foramen; V, trigeminal foramen. pig. J, distal pterygiophore (carti-
(After Parker and Bettany.) laginous.}

unossified and the hypobranchial absent. The fifth arch is reduced to a

single bone on each side. Small spine-like ossifications are attached in a
single or double row along the inner aspect of each of the first four arches.
These are the gill-rakers. They serve as a sieve to prevent the escape of food
through the gill-slits and to protect the gill filaments.
The comparison of this singularly complex skull with the comparatively
simple one of a dogfish or sting-ray is made easier by the examination of the
skull of a young trout or other bony fish. In the young salmon, for example,
at about the second week after hatching, the only ossifications are a few in-
vesting bones. When these are removed we get a purely cartilaginous skull
(Fig. zo6), exactly comparable with that of a shark or ray. The cranium is
 ZOOLOGY
devoid of replacing bones and divisible only into regions. The upper jaw is an
unossified palatoquadrate (Pl. Pt., M. Pt., Qu.), and the lower jaw (Mck.) a
large Meckel's cartilage. The suspensorium is an undivided hyomandibular
(H.M.), and the hyoid and branchial arches are unsegmented.
The first dorsal and the anal fins are supported each by a triple set of
pterygiophores, so that the fin-skeleton is multiserial, as in a dogfish. The
proximal series consists of slender bony rays-the interspinous bones (Fig. 207,
PTG. I}, lying in the median
plane, between the muscles of the
right and left sides. These are
more numerous than the myo-
meres of the regions in which
they occur. Their distal ends
are broadened, and with them
are connected the second series
(PTG. 2) in the form of small
dice box-shaped bones. To these,
finally, are attached small nodules
of cartilage (ptg. 3) forming the
third series of radials. The dermal
fin-rays, orlepidotrichia (D .F.R.),
which lie in the substance of the
FrG. zo8.-Salmo: Pectoral girdle and fin. Left, fin itself, are slender bones,
from inner aspect. CL. = cleithrum; COR. coracoid; jointed together, and mostly
D.F.R. dermal fin-rays; MS. COR. mesocoracoid;
P.CL., P.CL.', postclavicles; PTG. I , proximal, and branched in the sagittal plane.
ptg. 2 , distal pterygiophores; P.TM. post-temporal;
S.CL. supraclavicle; SCP. scapula. Each is formed of distinct right
and left pieces, in close contact
for the most part, but diverging below to form a forked and dilated end, which
fits over one of the cartilaginous nodules (ptg. J). In the caudal fin (Fig. 204)
the dermal rays are similarly seated on the broad h<emal arches of the posterior
caudal vertebr<e. The second dorsal or adipose fin (p. 299) has, as already
noticed, no bony support.
The shoulder-girdle (Fig. 208), like the skull, consists of a primary
shoulder-girdle (homologous with that of a dogfish), and, in addition, several
investing bones. The primary shoulder-girdle in the young ~sh is formed
of distinct right and left bars of cartilage, which do not unite with one
another ventrally. In the adult each bar is ossified by three bones. These are
I. a scapula (SCP.), situated dorsally to the glenoid facets, and developed
partly as a replacing, partly as an investing bone; 2 . a coracoid (COR.), situated
ventrally to the glenoid facet; and 3· a mesocoracoid (JlS. COR.), situated
above the coracoid and anterior to the scapula. Externally to these is a very
large investing bone, the cleithrum (CL.). This extends downwards under the
 PHYLUM CHORDATA

throat. Its dorsal end is connected by means of a sttpra-clavicle (S. CL.) to a

forked bone, the post-temporal (P.T1\II.), one branch of which articulates with
the epiotic, the other with the pterotic process. To the inner surface of the
cleithrum are attached two flat scales of bone (P. CL'.), with a slender rod-like
post-clavicle (P. CL.) passing backwards and downwards among the muscles.
The structure of the pectoral fin is very simple. Articulated to the posterior
border of the scapula and coracoid are four dice box-shaped bones, the proximal
pterygiophores or radials (PTG. r), followed by a row of small nodules of car-
tilage (ptg. 2) representing distal pterygiophores. The main body of the fin is
supported by dermal fin rays. These resemble those of the median fins and
have their forked ends articulated with the distal pterygio-
phores. The first ray, however, is larger than the rest, and
articulates directly with the scapula.
There is no pelvic girdle, its place being taken by a
large, flat, triangular bone, the basipterygium (Fig. 209, B.
PTG.). This probably represents fused proximal pterygio-
phores. To its posterior border are attached three partly PTG

ossified nodules, the distal pterygiophores (PTG.), and \vith

these the dermal fin-rays are articulated. The adipose
lobe of the pelvic fin is supported by a small scale-like
bone.
The muscles of the trunk and tail are arranged, as in
sharks, in zigzag myomeres (p. 86). There are small
muscles for the fins, and the head has a complex muscula-
ture for the movement of the jaws, hyoid, operculum, and
FIG. zog.- Salmo:
branchial arches. Pelvic fin and skeleton.
The crelom is divisible into a large abdomen (Fig. 210) Left, from dorsal aspect.
B. PTG. basiptery-
containing the chief viscera, and a small pericardial cavity gium ; D. F. R . dermal
fin-rays; PTG. distal
(containing the heart) situated below the branchial arches. pterygiophores.
Alimentary Canal and Associated Structures.-The large
mouth (Figs. 205 and 210) has numerous small, recurved, conical teeth, borne, as
already mentioned, on the premaxillre, maxillre, palatines, prevomer, dentaries,
and basihyal. They obviously serve chiefly to prevent the escape of the slippery
animals used as food. The food-small arthropods and fishes- is swallowed
whole with the assistance of an enzyme-free mucus. The pharynx (ph.) is
perforated on each side by four vertically elongated gill-slits, fringed by the
bony tooth-like gill-rakers. Each gill-slit is >-shaped, the epihyal being bent
upon the ceratohyal so that the dorsal and ventral moieties of the branchial
arches touch one another when the mouth is closed.
The pharynx leads into a short cesophagus (gul.) which ends in a sphincter
which probably prevents the entry of the respiratory water-current into the
stomach. The U-shaped stomach (st.) consists of a wide cardiac, and a narrow
 w
....
0

N
0
0
s
C"l
><:

FIG. 21o.-Salmo: Skeletal and Visceral relationships. a. bl. air-badder (opened); an. anus; au. atrium; b. a. bulbus
arteriosus; B.HY. basihyal; B.OC. basi-occipital; cd. a. caudal artery; cd. v. caudal vein; CN. centrum; crb. cerebellum; d. f. I,
first dorsal fin; D. F. R. dermal fin-rays; du. duodenum; FR. frontal; g. bl. gall-bladder; gul. gullet; H. SP. hremal spine; int.
intestine; kd. kidney; kd'. degenerate portion of kidney; lr.liver; N. SP. neural spine; opt. l. optic lobes; PA. SPH. parasphenoid;
ph. pharynx; pn. b. pineal body; pn. d. bristle passed into pneumatic duct; prsen. prosencephalon; pty. b. pituitary body; PTG.
pterygiophores; pv. f. pelvic fin; py. c. pyloric creca; S. ETH. supraethmoid; S. OC. supraoccipital; spl. spleen; st. stomach;
tg. tongue; ts. testis; 11. bl. urinary bladder; 11 . g. s. urinogenital sinus; ur. urinary duct; v. ventricle; v. ao. ventral aorta; v. df.
vas deferens; v.J. ventral fin; VO. vomer.
 PHYLUM CHORDATA 3II

pyloric, division. In the stomach true gastric digestion occurs. The entry
from the pyloric division of the stomach to the anterior end of the intestine is
guarded by a ring-shaped pyloric valve. The intestine passes at first forwards
as the duodenum (du.), then becomes bent upon itself (int.) and passes back-
wards, without convolution, to the anus (an.). Its posterior portion has the
mucous membrane raised into prominent annular ridges which increase its
absorptive surface but do not constitute a spiral valve such as occurs in elasmo-
branchs. Opening into the duodenum are about forty blind glandular tubes,
the pyloric creca (py. c.).
The liver (lr.) is imperfectly divided into right and left lobes. There is a
large gall-bladder (g. bl.). A diffuse and relatively inconspicuous pancreas
occurs. This exhibits the compound structure and function remarked on in
elasmobranchs (p. 239), producing both digestive enzymes and the hormone
insulin or its homologue. Attached by peritoneum to the fundus of the
stomach occurs a large spleen (spl.), a capacious sac of blood-storage, and other
functions that is functionally associated not with the alimentary canal, but with
the blood-vascular system (p. 312). The stomach, duodenum, and pyloric
creca are surrounded by loose folds of peritoneum loaded with fat. No rectal
gland occurs.
Lying between alimentary canal and kidneys, and extending the whole
length of the abdominal cavity, is the swim-bladder (a. bl.), a shiny thin-walled
sac. This is essentially a hydrostatic organ (p. 339). It contains air of high
oxygen content. In the trout it is physostomatous (as opposed to physoclistous)-
it is 'open'. Thus, anteriorly, its ventral wall has a small aperture leading,
by a short, thin pneumatic duct (pn. d.), into the resophagus on the dorsal side
somewhat to the right of the middle line.
Respiratory System.-There are four pairs of gills, each with a double row
of branchial filaments, united proximally, but having their distal ends free:
interbranchial septa are practically obsolete (Fig. 73, p. 106). The gills are
borne on the first four branchial arches, the fifth arch bearing none. On
the inner surface of the operculum is a comb-like body, the pseudobranchia,
formed of a single row of branchial filaments, and representing either the
vestigial gill (hemibranch) of the mandibular arch or the spiracular hemi-
branch. For the course of the respiratory current, See Fig. 76, p. 106.
Blood-vascular System.-Like that of the cartilaginous fishes, this consists
of a 'single circulation'. The heart (Fig. 210) consists of sinus venosus, atrium
(au.), and ventricle (v.). There is no conus arteriosus, but the proximal end of
the ventral aorta is dilated to form a thin-walled, elastic bulbus arteriosus (b. a.),
a structure which differs from a conus in being part of the aorta, and not the
heart. Although itself not muscular, the teleost bulbus dilates and contracts
as a response to the beat of the single ventricle, and by this means pressure
through the short aorta to the gills is maintained. As in the elasmobranchs,
3!2 ZOOLOGY
the heart action is regulated by a depressor nerve of vagal origin. There is no
corresponding sympathetic innervation.
In accordance with the atrophy of the hyoid gill, there is no afferent
branchial artery to that arch, but a hyoidean artery springs from the ventral
end of the first efferent branchial and passes to the pseudobranch (seep. 230),
which thus only receives oxygenated blood. Despite its gill-like structure, the
pseudobranch does not subserve any respiratory function. The right branch
of the caudal vein is continued directly into the corresponding cardinal. The
left breaks up in the kidney, forming a renal-portal system. There are no
lateral veins, but the blood from the paired fins is returned to the cardinals.
It will be seen that although the veins are wide, the elaborate system of venous
sinuses seen in the elasmobranchs is lacking. The exact means-muscular or
otherwise (p. no)-by which venous return is ~chieved are imperfectly known.
The red blood cells are, as in other fishes, oval nucleated discs carrying the red
respiratory pigment hcemoglobin (see, however, p. 343).
In addition, nucleated reticulocytes and thrombocytes, as well as coarse and
fine granulocytes and various lymphocytes, occur. No white cell, however,
appears to exhibit the characteristics of mammalian monocytes. Coarse
granulocytes commonly escape from the blood-vessels and are observed in large
numbers in the intestinal mucosa and submucosa, gill epithelia and peritoneum.
Hcematopoiesis occurs chiefly in bony fishes in the kidney intertubular tissue
and in the spleen. In some teleosts (e.g. Roach) this activity may be confined
to the kidney, and in others (e.g. Perch) to the spleen. In the trout both organs
are involved. In the maturation of the teleost erythrocyte there is a progressive
enlargement of the cell; in birds and mammals its size decreases.
Nervous System.-The brain (Fig. zn), though in general not unlike that
of elasmobranchs, is very different in detail and is in many respects of a dis-
tinctly more specialised type. The cerebellum (HH.) is very large and bent
upon itself. The optic lobes (MH.) are also of great size. There is experi-
mental evidence that this highly developed mid-brain region is of great im-
portance in connection with learning, association, and the performance of
relatively complex acts of behaviour. The diencephalon is much reduced and,
indeed, is indicated dorsally only as the place of origin of the pineal body
(G.p.). Ventrally occur large bean-shaped lobi inferiores. The infundibulum
between the lobi inferiores gives attachment to the pituitary gland (Hyp.).
Hence, seen from above, the small undivided prosencephalon (VH.) comes
immediately in front of the mid-brain. The original roof, an area equivalent
to the pallium of other vertebrates, has grown laterally, stretching the mid-
line into a non-nervous structure. Frequently olfactory bulbs are situated in
close apposition with the fore-brain, without intervening olfactory tracts such
as are present in Scyliorhinus, but they are nearly as large as the corpora striata.
Each contains a cavity (rhinoccele) communicating with the undivided ven-
 PHYLUM CHORDATA 313
tricle of the fore-brain. Three transverse bands of fibres connect the right
and left halves of the fore-brain, an anterior commissure joining the corpora
striata, a posterior commissure situated just behind the origin of the pineal
A

FIG. 21r.- Sabno: Brain and cranial nerves. Dorsal (A), v entral (B), and lateral (C) views.
BG., Bas. G. corpora striata; ell. crossing of optic nerves; G. p. pineal body; HH. cerebellum;
Hyp. pituitary body; I nf. infundibulum; L . ol. olfactory bulbs; Med . .spinal cord; MH. optic
lobes; NH. medulla oblongata ; Pall. non-nervous roof of prosencephalon (not equivalent to the
pallium of other groups); Sv. saccus vasculosus; Tr. opt. optic tracts; U L. lobi inferiores; VH.
prosencephalon; I- X , cerebral nerves; XII . I, first spinal (hypoglossal) nerve ; :2, second spinal
nerve. (After \Viedersheim.)

body, and an inferior commissure in front of the infundibulum. The pineal

body (G. p.) is rounded and placed at the end of a hollow stalk. No trace of
any optic structure remains within. A short er offshoot of the roof of the
diencephalon may perhaps represent a rudimentary pineal eye. There is
WLIT U
314 ZOOLOGY
considerable evidence of a direct photo-receptive function in the diencephalon
of certain fishes (p. 345). Behind the pituitary body is a sacctts vasculosus
(s. v.). This organ, as we have seen, is one of the cerebral specialisations that
is pronounced in both cartilaginous and bony fishes. It may be a pressure
receptor. The anterior part of the cerebellum (HH.) does not bulge outwards
in the way it does in a dogfish. Instead, it pushes forwards under the roof
of the mesencephalon to form the valvula cerebelli (Fig. 212), which is charac-
teristic of teleost fishes. The hinder part of the cerebellum bulges outwards in
the usual manner. The optic nerves do not form a chiasma, but simply cross
one another, or decussate (Ch.), on leaving the brain, the right nerve going to
the left, and the left nerve to the right eye.

FIG. 212.- Salmo: Brain and adjacentstructures. Median longitudinal section. Aq. aqueduc-
tua Sylvii; Bo. olfactory lobe; Cbl. cereb ellum; C. c. canal of spinal cord ; Cca. a nterior com-
missure; Ch. o. optic nerve ; Ci . inferior commissure; Glp. pineal body ; H y . Hy'. hypophy sis;
]. infundibulum ; N ol. olfactory nerve ; P a. roof of telencephalon; p. f . velum tra nsversum;
S. v. saccus vasculosus ; Tl. torus longitudina lis (of the cerebellum); T eo. pia m ater; tr. crossing
fibres of fourth nerve; V . c. valvula cerebelli; V. em. ventricle of telencephalon; V. q. fourth
ventricle; Vt. third ventricle. (From Goodrich, after Rabl-Riickhard.)

Autonomic Nervous System.-The autonomic nervous system is far more

advanced than that of cartilaginous fishes, and is in some ways suggestive of
the basic plan in tetrapods. It will be recalled that in elasmobranchs there is
no sympathetic nervous system in the head. In teleosts paired symmetrical
chains of sympathetic ganglia begin at the trigeminal level (Fig. 2n) and run
backwards to the end of the tail. A ganglion is connected with each cranial
dorsal root, but these receive, not the pre-ganglionic fibres from adjacent
segments, but others which emerge with the ventral roots in the trunk
region, and therefore pass forwards in the sympathetic chain. The sym-
pathetic trunk is connected with the mixed spinal nerves by rami communi-
cantes. Young, who studied the autonomic nervous system of the Mediter-
ranean Star-gazer (Uranoscopus), distinguished white and grey rami. (The
latter do not occur in selachians.) Each ganglion receives a white ramus
 PHYLUM CHORDATA

containing pre-ganglionic fibres from its spinal nerve and gives a grey ramus
to the same nerve. The latter ramus carries post-ganglionic fibres, some of
which are concerned with melanophore contraction.
Although there is a highly organised vagal arrangement, there is no
apparent sacral parasympathetic system (p. ng), nor have any antagonistic
fibres been demonstrated in other cranial nerves, although stimulation of
oculomotor nerve-fibres seems to cause an effect opposite to that controlled by
sympathetic fibres.
Apart from anatomical work there have been several physiological inquiries
(involving electrical and pharmacological stimulation) into the autonomic
organisation of teleosts. It would seem that in these fishes there has been
developed the relatively clear-cut, and essentially antagonistic, sympathetic
and para-sympathetic systems such as occur in the higher chordates (p. ng).
Organs of Special Sense.-The olfactory receptors are paired olfactory pits,
each of which opens to the external by means of two apertures. Water flows
into the anterior aperture, which is provided by a valve, and leaves by the
other. The water passes over a series of sensory folds equipped with chemo-
receptory cells. In bony fishes, unlike the elasmobranchs, the water current
is of purely olfactory significance.
Another kind of chemoreceptor, the taste buds, is found in many parts of
the body in various bony fishes. In Trigla, a gumard, chemoreceptors, in-
nervated by branches of spinal nerves, occur on their curiously modified
pectoral fins, enabling the animal to test the bottom-mud and debris over
which it slowly crawls. There is experimental and structural evidence that
certain modem bony fishes (e.g. Minnow, Phoxintts) have taste-receptors
located in their tails. The sense of touch is well-developed in bony fishes.
The eye (Fig. 213) has a very fiat cornea with which the globular lens is
almost in contact, so that the anterior chamber of the eye is extremely small,
Between the cartilaginous sclerotic and the vascular choroid is a silvery layer
or argentea (arg.), which owes its colour to minute crystals in the cells of which
it is composed. There are no choroid processes. In the posterior part of
the eye, between the choroid and the argentea, is a thickened ring-shaped
structure, the choroid gland. This surrounds the optic nerve. It is not gland-
ular, but is a complex network of blood-vessels, or rete mirabile. It is supplied
with blood by the efferent artery of the pseudobranch. Close to the entrance
of the optic nerve a vascular fold of the choroid, the falciform process, pierces
the retina, and is continued to the back of the lens. Here it ends in a muscular
knob, the campanula Halleri or retractor lentis. The falciform process with
the campanula Halleri takes an important part in the process of accommoda-
tion by which the eye becomes adapted to forming and receiving images of
objects at various distances. Accommodation in the trout is effected, not
by an alteration in the curvature of the lens, as in higher vertebrates, but by
 ZOOLOGY
changes in its position, by which it becomes more approximated towards, or
further withdrawn from, the retina. In bringing about these changes of posi-
tion the structures in question appear to play the principal part. In the trout,
and indeed in most bony fishes, the pupil size appears to alter very little or
not at all, but in others (as in the elasmobranchs) the iris is equipped with
muscles and is capable of
considerable adjustment in aperture. In some

FIG. 213.- Teleostei: Eye and optic nerve. Genera lised diagram of vertical section. The
argentea, shown in black, conta ins reflective and not absorptive pigment. Some structures sh own
above may be absent in different species. Thus (for example) a falciform process and a system of
h yaloid vessels are never simultaneou sly present, and where the falciform p rocess is lacking the
lentiform body is also absent (Walls) . Cf. figures on pp. 191, 273, and 413. (After Walls.)

teleosts contraction of the iris sphincter is controlled by the sympathetic

nervous system, and dilation is brought about by impulses carried by oculo-
motor fibres. There is evidence that in eels iris adjustment may be under the
direct control of light.
The organ of audio-equilibrium (Fig. 214) possesses three otoliths (ot. r - 3),
of which one, the sagitta (ot. r), is relatively large and may be as much as 6 mm.
in length. It almost fills the sacculus. Another otolith, the asteriscus (ot. 2), is
a small granule lying in the lagena or rudimentary cochlea, The third, the
 PHYLUM CHORDATA

lapillus (ot. 3), is placed in the utriculus close to the ampullce of the anterior
and horizontal canals. The above-described apparatus apparently shows a
great advance on that of selachians. Bony fishes undoubtedly hear with great
discrimination, despite the absence of the organ of Corti (p. 146). It would
seem that in bony fishes the pars inferior (of the labyrinth) as a whole is pro-
bably the sound-receptor. In a relatively small bony fish it is not easy experi-
mentally to assess the relative contributions of different auditory structures,
but there is evidence that both lagena and sacculus are involved in normal
performance. Destruction of either, however, still leaves considerable,
though defective powers. There is some evidence, too, that in some fishes the
utriculus plays a part in hearing, and it has
been suggested that the air-bladder may
be sometimes involved. Weberian ossicles,
which are modified vertebral elements,
connect the air-bladder with the auditory
apparatus in some fishes (Ostariophysi).
The lateral line system is very well shown
in Salmo, and indeed it reaches its highest
development in actinopterygians. Each
individual sensory organ lies in a pit.
Pores communicate with the surrounding
water. The pits are linked by canals and
are innervated by the lateral line branch of
nl.t
the vagus. The head, including the lower
jaw, is invested with shorter lines (p. 136). tion. Right organs from Audio-equilibra-
FrG. 214.- Salmo:
inner side. The
A considerable amount of experimental otoliths are shown separately below.
a. s. c. anterior semi-circular canal; aud.
evidence suggests that a primary function nv. auditory nerve; h. s. c. horizontal
of the lateral line system is to gauge canal; ot. 1-3. otoliths ; p. s. c. posterior
canal; sac. sacculus; ut. utriculus.
pressure-waves, and so no doubt apprise
the fish of the presence of other individuals (seep. 346).
Osmoregulation and Excretion.-Many teleosts, including trout, can live in
either marine or fresh water. There is much evidence that suggests that bony
fishes were originally fresh-water animals. Certainly the amount of inorganic
salts in the body-fluids of living forms is far less than that in the sea. The
ability to change from salt to fresh, or even merely to a brackish-water habitat,
imposes the necessity for physiological barriers to maintain the integrity of a
fish's internal environment. We have already noted the presence of a largely
non-vascular skin, mucous glands which secrete protective slime (p. 82), and
a well-developed resophageal sphincter which keeps the external medium
away from the stomach and highly absorptive surface of the gut. The
kidney tubules are equipped with a selective apparatus on the general craniate
pattern (p. 157) which allows the reabsorption of body-salts essential for
318 ZOOLOGY
osmo-regulation. Nitrogenous metabolites CJ.re excreted largely as ammonia
(ammonotelic mechanism), though some urea is formed and discharged. Much
of the nitrogenous excretion of aquatic teleosts probably occurs through the
gills.
Marine fishes, on the other hand, must keep their body-fluids at the appro-
priate concentration by swallowing sea-water (including that in their food), by
the reabsorption and retention of fluid via the tubules, and by the elimination
of salts by means of the gills, urine, and freces.
Glomeruli are fewer or may be entirely absent in some
species (see p. 347). This reduces the loss of fluid.
Marine teleosts, as with freshwater forms, still excrete
quantities of ammonia, some urea , and many rid
themselves of nitrogenous waste in the form of soluble,
non-toxic trimethylamine oxide (see also p. 253).
Here too an additional excretory mechanism is pre-
sent in the gills whereby chloride-secreting cells dis-
pose of excess salts which are inevitably absorbed
from the surrounding medium. Hoar has shown that
immature salmon (Oncorhynchus) go to sea only after
the development of salt-secreting cells in their gills.
The kidneys (Fig. 210, kd., and Fig. 215, R) are of
great size, extending the whole length of the dorsal
wall of the abdomen, above the air-bladder, and partly
fused together in the middle line. They are derived
FIG. 215 .- Salmo: Kid-
from the mesonephros of the embryo. Their anterior
ney and adjacent structures. ends are much dilated and consist in the adult of
d, precaval vein ; R (to the
right), kidney; R (to the left), lymphatic tissue, thus ceasing to discharge a renal
degenerate anterior portion function. The mesonephric ducts (ur.) unite into a
of kidney; rr, efferent renal
vein; s. subclavian vein; u, single tube, which is dilated to form a minary bladder
1w, mesonephric duct; v.
bladder. (After Gegcnbaur.)
(Fig. 210, ~t. bl., Fig. 215, v.), and discharges into the
urinogenital sinus. This, being mesodermal in origin,
is not homologous with the endodermal bladder of higher forms.
Endocrine organs.-See p. 346.
Reproduction.-The gonads are of great size in the sexually mature fish. The
testes (Fig. 210, ts.) are paired, often convoluted, organs, sometimes extending the
whole length of the abdominal cavity. These are the 'soft roes' of commerce.
Each is continued posteriorly into a genital dttct (v. df.) which opens into the
urinogenital sinus. The homology of this with the ducts of the primitive
nephridial system is uncertain. The spermatozoa are shed into the external
medium. The ovaries-the 'hard roes' - also run the full length of the ab-
dominal cavity. They are much wider than the testes and are covered with
peritoneum only on their inner or mesial faces. The numerous ova, which are
 PHYLUM CHORDATA

about 4 mm. in diameter, are discharged when ripe from their outer faces into
the crelom. The anterior wall of the urinogenital sinus is pierced by a pair of
genital pores through which the ova make
A 8
their way to the exterior where fertilisa-
tion occurs. It seems probable that these
pores are degenerate oviducts and there- bl
fore in no way homologous with the
abdominal pores of the Chondrichthyes
(p. 230).
Development.-Fertilisation is ex-
ternal. The male sheds its milt or semi-
nal fluid on the new-laid eggs. The
ovum is covered by a thick membrane,
the zona radiata. This is perforated by
an aperture, the micropyle, through
which a sperm finds access. A superficial em,h
layer of protoplasm surrounds a mass of
transparent fluid yolk of a pale yellow
colour. After fertilisation the proto-
plasm accumulates to form an elevated
area or germinal disc at one pole. Here
cleavage takes place (Fig. 216, A, B) in
much the same way as in Chondrich-
thyes, except that, owing to the smaller
proportion of yolk, the resulting blasto-
derm (bl.) and the embryo formed there-
from are proportionally much larger, FIG. 216. -Salmo: Development. A-H,
before hatching ; I, shortly after hatching. bl.
and the yolk-sac (y. s.) correspondingly blastoderm; emb. embryo; r. thickened edge
smaller, than in the two previous classes. of blastoderm; y . s. yolk-sac. (A-G after
Henneguy.)
Epiboly takes place as in Chondrich-
thyes, the blastoderm gradually growing round and enclosing the yolk (C-F).
The embryo (emb.) arises as an elevation growing forwards from the thickened
en...-rn..s
I

F rG. 217.-Salmo: Development. Blasto.

I

derm (in longitudinal section) at about the

stage represented in D of Fig. 216. ec.
ectoderm ; en + ms, infolding giving rise to
endoderm and mesoderm. (After Hertwig.)

edge of the blastoderm. As it increases in length it appears as a clear colourless

band (H, emb.) winding round the yellow yolk. It is kept in close contact with
the yolk by the enclosing zona radiata. There is no open medullary groove :
320 ZOOLOGY
the nervous system is formed, as in lampreys, from a fold of ectoderm the
walls of which are in apposition so as to form a keel-like ridge. The endoderm
and mesoderm are formed as a result of a process of infolding of the posterior
edge of the blastoderm (Fig. 217). Gradually the head and tail become free
from the yolk. At the time of hatching, the yolk-sac (I, y.s.) is a shoe-shaped
body sessile upon the ventral surface of the transparent embryo.

GENERAL ORGAN/SA TION OF ACTINOPTER YGIANS

External Form.-Teleosts never reach the great bulk attained by many of
the cartilaginous sharks and rays. They range in size from a tiny Philippine
goby (Mistichthys luzonensis), about half an inch long, to the spear-fishes (see
below) and the freshwater South American Arapaima gigas, which is said to
attain a length of rs feet and a weight of about 400 lb. On the other hand, the
chondrostean Httso lmso grows much bigger (p. 283).
A typical form of the bony fishes is represented by that of the Trout (Fig.
201)-a long, compressed body, nearly one third of which is formed by the
tail, pointed anterior and posterior ends, a large vertical tail-fin, a head of
moderate size, and a terminal mouth. Such a form is eminently fitted for
rapid progression through the water. The speed of fishes, incidentally (as of
most other animals, pp. 521, 559), is generally exaggerated. A goldfish's
maximum speed seems to be about 3·8 m.p.h. and that of a trout 8·5 m.p.h.
Salmon generally climb waterfalls by swimming up a more or less continuous
stream of water, but they can in fact leap about 6 feet high and rz feet forward
and probably leave the water with a velocity of about 14 m.p.h. A barracuda
can travel at some 27 m.p.h. (Gray). From the characteristic fish-form
described above there are many striking deviations. The body may be greatly
elongated and almost cylindrical, as in the eels, it may be lengthened and
strongly flattened from side to side, as in the ribbon-fishes. The head may be of
immense proportional size and strongly depressed, as in bottom-living fishes,
like the 'Fishing-frog'. In the beautiful reef-fishes the body may be as high
as it is long. The mouth sometimes has a ventral position, as in most chond-
rostei, with the snout prolonged over it. This is the case, for example, in the
sturgeons (Fig. 193). In the allied Polyodon the snout takes the form of a
horizontally flattened shovel-like structure, about one-fourth the length of the
body. On the other hand, in the ground-feeding star-gazers and some other
spiny-rayed fishes the lower jaw is underhung and the mouth is dorsal.
In the spear-, sword-, and sail-fishes (Istiophoridre) the snout and upper jaw
are forwardly prolonged into a sharp rostrum or beak. The British Museum
possesses a fragment of ship's planking through which a transfixing' spear' has
been thrust for a distance of 22 inches. Spear-fishes (Tetrapturus) grow to a
length of more than 14 feet and may weigh over r,ooo lb. Sword-fishes
(Xiphias) may be even bigger. Sail-fishes (Istiophorus) grow to about 12 feet
 PHYLUM CHORDATA 321

long; and their fin-rays support a tall, sail-like dorsal fin. In the tropical
half-beaks (Hemirhamphidre) the lower jaw projects and in the garfishes and
garpikes both elements do so. Such a projection is different from the snout
of sturgeons or Polyodon, for it is formed by the elongation of the bones of the
jaws (premaxilla, maxilla, dentary, etc.), whereas in the above two chondro-
stean forms it is the anterior region of the cranium which is prolonged. Still
another form of 'snout' is produced in many Teleostei by the great mobility of
the jaws, allo\ving their protrusion as a short tube. In the wrasses or 'lip-
fishes' the mouth is bounded by fleshy lips.
The bony fishes show many other interesting examples of structural and
physiological adaptation to their various environments. The results of this
adaptation can be viewed from two aspects. One result is to bring about a
resemblance between animals of different phyletic origin that have taken up
similar environments, a condition known as convergent adaptation. The other
aspect, known as divergent adaptation, is when related animals have entered
different environments and have come to differ superficially from one another,
sometimes to a surprising degree.
An illustration of convergent evolution is the superficial resemblance be-
tween the extinct holostean T lzoracopterus and the teleost 'flying fish', Exocretus.
By inference from its anatomy and morphology, Thoracopterus was a 'flying-
fish'. Both this species and the living Exoca:tus show in their exaggerated,
wing-like pectoral fins and hypobatic tails, a striking similarity in general body
form, while retaining, of course, the deeper-seated, fundamental osteological
differences of their respective classes. Further examples of convergent
evolutionary trends towards the 'flying' habit are found in two unrelated
teleost genera, Gasteropelews, a South American ostariophysian. and Pantodon,
an African isospondylid. Here again the species exhibit hypertrophy of the
pectoral fins, although the body-form of the two genera is different. Neither
Gasteropelecus nor Pantodon can be considered such efficient gliders as Exoca:tus,
but both can skitter across the surface of the water for fairly considerable
distances. Many fishes of very different families, or even classes, have acquired
similar body-forms adapted to particular needs, such, for example, as the deep
laterally compressed body shown by Microdon, an Upper Jurassic holostean, by
the extant teleosts Psettus sebce and Pterophyllnm (the 'Angel-fish') amongst
many others.
The teleosts, taken as a whole, in themselves afford an example of adaptive
radiation. While retaining their well-defined and diagnostic characters, they
have assumed an almost infinite variety of shape and construction in accordance
with the many different environments into which they have migrated. Thus
we may mention such forms as the flat-fishes, eels, pipe-fishes, porcupine-
fishes, sun-fishes, the curious sea-horses, and so on, a small selection out of a
vast range. On the other hand, the Ceratioidea ('angler-fishes'), give a good
322 ZOOLOGY

example of adaptive radiation over a small range of structure, and within the
limits of a single group (p. 234). Here can be observed the evolution of the
tentacle on the head in a diversified manner in various members of a family
which otherwise retain a general resemblance to one another.
It is possible that no class of animals exhibit a greater variety of peculiar
adaptations than the Actinopterygii. Almost anywhere we look-externally
or internally-we will see in one family or other some bizarre or otherwise
remarkable structural response to some special mode of life and environmental
niche. Notable among these are various devices associated with the eyes. For
example, in Anableps tetrophthalmus (Fig. 218), the 'Four-eyed Fish', each eye
is divided into an upper and lower section for vision above and below water
respectively. In Periophthalmus, the mud-skippers, or telescope-gobies, the
bulbous, independently moving eyes project above the surface of the head.

FIG. zz8.-Teleostei: Optic adaptation. Anableps tetrophthalmtts, a central and northern

South American cyprinodont, is often called the 'Four-eyed Fish'. All three such 'top minnow '
species possess two pupils in each eye. The upper (aerial) pupil is the larger and is generally out
of the water. The lower (aquatic) pupil allows the animal to use the single ovoid lens to look
under water. The retina is correspondingly modified. (From Cambridge Natural History.)

Tactile processes or barbels sometimes arise from the head (Figs. 193, 232).
An operculum (Fig. 205) is always present. This is supported by a variable
number of investing bones and is continued below into a branchiostegal mem-
brane, which, except in Crossopterygii (p. 353) and the sturgeons, is supported
by bony rays. In Polypterus a pair of bony gular plates (Fig. 196, B, jug.
pl., p. 209) are placed at the lower end of the branchiostegal membrane,
between the rami of the mandible. A mia has a single plate in the same posi-
tion. Spiracles are present only in Polypterus and some sturgeons.
Fins exhibit many modifications. This has been made possible because
fishes equipped with a hydrostatic organ (the swim-bladder) can largely dis-
pense with the more elaborate stabilising appendages (such as occur in the
elasmobranchs) and so free the fins to subserve other functions. 'Concomitant
with the loss of the heterocercal tail in evolution', Harris has explained, 'occurs
a rapid and tremendous adaptive radiation of the pectoral fin in form and
function.'
A common number of median fins is two dorsals, one caudal, and one
anal, but the number of the dorsals may be increased, or there is sometimes
a continuous median fin extending along the back and round the end of the tail
to the vent. The dorsal fin may be partly or wholly represented by a series of
 PHYLUM CHORDATA 323
small finlets (Fig. 196). The caudal fin may be diphycercal, hetcrocercal, or
homocercal. In the sea-horses (Fig. 219) it is absent. In these, the tail is
prehensile, and is used in the position of rest to coil, in the vertical plane, round
seaweeds among which the grotesque form of the fish is very difficult to dis-
tinguish. Other fishes have exaggerated appendages which serve as camouflage.
One such is the South American Angel-fish (Pterophyllum), the coloured stream-
ing filaments of which enable harmonious merging with surrounding water-
weeds. The brilliant reef-haunting so-called Butterfly-' cod' (Pterois) of the
warmer parts of the Pacific and Indian Oceans have colourful and greatly

A B

FrG. 219.-Teleostei: Exoskeletal and repro-

ductive adaptations. In this sea.horse (Hippo-
campus) there have been developed a beak-like
rostrum, a prehensile tail, and an extensive brood-
pouch (seep. 351). In B. the operculum has been
removed to show the gills. br. ap. branchial
aperture; brd. p. brood-pouch ; d. f. dorsal fin; g.
gills; pet. pectoral fins. (After Claus and Giinther.)

lengthened fins and a head beset with tassel-like appendages of several vivid
colours (p. 344). These trail through the water as the fish moves slowly along.
The fin-membranes do not cover the spines, which are capable of producing a
stinging wound. When approaching its victim, Pterois drifts almost imper-
ceptibly forwards with its long fin-rays waving to and fro. There is evidence
that instead of trying to escape, smaller fishes sometimes actually swim to-
wards the predator, and are then engulfed with a single action of remarkable
rapidity.
The dermal rays of the caudal fins of fishes are always jointed, as in the Trout.
In the acanthopterygians more or fewer of the foremost rays of the dorsal, anal,
and pelvic fins are unjointed, forming spines (Fig. 202, p. 297), sometimes
large and strong enough to recall the dermal defences of Holocephali.
The anterior dorsal fin may attain an immense size, and is subject to curious
variations. In the racquet-shaped Fishing-frog, Wide-gab, Allmouth, or
Angler (Lophitts piscatorius) the foremost rays are elongated, and bear lobes or
 ZOOLOGY
lures by which small fishes are attracted within reach of the jaws. The Angler
gets some of its names from the inordinately wide, frog-like gape at the extreme
anterior aspect of its flattened body. The outline of the lurking fish is partly
obliterated by a series of stumpy dermal appendages. The pale buccal cavity
is camouflaged by a remarkable fold of pigmented skin on the lower jaw.

Frc. zzo.-Teleostei: Bioluminescence and angling mechanisms in deep-sea forms. Angler.

fishes (Ceratioidea). The larger (nearly 19 inches long) has been referred to as Galatheathauma
axeli, the biggest angler yet caught. It is highly unusual in possessing a large light organ sus-
pended inside its jaws. The jaws are fringed with powerful stabbing and holding teeth.
The smaller animal is Lasiognathus saccostoma. The elongated rod is possessed only by females,
which (in many species) carry the dwarf males attached to them (p. 350), at least during the
breeding season. Some anglers are only ~- inch long. Many can swallow prey bigger than them-
selves. Note-The two animals shown above do not necessarily occur in the same environment.
(Redrawn after Bruun, N.B. Marshall.)

The Angler, aided by its weird specialisations, lies perfectly camouflaged and
motionless (except for the slowly-moving lure on its 'line') among weeds or
rock fragments. The Angler has a formidable armoury of teeth and is able to
engulf fishes, including small sharks, about half its own length. It also catches
surface-feeding gulls and other large birds.
Some Angler-fishes (Ceratioidea, Fig. 220) have evolved a luminous lure on
a sometimes extensible rod and line that is probably the modification of a fin-
 PHYLUM CHORDATA 325
ray. The luminous organ is attached to the end of an illicium-the rod-
which has a supporting basal bone and a musculature that raises and lowers it
and the terminal light. The rod is usually short, but in a few species it may be
several times longer than the fish that bears it. In some species (e.g. Ceratias
holbrelli) the basal bone, ensheathed in anterior and posterior sockets of tissue,
is mobile along a groove extending dorsally along head and body. Moved by a
special musculature the basal bone and its lure can be projected back and forth.
Bertelsen believes it possible that by such means prey may be lured into the
fish's cavernous jaws. In the recently described Galatheathauma the forked
luminous lure is dorsally situated inside the animal's gaping jaws.
In the remoras or sucking-fishes (Echeneis and other genera) the anterior
dorsal fin is modified into an adhesive vacuum disk by means of which the fish
attaches itself to the bodies of sharks, rays, sword-fishes, large bony fishes,
turtles, whales, or even boats. Although good swimmers, remoras allow them-
selves to be carried for long distances, periodically leaving their chosen vehicle
in order to capture smaller fishes. A remora is able to withstand the efforts of
the host-fish to dislodge and devour it. The sucker of large species can resist a
vertical pull of some 40 lb. The natives of some areas tie a line to a remora and
then liberate it. When it attaches itself to a turtle they haul both animals to
the boat. Occasionally small remoras have been found sheltering within the
gills, or even in the buccal cavity of other fishes. An African remora has been
seen to follow a man into the shallows in an attempt to fasten itself to his person.
Certain gobies, living in rock-pools from which they might be easily washed,
have their pelvic fins modified into a suctorial apparatus. Periophthalmus
can skip or waddle rapidly over sand and mud above the tide-mark by means
of its pectoral fins, which, operated by a special musculature, act as levers and
enable the fish to move at considerable speed. Its respiratory system is also
remarkably modified (p. 339). Certain gumards (Triglidce) 'walk' along the
sea-floor by means of elongated, innervated terminations to their pectoral fins
which are probably also organs of special sense. In very calm and shallow
waters gumard tracks can be followed as easily as those of a tortoise on a beach.
The Australian hand-fishes (Brachionichthyidce), related to the angler-
fishes, exhibit remarkable specialisations in the same direction. These may
occasionally startle novice fishermen by 'walking' along a trawler's deck on
their superficially hand- or leg-like fins. These fishes retain the dorso-frontal
'angler' appendage and, in addition (as Gregory has pointed out), have modified
the large and continuous pectoral fin (such as occurs in modem batrachoids or
frog-fishes) into paired and jointed appendages upon which they progress across
the sea-floor. The remarkable wing-like expansions of the pectoral fins in
flying-fishes have already been noted. Some species, in favourable air-currents,
can skim for several hundred yards over the wave-crests when menaced by
larger hunting fishes.
 ZOOLOGY
In bony fishes the portion of the paired fins visible externally is usually very
thin, and supported entirely by dermal rays. But in the chondrostean Poly-
pterus (Fig. 196, p. 290) the rays form a fringe round a thick basal lobe, which
is supported by endoskeletal structures. This condition is an approach to
the structure met with in elasmobranchs and holocephalans. The pectorals,
as we have seen, vary considerably in size. In many fishes the pelvics are
reduced to filaments or scales. The pectorals always retain their normal
position, just behind the gill-clefts, but the pelvics often become more or less
shifted forwards from beside the vent. The change in position is least in the
palreoniscoid derivatives 1 and in the Isospondyli and allied orders (p. 296), in

FIG. 221.-Teleostei: Morphogenetic adaptation to bottom-living. ·Flat.fishes' (Order Hetero-

somata) have the sighted upper surface obliteratively pigmented with chromatophores and
the lower surface usually pallid. Larvre at first swim near the surface and possess bilaterally
placed eyes. Soon an area of cranial cartilage (supraorbital bar) is resorbed on one side and the
adjacent eye migrates through a gap between the lateral ethmoid and the otic capsule over the
dorsal surface to the opposite supraorbital bar, both eyes taking up the positions shown. Ossifica-
tion now occurs. Other striking modifications occur. The animal surveys the sandy bottom
by raising its eyes slightly and moving them independently. The species shown is Glyptocephalus
cynoglossus (from right side). d. f. dorsal fin; 1. e. left eye; pet. f. pectoral fin; pv. f. pelvic fin;
r. e. right eye; v. f. ventral fin. (After Cuvier.)

which these fins usually lie between the middle of the abdomen and the anus,
and are abdominal in position. In a large proportion of the fishes in the remain-
ing teleost orders they come to be placed almost beneath the pectorals (Fig.
201, pv. f.), when their position is called thoracic, or on the throat, when they
are said to be jugular in position.
A very remarkable deviation from the typical form occurs in the flat-fishes
(Heterosomata). The body (Fig. 221) is very deep and strongly compressed:
the fish habitually rests on the bottom, in some species on the right, in others
on the left side, partly covering itself with sand, and occasionally swimming
with an undulating movement. The under side is usually unpigmented, the
1 The expression palaJoniscoid derivative will be used in place of the archaic and misleading
term • ganoid '. It should be remembered, of course, that all extant bony fishes (excluding
crossopterygians, p. 353) are probably ultimately derived from the Palreoniscoidea (p. 285).
 PHYLUM CHORDATA
upper side dark and camouflaged. The eyes (r. e., l. e.) are both on the upper
or dark-coloured side, and the skull is distorted so as to adapt the orbits to this
change of position. The abdominal cavity is very small, the anus placed far
forward, and the dorsal and anal fins are sometimes continuous.
Many shore-fishes exhibit protective characters, the tints and markings of
the skin being harmonised with those of the rocks and sea-weeds, among which
they live. The obliterative effect may be heightened by fringes and lobes of
skin resembling seaweed. The colours are often adaptable. Trout, for in-
stance, alter colour by the contraction or expansion of melanophores, according
to whether the streams in which they live have a muddy or a sandy bottom.
Teleost melanophores are innervated. Contraction of the pigment within the
melanophores causes a paling of the skin. A body of experimental evidence
suggests that darkening is caused by a secretion, from the posterior pituitary
gland. The precise mechanism by which the internal neurohormonal mechan-
ism is controlled by the environment is not understood (see however, p. 345).
In some shore-fishes, such as those of the coral reefs, the colours are of the
most brilliant description. Vivid reds, blues, and yellows, spots or stripes of
gold or silver, are common. It seems likely that the combination of colours
produce a disruptive or obliterative effect and thus, although some fish adorn-
ments have threat or recognitional functions, it is by no means certain that
coloured adornments appear to them as they do to higher vertebrates. But
even if certain fishes do not appreciate colour as such, protection may still be
gained if it produces an obliterative pattern. However, the female response to
the seasonal appearance of a patch of bright red on the belly of the male Stickle-
back certainly suggests that, in some teleosts, at least one colour registers in a
very positive manner.
In addition to chromatophores, fishes possess a second colour-producing
apparatus-the iridocytes or reflecting cells. These cells, which contain
guanin crystals, occur either immediately above or below the scales. Below
the scales they are responsible for the silvery or white argenteum layer. When
above the scales they alter light by interference and give rise to a characteristic
iridescence. Some colours are produced by the combination of iridocytes and
chromatophores.
Many deep-sea Teleostei are luminescent. In some species definite luminous
organs (Fig. 222) are arranged in longitudinal rows along the body, each pro-
vided with a lens and other accessory parts, not unlike those of the eye, the
whole organ having the character of a minute lantern. Pachystomias has a pair
of relatively complex luminescent organs under each eye. An arrangement
such as this occurs in several forms, giving the impression of a small searchlight.
The Hatchet-fish (Argyropelecus) presents a dazzling spectacle, with much of its
lateral surfaces illuminated by light from rows of photo-organs. The appear-
ance of light can be stimulated by adrenalin injections, electric shocks, or other
328 ZOOLOGY
interference. Some fishes depend for the production of light on the presence of
photogenic symbiotic bacteria in specialised areas. Most deep-sea angler-
fishes possess luminescent lures (p. 314). There is some evidence that the
luminous chin-barbels (Fig. 223) of some of the stomiatoid fishes are not only
lures, but are also sensory-that by their means fishes may detect the precise

FIG . 222.- Teleostei: Biolumi-

nescence in deep-sea forms. A
hatchet. fish (A rgyropelecus gigas)
with light organs shown in white.
Although only 3k inches long, this
is the largest member of the genus.
Hatchet.fishes (Sternoptychida-)
a re laterally compressed. (After
N . B . .Marshall.)

whereabouts of moving prey. Many fishes (stomiatoids and others) have, in

addition, anterior luminous organs which act as flashing headlights and illum-
inate the copepods that the fishes engulf as food. The luminescence of some
species is at least partly recognitional in function. In others, quick flashing
may be defensive, and in yet others the function may be sexual. Teleost
luminescence, although sometimes bacterial in origin, is more often produced

FIG. 223.-Teleostei: Tactile, buccal, and luminal specialisations in deep-sea forms. Stomias, a
deep-sea form, possesses a barbel and can swallow prey larger tha n itself. The white spots are
luminous organs. (From Hickson, after Filhol.)

by glandular cells (backed by black pigment and a silvery reflector), and passes
through, and is altered by, a filter (e.g. red, violet, or green), finally emerging
through a lens. A single fish may have thousands of tiny photo-spots. Photo-
phores are richly vascular and there is evidence that some are under hormonal
control. In some species innervation has been demonstrated.
Many species of fish possess poison-glands. The Stargazer (Uranoscopus) is
equipped with poison spines on the operculum and dorsal fins, as is also its
 PHYLUM CHORDATA 329

relative, the Weever (Trachinus). The anglers (Lophius) and the Trigger-fishes
(Balistidce) are other teleosts with a dorsal armament. Cat-fishes (Siluroidea),
of wide distribution, possess a dangerous spine on each pectoral fin. The most
formidable of all poisonous fishes is Synanceja lzorrida, the notorious Stone-fish
of tropical reefs. This has inflicted a foot injury in Man causing total incapacity
for months and necessitating an extensive skin graft.
Exoskeleton.-In many bony fishes, such as Polyodo11 (a chondrostean) and
many eels, the skin is devoid of hard parts, but in most cases a dermal exo-
skeleton is present. In Amia (Fig.
199, p. 293) and in the majority of
Teleostei this takes the form, as in
the Trout, of scales, rounded plates of
bone imbedded in pouches of the der-
mis and whose exposed areas overlap
one another. vVhen the free border
of the scales presents an even curve,
as in A mia and most physostomcs and FIG. 224.-Actinopterygii: Scales. Left :
Cycloid scale of Salnzo. a. anterior portion
the cods (Anacanthini), they are called covered by overlap of preceding scales; b. free
cycloid scales; when, as in most portion covered only by pigm ented epidermis.
Such clements occur in som e h olesteans and
' spiny-rayed' fishes, the free edge is m ost teleosts. R ight: Ctenoia scale in which
produced into small spines (Fig. 224), the free edge is a rmed with minute spines.
Unlike the placoid scale of selachians (Fig. 59,
they are distinguished as ctenoid scales. p. 83) these are generally covered by epidermis.
(For cosmoid and ganoid scales see Figs. 6o
Usually the integument is continued and 61. )
as a thin layer over the surface of the
scales, but in a good many cases this investment is absent. In exceptional
cases the scales by a redeposition of bone may be so large and strong as to
form a rigid armour. In the sturgeons (Fig. 193) there is a strong armour,
formed of stout bony plates, or scutes, produced into enamelled spines and
articulating with one another by suture. Scutes are also found in many
siluroids and in sea-horses and pipe-fishes (Fig. 219) and some Plectognathi;
while in other Plectognathi the exoskeleton takes the form, as in globe-fishes
(Diodontidre), of long, outstanding, bony spines. In these so-called Needle-,
Porcupine-, or Puffer-fishes, a thin-walled inflatable gastric diverticulum allows
the whole body to be puffed up in globular shape, so that the remarkable
armoury of dagger- or thorn-like spines can be defensively erected. Such
spikes make it difficult for a fish to be swallowed.
Interesting experimental evidence is available on the relative vulnerability
to predators of the Three-spined Stickleback (Gasterosteus aculeatzts), with large
spines, the Ten-spined Stickleback (Pygosteus pungitius) , with small ones, and
the Minnow (Phoxinus phoxinus), with none at all. Phoxinus, Pygosteus, and
Gasterosteus were preferred by P erch and Pike in the order named, but if de-
spined sticklebacks were offered, these were treated as minnows. Of the two
VOL. II X
330 ZOOLOGY

sticklebacks, the more formidably armed Gasterostetts is the bolder, and nests in
open situations (Morris).
In Polypterus and Lepisosteus are found ganoid scales in the form of thick,
close-set, rhomboidal plates formed of bone, covered externally by a layer of
enamel-like material (ganoin) and joined together by pegs and sockets. In
some of the palreoniscoid derivatives the anterior fin-rays of both median and
paired fins bear a row of spine-like scales called fulcra (Fig. r g8, fl.). True
dermal teeth similar to those of the Chondrichthyes occur scattered over the
scales and lepidotrichia in some of the bony fishes (e.g. Lepisosteus, Polypterus).
Endoskeleton.-In the sturgeons the vertebral column (Figs. 194, 225)
consists of a persistent notochord with cartilaginous arches, and is fused

·····t!
PU········ ·

COp
FIG. 225.-Acipenseroidea: Skull. Sturgeon. The investing bones have been removed. a.
pharyngobranchials. A F. antorbital process; A R . articular; b. epibranchial; c. cerat o-
branchial; C. n otoch ord; Cop. basibranchials; d. hypobranchial; D e. dentary; GK. auditory
capsule ; H l'vf. hyomandibular; hy. hyoid cornu; Ih. interhyal; Md. m andible; Na. nasal
capsule; Ob. n eural arches; PF. postorbital process; PQ. palat oquadrate ; Ps. Ps'. Ps". para-
sphenoid; Psp. n eural spines; Qu. quadrate; R. rostrum; R i. ribs; Sp . N. foramina for spinal
nerves; Sy. symplectic; WS . vertebral column; x . vagus foramen ; 1- V, branchial arch es.
(After Wiedersheim.)

anteriorly with the cranium. In the remammg orders bony vertebrre are
present. The centra are biconcave, except in some eels, in which the anterior
face is flat or even convex, and in L episosteus, in which the anterior face is
distinctly convex. Vertebrre of this form, i .e. having the centrum convex in
front and concave behind, are called opisthoccelous. Ribs are usually present :
in Polypterus each vertebra has two pairs: a dorsal pair of considerable length,
running between the dorsal and ventral muscles, and a short ventral pair be-
tween the muscles and the peritoneum. The former answer to the ribs of elasmo-
branchs, the latter to the ribs (pleural ribs) of the remaining bony fishes. There
may be one or more sets of intermuscular bones, attached either to the neural
arch (epineurals), or to the centrum (epicentrals), not preformed in cartilage,
but developed as ossifications of the intermuscular septa. The posterior end
of the vertebral column is turned up in the sturgeons, Lepisosteus, and Amia,
resulting in a heterocercal tail-fin. In Amia, however, the fin-rays are so
 PHYLUM CHORDATA 33!

disposed that the fin appears almost symmetrical. Among Teleostei the tail-
fin is very usually homocercal, as in the Trout, with a more or less disguised
asymmetry: in many cases in the adult the development of the large, fan-
shaped, posterior hcemal arches (hypurals) completely hide the upturned end
of the notochord. In some, the spinal column ends simply in a somewhat
compressed centrum around which the fin-
rays are symmetrically disposed. Such
truly symmetrical tail-fins are diphycercal.
In the structure of the skull, the
cranium of the Acipenseroidea (Fig. 225) is
an undivided mass of cartilage with a few
isolated replacing bones. The roofing in-
vesting bones lie in the dermis, so as to
be practically superficial, and behind pass
insensibly into the scutes covering the
trunk ; the fact thatthese bones (parietals,
frontals, etc.) are exoskeletal structures is
here perfectly obvious. The same is the
case in Polypterus (Fig. 226), in which,
however, the replacing bones are better
developed. In Lepisosteus and Amia, and
especially the latter, the skull resembles
that of the Trout in all essential respects,
the main differences consisting in the
absence of certain bones, such as the supra-
occipital, and in the presence of additional
investing bones. Among Isospondyli the
investing bones remain separable from the
chondrocranium in the adult. In most
other orders (e.g. cod, haddock, or perch) FrG. zz6.- Polypterini: Skull. Poly·
they become grafted on to the chondro- pterus. In dorsal view. Pmx. premaxilla;
Na. external nostril; N. nasal; Sb, Sb 1 •
cranium and so closely united with the anterior and posterior suborbital: Orb.
replacing bones that they can be removed orbit ; M. maxilla; Sp. spiracular bones;
PO. preoperculum (?) ; SO. subopercu-
only by disintegrating the skull. Most lum; Op. operculum; F. frontal: P .
parietal; a, b, c, d, supra-occipita l shields.
of the original cartilage frequently disap- The two arrows pointing downwards
pears in the adult; the cranium becomes a under the spiracular shields show the
position of the openings of the spiracles on
bony mass in which replacing and investing to the outer surface of the skull. (After
Wiedcrsheim.)
bones are indistinguishable.
The varying size of the gape, which is so noticeable a feature in the fishes,
depends on the inclination of the suspensorium. In wide-mouthed fishes
the axis of the hyomandibular and suspensorium is nearly vertical or even
inclined backwards. In small-mouthed forms it is strongly inclined forwards,
332 ZOOLOGY

and the length of the jaws is proportionately reduced. In the branchial

arches the pharyngobranchials of each side are very commonly fused, and
constitute what are called the superior pharyngeal bones. The reduced fifth
branchial bars, or inferior pharyngeal bones, bite against them. Some percoid
fishes are distinguished by having the inferior pharyngeal bones united into a
single bony mass of characteristic form. The gill-rakers are often very highly
developed, and may form a mesh capable of retaining even microscopic
organisms.
Several species of teleosts are able to swallow prey much bigger than them-
selves. When certain deep-sea stomiatoid fishes do this they are aided by a

FIG. 227.-Teleos-
tei: Deglutition spec-
ialisations. Cltauliodus
swallowing spiny prey
(see text). Although
this animal a ngles, it is
a stomiatoid (see also
Fig. 223), notaceratoid
angler-fish. (From
N . B. Marshall, after
Tchernavin.)

remarkable deglutition mechanism that enables the large struggling fish to be

engulfed without damage to the hunter. Thus in Chauliodus (Fig. 227) the
heart and ventral aorta are anteriorly placed between the lower jaw-bones.
Parts of the fragile gill-arches, too, are directly adjacent. Tchernavin has
shown, however, that when Chauliodus attacks, the powerful muscles extending
from cranium to column contract and so swing the skull and anterior vertebrre
upwards. At the same time the articulatory apparatus of the jaws is thrust
forward. Concurrently, the wide gape is opened and sets of special muscles
depress the pectoral girdle (and attached pericardium and heart) and the gill
arches. Thus, each delicat e and vital organ is pulled out of harm's way while
the struggling, spiny catch is thrust by a 'stabbing' movement of sharp,
elongated teeth down the pharynx into the widely distensible stomach. The
 PHYLUM CHORDATA 333

deglutition process probably takes place very quickly and so respiration is only
momentarily impeded.
In the shottlder-girdle of the Chondrostei there is a primary girdle consisting
of large paired cartilages, not united in the middle
ventral line, and unossified : each is covered ex-
ternally by a large scute-like investing bone, the *:
cleithrum. In Polypterus a clavicle and cleithrum
are also present, but in the remaining palreoniscoid
derivatives (p. 326) and in Teleostei the primary
shoulder-girdle is reduced in size and is usually
ossified as two bones, a dorsal scapula and a
ventral coracoid : sometimes, as in the Trout,
there may be an additional ossification, the
mesocoracoid. Additional investing bones-
sztpracleithrum, postcleithrum, etc.-are added,
and one of them, the post-temporal, serves to
articulate the shoulder-girdle with the skull (Fig.
208, p. 308). The skeleton of the pectoral fin of
Polypterus (Fig. 228) differs substantially from
that of other orders, including teleosts (Fig. 208) FJG. 228.- - Polypterini: Pect-
oral fin. The presence in P oly-
(see Daget). pterus of three elements- dorsal
In the Chondrostei Pr. and ventral1VJI. margins, and
ossified media plaque Oss.-gives
(e.g. Polypterus) there this fin a superficial resemblance
to the pectoral fin of selachians
occurs a vestigial pelvic (p. 235). There is no phylo-
girdle (Fig. 229, BP) in genetic significance in this simil-
arity. F. 5 . dermal rays ; N . L.
the form of a small nerve foramina ; Ra. first radia ls;
rhomboidal cartilage to bony Ra1 • second radials; At * the
marginal rays meet and
which the anterior ends shut off the middle region from
the shoulder girdle. (After
of the basalia (Bas 1) are Wiedersheim.)
attached. In all the re-
maining orders the pelvic girdle appears to be atrophied.
The pelvic fin is supported by a single bone of variable
form (Fig. 209, B. PTG) and apparently arising from
FIG. 229.-Po!ypterini:
the fusion of proximal pterygiophores. Between its
Pelvic fin and cartilages. posterior end and the dermal rays irregular nodules,
Young Polypterus. Ap.
part of basal; Bas1 . basal; representing radials, may be interposed.
BP. p elvic cartilages (fused The distinction between hard or unjointed fin-rays,
in adult); Rad. radials.
(After Wiedersheim.) or spines, and soft or jointed fin-rays has already been
referred to. The major ray of the dorsal and pectoral
fins sometimes, e.g. in trigger-fishes (Balistidre), has the form of a very strong
spine which can be 'locked' into its erected position, and only lowered by
means of a complicated trigger mechanism derived from other bony elemente
334 ZOOLOGY

of the fin (dorsal fin-spines), or by specially inserted muscles moving the spine
base over an elaborate articular surface (pectoral spines in certain siluroids).
In some cases the first dorsal spine springs from the skull.
The texture of the bones is subject to wide variation : in some' spiny-rayed'
fishes they are very thick and strong, in some places almost like ivory ; while in
the Lump-fish (Cyclopterus), the huge Sunfish (Mala), and in many deep-sea
forms, such as the ribbon-fishes (Regalecus and Trachypterus), the amount of
mineral matter is so small that the
bones are easily cut with a knife and
weigh astonishingly little when dry.
Electric Organs.-Members of
four families of teleost fishes have
independently developed organs
which produce electricity. These
are used for defence, the capture of
prey, and perhaps as direction-
finders. The best-known is the so-
called Electric-' eel' (Electrophorus
=Gymnotus) (Fig. 230)-a blind,
superficially eel-like fish which grows
to a length of about 8 feet, and is
almost as thick as a man's thigh and
lives in shallow muddy parts of the
Amazon, Orinoco, and other South
American rivers. On land Electro-
phorus can discharge about 500
volts. In water the charge is partly
FIG. ZJo.- Teleostei: Electric organs. Elec.
trophorus electricus. Above: showing the extent
short-circuited and the shock (from
of the electric organ (E). Fl, ventral fin. Below:
an unexhausted fish) is about 250
small portion of tail, in section. DM, DM'. dorsal
muscles. E, E'. electric organ ; Fl. ventral fin ;
volts, but still sufficiently strong to
H. skin; LH. caudal canal; S ep . fibrous septum;
VM, VM' . ventral, muscles; WS, WS'. vertebra l
cause great discomfort to Man or
column, with spinal nerves. (After Wiedersheim.)
to activate an electric buzzer or light
a neon sign (as one does in the New York Zoo). The electric organ consists of
some seventy columns, each of which contains a series of about six hundred
disc-shaped syncytial electroplaxes (electro-plate cells). The cells are probably
formed from striated muscle-cells and together constitute a jelly-like
mass located in the postero-ventral four-fifths of the body, and tail. This
tissue is responsible for about half the total body-weight. Each electro-
plate is innervated, and shocks are normally transmitted only when the fish
is molested, or when it comes within range of its prey, which is stunned and
then swallowed whole. The delaying mechanism which, after the reception
of external stimuli, ensures the electric potential of all the widely separated
 PHYLUM CHORDATA 335

electro-plates discharge at about the same time has not been satisfactorily
explained.
Two star-gazers-the Mediterranean Uranoscopus and the American Astra-
scopus-also have muscle tissue modified into electric organs, but in each case
these, innervated by the oculomotor nerves, are restricted to the region of the
eyes. A third group of electric fishes is the African Mormyridce, which produce
weak shocks from modified caudal muscles (see below). A fourth family is
represented by the African Cat-fish (M alapterurus).
Normal nerve-transmission is essentially an electrical phenomenon, but
there has been for many years conjecture concerning the evolution of electric
organs, since it was difficult to imagine what use they subserved until develop-
ment had proceeded to a degree sufficient to generate power sufficiently high to
deter an enemy or immobilise food. There is now evidence, however, that the
African Gymnarchus niloticus gives out weak electrical impulses at a rate of
about 300 per second throughout life and may detect changes in its electric
field caused by obstacles or other animals in the surrounding murky river-
water. If such a mechanism exists it might provide an explanation, as
Lissman suggests, of the initial steps in the evolution of powerful electric organs.
If one picks up a wet individual Mormyrus, only a faint tingling can be
detected. M. kannume, for example, discharges a continuous stream of weak
impulses at a variable frequency. The discharge frequency is lowest when
the fish is motionless. The rate increases rapidly to So-roo impulses per
minute if the animal is alarmed. The fish responds immediately any conductor
enters its electromagnetic field. The precise means by which M ormyrus
perceives its electrical disturbances is still unknown. M ormyromasts, which
are neuro-glandular epidermal cells of a special kind, may be involved. It is
perhaps significant that the cerebellar and acoustico-lateralis areas of the
M ormyrus brain are remarkably developed, helping to make up a brain-weight
between I/52 and rj82 of the total body-weight, 'a thing unparalleled among
lower vertebrates' (Boulenger). It is not unlikely that this unusual develop-
ment is somehow related to the co-ordination of impulses concerned with
direction finding in this fish, which also lives in muddy waters providing poor
visibility. Substantial electric potentials are also generated by certain elasmo-
branchs (p. 271) but (as in the case of star-gazers and M alapterurus) there is no
evidence of their contemporary use in navigation (p. 272).
Alimentary Canal and Associated Structures.-Some fishes are toothless ;
but in most instances teeth are present, and may be developed on the pre-maxilla,
maxilla, palatine, pterygoid, prevomer, parasphenoid, dentary, basihyal, and
bones of the branchial arches. It is characteristic of most Teleostei, with the
exception of Isospondyli, that the maxilla is edentulous and does not enter into
the gape. The teeth may be either simply embedded in the mucous membrane
and can be detached when the bones are macerated or boiled, or they may be
 ZOOLOGY
implanted in sockets of the bone, or ankylosed to it. They are formed of some
variety of dentine, and are often capped with enamel. Their succession is
perpetual, i.e. injured or worn-out teeth are replaced at all ages.
In a very large majority of teleost species the teeth are small, conical,
and recurved, suitable for preventing the struggling prey from slipping out of
the mouth, but quite unfitted for either tearing or crushing. In some fishes,
such as the pike, the teeth are hinged backwards so as to offer no resistance to
the passage of the prey towards the gullet, but effectually barring reverse
movement. In many deep-sea fishes (Fig. 227) the teeth are of immense size
and constitute a very formidable armature to the jaws. A number of instances
occur in which there is a marked differentiation of the teeth, those in the front
of the jaws (Fig. 231) being pointed or chisel-edged, and adapted for seizing,
while the back teeth have spherical surfaces adapted for crushing. In wrasses
strong crushing teeth are developed on the
pharyngeal bones. In globe-fishes the teeth are
apparently reduced to one or two in each jaw,
but each 'tooth' in this case really consists of
numerous calcified plates fused together.
Sniper-, rifle-, or archer-fishes (Toxotes) of
Indo-Australian rivers exhibit remarkable modi-
fications of the buccal cavity related to the pro-
pulsion of water at insects in foliage overhanging
FrG. 231.-Teleostei: Dental the river-banks. These surface swimmers com-
specialisation. In Sargus (Spa - press their opercula, sharply elevate the floor of
ridt~~) premaxillary and maxil-
lary elements are diversified in a the buccal cavity, and propel a jet of water through
manner reminiscent of tetra-
pods. (After Owen.) a nozzle-like groove in the dorsal aspect of the
mouth. This jet may travel, with startling
accuracy, for several feet, knocking a fly into the water where it can be devoured.
Some species are about one foot in length; young fishes only an inch long also
have the shooting habit.
In some bony fishes the alimentary canal shows little differentiation into
regions, but, as a rule, gullet, stomach, duodenum, ileum, and rectum are more
or less clearly distinguishable at least histologically. The stomach is generally
¥-shaped, but its cardiac region may be prolonged into a blind pouch. This
is often very distensible, allowing some of the deep-sea Teleostei to swallow
fishes as large as themselves. In many genera of several families the stomach
is entirely absent (e.g. Blennius, Cyprinus, Fundulus, Gambttsia, and Labrus).
This is also the case in the Holocephali (p. 256) and is no doubt a secondary and
adaptive condition.
Globe-fishes can inflate the gullet with air or water, as a result of which they
can float upside down. A spiral valve is very well developed in Polypterus and
the sturgeons, vestigial in Lepisostetts and Amia. It appears to be absent or
 PHYLUM CHORDATA 337

vestigial in all Teleostei, except possibly in Chirocentrus (Isospondyli). A trace

occurs in the herrings. The liver is usually large. A pancreas may be present
as a compact gland, as in Chondrichthyes, or may be widely diffused between
the layers of the mesentery, or in part surrounded by the liver. Pyloric cceca
are commonly present, and vary in number from a single one to two hundred.
The anus is always distinct from, and in front of, the urinogenital aperture.
Respiratory Organs.-The gills are usually comb-like, as in the Trout, the
branchial filaments being free, owing to the atrophy of the interbranchial
septa. In the sturgeons, however, the septa are fairly well developed, reaching
half-way up the filaments, so that the latter are free only in their distal portions.
This arrangement is obviously intermediate between the chondrichthyan and
teleostean conditions. Perhaps the most striking deviation from the charac-
teristic structure occurs in sea-horses, in which the gill-filaments are replaced
by curious tufted processes (Fig. zrg, B, g.). As a rule gills (holobranchs) are
developed on the first four branchial arches, but the fourth is frequently reduced
to a hemibranch, and further reduction takes places in some cases. The
pseudobranch or vestigial mandibular gill may either retain the characteristic
comb-like structure, as in the Trout (p. 3rr), or may be reduced, as in the Cod, to
a gland-like organ formed of a plexus of blood-vessels and called a vaso-ganglion
or rete mirabile. In most teleosts the mechanism of respiration is similar to
that already described in the Trout, and respiratory valves are developed in the
mouth-cavity. But there are considerable differences in details, more especially
as regards the relative importance of the opercula and the branchiostegal
membranes in carrying on the movements of inspiration and expiration.
A most complicated accessory respiratory device is the paired supra-
branchial organ possessed by the catfish, Clarias. This is divided into two parts:
(r) a highly branched arborescent structure associated with the second and
fourth gill-arches on either side and (2) a heavily vascularised diverticulum of
the branchial chamber which encloses it (Fig. 232). The epithelium of both
the chamber and the arborescent organ has the same histological structure as
gill-filaments, i.e., thin outer epithelial layers and large intercellular spaces
separated by 'pilaster' cells. Afferent and efferent blood-vessels, derived
from those of the gill-arches, supply the intercellular spaces of the organ and
its chamber. The entrance to the suprabranchial chamber is closed by a
succession of fan-like structures developed from coalesced neighbouring gill-
filaments on the dorsal aspect of the gill arches. These fans can seal the
respiratory suprabranchial cavity. Air gulped through the mouth is then
temporarily retained within. Greenwood reports that even before the arbores-
cent organs arise, young Clarias mossambicus are able to survive in water of
greatly reduced oxygen tension and, in fact, can live for as long as r8 hours
when taken from the water, provided their environment is kept damp. Since
the highly vascularised suprabranchial cavity is developed in fishes of this
 ZOOLOGY

size, it is presumed that it can function as a lung. The early post-larval life is
spent in poorly oxygenated pools and swamps. Adult Clarias are able to
progress from one swamp to another across damp grass-land. In the neotenous
Clariallabes petricola the supra-branchial organ has almost disappeared.
Certain other probably neotenous clariid genera show various stages in the
reduction of this organ.
In several fishes (e.g. Loach) respiration can take place t4rough the epithelium
of the gut. In Amphipnotts, an Indian symbranchid, the gills are poorly

resp . epith.

FrG. 232.- Teleostei: Extra-branchial respiration. Supra-branc hial organ of an air-breathing

freshwater catfish . Clarias mossambicus gulps a ir at the surface in a manner not unlike that of
lung-fishes (Fig. 251, p . 365). but more swiftly. The gill fans (derived from the upper gill filaments)
are mobile and probably control the flow of air or water into the supra-branchial cavity, but
exact details of the respiratory process are unknown. Both aborescent organ a nd adjacent
epithelium (reflect ed) are respiratory in function. There is no connection b etween nostrils and
either the branchial or pharyngeal cavity. Nostrils are purely olfactory in function, allowing the
flow of water over the olfactory epithelium (Fig. Sg, p. 138). The barbels are tactile. (From a
dissection by Greenwood and Marshall.)

developed and functionally replaced on each side of the body by a vascular sac
developed as a diverticulum from the pharynx and which opens anteriorly
into the first (hyobranchial) gill-cleft. Such sacs are physiologically, though
not morphologically, lungs. In the so-called Climbing Perch (Anabas) of Asia
(Fig. 233) folded plates (B) are developed from the first epibranchial bones and
covered with vascular mucous membrane. The fish is able to progress on land,
holding on alternately by the spines of its pre-operculum and of its ventral
fins. Anabas is so dependent on atmospheric oxygen that it is asphyxiated if
kept in water- and denied access to the surface. The \Videly held belief that
this species can climb trees is probably the result of faulty observation. Anabas
 PHYLUM CHORDATA 339
are certainly found in the forked branches of trees. But they are dropped or
placed there by kites which catch stranded fishes in drying pools (Das).
The Common Eel (Anguilla anguilla), which is notable for its overland
journeys, is able to breathe cutaneously both in water and on land. Krogh
long ago stressed that even in water more than half the total oxygen absorption
is cutaneous, although carbon dioxide excretion is principally through the gills.
Ashore, on the other hand, the eel excretes more than go per cent of its carbon
dioxide through the skin. Oxygen uptake by the skin decreases sharply with a

F1c. 2JJ.- Teleostei: Extra-

branchial respiration. A11abas
scandens. (See t ext.) A . Anabas
(Family Anabantid<e) is not a true
B perch nor does it climb. The
specific name (scandens), however,
must be (very p roperly) per.
pctuated on grounds of priority.
T he even less suitable veracular
na mes will p robably continue to be used in
accordance with c ustom. B. d issection of
head, showing accessory respiratory organ.
(A, after Cuvier; B, after Gunther.)

rise in external temperature: perhaps a reason, as Carter suggests, why eels

usually are found ashore only at night. In the mudskippers (Periophthalmus,
p. 325) the oral and pharyngeal epithelium is highly vascular and subserves
respiration.
In a few t eleosts, notably the Stargazer, Astroscopus, there have arisen intern-
al nares. Paired passages lead from the nasal capsules into the buccal cavity,
perhaps facilitating aquatic respiration (and possibly olfaction), while the animal
lies buried in the sand. The channels do not perforate any bony structure (Atz).
They are absent in related genera; and are of course not homologous with
comparable structures in lung-fishes and tetrapods (pp. 366, 383).
Swim-bladder.- It is now generally held that the swim-bladder arose from
the primitive lung of ancient fishes (e.g. p. ro8) and reached its present efficiency
only when actinopterygian ancesters took to the sea. Swim-bladder and lung,
then, are homologous. The swim-bladder (Figs. 234, 235) lies between the
340 ZOOLOGY
alimentary canal and the kidneys. Although essentially a hydrostatic organ
(see below), it may function also as an auxiliary re-
spiratory device (see also p. 290), and is sometimes
connected with the production or amplification of
sound. It may be single or multi-chambered, and is
generally a tough sac overlain by a capillary network.
Below the capillary system is a tunica externa of con-
nective tissue. Beneath this is a tunica interna con-
sisting of smooth muscle-fibres with inner epithelial
gas-gland (Fig. 235), which, in general, is specialised
anteriorly for gas secretion and posteriorly for
diffusion. It may be open (physostomatous) or closed
(physoclistous). In the latter case the pneumatic duct,
connecting the organ with cesophagus or pharynx,
may be reduced to a mere thread or absent altogether.
In Polypterus the swim-bladder consists of two lobes,
the left one large, the right smaller (see Fig. 255,
p. 371). The swim-bladder in some species is divided
into compartments, or may be produced into lateral
diverticula. In Amia and Lepisosteus (Fig. 234, a. b)
its wall is sacculated or raised into anastomosing
ridges, enclosing more or less well-marked chambers
and thus resembling a primitive lung. In Polypterus
its lung-like character is enhanced by the ventral
position of the opening and by the blood being con-
veyed to it (as is also the case in Amia) by a pair of
pulmonary arteries given off from the last pair of
epibranchial arteries, as in the Dipnoi (p. 372). Even
in some modern fishes the swim-bladder seems to be
capable of acting as an accessory respiratory organ.
It has been found that in a perch asphyxiated in stag-
FrG. :!34.-Holostei: nant water, the oxygen in the bladder, which normally
Swim-bladder and its general amounts to 20 or 25 per cent. , is entirely absorbed
relationships. L episosteus. a.
anus; a. b. swim-bladder; a. and replaced by nitrogen and carbon dioxide. The
b'. its aperture in the phar-
ynx; b. d' . aperture of bile- primary function of the bladder, however, is hydro-
duct; c. pyloric c<Eca; g. b. static, i.e. it serves to keep the fish at the same specific
gall-bladder; hp. d. hepatic
duct; lr. liver; py. pyloric gravity as the water. The specific gravity of the fish
valve; s. spleen; sp. v.
spiral valve ; st. stomach. as a whole, rising or falling because of the increase or
(From Wiedersheim, after decrease of pressure at various depths as it descends
Balfour and Parker.)
or ascends, and causing greater or less compression of
the gases in the swim-bladder, can be brought to approximately that of the
surrounding water by increase or decrease in the quantity of the contained
 PHYLUM CHORDATA 341

gas. Although physostomes (with 'open' swim-bladders) can replenish lost gas
by swallowing at the surface, physoclists and some physostomes can transfer
gas of high oxygen content direct from the blood-stream. In these animals,
the epithelium of the bladder is differentiated into
a gas-gland which is generally composed of many layers sac.
, !.c.

of specialised gas-producing cells which are supplied

- sin.end.
with blood-vessels from the retia mirabilia or wundernetz __ .sin.imp.
mentioned below (Fig. 236). Part of the arterial and - - - St>p.

venous system supplying the swim-bladder breaks up

into minute vessels to form the retia mirabilia, and so -·mal.

comes into functional relationship with the cells of the

gas-gland. Arterial capillaries bring in blood gases,
and the gland secretes oxygen, carbon dioxide, and
nitrogen into the lumen of the bladder, according to
the needs of the moment. If gas is experimentally
withdrawn, the gas-gland undergoes a compensatory
hyper-secretion. Likewise, if the fish is experiment-
ally burdened with a weight, it will at first sink, but
later, with increased gas secretion, it recovers buoy-
ancy by swim-bladder expansion. Gases can, on the - posc.ch,
other hand, diffuse from the swim-bladder into the
capillaries and, in fact, some fishes (e.g. Myctophid~.
Gadid~. Percid~. Mugilid~. etc.) possess an oval (Fig.
236), which is a thin-walled, richly vascular area of
swim-bladder modified for the resorption of gases.
The oval is equipped with an arrangement of circular Swim-bladder FIG. 235. - Teleostei:
and its rela-
and longitudinal muscles that can either expose the tionships with apparatus of
audioequilibration. (In a
capillary area to, or mask it from, the internal surface cyprinoid.) ant. ch. and
of the swim-bladder and so reflexly control the gas tpost. ch: anterior and pos-
erior cha mbers of swim-
pressure within. This secondary specialisation is no bladder; W eberia n ossicles :
doubt of immense benefit to fishes that undergo inc. incus; mal. malleus
a nd stap. stapes ; o. s. os
relatively rapid vertical migrations to spend t he night suspensorium ; sac. sac-
culus; sin. end. sinus
hours near the surface and to avoid daylight in the endolymphaticus ; sin. i mp.
'twilight' reaches of the depths. By altering the in- sinus impar ; t. c. trans-
verse canal. 1-4, vertebrae .
ternal pressures and volume of its swim-bladder, the fish (From Harden J ones and
N. B . Marshall after
remains in hydrostatic equilibrium with the changing Chranilov.)
water-pressure, and so can move up or down with a
minimum of muscular effort. The result of the sudden removal of pressure can
be seen in the grotesque buccal protrusion of the swim-bladder from fishes
suddenly hooked or trawled up from even moderate depths.
In some fishes with a functional pneumatic duct the gas glands are absent,
but in the eels and others their place is taken by red bodies of similar appearance,
342 ZOOLOGY
but with non-glandular epithelium. In some forms with an enclosed swim-
bladder the anterior end of the organ is forked, and each branch fits closely
against a membranous space in the posterior wall of the auditory capsule,
while laterally it extends outwards in the region of the shoulder-girdle, and
comes to lie immediately beneath the skin. Any change in volume of the
swim-bladder is thus communicated to the auditory organ. A more sensitive
apparatus exists in the carps and siluroids (order Ostariophysi), in which a
chain of bones connects the swim-bladder with the ear, forming the W eberian
apparatus (Fig. 235). There is reason to believe that this apparatus, modified
from processes of certain vertebrc:e, carries pressure stimuli anteriorly to the
perilymph (p. 145).
p.c.v. k . d.a.
'' '' '

c.m""
..il.

FrG. 236.-Teleostei: Swim-bladder, its blood supply and intrinsic circulation. The gas gland,
oval and spatial relationship with the kidney are indicated. The arteries are sh own white and the
veins black. c. capillary layer; c. m. a. creliaco-mesenteric artery; d. a. dorsal aorta; g . gl. gas
gland; II. p. v. h epatic portal vein; k . kidney ; ov. oval; p. peritoneum; r. m. retia mirabilia.
(From Harden Jones a nd N. B. Marshall, after Saupe.)

The presence or absence of the swim-bladder is often closely correlated with

the ecology of the animal (see Jones and N. B. Marshall). It may be lost in
bottom-living, shallow-water fishes, and in those inhabiting· the extreme ocean
depths where it is sometimes converted into a fat-storage organ (see alsop. 359).
The efficient swim-bladder shown in so many modern teleosts is a relatively
recent development. It has conferred upon them incalculable advantages.
Blood-Vascular System.-The structure of the heart forms one of the most
striking differences between the palc:eoniscoid derivatives and the teleosts. In
the former there is a muscular conus arteriosus with rows of valves, as in the
Chondrichthyes. In teleosts a vestige of the conus containing two rows of
valves has been found in Albula (Isospondyli), and similar vestiges occur in
several other genera of the same order, but in all the rest of the teleosts it is
entirely unrepresented. On the other hand, teleosts always have a large bulbus
arteriosus- a fibrous non-contractile ring of tissue at the base of the ventral
aorta.
 PHYLUM CHORDATA 343

The flesh of most fishes is pale, but in fast-swimming teleost tunnies

(Thunnus and allied genera) and certain others there occurs highly pigmented,
fat- and glycogen-containing red muscle, which, according to its anatomical
situation, appears not to take part in the principal work of the body. In
Thunnus and mackerel (Scomber) disproportionately large amounts of vitamins
of the B-complex (excluding niacin) occur in red, compared with ordinary
skeletal, muscle, leading to the view that it has functions somewhat comparable
with certain of those of the liver.· The circulatory arrangements of tunnies
are of a complexity unique among fishes and these, in themselves, differ curi-
ously between genera. The dorsal aorta gives off a series of cutaneous arteries
which run laterally through the kidney tissue to emerge in the superficial
musculature. Dorsal cutaneous arteries give off dorsal segmental arteries.
The corresponding ventral vessels usually do likewise. The cutaneous arteries
in turn lead to rows of arterioles. This arrangement is accompanied by a
correspondingly rich venous vascularity. The visceral circulation, too, is
noteworthy: certain branches of the cceliaco-mesenteric artery terminate in
unusually rich and complicated vascular plexuses in the liver. The heat
generated by metabolic activity raises tlle temperature of fishes generally to
a level slightly in excess of that of the surrounding sea-water. In Thunnus
the temperature may be as much as I0° C. more than that of the surrounding
medium.
Bony fishes possess, as with all vertebrates, erythrocytes, and various kinds
of leucocytes. The possession of red respiratory blood-cells (containing
hcemoglobin) is a characteristic of vertebrates, although the leptocephalus larva
of the eel does not acquire the pigment until it becomes an elver, and the pig-
ment of Petromyzon (p. 176) is somewhat different from that of other vertebrates.
There is evidence, too, that Antarctic members of no less than three genera
of marine teleosts (Chamocephalus, Champsocephalus, and Pseudochcenichthys)
possess colourless blood. They have yellowish-white gills, and Norwegian
whalers call them blodlaus-jisk (Ruud). Investigation has shown that their
blood is in fact essentially a transparent, yellowish-white plasma containing
leucocytes as well as unknown principles that enable it to clot. No red cells
whatever are found in Chcenocephalus and no absorption bands can be dis-
cerned in the spectrum. The iron content of the plasma is relatively minute
compared with that of other teleosts, and its oxygen capacity is correspondingly
low. These probably comparatively sluggish fishes are relatively large ; they
sometimes attain a length of 6o em. and a weight of more than I kg. Generally
they are caught in water that is somewhat colder than 2° C.
Experimental evidence is available that carp, pike, and eels can remain alive,
apparently little incommoded, after their blood is almost saturated with carbon
monoxide bubbled through their tank. Such a state of carboxylation would
kill quickly a mammal, and Fox has suggested that many fishes, including
344 ZOOLOGY
certainly the above Antarctic forms, probably carry adequate oxygen in their
blood plasma for their needs while swimming quietly. The hremoglobin in
many teleosts is possibly used only in an emergency. On the other hand, it
has been claimed that mackerel, for example, are unable to obtain sufficient
oxygen unless they are swimming rapidly.
Brain.-The cerebellum and optic lobes are usually large. The dience-
phalon is well developed in palreoniscoid derivatives (p. 326). In the Teleostei
the prosencephalon has the general features which have
been described in the account of the brain of the Trout.
It is not divided into hemispheres and has a roof which,
except in Amia, is completely non-nervous. Its floor
consists of a pair of massive corpora striata (Fig. 237,
prs
prs., and Fig. 2II, p. 313, BG.). In most instances the
olfactory bulbs are in close apposition with the olfactory
region of the prosencephalon without the intervention of
olfactory stalks or tracts; but in some cases, as in cod
(Fig. 238, olf. p.), they are borne on long olfactory peduncles
or olfactory tracts. The palreoniscoid derivatives agree with
Chondrichthyes in that the optic nerves form a chiasma,
whereas in teleosts they simply cross one another or
decussate. Here also, however, the distinction is not
FIG. 237.-Holostei: quite absolute, since in herrings and some other isospondy-
Brain. Lepisosteus in lids one nerve passes through a slit in the other. In some
dorsal view. chl. cere.
bellum; c. h. olfactory plectognaths the spinal cord undergoes a remarkable
part of prosencephalon ;
di. diencephalon; m. o. shortening. In a sun-fish 21- metres in length and weigh-
m ed ulla oblongata;
olf. l. olfactory bulbs;
ing a ton and a half, the cord is only rs millimetres long.
opt. l. optic lobes; prs. It is actually shorter than the brain.
corpora striata. (After
Balfour and Parker.) Exteroception.-Walls is of the opinion that the eyes of
deep-sea fishes are 'probably by far the most sensitive in
existence'. It is certain that, like those of owls and other nocturnal land
vertebrates, the eyes of certain deep-sea teleosts receive enough light in order
to function in situations in which Man is completely blind. Certain deep-sea
fishes have a retina containing up to 2o,ooo,ooo rods per square millimetre.
Whole 'blocks' of rods are linked (via bipolar ganglion cells) to a single fibre
of the optic nerve. Thus even though the infinitesimal amount of light imping-
ing on a few rods may be insufficient to evoke a neural response, the combined
(though withal weak) reaction of many may easily do so in an abyssal environ-
ment. Among fishes, large eyes are prevalent down to about soo metres.
Some species with wide pupils and big lenses- adaptations to marine twilight-
live as deep as r ,ooo metres. Below this depth the eye seems generally to
become small, related essentially to the mere perception of light; and sometimes
extremely degenerate, or merely vestigial, or even absent altogether. Light
 PHYLUM CHORDATA 345
falling on the sea is absorbed and scattered: Beebe, in his bathysphere, found
that under clear Bermuda waters a coloured book illustration showing bright
red prawns appeared almost black at about 18 metres. Yellow light almost
disappeared at about roo metres and much of the green-blue was gone at 240
metres below. From 520 metres downwards the sea was black to the human
eye.
A secondary photoreceptor mechanism exists in the pineal and adjacent
areas of the dienceph?Jon of some (and perhaps many) fishes (see also p. rg6,
and cf. p. 546). In Phoxinus (a cyprinid) it has long been known that chromato-

FIG. 238.-Teleostei:
Brain and membranous
labyrinth. Physiculus
bach us (Moridae : cods).
a. thickened portion of
swim-bladder fitting into
fenestra in posterior wall
of auditory capsule; a. bl.
swim-bladder; au. cp.
outer wall of auditory
capsule; au. cp'. inner
(membranous) wall; b.
hollow offshoots of air-
bladder; cp. str. corpora
striata; crb. cerebellum;
memb. lab. membranous
labyrinth; olf. l. olfactory
bulbs; olf. p. olfactory
penduncles (olfactory
tracts); op. operculum;
opt. l. optic lobes ; vs. gn.
vaso-ganglia.

phore responses are linked with the pineal region. There is evidence, too, that
this area is light-sensitive in blind cave-fishes (Anoptichthys), as well as in
other species with translucent tissue covering the diencephalon. Hoar has
now shown that in the smolt of the Sockeye Salmon (Oncorhynchus nerka) light
sensitivity is dependent on the pineal organ as well as the eyes. Destruction
of the pineal area 'affects the distribution of pigment in the chromatophores
and the phototactic response of the fish'.
An auditory apparatus (Fig. 93, p. 144; Fig. 214) occurs in all groups of
fishes. It was for many years generally believed that fishes are relatively
deaf and that few indeed are able to emit sounds. Water, of course, carries
sound very efficiently. The speed of submarine sound is modifiable by density
and hydrostatic pressure, but, as a general rule, it can be said that sound
VOL. II Y
 ZOOLOGY
travels underwater at about r,soo metres per second, compared with an average
aerial speed of about 340 metres per second at sea-level.
By the use of sensitive hydrophonic apparatus developed during the 1939-
1945 war much valuable information is becoming available concerning the
noises made by bony fishes. Sound is produced by movements of various
skeletal bones, spines, teeth, and the swim-bladder. The whistling 'song of the
Sirens', which can be heard above the surface, is probably produced by members
of the family Scirenidre. Gurnards (Triglidre) produce snoring noises by the
muscular vibration of the swim-bladder, and because of this have been called
'crooners', 'pipers', and several onomatopreic names in various countries.
Porcupine-fishes (Diodon and allies) produce grating sounds by movements of
the jaws (and possibly palatal ridges), heightened perhaps by the resonatory
capacity of the swim-bladder. On the other hand, some fishes are said to be
deaf, and others incapable of making a noise. There is no reason to disbelieve
that fish noises are important components of behaviour patterns concerned
with threat, recognition, and courtship, as in other vertebrates.
The lateral line organs (pp. 136, 449) may be uncovered, but are usually
canals which open to the environment by means of pores or tubules. Some
abyssal fishes lack actual canals, but possess lateral line organs as sensory
terminations of variously-shaped papillre swinging free in the water. There is
experimental evidence that when another fish swims by, the neural impulses
from the lateral-line system are intensified. Such organs seem to respond only
to low-frequency vibrations, and so apprise the fish of the approach of predators
or prey, and probably help keep members of a shoal together. The disappear-
ance of eyes in the deep-sea bottom-feeding brotulid fishes is accompanied by a
remarkable compensatory development of the lateral-line system of the head.
Brotulids inhabiting higher reaches have functional eyes, and less powerfully
developed pressure receptors. In the scaleless Miripinna of the recently erected
teleost order Miripinnati (Bertelsen and N. B. Marshall), the lateral-line organs
are borne on projections. In one species (M. esau) the body is covered with
branched, hair-like processes bearing glandular cells of uncertain function.
Endocrine organs.-These fall into the characteristic vertebrate pattern
(p. 147). The pituitary varies considerably in structure from one group to
another. 'Anterior' and neural lobes are readily recognised. There is some
evidence that the adenohypophysis (Fig. 96, p. rso) in teleosts is itself made up
of anterior, median, and posterior parts, the median section being homologous
with the pars distalis, and the posterior, glandular portion with the pars inter-
media of mammals. There is experimental evidence that the hypophysis
produces growth, gonadotrophic, thyrotrophic, and adrenocorticotrophic
hormones possibly from the median part of the adenohypophysis. Intermedin,
of melanophore dispersing function, occurs and may come from the posterior
part; and it has been claimed that an agent with an opposite effect is elaborated
 PHYLUM CHORDATA 347
at the anterior end of the gland. The pars nervosa is believed to produce
vasopressin and oxytocin. In some bony fishes the thyroid occurs in a com-
pact organ, but in many others thyroid follicles are dispersed in the region of
the ventral aorta. There is much evidence suggesting that thyroxin or its
homologue has at least a morphogenetic function. Most investigators have
failed to find parathyroid tissue, but there is recent evidence that the ultimo-
branchial cells of the pharyngeal epithelium (Fig. 73 A, p. ro6) may be significant
in this respect. The adrenocortical homologue or inter-renal tissrte is usually
confined to the generally lymphoid pronephros and may be congregated around
the posterior cardinals and their tributaries. In many, if not all, groups
the chromaffin tissue lies in close association within the pronephros or in the
walls of the adjacent cardinals. The function of neither tissue has been proved
in teleosts; but a relationship between the pituitary and inter-renal tissue
exists. Much of the islet tissue may often occur outside the diffuse pancreas
in the region of pyloric creca, spleen, and gall-bladder. A hormone similar in
effect to insulin is discharged. The bud-like corpuscles of Stannius (pronephric
region of Holostei and Teleostei) may be secretory. In teleosts (Lebistes) the
pineal has possibly a secretory function related to growth.
Urinogenital Organs.-The kidney (Fig. 210, kd.) is formed from the mesone-
phros of the embryo. The pronephros usually atrophies. The urinary duct
(ur.) is the undivided pronephric duct: it unites with its fellow of the opposite
side before opening either directly on to the exterior or into a urinogenital
sinus. A urinary bladder is formed as a single or double dilatation of the duct.
The right and left kidneys undergo more or less fusion, and their anterior ends
are usually converted into lymphatic tissue (kd.'), so that, while resembling the
rest of the organ in external appearance, they do not discharge a renal function.
In general, marine teleosts possess fewer glomeruli than fresh-water species.
Some, in fact, have become wholly aglomerular (e.g. the toadfish, Opsanus tau).
This specialisation was no doubt primarily associated with a constant osmotic
water-loss, which in turn reduces the amount of fluid traversing the glomeruli
and, in addition, the utility of urine-formation. However, at least two
species of aglomerular fishes can live in fresh water: 0. tau ascends streams,
and a tropical freshwater pipe-fish (Microphis boaja) breeds in them.
The male organs of Lepisosteus may be taken as an example of those of
palreoniscoid derivatives (p. 326). The testis (Fig. 239, ts.) is a paired, lobulated
organ. Spermatozoa from the seminiferous crypts (lobules or tubules) are
carried by a large number of vasa efferentia (v. ef.) into a longitudinal canal
(l. c.) lying alongside the urinary duct (ur.). From this canal tubes are given
off which communicate with the urinary tubules of the kidney or open directly
into the duct, so that the seminal fluid has to traverse the latter in order to reach
the urinary bladder (bl.) and make its escape by the common urinogenital
aperture (u.g. ap.). In teleosts there are no vasa efferentia. However, the
 ZOOLOGY
posterior end of the testis is directly continued into a duct (Fig. 210, v. d.)
which unites with its fellow of the opposite side and opens either into a urino-
genital sinus, as in the Trout, or directly on the exterior, between the anus and
the urinary aperture, as in cods. In eels the spermatozoa escape into the
crelom and are discharged by genital pores.

FIG. 239.-Holostei: FIG. 24o.- Holostei: Urinogenital system in

Urinogenital system in male. female. A. L episosteus and B. Amia. a.
Lepisosteus. bl. bladder; degenerate anterior portion of kidney; bl. blad-
l . c. longitudinal canal ; ts. der; kd. kidney ; ovd. oviduct; ovd'. aperture of
testis; u .g. ap. urinogenital oviduct into bladder ; ovd". peritoneal aperture;
aperture ; ur. urinary duct; ovy. ovary; p. peritoneum; tt.g. ap. urinogenital
v. ef. vasa efferentia. (After aperture; ur. urinary duct. (A , after Balfour
Balfour and Parker.) and Parker; B, after Huxley.)

The secretory elements of the testis of teleosts fall into two distinct types,
the phylogenetic distribution of which is not yet determined. In many
teleost fishes there are Leydig cells (p. 152) of the typical vertebrate t ype
such as we saw first in Petromyzon and which occur in selachians and in
lung-fishes and Latimeria (p. 356). In others (e.g. Esox ) there is no true
interstitium. Instead, connective tissue cells of the lobule walls are seasonally
 PHYLUM CHORDATA 349
modified to an apparently comparable function (Fig. 97). Such a phenomenon
occurs also in certain urodeles (p. 452) but not in anurans.
In most paheoniscoid derivatives (p. 326) the oviducts (Fig. 240, B, ovd.) have
funnel-like anterior ends (ovd".) opening into the ccelom, while posteriorly
(ovd'.) they discharge into the dilated urinary duct (bl.). A similar arrange-
ment occurs in the Smelt, one of the Salmonidce, in which the eggs are
discharged from the outer or lateral face of the ovary into the open end of the
oviduct. But in most teleosts and in Lepisosteus (Fig. 240, A) the ovary (ovy.)
is a hollow sac continued posteriorly into the oviduct (ovd.). The eggs are set
free into its cavity from the folds into which its inner surface is produced, and
so pass directly into the oviduct without previously entering the ccelom. The
lumen of the ovary can be looked upon as a shut-off portion of the ccelom in
Lepisosteus and the Teleostei. In the embryo a longitudinal foldgrows from
the ventral edge of the then solid ovary, and turns upwards along the lateral
face of the organ. This is met by a descending fold of peritoneum from the
dorsal wall of the abdomen. By the union of the two folds a cavity is enclosed
and becomes the lumen of the ovary. The oviduct is developed as a backward
continuation of these folds of peritoneum, and appears to be quite unconnected
with the embryonic renal system, and therefore not to be homologous with the
oviducts of selachians and Holocephali, which, as we have seen, are Mullerian
ducts. In the Salmonidce and the eels oviducts are absent. The ova are
discharged by genital pores, which are probably to be looked upon as degenerate
oviducts. True abdominal pores are present in palceoniscid derivatives and in
some Isospondyli.
In two families, Sparidce and Serranidce, functional hermaphrodites are com-
mon: the gonads rna y contain both male and female elements. Such a condition
is distinct from the occasional intersexuality that probably occurs in all verte-
brate species. Serranus and Lebistes have been reported to be self-fertilising.
Seranellus subligarius has been experimentally shown to be so (Clark).
Reproduction.-Most bony fishes are oviparous, the eggs being fertilised
after they are laid. The eggs are always small by comparison with those of
cartilaginous fishes, never exceeding 5-ro mm. in diameter. Usually they are
much smaller. Externally shed eggs can be roughly arranged in two categories :
(r) floating, pelagic eggs and (2) demersal eggs which sink to the bottom of
the sea. Pelagic eggs generally contain an oil-globule which aids buoyancy.
Their existence is hazardous and they are usually produced in stupendous
numbers, e.g. a single turbot may shed over g,ooo,ooo eggs, and certain other
species a great many more. Demersal eggs are usually covered with an adhesive
substance and cling to each other and to weeds, rocks, and debris. Some
species (e.g. sticklebacks) build nests. Some cat-fishes, and most members of
the percomorph family Cichlidce, carry eggs and young within their own
buccal cavity (see below). The female Bitterling (Rhodeus) deposits its eggs in
350 ZOOLOGY
the siphon of a mussel by means of a seasonally developed tube-like ovipositor.
Spermatozoa shed by the male are probably carried to the eggs in the siphon-
current. The male Nursery-fish (Kurtus) possess a cephalic hook in the crook of
which is anchored and carried a grape-like aggregation of eggs deposited by the
female. An even more bizarre nuptial specialisation is exhibited by certain
deep-sea angler-fishes. Regan has shown that in Edriolychnus and Ceratias one
or several diminutive males are carried firmly attached to various parts of the
body of the female. The blood-vascular system of each parasitic male and that
of the female host become joined. The normal male mouth-parts are degener-
ate. The teeth and much of the digestive apparatus are sacrificed, but at the
same time a vascular union of such intimacy is formed that it is often not easy

FIG. 24 r .- Teleostei: Exoparasitism. A fema le angler

Edriolyclmus schmidti wit h t hree attached dwar f males (sec
t ext) . T he m ales of some anglers are n on.parasitic. Th
p resent fem ale is only zi inches long. and the rostra l protu-
b era nce is a lure. (For vertebrate exampleR of internal
parasit ism see p. 353.) (From Coleman. after Dana Report,
1932 .)

to see where male tissue begins and that of the female ends (Fig. 241). Respira-
tion in the male takes place through lateral slits.
In teleosts, as well as in elasmobranchs (p. 278), true vivipary, as opposed
to ovovivipary, has independently arisen. It occurs in several orders, and
is widespread in the Microcyprini. Families of this order show an extra-
ordinary range of phenomena, exhibiting on the one hand the birth of living
young associated with little change from the typical oviparous structures (e.g.
Lebistes) and, on the other, vivipary associated with a degeneration of the yolk-
sac and the development of special structures that enable gaseous and nutritional
exchanges with the maternal tissues.
In the Preciliidre the fertilised eggs remain lodged in the single ovary, and
the embryos are held in the follicles until parturition. The epithelium of each
follicle becomes a vascular syncytium. The embryonic pericardia! sac of each
embryo expands, becomes invested with capillaries, and, with the development
 PHYLUM CHORDATA 351
of epithelial folds, the two tissues come in contact as a pseudo-placenta not
unlike the epitheliochorial placenta of certain mammals.
In the Anablepidre (e.g. Anableps) a grotesque extension of this follicular
pseudo-placentation occurs when the embryonic hind-gut temporarily expands
and elongates. There are developed vascular bulbs on the tributaries of portal
veins (Fig. 242). Turner suggests the possibility that additional nutriment can
be absorbed through these structures from the parent to the gut of the embryo.

FIG. 242.-Teleostei:
Viviparity. Partly dis-
sected embryo of Ana-
bleps dowei, showing re-
lationship of pericardia!
and ccelomic cavities,
and expanded posterior
intestine. The embryo
is retained until parturi-
tion within one of the
ovarian follicles, the epi-
thelium of which is trans-
formed into a follicular
pseudo-placenta and an
association between ma-
ternal and embryonic
circulations comparable
with the mammalian
epitheliochorial placenta
(p. 904) is formed .
(From Amoroso, re-
drawn after Turner.)

Vascular bulbs

In one cyprinodont family-the Jenynsiidre-the embryos are retained not

in the follicles, but are ejected into the adjacent ovarian cavity. Here the
epithelium becomes first secretory, and next sheds huge numbers of cells. The
products of these, together with the remains of autolysed non-viable embryos,
are ingested by the surviving young. As embryonic development proceeds,
the ovarian tissues regress, and there are finally developed in the cavity a
series of vascular folds which come in contact with the embryonic gills, and
thereby allow a gaseous and nutritional exchange between ovary and the gills
of the embryo.
Many instances of parental care of the young are known. A few species
such as the Sea-horse (Hippocampus) and the pipe-fishes (Syngnathidre), develop
their young in a pouch (Fig. 219, brd. p.) on the abdomen of the male. In the
siluroid Aspredo the eggs are pressed into the soft, spongy skin of the female
(and thus carried about). Parental care is highly developed in the fresh-
water family Cichlida:, of which Tilapia, an African genus, is an example.
When the breeding season approaches the male becomes aggressive and
352 ZOOLOGY
selects and defends a territory of lake-bottom. Next he excavates a small pit
with his mouth. There now follows an active courtship, after which the eggs
are laid in the nest and then fertilised by the extrusion of spermatozoa. Now
occurs a most extraordinary thing in many, but not all, species of Tilapia.
The female gathers up the eggs into her mouth, where they are brooded. When
hatched, the very small young still take refuge in the mother's mouth, ventur-
ing out as they grow older, and retreating in times of danger. When they are
about twenty days old, and still only a few millimetres long, they do not return
and thereafter fend for themselves.
Tilapia nilotica, a fish that is common in the Sea of Galilee, is one of the
mouth-brooding species of Tilapia. If a brooding female is picked up, the eggs
often fall from her mouth. It is not impossible that this phenomenon inspired
the command to St. Peter, attributed to Jesus: 'Go thou to the sea, and cast a
hook, and take up the fish that first cometh up; and when thou hast opened
his mouth, thou shalt find a piece of money: that take and give unto them for
me and thee.' T. nilotica does in fact lay golden eggs. Those of T. gallilea
and a few others lay green ones. All species of Tilapia possess, when young,
a dark spot near the posterior end of the dorsal fin. This is often called the
'Tilapia-mark '. It is sometimes called' St. Peter's mark', allegedly left by the
fisherman saint when he picked the animal up. In most species the St. Peter's
mark disappears when the fish gets older.
Development.-In all pala:oniscoid derivatives hitherto investigated, with
the exception of Lepisosteus, cleavage is complete. In Acipenser and Amia it is
very unequal, the macromeres being immense as compared with the micro-
meres. In Polypterus it is equal at first, becoming unequal later: the process
may be said to be intermediate between the holoblastic and meroblastic types.
In Lepisosteus the segmentation is meroblastic, the fissures not extending much
beyond the equator of the egg. In Teleostei segmentation is always partial
and discoidal. The general features of development are much the same as in the
Trout, except that in sturgeons and Polypterus, as in craniates in general, there
is an open medullary groove which becomes closed in to form a medullary canal.
There is frequently a metamorphosis. In Lepisosteus, for instance, the newly-
hatched young is provided with a sucking-disc, and the proportions of the head
are quite different from those of the adult. In the larval sturgeon provisional
teeth are present, and in many teleosts the young differ from the adult in the
presence of large spines which by increasing the surface area probably aid in
flotation. The pelagic larva: of eels are strongly compressed, transparent, and
have colourless blood (see p. 343). They are sometimes known as 'glass-fish ',
and until their real affinity became known were placed in the 'genus'
Leptocephalus. One such leptocephalus larva, the adult of which is probably
still unknown, is 184 em. long. This, the biggest larva ever described, was
taken by the Dana Expedition west of the Cape of Good Hope in 1930 (Taning).
 PHYLUM CHORDATA 353

The young of Polypterus possess external gills (Fig. 243; see also Fig. 262).
An especially remarkable developmental sequence occurs in Carapidre or
fierasfers. Carapus acus, for example, produces planktonic eggs, from which
hatch vexillifer larvce, each equipped with a long dorsal vexillum or banner.
This degenerates, and the larva sinks to the sea-bed and enters the anus of a
holothurian. The larva now metamorphoses into the temtis stage, which feeds
on the holothurian tissues and next changes into a juvenile and later into an
adult.

FrG. 243.-Holostei: Development.

External gills (E.G.) in advanced larva
of Polypterus bichir. (From De<tn,
after Steindachner.)

Internal parasitism.-The adult Caraptts is capable of feeding both inside

and away from the host. As many as seven parasites may occur in a single
holothurian. A second group of internal (and external) parasites are the
slender South American freshwater pygiidid candiru (e.g. Vandellia spp.)
which enter the gill-cavities of bigger fishes and feed on the soft tissues within.
V andellia sometimes penetrates the urethra of Man and cow if they micturate
while in the water.

SUB-CLASS CROSSOPTERYGII (CHOANICHTHYES)

INTRODUCTION
It is appropriate to deal with these, the crossopterygians (lung-fishes and
allies) , last and therefore immediately before the Amphibia. At the same time
it must be clearly understood that even though they show certain undeniable
features in common with t etrapods, they merely represent one branch of three
widely different branches of piscine evolution and, in themselves, cannot be
thought to be' higher' in the scale of animal classification than either the elasmo-
branchs or actinopterygians. We will see, however, that the development by
this stern of the aerial respiratory apparatus and other specialisations made
possible the revolutionary movement of fish-like animals to dry land, and the
consequent evolution of the tetrapods (p. 381). On grounds of microscopic
anatomy as well as general morphology there is something to be said for
separating the crossopterygians as a class by themselves.
It is probable that this sub-class, though once abundant in regard to in-
dividuals, has never been very extensive. To-day it is probably reduced to
one recently discovered ccelacanth (p. 356) and to a few species of Dipnoi (p.
361), themselves aberrant members of an ancient side-line.
354 ZOOLOGY
The earliest fossil appearance of the Crossopterygii is during the Middle
Devonian period, by which time three separate groups, the Osteolepidoti,
Crelacanthini, and Dipnoi, were already clearly established. Weston has
provided good evidence that the early Dipnoi were closely related to contem-
poraneous Rhipidistia (see below). From the latter the Actinistia ( = Crela-
canthini) are an ancient branch. The extant Latimeria, and lung-fishes as well,
preserve many archaic characters. Because the Crelacanthini and the Osteo-
lepidoti are far more closely allied to each other than to the Dipnoi (lung-
fishes), Trewavas et al. have proposed the classification which is here adopted:

SUB-CLASS CROSSOPTERYGII
Order Rhipidistia
Sub-orders Osteolepidoti (Devonian-Permian)
Ccelacanthini (Actinistia) (Devonian-Recent)
Order Dipnoi (Devonian-Recent)
ORDER RHIPIDISTIA
Although the members of this group are in some respects too specialised to
be themselves ancestral to the crelacanths, lung-fishes, or tetrapods, the order
may nevertheless be considered as the central one, from whose earlier, less
specialised, roots the three above-mentioned branched off on their own lines of
evolution. We shall see later (p. 382) that there is much reason to believe that
the prototetrapod came from a generalised rhipidistian crossopterygian which
was already well differentiated 'in earliest late Devonian times' (Westoll).

SUB-ORDER OSTEOLEPIDOTI
Osteolepis macrolepidotus (Fig. 244), the earliest well-known rhipidistian
species, illustrates all the main features of the sub-class, as well as the particular
features of its group. Compared with a primitive actinopterygian, such as
Palceoniscus (p. 285), there were several points of variation in addition to
such absolutely fundamental differences as the internal nostril and the cosmoid
structure of the scales and head-bones. Among these were the two dorsal fins
instead of one, the smaller eye with numerous sclerotic plates, as well as paired
fins of the archipterygial type with a single basal element articulated with
the shoulder-girdle. The dentition showed two peculiar features. The pre-
maxilla, maxilla, and dentary were supplied with a number of small, sharp,
conical teeth, such as occur in many fishes, but the prevomer, palatine, and
ectopterygoid had, in addition, a series of larger teeth of a characteristic
structure and method of replacement. The dentine was much folded, and
replacement was alternate, a second tooth growing up beside the first, so that
there was a large and a small socket side by side. Such an arrangement does
not occur in other fishes. It is of high interest that both the above features
 PHYLUM CHORDATA 355
have also been transmitted from some earlier ancestor to the tetrapods, as is
shown by the 'labyrinthodont' teeth of the early Amphibia (p. 422). The
arrangement of the jaws was autostylic. The suspension therefore resembled
that of the earliest gnathostomes (p. 203), as well as that of the Amphibia. I
Gills, guarded by opercula, were present.

FIG. 244.- 0rder Rhipidistia, Sub-order Osteolepidoti, Family Osteolepidre. Os teolepis. Above:
Reconstruction of t he Devonia n 0. macrolepidotus (after Traquair). Osteolepiform fish a lready
possessed a n interclavicle w h ich is retained by primitive amphibians (see p . 425) . The hinge
between p a rietals and post-parietals was lost (Fig. 285, p . 427). B elow: Skull, in dorsal a nd
latera l views (modified after Save-Soderbergh ). Ext. l . an d Ext. m . extra-scapula rs ; Fr.
parietal; G. gula r; It. sup ra -temporal ; La. L achrymal ; Md. dentary ; Mx. maxilla ; Op.
opercular ; P a. post-parietal ; P o. post -orbita l ; P. Op. pre-op ercula r ; Qtt. ]. q u adra t ojugal;
5 . op. sub-opercular; Sq. squamosal. (Aft er Save-Soder bergh.)

The t ail of the Rhipidistia, having a small epichordal lobe, was partly
het erocercal. The compositicn of the ossified skull was, in general, not unlike
that of a palCPoniscid, but t here was one important and characteristic point of
difference : in all rhipidist ians (Fig. 244) there is a line running t ransversely
1 J ar vik's (1954) reconstruction of E ustlzenopteron shows a h yoid a rch which a lmost certainly
bore a norm a l poster ior hem ibranch . T here is a lso evidence of a vestigia l p respiracular pouch .
356 ZOOLOGY
across the skull between the parietal (frontal in older nomenclature) and
postparietal (parietal) bones. This divides the skull into two portions. This
feature alone would prevent any known form from being regarded as ancestral
either to the dipnoan or tetrapod lines.
There is evidence that in early members of both orders there is an alter~
nate deposition and resorption of the outer layer of cosmine in the skull-bones
and scales during the life of the individual fish. This periodical absorption
may have allowed the bones to grow and expand. As much of the detailed
classification of extinct forms has been based on the structure of the scales,
the recognition of this phenomenon may alter considerably the arrangement
within the order (Westoll). Genera are as follows: Osteolepis, M egalichthys,
Thursius, Eusthenopteron, Rhizodus.

SUB-ORDER C<ELACANTHINI (ACTINISTIA)

The Ccelacanthini, represented to-day by a single family, are a side line of the
Crossopterygii which appear first in Upper Devonian deposits. Fossils occur
both in fresh- and salt-water deposits. In the Carboniferous period (p. 3)
Ccelacanthus, for example, was a fresh-water animal; whereas Undina (Fig. 245)
and other Mesozoic ccelacanths were marine. No post-Mesozoic ccelacanth
fossils have been found, and until 1939 it was thought that the Ccelacanthini
had been extinct for some 5o,ooo,ooo years. Then a live specimen was trawled
up off the east coast of South Africa. This fish, Latimeria chalumna: (Fig. 246),
was caught at about 40 fathoms and was some 5 feet long and weighed 127lb. It
was steel-blue in colour. Subsequent living specimens have been described as
possessing eyes of a greenish-yellow luminescence. Unfortunately, before their
extraordinary value was fully appreciated the viscera of the first specimen
was destroyed; only the skin was preserved. ]. L. B. Smith, who described
Latimeria, now began an intensive effort to get further specimens with soft
parts intact, and due to his initiative a second ccelacanth was recognised
in a native fish-market on the remote island of Anjouan in the Comoro Archi-
pelago late in 1952. This specimen, ironically enough, was caught with a
fish-baited hook at about 8 fathoms some 200 metres off-shore. It was
abnormal, having lost its dorsal fin and supplementary tail-lobe (no doubt in
youth), and was described as a new genus and species. Subsequently, several
more Latimeria, including one alive, have been captured and extensively
studied by Millot and his associates. The new generic name proposed by Smith
must be relegated to synonymy.
Fossil specimens (Ccelacanthus, Macropoma, Undina, etc.) had long revealed
that the Ccelacanthini has the same general organisation of skull and scale
structure as had the Osteolepidoti, together with some very definite specialisa-
tions of its own. It would seem that internal nostrils, if they ever existed,
were lost very early in the evolution of the group (see Fig. 248). The structure
 PHYLUM CHORDATA 357
of the fins and tail, and the peculiar, calcified swim-bladder are highly character-
istic. The paired fins have the superficial appearance of those of an actinopt-
erygian. There is a short, blunt, and scale-covered lobe, around which the
fin-rays are attached as a fan. The fins are pedunculate, their soft-parts
affording important information concerning the evolution of limbs, although
it must be emphasised that crelacanths are off the main line of tetrapod
descent. Such a narrow-based paddle, mechanically imperfect for aquatic
progression, was adapted for locomotion on a solid or semi-solid sub-stratum.
Observations by Millot on a captive crelacanth have shown that it swims
by curious rotating movements of its pectoral fins, while the second dorsal
and anal fins, also highly mobile, serve together with the tail in steering.

FIG. 245.-0rder Rhipidistia, Sub.order Cc:elacanthini, Family Cc:elacanthidre. Undina.

(Triassic a nd Jurassic.) j, gular plates. (After Smith Woodward.)
U. penicillata.

Millot has shown, too, that a remarkable limb flexibility exists. The orienta-
tion of these lobe-like fins varies greatly (by as much as 180°) even between the
two sides of one individual. These data support the palceontological work of
Gregory, who emphasised the probable tetrapod descent from rhipidistian
ancestry involving the development of land-limbs from muscular, and hypo-
thetically flexible, lobe-like paddles (Fig. 246).
The internal skeleton of the fins of Latimeria consists essentially of a single
basal plate. This is attached to the shoulder-girdle in the case of the pectoral
fins. Each basal plate is followed by a few axial pieces around which the fin-
rays are arranged, so that the fin appears to be essentially mesorhachic or
archipterygial. The pelvic fin in extinct forms (e.g. the Triassic Laugia and
Jurassic Coccoderma) has come to be attached near and below a tiny true
pectoral fin . This anterior migration was repeated in the unrelated actino-
pterygian t eleosts 100 million years later.
Of the two dorsal fins, the anterior is supported by a bony plate, and the
358 ZOOLOGY
posterior by an osseous structure not unlike that of the pectoral fin. The tail is
diphycercal, with a small central projecting lobe. The fin-rays are attached
each one singly to a dorsal or ventral spine of a vertebra. This is another point
of convergence with the later teleosts. The vertebral column, apart from the
spines, is unossified. There is a massive, unconstricted, and wholly cartila-
ginous notochord. Neural and hremal processes occur.
The skull, while showing a number of specialisations, is like that of the

FIG. 246.- 0rder Rhipidi3tia, Sub-order Crelacanthini, Family Latimeriidre. Latimeria

clmlumnre. For cephalic details, see Figs. 247, 248. (Redrawn after Millot.)

Osteolepidoti in possessing a powerful and freely movable hinge on the dorsal

surface between the parietals and post-parietals (p. 355). This feature, together
with the traces of cosmoid covering of the bones and scales, is further evidence
of the common ancestry of the two sub-orders.
There are five gill-arches. Four only of these are attached to the copula.
The smallest and fifth gill-arch has no slit behind it. The top of each arch bears
a series of small plates which are covered with sharp spines of variable size.
Two extensive sensory systems occur in the rostral region (Figs. 247, 248).
They are quite distinct from each other. One consists of a large, single-
median chamber from which three paired rostral tubes communicate with the
 PHYLUM CHORDATA 359
exterior. Of these, the postero-superior and postero-inferior rostral tubes
emerge near the orbit and the anterior rostral tube near the point of the snout
(Fig. 247). There is no cir-
culation of water through
ant. rost.
this apparatus, which is tube
permanently full of a jelly-
like material recalling that
ant. nas.
of the ampullre of Lorenzini canal
(p. 251). Its function is post. nas.
uncertain. The second canal
apparatus is olfactory in
function. It is paired,
contains a nasal capsule,
and communicates with the
exterior by means of
typical nasal tubes. Each
anterior nasal canal em-
erges through one of the FIG. 247.-Latime1·ia: Bead and sensory organs. (See
text and Fig. 248.) (Redrawn after Millot and Anthony.}
paired rostral papillre near
the extremity of the snout. Each posterior nasal canal, on the other hand,
opens to the cheek by a brief inconspicuous slit between the postero-inferior
rostral tube and the pseudo-
maxillary fold. In general, the
nasal apparatus resembles that
of Polypterus.
After the first (eviscerated)
specimen of Latimeria was de-
scribed, there was much specula-
tion concerning the probable
form of the swim-bladder.
Millot has no\v shO\vn that the
organ is variable and sometimes
vestigial. A mere tube, two or
three inches long, extends from
the ventral aspect of the reso-
phagus, and is prolonged by a
FIG. 248.-LafimeJ•ia: Sensory organs. (See text
and Fig. 247.) (Redrawn after Millot and Anthony.) long, fat-sheathed filament that
divides to an attachment with
the dorsal wall of the abdominal cavity. This reduction suggests that its original
function in Latimeria stock may have been more pulmonary than hydrostatic, as
might be expected. Further, no essentially pulmonary sac would be likely to
survive the retreat into relatively deep water and the assumption of wholly
 ZOOLOGY
aquatic respiration, but it has also to be remembered that the functional
hydrostatic organ is at times most erratic in its distribution even within a genus.
Both the duct and its filament are surrounded by dense layers of homo-
geneous fat, constituting altogether a large fatty cord more than an inch in
diameter. The flesh of Latimeria, too, is extremely oily, no doubt making for
increased buoyancy (p. 349). The heart is exceedingly primitive-even more
so than in the selachians (p. 240). The student is already familiar with the
original simple longitudinal pump plan of the four chambers-bulbus cordis,
ventricle, atrium, and sinus venosus-and how the characteristic $-flexure
has become increasingly elaborate in tetrapod evolution. The crelacanth
heart remains relatively unfolded ; in fact, the atrium has for the most part
remained posterior to the ventricle. 'These fishes have thus retained up to the
adult stage the mechanism most akin to the original vertebrate heart' (Millot).
From the buccal cavity, a remarkably wide, fluted, and muscular resopha-
gus leads imperceptibly into the long, spacious, double-limbed stomach, the
posterior end of which is well behind the anus. Fish-prey can be swallowed
whole. The transition between resophagus and stomach can be determined
only by its microscopic glandular structure ; the long, tubular fundus glands are
very well-developed. The gastric mucosa is dense. There is no cardia. A
prominent annular sphincter allows ingress to a short chamber, probably an
adumbration of the bursa entiana which occurs in many fishes and which opens
into a 'sub-fusiform spiral intestine en rouleaux showing nearly a score of
concentric turns' (Millot). This leads to a rectum and cloacal pouch-the
latter bearing a single urinogenital papilla. The cloaca receives also secretions
from a nearby encapsulated, median, nodular gland, which, though different
from it, is possibly homologous with the digitiform (rectal) gland of selachians
(p. 239)·
The liver is hi-lobed, the left lobe being considerably the larger. A capa-
cious gall-bladder occurs. The pancreas is prominent. The kidneys are fused
into a single median organ which is applied to the ventral wall of the abdominal
cavity. The ureters are symmetrical, and expand dorsally each into a volu-
minous bladder : thus the bladder is paired, and the kidney unpaired. The
urethras are symmetrical as they enter the urinogenital papilla. The testes
are paired, partly segmented, and held posterior to the anus. The large urino-
genital aperture lies immediately behind and between the bases of the paired
pelvic fins.
Despite profound changes in the swim-bladder (a notoriously adaptive
organ), and the fact that only a bare outline of the anatomy is yet available, it is
nevertheless obvious that in Latimeria we have an animal of extraordinary
stability. The fish has persisted relatively unchanged from an era in which
profound changes took place (p. 383), and became stabilised at a particularly
instructive period of vertebrate history.
 PHYLUM CHORDATA

ORDER DJPNOI (DIPNEUSTI, DIPNEUMONES)

The peculiar lung-fishes are a Devonian offshoot from the crossopterygian
stem. They still survive as three degenerate and specialised forms in widely
separated tropical, or sub-tropical, fresh-water habitats of a special kind
(Fig. 249). Lung-fishes appear first in mid-Devonian deposits, and were well

FIG. 249.~Lung-ftshes: Discontinuous

distribution. A. Protopterus, B. Lepido-
siren, C. Neoceralodus. These dipnoan
survivors are to-day confined to near-
tropical (Neoceratod1ts) and tropical (Pro-
topterus, Lepidosiren) fresh waters. N eo·
ceratodus is restricted to two adjacent
rivers: the black area shown is much
too extensive. (After Norman.)

differentiated even then. They flourished moderately in the Permian and

Triassic, and then became rare.
The living dipnoans are Neoceratodus of south-eastern (not tropical) Queens-
land (once called the' Burnett Salmon'), 1 Protopterus of Africa, and Lepidosiren
1 This singular fish was formerly casually eaten by people who were unaware of its interest.
It is now rigidly protected by law. After Krefft in a newspaper of r87o described what settlers
called the 'amphibious fresh-water-salmon', he wrote some doggerel, probably for the amusement
of his small son, from which the following is an extract :
Lucullus ate Mur<ena rare,
In Rome the daintiest dish,
But Squatters on the Burnett dined,
On geologic fish.
(A' squatter' in the original early nineteenth-century Australian sense was usually, but not always,
a reputable immigrant who took up vacant grazing-land, often under fee and licence. The term
there has become respectable.)
VOL. II Z
 ZOOLOGY
of South America. The name of the order, given before the structure of
the extinct types was fully known, refers to the two internal nostrils (choance)
which henceforth becomes a feature of the tetrapods. A study of living lung-
fishes throws considerable light on the probable anatomy of the soft parts of the
extinct forms. Each extant form not only breathes by means of gills but has a
highly-developed apparatus for the respiration of air-a lung (Neoceratodus)
or lungs (Protopterus, Lepidosiren)-with a co-ordinated arrangement of the
circulation. Among other groups a comparable arrangement is found only in
Polypterus and Amia. Neoceratodus is able to live in stagnant water that is
toxic to certain other species, but nevertheless dies if removed from water.
Protopterus and Lepidosiren can survive the drying of the environment by
<estivating in mud with a small aperture to allow respiration. The former
secretes a slimy protective cocoon. In both Protopterzts and Lepidosiren an
epiglottis (p. 668) occurs. The lung-fishes have bony scales which differ
among genera, but their relationship with the ancient cosmine scale (p. 83)
seems well established. Dermal fin-rays occur. The paired fins, however,
unlike those of any other fishes (with the exception of certain extinct Chondrich-
thyes and crossopterygians), are constructed on the biserial plan (' archiptery-
gium', seepp. 99, 224).
With special features of their own, the Dipnoi combine also characteristics in
which they resemble now one, now another, of the other groups of fishes. The
notochord is persistent. There are no vertebral centra, and the primary cra-
nium persists with little ossification, but has added to it a number of investing
bones (p. 302). There are four to six cartilaginous branchial arches. The
dermal fin-rays are slender more or less ossified fibres and are supported by
numerous cartilaginous or ossified pterygiophores. The caudal fin is often
apparently diphycercal. The pectoral arch is a single cartilage with a pair of
superficial investing bones on each side. The pelvic arch is well-developed and
cartilaginous. The gills are covered by an operculum. There is a dermal skele-
ton formed by over-lapping cycloid scales. The heart is peculiar and to some
degree approaches that of the Amphibia. The atrium and the sinus venosus are
each imperfectly divided into two parts. The contractile conus arteriosus,
which has a spirally twisted form, is partly or completely divided by a longitu-
dinal partition. A pulmonary artery is given off from the efferent branchial
system on each side, and a pulmonary vein opens into the left division of the
atrium. In Lepidosiren and Protoptertts the larva possesses both suckers and
external gills. In the latter genus, remains of these gills may persist in the
adults of some species. The stomach has been secondarily lost in so far as
histological structure is concerned.
The central nervous system is interesting in its undivided, or almost
undivided, mid-brain ; and the pallium of the cerebral hemispheres, with its
layers of nerve-cells, has no parallel among the lower vertebrates so far con-
 PHYLUM CHORDATA

sidered. The paired oviducts open anteriorly into the crelom, and male
vasa efferentia carry sperms through the excretory part of the mesonephros
(Fig. 260, p. 376).
The sub-class was never a very large one, but a succession of extinct genera
is known that leads clearly to the modern species. A number of characters is
common to all members of the order. The internal nostrils, cosmine covering of
the bones and scales, and the mesorhachic fin all show that the Dipnoi evolved
originally from an ancestor in common with the other crossopterygians. The
jaw suspension is autostylic, the lower jaw articulating with a palatoquadrate
process that is immovably fixed to each side of the skull. The characteristic
pattern of the teeth is peculiar to the order. During the long history of
the group its component members, while retaining all the essential anatomical
features, show a gradual degeneration of certain parts. This can be seen in the
bones of the skull, in a loss of cosmine, and in an alteration from the normal two
dorsal fins and heterocercal tail to the almost eel-like body of Lepidosiren.

FIG. 250.-0rder Dipnoi (D. valenciennesi). Family Dipteridre. Dipte1·us. From something
like this Middle D evonian form a 'line ' is traceable through Ceralodus (Triassic) to Neoceralodus
(Fig. 249). Protopterus and Lepidosiren represent an ancient side-line. (After Forster -Cooper.)

This fish has fused dorsal and anal fins, a gephyrocercal tail, thread-like paired
limbs, and reduced scales (Fig. 249).
Dipterus valenciennesi (Fig. 250), the most primitive known form, had
a body protected with stout cycloid scales covered, as were the bones of
the head, with cosmine. The archipterygial paired fins were moderately long;
there were two small dorsal fins, a heterocercal tail with a small epichordal lobe
and an anal fin. The skull was completely roofed in by bones so numerous
that their homologies, especially of those in the front region, with the head
bones of other fishes are difficult to determine. There were no premaxilla:
or maxilla:, and the bones of the palate were firmly fixed to the neurocranium
and so produced the autostylic condition. There was a double series of gular
bones between the rami of the lower jaws. The upper teeth were borne on the
prevomers and palato-pterygoids, the former small, the latter a larger series of
tubercles arranged as a pair of fans. These palatal teeth were in later forms
fused into characteristic tooth-plates. A single pair of tooth-plates of corre--
sponding pattern are carried on the lower jaws by the splenials.
Dipterus showed an interesting feature in the life-history of individual
 ZOOLOGY
animals. Originally there were described two species of the genus, D. platy-
cephalus and D. valenciennesi, both found preserved in the same horizons. The
specific differences between the two lay in the fact that D. platycephalus had
a completely bony snout, and the bones of the head and the scales are covered
by a thick, smooth, and finely pitted outer layer of cosmine, which hid the
underlying pattern of the scales, whereas D. valenciennesi had the front part
of the snout without bone and the scales lack the outer layer, so that the
pattern is clearly visible. It now appears that this apparent diversity may
be explained as seasonal differences of a single species (D. valenciennesi).
At recurring intervals during the life of the fish the outer layer of cosmine is
resorbed to allow growth to take place. It is then re-deposited. Specimens
have been found in the intermediate condition.
Representative genera of Dipnoi were Dipterus (Middle Devonian), Pent-
landia, Phaneropleuron, Scaumenacia, and Fleurantia and Rhynchodipterus
(long-snouted fishes) of the Upper Devonian. Others were Uronemus (Lower
Carboniferous), Conchopoma (Lower Permian), Ctenodus, Sagenodus (Carboni-
ferous and Permian). Of the living forms, Protopterus (Fig. 249) is known
from the Miocene (with close allies from the Triassic onwards). Neoceratodus
(Fig. 249) has apparently changed relatively little during the last 150 million
years. Its widespread Mesozoic ancestor, Ceratodus, was described long before
the extant Australian fish, dealt with below, was discovered.
Living Dipnoi belong to two sub-orders arranged as follows :

SUB-ORDER MONOPNEUMONA
Dipnoi in which the lung is single, and the lateral jointed rays of the
'archipterygium' are well developed. There survives only the Australian
Neoceratodus forsteri (Fig. 249). 1
1 A brief note may be made concerning 'Ompax spatuloides' (Jordan, D. S. (1919), Genera of
Fishes, iii, 399), which was described by Count F. de Castelneau from drawings and other informa-
tion in 1879. This interesting and perplexing specimen was described as' a ganoid nearly allied to
Atractosteus', and possibly to Polyodon, constituting a new species, genus, 'and probably a new
family'. Very soon after its description there arose not inconsiderable doubt as to its authen-
ticity, particularly as Count de Castelneau related that its discoverer, a museum director visiting
the Burnett River, had been presented with the fish cooked, whereupon it had been consumed for
breakfast. However, being impressed by its shape, the visitor had first had the animal drawn by
a passing road-inspector. The sketches were despatched to the Count, together with field data
suggesting that the specimen had been caught by aborigines in a water-hole where it lived with
Neoceratodus. Many unsuccessful searches for further specimens were made. Not until half a
century later, however, was the full story of 'Ompax' revealed. A group of lung-fish conscious
stockmen, hearing that a museum man was visiting the area, had determined that he should meet
with a second 'living fossil'. They thereupon made him one: it was composed of several fishes,
including perhaps N eoceratodus. From that time onwards, whenever anything peculiar in the
locality was described, bushmen would say sardonically: 'It must be an Ompax.' (See Whitley.)
'Ompax spatuloides' arose out of the almost universal desire to make a fool of the expert.
More such hoaxC's (as distinct from serious frauds, pp. 454, 746) have found their way into zoological
literature than is generally realised.
 PHYLUM CHORDATA

SUB-ORDER DIPNEUMONA
Dipnoi in which the lung is double, and the lateral rays of the 'archi-
pterygium ' are vestigial or absent. This group includes Protopterus (Fig. 251)
of Africa and Lepidosiren paradoxa of South America (Fig. 249).

EXAMPLE OF THE SUB-CLASS.-PROTOPTERUS IETHIOPICUS

The genus Protopterus is confined to the fresh waters of tropical and sub-
tropical Africa and is composed of the following four species: P. cethiopicus
(Fig. 251), P. annectens, P. amphibius, and P . dolloi. The fossil history of the

FIG. 251.- 0rdcr Dipnoi, Sub-order Dipneumona, Family Lepido.sirenidre. Protopterus

retlliopicus. Aerial respiration in the East African Lung-fish. Hespiration is essentially puJ.
monary from a very early age. Degenerate internal gills are retained (Fig. ·258) but the fish must
surface periodically. It inhales for a few seconds, then tlips its tail above the surface and plunges
downward again.

genus, unlike that of Neoceratodus, is obscure. P. cethiopicus occurs in the

tropical Lakes Victoria, Kyoga, Edward, and Albert and in the River Nile.
Within the area of its present-day distribution, tooth-plates have been found
in Miocene and Pleistocene deposits (p. 3).
P. cethiopicus attains a length of about seven feet and a weight of go lb.,
but a fish longer than five feet is exceptional. It is widely distributed within
 ZOOLOGY
the lakes, occurring most abundantly in shallow water. It thrives in swamps
where other fishes are not frequently found. It is able to rise to the surface and
gulp air through the mouth into its lungs, thus obtaining enough oxygen to
respire and so gain independence from dissolved oxygen in the water (Fig. 251).
The West African P. annectens is able to spend long periods of ::estivation under
the mud in cocoons formed by dry mucus when the temporary swamps and
rivers become periodically dry. The bulk of the population of P. cethiopicus is
not compelled to restivate and cocoons are not commonly found in nature.
The species has been induced to restivate under laboratory conditions (Homer
Smith). As the water recedes, Protoptems burrows head-first into the ooze,
and then reverses its position until the snout is just beneath the surface of
water. As long as water remains, the fish surfaces regularly to breathe.
When the mud-flat becomes exposed the fish coils upright in ooze, up to about
18 inches below the surface, with its tail covering the eyes. The epidermis
secretes mucus, which hardens into a waterproof cocoon as the surrounding mud
solidifies (Fig. 259). The cocoon is perforated by a breathing tube which
connects the fish's mouth but not the nostrils with the exterior (see. p. 375).
If exposed naked during this period, its skin shrivels and the fish dies within
twenty-four hours. Inside its protective, fluid-retaining cocoon P. cethiopiws
has lived for as long as three years in experimental ::estivation.
The food of P. cethiopicus consists mainly of molluscs, although insects and
small fishes are also eaten. Its movements are generally sluggish. The
filamentous and highly mobile paired fins are well-supplied with receptor-
organs which probably assist in the location of food. Protopterus moves in two
different ways. Swimming is of an anguilliform type. In addition, it is able
to' walk' along the bottom and through weeds by using its pectoral and pelvic
fins as ' legs '.
External characters.-The eyes are small. The body is elongate and
cylindrical, and covered with thin, bony imbricating cycloid scales. There is a
continuous dorsal, anal, and caudal fin, supported by fibre-like, partly calcified
dorsal rays (camptotrichia) which are more numerous than the endoskeletal
radials. The limbs (pectoral and pelvic fins) are elongate and filamentous and
are devoid of fin-rays.
The external nares differ from those of other vertebrates in being situated
immediately outside the aperture of the mouth enclosed within the upper lip. A
pair of internal nares open not far behind them into the anterior part of the
mouth cavity, immediately in front of the upper tooth-plate. About two-
thirds of the distance from the snout to the tail extremity there is a cloacal
aperture. Two abdominal pores, opening into the cloaca, may be present, but
usually they are confluent and have a single external opening.
There is an operculum, similar in appearance to that of other bony fishes,
covering the branchial opening and bordering the single, slit-like branchial
 PHYLUM CHORDATA

aperture of each side. No spiracles occur. The lateral-line system is well

developed, especially on the head (Fig. 251).
Endoskeleton.-The spinal column is represented by a persistent noto-
chord enclosed in a thick, fibrous sheath, together with cartilaginous neural and
hcemal arches. The notochord-there are no vertebrre in modern Dipnoi-
extends to near the pituitary region, and there is no distinct joint between the
skull and vertebral column, which pass gradually into one another.

FIG. zsz.-J•mtotJiet·us: Skull,shouldergirdle,andpectoralfin. A,A 1 • splenial; A,F. pre-

orbital process; a and b (on lower jaw) , and SL. t eeth ; b. basal cartilage of pectoral fin ; B .
ligamentous band connecting the mandible with the hyoid; co. ligamentous band connecting the
dorsal end of the pectoral arch with the sk u ll ; D. angular; F P. frontoparietal; HI. membra nous
fenestra perforated by the foramen for the optic nerve (II); H y . hyoid; K . external gills ; Kn.
Kn 1 • cartilage of the pectoral arch ; KR. occipital rib; LK and MK. investing bones of t h e pec-
toral arch; NK. olfactory cap su le; Ob. auditory capsule; Gee. supraoccipital; Op. and Op'.
rudimentary opercular bones; PQ. palatoquadrate; Psp. Psp1 • spinous processes of the anterior
vertebr<e; SE. supra·ethmoid bo ne; SK. roofing investing bones; T r . palatoquadrate cartilage;
WW 1 • anterior vertebra:~ coalescent with the skull; 1- V. branchial arches (that marked I is
forked and the anterior bar may represent the first, in which case there are six b ra n chial arches) ;
I, 2 , 3, segments of axis of pectoral fin; *,*,vestigial lateral rays o f p ectoral fin. t, t, projecting
processes of free portion of Meckel's cartilage. (After \Viedersheim .)

A series of neural or basidorsal cartilages form the bases of the neural arches.
These basidorsals extend upwards and form partly ossified neural arches which
meet above the spinal cord. A longitudinal ligament runs along the top of the
arches, and above it are a series of neural spines. The last-named fuse with the
basidorsals in the posterior part of the column. This region is provided with
hcemal or basiventral cartilages. Each neural and hremal spine forms the
proximal segment of a continuous, three-jointed, ossified rod. The two other
segments of the rod are the radials. No centra are formed. Although the
notochord is impressed by the basidorsal and basiventral cartilages, it is not
constricted as in bony-fishes.
 ZOOLOGY
Pleural ribs are present, and extend downwards and somewhat posteriorly
in the body-wall immediately outside the peritoneal membrane. The arrange-
ment resembles that of the pleural ribs in other bony fishes. The first pair
(the occipital ribs) are thicker and more massive than the others and are
connected with the skull.
In the skull (Fig. 252) the cranial roof and walls, internal to the jaw muscles,
are largely formed from the so-called parietal bones. Lying above and lateral
to the parietals are the
lrontal paired, posteriorly directed
frontals. The cranial floor
is formed by the expanded
parasphenoid. The homo-
logies of these investing
bones are not understood:
the terms employed above
are used for the sake of
convenience only. A sys-
tem of letters and numbers
is usually employed for
their identification (West-
tooth
oll). Two exoccipitals are
present. A palatopterygoid
cartilage, firmly fixed to the
coronoid
cranium, gives attachment
to the mandible, so that the
1urangu/ar
jaw suspension is autostylic
ceratohyoid
(p. 94). Hyomandibulce
are absent. The upper
FIG. 253.-P·rotopfet·us: Dentition. Upper and lower tooth-plate is attached to
t ooth-plates in occlusion and their relationships with adja-
cent bones. The above clear-shown distinction b etween the prevomers and to the
dental and osseus structures can be seen only in dry or
fossil specimens. (Partly from a specimen in University palatopterygoid. Premaxil-
College, London.) lre, maxillre, and nasals are
absent.
The lower jaw consists of paired Meckel's cartilages, with large, tooth-bearing
coronoids ( = ' splenials ') situated on their medial aspects, and supra-angulars
placed laterally. No true dentaries or splenials are developed. The mandible
articulates with a large intertemporal cartilage ( = pterotic, squamosal, of
different authors). Fenestrated, cartilaginous nasal capsules are present, and
small opercular and interopercular bones support the opercular fold.
The hyoid and branchial arches are cartilaginous. Only a ceratohyal is
retained in the hyoid arch. There are five branchial arches and, in front of the
first gill-slit, a rod, probably formed from the fused bases of the hyal rays.
 PHYLUM CHORDATA

The.re are no branchial rays, but all branchial arches bear a series of gill-rakers
with cartilaginous supports.
inferior
The pectoral girdle (Fig. 252) vena cava
is a stout cartilage overlain by
a pair of investing bones. It is
attached to the skull by a
ligamentous band. The pelvic
girdle is a single, lozenge- gall
shaped, cartilaginous plate bladder
produced anteriorly into a long
epipubic process. On each side
of this projection are a pair hepatic ---lfi~
duct
of forwardly directed prepubic
processes. The posterior aspect
of the plate bears short pro- and bile duct _ _--+
cesses for the basal cartilages aperture
of the pelvic fins.
The skeleton of the pectoral
fins consists of an elongate entiana
basal cartilage followed by a
central axis of small, cartila-
ginous rods. On one side (pre-
axial face) of the central axis
there is a row of jointed car-
tilaginous rays which support
the somewhat expanded part
of the fin. The pelvic fin
skeleton is like that of the
pectoral fin, except that
preaxial radials are absent.
Alimentary Canal and Asso-
ciated Structures.-The teeth
(Fig. 253) consist of upper and
lower tooth-plates derived dur-
ing ontogeny from the coales-
cence of a number of small pore
aperture
denticles. These become fixed
to independently developed FIG. 254.- Protopterus: Alimentary canal and assoc-
iated structures. The cystic ducts are not labelled.
bony plates-the prevomer and (:\lodified after W. N. Parker.)
palatopterygoids in the upper
jaw and the coronoids in the lower. The occlusal and labial surfaces of the
tooth-plates are thrown into three major folds or cusps. In addition, there
370 ZOOLOGY
is a lingual ridge lying medially to the outer cusps. The shape of the tooth-
plates and the disposition of their cusps are essentially adaptations to crushing
and shearing molluscs. Paired vomerine teeth also occur. A chief feature of
the alimentary canal (Fig. 254) is the presence, throughout the length of the
intestine, of a spiral valve. It will be recalled that a spiral valve is present in
elasmobranchs (p. 237), palreoniscoid derivatives (p. 326), and in Latimeria
(Ccdacanthini, p. 360). The rectum opens into a small cloaca. The liver is
large and unequally bilobed. The gall-bladder lies between the small anterior
lobe and the much larger posterior lobe. A well-developed pancreas is em-
bedded in the wall of the stomach and intestine, internal to the peritoneum.
The pancreatic ductules unite and open into the bile-duct immediately before
the latter enters the intestine. Oesophageal, gastric, and intestinal epithelia
are little different. All are equipped with ciliated, columnar, and goblet cells.
Respiratory System.-The extent to which the adult Protoptertts combines
aquatic and aerial respiration has not been fully determined. There are six
branchial arches, with five open gill-clefts. Gill filaments are developed only
on the fourth and fifth branchial arches. There is also an anterior hyoidean
hemibranch and another posterior hemibranch associated with the sixth arch.
Field and laboratory studies indicate that the adult P. cethiopicus is dependent
on aerial respiration, since individuals prevented from breathing at the surface
die of asphyxiation. Doubtless the gill-filaments play some part in respiration,
but their main function is probably associated with carbon-dioxide excretion.
Four pairs of functional external gills occur in larval lung-fishes. The external
gills are atrophied or lost in the adult.
The lungs (Figs. 255, 256) are paired, elongate median dorsal sacs which
open into a common pneumatic duct. This duct is provided with a muscular
chamber which opens into the ventral side of the resophagus through a slit-like
glottis. The margins of the glottis are muscular, and there is an epiglottis-like
fibro-cartilaginous plate. Internally the cavity of each lung opens into a series
of alveoli. Each alveolus opens into lesser cavities, which in turn communicate
with still smaller sacculi. Hence, in structure the lungs basically resemble
those of higher vertebrates (p. 107). Phylogenetically, the lungs of Protopterus
almost certainly correspond to the swim-bladder of bony fishes (p. 339). They
differ, however, in their blood supply (see below). Blood reaches the lungs by a
pair of pulmonary arteries which originate from the roots of the paired dorsal
aortre near the point of their junction to form the median dorsal aorta.
Blood-vascular System.-Correlated with the existence of lungs and a
distinct pulmonary circulation (Fig. 256) there is a complication in cardiac
structure. The heart lies in a stiff-walled pericardial cavity (Fig. 257) situated
below and somewhat behind the gills. The sinus venosus is imperfectly divided
into two parts, \Vhilst the cavity of the atrium is divided by an almost complete
septum.
 PHYLUM CHORDATA 371

Venous blood enters the right-hand division of the sinus venosus and passes
through the right-hand chamber of the atrium to the partly divided ventricle.
The pulmonary vein, by which oxygenated blood is returned to the heart from
glottis and
pharynx
glottis

oesophagus

pulmonary
vein

alveolar
sacs

FIG . :'55-- 0steichthys: Lungs of PI'DIOJJle•·w" and

L eft: Lung and its relationships in Proto·
Pol!fiJie t'tfS.
ptems from ventra l aspect. (Parts arc removed to show
internal structure.) Centre (above): Alveoli from dorsal
aspect. Centre (below ): Alveolus. R ight: Primitive
a symmetric a rrangement in Polyptcrus (see p . 290).
Although the lungs are d isproportionate in size, both (in
Erpetoichthys at least) arc fully functional. (Hedrawn
from Cambridge Nat ural History, alter various authors.)

the lungs, passes through the sinus venosus and opens directly into the left
side of the atrium. The large atrio-ventricular opening is fitted by a fibrous
plug developed from the posterior margin of the opening. This characteristic
structure, found only in the Dipnoi, can be raised or lowered to close or open the
372 ZOOLOGY
atrio-ventricular passage. No true atrio-ventricular valves are developed. A
spirally twisted and weakly contractile conus arteriosus is present, but the
ventral aorta is greatly reduced. Internally, the conus is divided into two
longitudinal channels. This division is achieved by a longitudinal compound
valve that abuts on one side of the conus against a longitudinal ridge on the

lung

p.v. cav.

I II Ill v v. ao.
(all. h.)

FIG. 256.-Protopie rus: Blood vascular and respiratory systems. (From left side. ) The
blood from the lungs goes through the pulmonary veins into the left-hand division o f the atrium.
It is then directed by the valves of the heart into the ventra l chamber of the short, horizontally-
divided v entra l aorta, thence probably proceeding direct to the a nterior branchial vessels from
which it is distributed t o the bra in and oth er cephalic structures. (These, t herefore, probably get
the most highly oxygenated blood.) The blood from the posterior vena cava is direct ed by t he
valve-system of the h eart so as to p ass into the upper channel of the ventral aorta , whence it is
probably directed mainly into the m ore posterior branchia l vessels. Here som e slight gas ex-
change may take place in the reduced gills. Some of this blood probably returns t o the lungs and
some finds its way into the dorsal aorta to be distributed posteriorly (see Fox on). I (aff. hy.),
afferent hyoidean followed b y a fferent branchial arteries (II- V) ; cer. a., cerebra l artery; ccel. a.,
creliacomesenteric artery; d. ao., d orsal aorta; d. Cuv., ductus Cuvieri; eff. h., efferent h yoidean
artery; ' e.'rl. car.', 'external carotid' (continuous with branchial arch II) ; h. hem., hyoidean
h emibranch ; II. br. d. ao. , le ft branch of dorsal aorta ; nas. a., nasal artery; pit., position of
pituita ry gla nd ; p. v. cav., posterior vena cava ; pul. a., pulmonary a rtery; put. v., pulmonary
vein; v. ao., ventral aorta ; *r. m arks point a t which t he nasal a rtery goes through the crania l
wa ll; *2. point at w hich the ' external carotid' goes through squamosal area of the cranium to
supply facial musculature and jaws. (Greenwood and Marshall, unpubl.)

opposite side. Oxygenated blood from the pulmonary vein flows almost
directly into that division of the conus which is connected with the first two
arches, and is thus carried into the carotid and cerebral circulation. Venous
blood, entering the right side of the heart, flows into the channel connected with
the more posterior aortic arches, and is thus ultimately carried to the pulmonary
circulation.
The four afferent branchial arteries originate close together, immediately in
 PHYLUM CHORDATA 373

front of the conus, so that a ventral aorta scarcely exists. The afferent vessel
to the hyoidean hemibranch arises from the first arch. A recurrent branch
from the fourth arch supplies
the posterior hemibranch. per.

In the two gill-less arches

the aortic vessel is continuous, ant. card.
and not interrupted to form sb. d.
afferent and efferent vessels.
In the hyoidean hemibranch,
the posterior hemibranch, and
in the two gill-bearing arches
there are distinct afferent and
efferent vessels. The epi-
branchial arteries from the gill-
bearing arches are bifurcated 1+---- l.p. card.
and drain the hemibranchs of hep. port.--f*-.;~~~~ (
each arch. An efferent vessel (
[
from the posterior hemibranch
(
joins that of the fifth (poster-
ior gill-bearing arch). The
four epibranchial arteries unite ov. v.
at about the same point to
form a short common trunk on
each side (the paired dorsal
aortce). In tum, these unite to
form the median dorsal aorta.
ren. p.v.
The pulmonary arteries arise
from the paired aort~ slightly
proximal to the union of the
most posterior epibranchial;
each divides into branches
supplying the dorsal and ven-
tral aspects of the lungs.
ren. port. lf=--caud. v.

FIG. 257 .-J>rotopterus: Visceral relationships. Dia-

There are two precavals grammatic. ant. card. anterior cardinal vein; an. v.
(dttctus Cuvieri) (Fig. 257) as in anastomotic veins; atr. atrium; caud. v. caudal vein;
f. v. femoral vein; g. bl. gall bladder; hep. port. hepatic
dogfishes (p. 240). The right portal vein; h. v. hepatic vein; inf. j. v. inferior jugular
int. intestine; i. v. c. inferior vena cava; kid.
precaval is formed by the vein; kidney; I. p. card. left posterior cardinal vein; ov. v.
junction of the inferior jugular, ovarian veins; p. v. parietal veins; per. pericardium;
ren. p. v. renal portal veins; sb. cl. subclavian vein; st.
anterior cardinal, and sub- stomach. (Modified after W. N. Parker.)
clavian veins. The left pre-
caval receives, in addition, the large posterior cardinal vein. There is no right
posterior cardinal. The greater volume of blood from the posterior part of the
374 ZOOLOGY
body is brought back to the heart by the inferior vena cava (Fig. 256), which
discharges directly into the sinus venosus. Alone among fishes an inferior
vena cava occurs in Dipnoi. It is invariably present in tetrapods (p. 404) .
The radicles of the inferior vena cava and the left posterior cardinal are formed
by the renal veins; in their
course l:o the heart both veins
receive genital and segmental
veins. Several hepatic veins
open into the inferior vena cava
during its passage through the
cerebral
hemisphere
liver. Posteriorly, the caudal
vein divides into left and right
choroid e~;~==~~~~,-- lobus renal portal veins, neither of
>c hippocampus which is, however, continuous
with the inferior vena cava or
transversum
mesencephalon
the left post-cardinal vein.
pineal stalk
(fused optic ---+.;~:~; Each renal portal vein is joined
lobes)
by several segmental veins and
by the femoral veins. There is

saccus
endolymphaticus
an intra-intestinal vein. The
pulmonary veins unite before
discharging directly into the
left auricle. It will be seen
from the above that the plan of
the blood-vascular system is in
some respects intermediate be-
tween that of the Chondrich-
thyes (p. 219) and that of the
Amphibia (p. 381).
spinal nerve 1 --~ A spleen is prominently
(dorsal root)
spinal cord present, producing both red
cells and lymphocytes. The
FIG. 258.- Protopterus: Brain. The significance of blood ' contains all the cellular
the peculiar saccus endolymphaticus, associated with the
inner ear, is unknown. The central nervous system, varieties of higher forms '
like many other features, is reminiscent of the amphibian
condition . (Modified after Burcl<hardt.) (Downey).
Nervous System.-The brain
(Fig. 258) is elongate and narrow, and shows great development of the cerebral
hemispheres, which are dist inct anteriorly but united behind. In their pro-
nounced evagination the cerebral hemispheres recall those of amphibians, as
does also the small size of the cerebellum. The olfactory lobes are sessile. The
optic lobes form a single, o\·al body lying partly in front of the small cerebel-
lum. A pineal body is present, and overlies a peculiar conical projection, the
 PHYLUM CHORDATA 375
so-called pineal pillow, developed from the roof of the third ventricle. A
choroid plexus covers the roof of the fourth ventricle, and an anterior plexus, in
connection with the roof of the thalamencephalon, projects downwards into the
third ventricle.
A sympathetic system, consisting of a very fine nerve-cord on each side of the
dorsal aorta (below the notochord), has been described, associated with the
vagus, in P. annectens by Giacomini.
General Metabolism and Ex-
cretion.-The elongated kidneys are
separate anteriorly, but fused behind.
They extend throughout the greater
length of the visceral cavity. From
each kidney a thick-walled duct
opens separately into the cloaca. A
cloacal ·urinary bladder is present.
Metabolism and excretion in P.
mtltiopictts have been studied ex-
tensively by Homer Smith. During
the first week or so of ::estivation
(Fig. zsg) the metabolic rate drops
by about so per cent. It then de-
creases slowly until, after some three
months, it is only about 10 or IS
per cent of that at the normal active
feeding level. Under normal con-
ditions the fish gulps air every ten or
fifteen minutes; in ::estivation it in-
spires at a minimum rate of once FIG. zsg.- P1'otopteJ•us: Aestivation. While
buried in mud P. annecten s is curled inside a
per hour and may, in fact, not parchment.Jike, waterproo f cocoon of dried
breathe for several hours at a time. torymucus that is perforated only by a short respira .
funnel from mouth to t unnel. The cocoon
The cardiac rate slows to about three prev ents desiccation. Sp ecial excretory mech-
contractions per minute. The lung- anisms prevent autointoxication (see text).
(Modified after W. N . Parker.)
fish may be relatively lean when it
goes into ::estivation, and after the first few days the reserve carbohydrate
is exhausted, and it then metabolises fat and protein in about equal
degree. The cloacal aperture is sealed by the cocoon, and nitrogenous meta-
bolites are stored essentially as comparatively non-toxic urea, so that after
about three years the urea may constitute as much as 3 per cent of the total
body-weight. Inorganic sulphate, too, is stored, but there is no evidence of the
formation of inorganic phosphate or the production of normal by-products such
as creatine, creatinine, ketone bodies, uric acids, and ammonia. Active lung-
fishes excrete between 30 and 70 per cent of their nitrogenous metabolites
 ZOOLOGY
through the gills as ammonia (and the remainder mostly as urea). Ammonia, a
highly toxic substance, is entirely absent in the buried fish, which thus avoids
autointoxication. Upon its return to water the animal quickly discharges the
stored urea through its gills, and after a brief period of osmotic inbalance
(during which it appears somewhat waterlogged) .normal excretory function is
re-established.
Reproduction.-The paired ovaries are separate and in close proximity to
the kidneys. The oviducts are paired tubes that open anteriorly into the

Mul. d.----/

testis
l.t .d.

reet.

FIG. 260.-Protopterus: Urinogenital system. The similarity to the amphibian arrangement

is striking. Left: Female. ab. p. abdominal pore; gen. p. genital papilla and pore; kid. kidney;
mes. d. mesonephric duct; ov. ovary; ov. f oviducal funnel; rect . rectum; ur. p. urinary
papilla. (After Ayers and \V. N. Parker.) Right: cl. cloaca; cl. b. cloacal bladder; kid. kidney;
l. t. d . longitudinal testis duct; Mul. d. vestigial Mullerian duct; v. d. vas deferens. The associa-
tioiJ between gonad and kidney is much closer than shown in the present diagram. {After W. N.
Parker.)

visceral cavity (Fig. z6o). Posteriorly the oviducts coalesce before opening
into the cloaca. A well-defined genital papilla and pore is present in the cloaca.
At the approach of spawning the oviducts of a 'ripe' female may contain as
many as z,ooo eggs. The ovaries in addition hold about 4,000 in an advanced
stage of development (Greenwood).
The testes (Fig. 260) are elongate and, like the ovaries, are closely associated
with the kidneys, lying along their lateral border. Each testis is differentiated
into an anterior spermatogenic portion and a posterior tubular part which acts
 PHYLUM CHORDATA 377
as a vesicula seminalis. There is a great reduction in the number of vasa
efferentia, which are confined to the posterior part of testis and open into the
kidney duct. Vestigial Mullerian ducts are present.
Development.-The following information refers to the West African P.
annectens. In general, however, it covers also the embryology of P. cethiopicus,

micromere animal edge of blastopore

pole

A 8

megamere

mega mere

(ore brain
vegetal pole

hind brarn branchial

eminence

spiracle

sucker blood vessels

FIG. 261.- Protopterus: Development. Note points of similarity (e.g. total cleavage a n d the
formation of the yolk-plug) with the Amphibia. A . one day after fert ilisation; B . two days,
showing beginning of gastrulation ; C. three days, showing late stage in closure of b lastopore;
D . four days; showing neural folds encircling blastopore; E . late on fourth day, showing differen-
tiation of head region, branchial region, and pronephros ; F. fifth day, showing early differentiation
of external gills and sucker; G. larva 10 days after fertilisation and two days after hatching.
(Modified after Budgett.)

except that the larvre of the latter remain in the nest for about fifty days and
breathe atmospheric air at an earlier stage. The slower rate of development in
P. cethiopicus is perhaps related to the lower water temperatures at which t he
embryos develop (Greenwood). The eggs (Fig. 261) are about 3·5-4 mm. in
diameter. Each egg becomes broken up during cleavage into large, pale green,
yolky cells at the vegetal pole (megameres), and smaller, pink cells at the
VOL. II. AA
 ZOOLOGY
animal pole (micromeres) (Fig. z6r A). During gastrulation the yolky cells
become invaginated through a blastopore, the pink cells gradually extending
until they cover the whole egg (Band C). The blastopore is the place where the
hind end of the embryo will be. The neural folds arise at the head end first,
and then gradually develop along the trunk and tail regions until they even-
tually enclose the blastopore (D). The various regions of the brain and
branchial arches are soon distinguisha ble externally (E and F).
The four pairs of external gills are well developed at hatching, and a tail
has begun to form. An auditory capsule is present. The young larv<e are at
first non-pigmen ted, and they remain in the nest for about three weeks (Bud-

·. ·~
Frc. 262.-PTotopt erus: Development. Young P. cethiopicus with reduced external gills
shortly after leaving the nest. They are found among the protective matted roots of papyrus and
other lake-side plants where they feed on oligoch<ctes and small bivalYes. (From photograph and
specimens.)

gett). They attach themselves to the walls of the nest with a ventral sucker
(G). They probably do not feed, but obtain nourishmen t from their yolk
stores. At this stage a network of blood-vessels is visible on the yolk-filled
belly.
When the larv<e leave the nest they have become pigmented, have developed
limbs, and are practically adult in form (Fig. 262). Their yolk supplies are
almost gone and they soon start to feed on animal material. Their external
gills become reduced and about a month after leaving the nest they begin to
breathe air. The lung develops from a downgrowth in the floor of the pharynx.
The rudiment grows backwards and eventually becomes hi-lobed. The lungs
of Protopterus therefore have the same origin as those of terrestrial vertebrates,
although they subsequentl y move around to the dorsal side of the alimentary
 PHYLUM CHORDATA 379
canal and come to lie under the vertebral column. Pinllus' organ develops
from the spiracular rudiment. This structure, of doubtful function, is known
to occur only in Protopterus and Lepidosiren.

GENERAL ORGANISATION OF DIPNOI

The three genera of living Dipnoi are allied in all the essential features of
their structure. Therefore it is only necessary to mention here the principal
points in which N eoceratodus and Lepidosiren differ from Protopterzes. Protop-
terus and Lepidosiren resemble one another more closely than either resembles
N eoceratodus and are placed in the same family, the Lepidosirenidre.
The limbs of Lepidosiren are similar to those of Protopterus, but in Neo-
ceratodus they are broad and leaf-like, with a correspondingly modified skeleton.
The tapering central axis is composed of a stout basal cartilage and a number of
short, cartilaginous segments, with serially arranged rows of jointed, cartilag-
inous radialia projecting on either side. The skull of Neoceratodus differs some-
what from the lepidosirenid type, being an undivided mass, devoid of fontan-
elles. In many respects it resembles the skull of the fossil dipnoan Ceratodtts,
and is more primitive than that of lepidosirenids. Furthermore, true splenials
and vestigial dentaries are present.
There are two lungs in Protopterus_ (Fig. 255) and Lepidosiren, but only one
in Neoceratodus. The monopneumonous condition in Neoceratodus is possibly
due to the suppression of the original left lobe (Goodrich). The internal
structure of the lung, too, is simpler in this species. In Neoceratodtts only four
pairs of gills are developed, but each gill arch carries a double row of filaments.
The hyoidean ' gill ' is a true pseudobranch. A peculiarity in this genus is that
the branchial lamellre of each gill arch extends on to the dorsal and ventral
walls of the gill cleft, so that the hemibranchs of each cleft are continuous,
both dorsally and ventrally. The fifth arch lacks gills.
Differences in the branchial systems of Protopterus, on the one hand, and
Neoceratodus, on the other, are probably correlated with the more aquatic
habitus of the latter. Neoceratodus does not restivate, and is said to inhabit
only permanent waters. It resorts to aerial respiration when the water becomes
foul during the dry season. Although able to live in stagnant water that is
toxic to other fishes, N eoceratodus dies if removed from water. It is not
asphyxiated by prolonged immersion.
External gills are developed in the larvre of Lepidosiren (Fig. 262), but not
in Neoceratodus. Vestiges of the external gills persist in adult Protopterus
annectens and P. amphibitts. During the breeding season vascular filaments
develop on the pelvic fins of male Lepidosiren. The function of these filaments
is unknown, but it is thought that they may serve as accessory respiratory
organs.
 ZOOLOGY
The vascular system of Lepidosiren closely resembles that of Protopterus
(Fig. 256). In Neoceratodus, however, there are several points of difference.
The heart in this genus is less specialised, in that the longitudinal septa of
the conus are incomplete and represented by longitudinally arranged valves.
Consequently there is more mixing of oxygenated and deoxygenated blood
supplied to the anterior afferent vessels. Since there is a hyoidean pseudo-
branch (as opposed to a hemibranch in Protopterus and Lepidosiren), the anterior
afferent vascular system is different. The pseudobranch is supplied by a vessel
from the efferent branchial artery of the first gill-arch. Anterior and posterior
cerebral arteries originate from the pseudobranch.
The venous system in Protopterus and Lepidosiren is very similar, and
represents in general a more advanced evolutionary stage than that of Neo-
ceratodus. An abdominal vein, derived from the union of the pelvic veins, is
present in Neoceratodus, but not in the other genera.
The brain of Neoceratodus differs from that of Protopterus (and Lepidosiren)
in having paired optic lobes and a clearly demarcated bulbus and lobus in the
olfactory tract. In each genus the alimentary canal is similar in most respects
except for differences in dentition.
In the urinogenital system the kidneys are relatively more elongate in
Protopterus and Lepidosiren than in N eoceratodus. In Lepidosiren the posterior
aspects are unfused.
The testes of Protopterus and Lepidosiren are elongate, and divisible into an
anterior, longer spermatogenic part and a posterior vesicula seminalis. In
N eoceratodus and Lepidosiren about six vasa efferentia rise from the posterior
region and enter the mesonephric tubule, the spermatozoa then passing through
the mesonephric ducts. This arrangement shows considerable affinity with the
Amphibia.
In Protopterus it has been demonstrated that the homologue of the adrenal
medulla occurs in the walls of the intercostal branches of the dorsal aorta
(Giacomini) and its chromaffin cells are innervated from the sympathetic chains
(Holmes). It has been further suggested by Holmes that the cortical homo-
logue may be widely distributed as diffuse specialised tissue identifiable in
various organs throughout the body cavity.
 CLASS AMPHIBIA
INTRODUCTION
The Amphibia, the first vertebrates to become adapted to life on land, may
be distinguished from the choanate fishes, their predecessors, chiefly by their
pentadactyle limbs, the absence of fin-rays in the unpaired fins (when these are
present), and by the presence of a middle ear (seep. 144). The class falls into
two main stems, one of which includes the frogs and toads (Anura) and the other
the newts, salamanders (Urodela), and superficially worm-like Apoda (Gymno-
phiona. There were also a number of extinct orders, dating from the late Devon-
ian, which were particularly numerous in Carboniferous and Permian times.
The earliest known anuran ancestor, Protobatrachtts, 1 did not appear until the
early Mesozoic. This animal, u:1questionably frog-like, possessed a tail in the
adult, but the skull was similar to those of modern frogs, except that an opis-
thotic was retained. The ribs were short, and the skin impression shows it had
a frog-like waist. The presacral vertebrre were reduced in number, although
there were more than eight. There were free caudal vertebrre. The animal was
frog-like in having an elongate ilium and a fairly long femur, but the radius and
ulna, and the tibia and fibula, were separate. The earliest true frog remains
occur in the Upper Jurassic. The urodeles, too, have not been found before
that period. No fossil Apoda has been discovered. To-day the most ter-
restrial Amphibia are the Anura. The Urodela have retreated once more to
the water, and the degenerate Apoda into moist holes in the ground.
Typically, amphibians breathe by means of gills in the various larval
stages, and by lungs when adult. The gills are retained by the adults of some
urodeles, and the skin, which is usually naked, often plays an important part in
respiration. The skull is autostylic (p. 94), and the free hyomandibula has
been converted into a columella auris, which lies in the upper part of the spiracle
between the inner ear and the tympanic membrane that is stretched on the outer
part of the skull. In all but a few of the early forms there is an opening in the
side of the ear capsule, the fenestra ovalis, through which the columella auris
conveys sound-vibrations to the inner ear. The above elaboration is another
A number of equally important points should be briefly noted with regard
1 Hecht (1962) believes that the resemblances between Protobatrach~ts and the Anura may be
the result of convergence.
 ZOOLOGY
to skeleton and musculature. The skull has become movably attached to the
vertebral column by means of either one, or two, condyles. An interclavicle,
already present in osteolepiform fish, serves to brace the two halves of the
shoulder-girdle together, and the pelvis became attached to the vertebral
column, typically by a single sacral vertebra. In modern forms only ten intra-
cranial nerves occur; some of the Labyrinthodontia (p. 388) had twelve.
Although many of the old comparisons between the amphibian limb of
to-day and the paired fins of selachians and Polypterus are of only historical
interest, gross anatomical, and embryological, studies have shown that the
complex system of tetrapod limb-muscles are arranged in two series that are
derived from the simpler musculature of the upper and lower aspects of fins.
Again, a comparison of the bony and skeletal structures of crossopterygians and

FIG. 263.-The fish-amphibian transition: Shoulder girdle and fore-limb. Comparison

between a Devonian crossopterygian (Sauripterus) (left) and a primitive tetrapod. c. cleithrum;
cl. clavicle; h. humerus; icl. interclavicle; r. radius; sc. supracleithrum; u. ulna. (From
Romer, partly after Gregory.)

early (fossil) amphibians shows how closely allied the two limbs really are
(Fig. 263). The earliest tetrapods were, in essence, highly specialised walking
fishes with lungs. This enormously interesting and important initial move-
ment from water to land is now understood in broad outline.
There has been accumulated considerable evidence that suggests a mono-
phyletic origin of all tetrapods from 'a single osteolepid type in the latter half
of Devonian time' (Westoll). The gradual colonisation of dry land depended
not merely on the development of efficient lungs and walking legs. Almost
every part of the animal body was involved-the integument, skeleton, nervous
system, special senses, blood vascular system, and so on. Not only did animals
require to breathe atmospheric oxygen and to walk : they had to withstand
desiccation, rid themselves of the lateral line (in the adult), detect air-borne
substances of a much greater dilution, and see and hear predators and prey at
much greater distances-to mention only a few very obvious lines of adaptation.
At the same time no new organs were formed.
Therefore it is possible that the changeover process may, under special
environmental conditions that favoured the selection of special characteristics,
 PHYLUM CHORDATA

have occurred relatively quickly in terms of geological time. It would seem

that all the changes, however profound they appear at first sight if we compare,
say, a crelacanth and a frog, are readily explicable in terms of Darwinian-
Mendelian inheritance. It is to-day a commonplace that genic segregation,
recombination, and (rarely, but commonly enough) mutation constitute a
mechanism which can generate, in conjunction with environmental selection
pressure, changes and characters of a 'high degree of improbability' (Fisher).
The state of the environment, and therefore natural selection, dictates probably
the tempo and certainly the direction of subsequent events.
The Devonian period (p. 3), when fish were the dominant large animals,
was an age of great climatic instability. There is evidence that the fresh-
water streams and lagoons of that time were alternately filled and dried out in
violent extremes of flood and drought. Such conditions of periodic desiccation
theoretically should result in the extinction of numerous species, and at the same
time favour the survival of the few possessed of physiological and structural
adaptations suitable to function in the new conditions.
We have already seen that a bony supporting skeleton, osmotic homeostasis,
internal nares, and lungs (to mention only three fundamental attributes) had
been developed by ancient bony freshwater fishes. In the crossopterygians
the appearance of two pairs of highly mobile, muscular, lobe-like lateral fins,
supported by bones (Fig. 263), gave an indication of later possibilities in the
development of walking legs as we know them in modem tetrapods. This is a
classic example of pre-adaptation-the possession by an organism of characters
that are conducive to its survival under altered conditions.
Buoyant progression in water, however, is a vastly different matter from
terrestrial locomotion with its additional gravitational complications. The
problems created by the gradual removal of the tetrapod body from the water
were met by profound, but not radical, skeletal, muscular, and other changes in
all parts of the body. Gross mechanical and structural adaptations (including
new facets for the attachment of modified muscles) occurred particularly in the
occiput, the central cantilever vertebral column, and the pectoral and pelvic
girdles. The head, no longer largely supported by water, now required a more
powerful musculature and corresponding elaboration of articular surfaces in
the skull and adjacent endoskeleton. The lower jaws, no less than the neuro-
cranium, required an elaborated musculature for their operation and support.
The vertebral column had increasingly to support body weight : it gained in
ossification and strength. It developed elaborate articulatory mechanisms,
and serial apophyses for the anchorage of the modified musculature. The
girdles had now simultaneously to provide support in locomotion and also to
protect vital viscera from the injury threatened by the new upward thrusts
involved. In fishes, each dermal girdle had constituted the posterior wall of
the nearby gill-chamber and, in addition, had provided anterior anchorage
 ZOOLOGY
for the locomotory myotomal muscles. With the advent of heavy upward
pressures consequent upon walking, the dermal skeleton declined, and there
arose anteriorly a powerful scapula (shoulder-blade) which became bound to
the front ribs (of the thorax); and, posteriorly, there arose the triradiate
pelvic apparatus, including the elaboration of the ilium with which we shall
become familiar. In each girdle region arose supplementary modifications-
such as endoskeletal processes for the firmer attachment of muscles, and (later
on) the developed specialised limb-bones, including digits and other refine-
ments.
Equally, the conquest of land depended on at least a primitive means of
aerial respiration. It was formerly generally believed that the swim-bladder
(p. 339) gave rise to the lung-a not unnatural conclusion, because the air-
bladder at first sight appears as typical of bony fishes as is the lung of tetrapods.
We now know that the primitive lung is a characteristic of ancient fishes; and it
is usually held that it came first and that from it evolved the swim-bladder, an
organ of specialised hydrostatic and other function found only in bony fishes.
It seems certain that the swim-bladder reached its present efficiency only when
the ancestors of the actinopterygians took to the sea. In the Devonian tl:te
development of a respiratory sac, however primitive, but at least capable of
absorbing atmospheric oxygen, would be of immense importance to early
fishes (as it is with some extant lung-fishes (p. 375)) that were compelled to
live in water that became periodically low in level and clogged with rotting
vegetation.
This periodic drying up of swamps placed a premium on the possession of
the means to move up or downstream or across country from a drying locality
to one that remained moist. Limbs thus paradoxically may have been a means
of survival because they first allowed their possessors to remain in water, not
to escape from it. Certainly some of the early tetrapods still retained the
lateral line. So much granted, it is easy to imagine that types which possessed
locomotory, respiratory, integumentory, excretory, and sensory specialisations
related to drought survival would prosper and reproduce. Those whose
adaptations were directed towards purely aquatic efficiency would fail.
More or less direct, and relatively unchanged, descendants of certain of the
survivors are to be seen in the Dipnoi (lung-fishes) (p. 361). It should be
emphasised, however, that there is no suggestion that the tetrapods are des-
cended from even the most ancient Dipnoi. Both groups probably owe their
ancestry either to ancient osteolepid, or to even earlier, rhipidistian freshwater
fishes (p. 354) that flourished in the mid-Palreozoic (p. 3).
A comparison of the earliest Amphibia with Palreozoic fishes shows many
resemblances between the embolomerous labyrinthodonts and the osteolepids of
the Devonian. This is particularly marked in the general structure of the skulls
(Fig. 285 B); in the similar 'labyrinthodont' pattern of the teeth (Fig. 280);
 PHYLUM CHORDATA

in the possession of large palatal 'tusks', and, to a less degree, in the pectoral
and pelvic girdles. The comparison of the two types of archaic animals throws
light on the probable mode of origin of certain tetrapod structures.
Along with the development of limbs, and increased efficiency of aerial
respiration, the Amphibia suffered a remarkable loss of true biting, as apart
from holding, teeth (though at least one genus, Amphignathodon, appears to
have re-developed some). The earliest amphibians were probably piscivorous,
but a drastic alteration in diet was compelled by their change from aquatic
to terrestrial life. They were forced, by virtue of their imperfect adaptation,
to confine their feeding to slow-moving prey such as snails, slugs, and later
(some of them) to insects and other creatures that could be reached, and
anchored, with a sudden flick of the muscular, protrusible, and sticky tongue
which they later developed. The lateral line system, too, was soon lost in the
early land-living adult amphibians, but was retained in the Jarvre. Along
with the loss of the lateral line in adults there have been remarkable modifica-
tions of other sensory organs, as will be seen later.
The amphibian skin became heavily keratinised. This assisted water
retention, but at the same time allowed cutaneous respiration (p. 403). Yet,
although certain toads inhabit relatively dry areas, amphibians in general
cannot live away from moist situations. It is true that a very few anurans can
survive in deserts but these (e.g. Cyclorana of Central Australia) do so by means
of peculiar adaptations such as restivation under dry caked mud, assisted by
water storage allowed by the partial loss of kidney glomeruli. The water
is held in the urinary bladder, peritoneal cavity, and subcutaneous tissues.
If a lean Cyclorana is put into a beaker with two inches of water it resembles 'a
somewhat knobbly tennis ball' within two minutes (Launcelot Harrison, cited
by Buxton). Certain desert tribes of Australian aborigines know of this water
storage and dig the toads out in times of extremity.
The permeability of the amphibian integument, then, has imposed serious
limitations on the choice of habitat, as well as upon geographical distribution.
A frog placed in sea-water dies very quickly-a moment's reflection will show the
osmotic consequences of such a transfer. 1 This limitation reduces the possi-
bilities of (but has not wholly prevented) the successful chance-transfer from
island to island which has been an important factor in the geographical
distribution of the higher groups (e.g. the migration on driftwood of rodents to
Australia and to other islands). Again, the inevitable failure of the Amphibia
to develop temperature control further limits their chances of land colonisation.

1 Nevertheless, at least two euryhaline frogs have evolved. Both the mangrove-dwelling.
crab-eating Rana cancrivora of Thailand, and the green toad (Bufo viridis) of Europe and the
Middle East, can live in a highly salty environment. Adult R. cancrivora can tolerate salinity as
high as 28%, and tadpoles 39%, at 30° C. For remarks on the striking physiological convergence
between R. cancrivora and the ureotelic elasmobranchs see Gordon, et al. (1961); also Gordon
(1962), in connection with B. viridis.
 ZOOLOGY
Fishes, although poikilothermous ('cold-blooded'), 1 can live in waters below
the pack-ice of the coldest seas. Air is a much less efficient heat-conductor than
water, and terrestrial animals are therefore subjected to violent fluctuations of
temperature in deserts and, particularly, at high altitudes. Thus, although
amphibians could exist comfortably in summer time in the high Arctic (as do in-
sects), they could not survive winter there: they would be frozen hard for months.
Of the three surviving amphibian groups, the Anura (frogs and toads) are
abundant in all the greater zoogeographical regions, but are represented by only
one native genus in New Zealand and are absent from most oceanic islands.
The Urodela (newts and salamanders) are almost exclusively Pal~arctic and
Nearctic forms, occurring in North America, Europe, Asia, and North Africa.
A few species extend southwards into the Neotropical and Oriental regions.
The Apoda (Gymnophiona, C~cilia), on the other hand, are mainly tropical,
occurring in the Neotropical, Ethiopian, and Oriental regions. They are
absent from Madagascar, Australasia, and the Pacific Islands.
Dependence on relatively warm conditions affected the distribution of
amphibians in past times as well as to-day. There is much fossil evidence that
frogs, toads, and newts inhabited Britain before the last ice age. As the ice
sheets swept over Europe the Amphibia perished, but during interglacial
periods they gradually re-established themselves. Only a few species reached
Britain before it was separated from the rest of the Continent. Fewer still
reached Ireland, which was separated even earlier (see alsop. 460).
Although amphibians were the dominant land-fauna in the Carboniferous,
little more than 2,000 species live to-day, although some remain plentiful in
appropriate areas. From the time of their emergence they have remained
imperfectly adapted to terrestrial life : most land-going species remained de-
pendent on fresh water for reproduction. The primitive tunnelling Apoda
generally lay their eggs in damp burrows. Among the Anura, even the dry-
skinned toads need water or damp earth to breed. Tree-frogs lay their eggs in
water-filled notches and hollows. Others carry their eggs in pits and pouches
in the skin, and another hatches its eggs in the damp vocal sacs of the male (p.
454). The male Obstetric Toad (Alytes) envelops his body and hind legs with
strings of eggs, and thus keeps them moist. The Egg-pill Toadlet (Metacrinia
nichollsi) of Western Australia lives under stones and logs, and in the galleries
of the nests of a savage bull-ant (Myrmecia regularis) as a tolerated guest.
Brooks found large eggs so coated in dust that they resemble pills covered with
powdered cinnamon. This might retard evaporation. The tadpole stage passes
before they hatch, and if put in water the young toadlets sink helplessly to the
bottom. In another anuran genus, the African bufonid N ectophrynoides (p.
430), well-yolked eggs are developed but the young are retained within the
1 The term 'cold-blooded' is not altogether desirable. The blood of a reptile in a desert in
mid-summer is probably much warmer than that of warm-blooded mammals nearby.
 PHYLUM CHORDATA

oviduct, from which they emerge as perfect froglets. The elongated tadpole
tails are highly vascular and effect respiratory transference from the maternal
circulation. Up to 200 young may be borne by each female.
These and other bizarre adaptations (p. 453) are an expression of the
amphibian inability successfully to colonise a wider, dryer world. The vast
majority of Amphibia require a damp environment in which to breed because
their eggs and embryos must extract oxygen and food from the surrounding
water and at the same time excrete waste material directly into it. They have
developed no protective shell, as have reptiles (p. 457), birds (p. 645), and primi-
tive mammals (p. 693), and they lay down little yolk for the nourishment of the
growing young. We will see later (p. 457) how the development of the amnion,
allantois, yolk sac, and a horny or calcareous shell has enabled higher tetrapods
to develop in a fluid environment untroubled by most of the hazards of dry land.
The need of a damp breeding environment, the permeability of the skin,
and the imperfections of the blood vascular system rigidly limited the chances of
survival away from water, and the development of amphibians as large terres-
trial animals capable of effective competition with reptiles and birds. In the
Carboniferous the Amphibia were able to flourish : to-day they are phylogenetic-
ally senile. It remained for the reptiles to escape completely from the water
(though some have returned to it) and to give rise to the higher tetrapods which
later came to dominate earth and air.
It is plain that the Permian amphibians had already become far too
specialised to have been ancestral to the reptiles (traces of which are found in
the Carboniferous), but the earliest reptiles are so similar to the Carboniferous
labyrinthodonts that it is often hard to distinguish between them. A principal
difference lies in the composition of the vertebrre, for, while amphibians tended
to reduce the posterior pleurocentra, the reptiles reduced the anterior intercentra
and evolved gastrocentrous vertebrre.
Although the Amphibia are, as mentioned above, a relatively unsuccessful
class to-day, many individual species in special habitats achieve a considerable
biomass, particularly in damp tropical regions. Even in temperate countries
frogs can be extremely numerous. In one night alone 500 lb. of frogs' legs
(Rana pipiens) were gathered in one half-mile stretch in the eastern United
States (Noble). A few amphibians have become adapted to relatively severe
climatic conditions-a species of Bufo, for example, lives on the Himalayas at
14,000 feet. As regards longevity, a salamander (Megalobatrachus) survived for
fifty-two years in the Amsterdam Aquarium. The numerous stories of anurans
living for centuries entombed in rocks are of course untrue. Such tales have
currency in every country because eggs are occasionally washed into cavities and
develop there ; and the adult sometimes survives for a time by means of food
particles washed in by the same minute aperture that admitted the original egg.
A frog is generally used as an elementary type in the study of vertebrate
 ZOOLOGY
morphology largely because of an almost universal availability and because
many points of internal anatomy can be demonstrated merely by opening the
body cavity. It need hardly be said that frogs (or Necturus, the urodele 'mud-
puppy' used in many North American universities) cannot be considered truly
typical of the Amphibia, as a glance at the condensed classification below
will show.
This classification, while being 'vertical' and provisional, is at least 'an
attempt to establish two major lines of descent each of which includes both older
and younger elements' (Romer). These are, firstly, the Apsidospondyli, with
several long-extinct labyrinthodont (p. 422) orders, including probably the
Seymouriamorpha and the frogs; and secondly, the Lepospondyli (p. 430),
including extinct orders as well as salamanders and the Apoda.
Some may regret the abandonment of the older classification involving
the recognition of the Stegocephalia but this, as emphasised both by Watson
and Romer, was an unnatural grouping. It mainly distinguished between
geologically older and younger amphibians and had relatively little regard to
their true relationships. Little virtue exists in perpetuating a useless and to
some extent misleading concept merely on grounds of antiquity. It is now
abundantly evident, for example, that modern amphibians are not a simple
monophyletic assemblage but at least diphyletic. The many resemblances
between the highly specialised frogs and salamanders of to-day are probably
due to convergence as much as to their common origin perhaps nearly 300
million years ago.

CLASS AMPHIBIA
Sub-class Apsidospondyli
Super-order Labyrinthodontia
Orders lchthyostegalia (Upper Devonian-Upper Carboniferous)
Rhachitomi (Lower Carboniferous-Permian)
Stereospondyli (Triassic)
Embolomeri (Upper Carboniferous-Permian)
Seymouriamorpha (Upper Carboniferous-Upper Permian)
Super-order Salientia
Orders Eoanura (Upper Carboniferous)
Proanura (Lower Triassic)
Anura (Salientia) (Upper Jurassic-Recent)
Sub-class Lepospondyli
Orders Aistopoda (Carboniferous)
Nectridia (Upper Carboniferous-Permian)
Microsauria (Adelospondyli) (Lower Carboniferous-Permian)
Urodela (Caudata) (Cretaceous-Recent)
Apoda (Gymnophiona, Crecilia) (? Recent)
 PHYLUM CHORDATA

EXAMPLE OF THE CLASS.-A FROG (RANA)

Rana temporaria is the most common British frog, and is found in ponds and
damp situations all over the country. R. esculenta is the large green edible
frog found on the continent of Europe, where its hind-legs, garnished with
spices, are essentially a delicacy. It has been introduced into England. R.
pipiens and R. catesbiana are common North American members of the genus.
Other species of the same genus occur in all parts of the world except New
Zealand, the southern part of South America, and various oceanic islands.
The Ranidre, along with other anurans, are peculiarly specialised both in
skeletal structure and muscular arrangement for the thrusting movements of
the elongated hind-legs by which they jump and swim. In fishes, the vertebral
column is essentially a pliable rod which is resistant to the tendency towards
body-shortening caused by the serially contracting swimming muscles (p. 86).
The new modes of locomotion, sometimes in a new environment-land-have
given the vertebral rod a new task : it must now act as a central girder or
ridge-pole for the support of the weight-bearing limbs. In conjunction with
these are developed other structures in the pectoral and pelvic girdles to ensure
the smooth articulation and functioning of the limbs. We shall see, equally
allied to changed mode of locomotion, considerable departures in the arrange-
ment of dorsal, lateral, and ventral muscles as well.
External Characters.-There is no exoskeleton. The trunk is short and
stout, and is continued, without the intermediation of a neck, into the broad,
depressed head. There is no trace of a tail, the cloacal aperture being terminal.
The mouth also is terminal, and is characterised by its extraordinary width, the
gape extending considerably behind the eye. On the dorsal surface of the snout
are the small nostrils. The eyes are large and prominent, and each is provided
with an upper eyelid in the form of a thick fold of skin and a nictitating mem-
brane. This is a much thinner fold, which arises from the lower margin of the
eye and can be drawn up over it. Close behind the eye is a circular area of
tensely-stretched skin, the tympanic membrane, a structure not met with in any
fish, and absent in the two other orders of living Amphibia. As we shall see,
this is an accessory part of the auditory organ. There is no trace of branchial
apertures.
The back has a peculiar bend or hump in the sitting posture which marks the
position of the sacral vertebra. The limbs are of very unequal size. The fore-
limbs are short. Each consists of an upper arm, or brachium, which, in the ordi-
nary position, is directed backwards and downwards from the shoulder-joint;
a fore-arm, or antebrachium, directed downwards and forwards from the elbow ;
and a hand, or manus, ending in four short, tapering digits, directed forwards.
The hind-limb is of great size. In the usual squatting posture the thigh or
femur is directed downwards, outwards, and forwards from the thigh-joint, the
390 ZOOLOGY
shank or crus, inwards, backwards, and upwards from the knee. The foot (pes)
consists of two parts : a tarsal region directed downwards from the heel-joint,
and five long, slender digits united by thin webs. Thus the limbs are placed
in such a way that the elbow and knee face one another. The first digit of the
frog's hand represents the index finger of Man. That of the foot represents
the hallux or great toe, and is turned inwards or towards the median plane of
the body.
The skin is variable in colour (see below). Generally it .is greyish-brown in
R. temporaria, greenish in R. esculenta, and is mottled, in both species, with
dark brown or black. In R. temporaria there is a large black patch over the
tympanic region. Sexual differences occur in both species. In R. temporaria
there is a large, black, glandular swelling on the inner side of the hand of the
male. This nuptial pad, which is less prominently developed in R. esculenta,
facilitates the male's grip during amplexus (p. 4r8). In R. esculenta and R.
ridibunda the male has, at each angle of the mouth, a loose fold of skin, the
vocal sac, which can be inflated from within the mouth into a globular form
(Fig. 298, p. 443).
The skin is attached to the body-wall only at certain definitive points on
ridges of connective tissue which are also the boundaries of subcutaneous lymph
spaces (p. IIS), of a semi-permeable character. The skin is of great importance
as a respiratory organ on land: there is evidence that lung respiration alone is
insufficient to support life ashore and that the skin may excrete a greater
amount of carbon dioxide than the lungs. Further, during <estivation and
hibernation the animal respires entirely by means of the skin.
The external respiratory surface is kept moist by the colourless secretions
from mucous and serous glands. The mucus makes the frog slimy and difficult
for its enemies to hold and is also toxic in some species (p. 437). The
characteristic odours of frogs have their origin in glandular secretions.
Although the basic colour-pattern does not alter, the ground-colour of the
skin of frogs can change slowly but completely as a result of environmental
stimuli. Placed in a dark, moist environment a frog darkens within a few
hours and becomes black within a few days. Removed to a dry, very light
environment it pales within a few hours, and within a couple of days turns
light yellow. Unusually low temperatures cause darkening. The neuro-
endocrine basis of colour change, involving the melanophores of the skin, is
discussed later (p. 450). In addition, the melanophores can be directly influ-
enced through the epidermis. As well as the deeply situated black melano-
phores, the skin also contains layers of guanophores, enclosing almost colourless
crystals of guanine, and yellow lipophores which lie close below the epidermis.
The various colours of the skin of frogs are produced both by pigments and purely
physical phenomena. The melanophores are responsible for the appearance
of black or dusky hues; the lipophores produce yellow or gold. When a frog's
 PHYLUM CHORDATA 39I
skin appears green it is because all other visible components of light are elimi-
nated in one way or another. The melanophores absorb light of certain wave-
lengths; and the guanine crystals produce by diffraction a blue-green colour
from which the blue or near blue are filtered by lipophores. Rana temporaria
sometimes exhibits a certain amount of red which is thought to come from red
pigment granules in the lipophores. Blue, which sometimes appears in R.
escttlenta (and in several tree-frogs), results from the absence of the yellow of
lipophores which, in most species, screens it out.
Endoskeleton.-The vertebral column (Fig. 264) is remarkable for its extreme
shortness, and, compared with fishes, its inflexibility. It consists of only nine
vertebrre (V. I-V. 9), the last followed by a slender, bony rod, the urostyle
(UST.). The second to the seventh vertebrre have similar characters. The
centrum (B. en.) is somewhat depressed and has a concave anterior and a convex
posterior face-a form known as proca:lous. Each half of the neural arch con-
sists of two parts: (r) a pillar-like pedicle (pd.) springing from the centrum and
extending vertically upwards, and (2) a flat, nearly horizontal lamina (lm.),
forming, with its fellow, the roof of the neural canal. When the vertebrre are
in position, wide gaps are left between successive pedicles. These are the
intervertebral foramina, and serve for the passage of the spinal nerves. The
zygapophyses (a. zyg.) or yoking processes are far better developed than in any
fish. They spring from the junction of pedicle and lamina, the anterior zyga-
pophysis having a distinct articular facet on its dorsal, the posterior on its
ventral surface. Thus when the vertebr::e are in position the posterior zyga-
pophyses of each overlap the anterior zygapophyses of its immediate successor.
Laterally the neural arch gives off on each side a large outstanding transverse
process (tr. pr.). Its crown is produced into a very small and inconspicuous
neural spine (n. sp.).
The first or cervical vertebra (V. I) has a very small centrum and no transverse
processes. There are no anterior zygapophyses, but at the junction of centrum
and arch there occurs on each side a large oval concave facet for articulation
with one of the condyles of the skull (see below). The eighth verfebra has a
biconcave centrum. That of the ninth or sacral vertebra (V. 9) is convex in
front and presents posteriorly a double convexity articulating with a double
concavity on the anterior end of the urostyle. The latter (UST.) is entirely
hypochordal in nature and has nothing to do with the vertebral column or the
perichordal tube.
The skull (Figs. 264 and 265) of modern Anura shows striking departures
from the massively ossified early amphibian plan (Fig. 265), which has much in
common with that of the rhipidistian fishes (see p. 354). In the Ranidre it
consists of a narrow brain-case, produced behind into great outstanding auditory
capsules, and in front into large olfactory capsules. The whole of the bones of
the upper jaw are immovably fixed to the cranium, so that the only free parts
392 ZOOLOGY

UST- -M---+1

FrG. 264.-Class Amphibia, Order Anura, Family Ranidre. Ilana: Endoskeleton. A. from
the dorsal aspect. The left half of the shou lder.g irdle and the le ft fore- and hind.limbs are
r emoved, as also are the investing bones on the left side of the skull. Cartilaginous parts dotted.
Names of replacing bones in thick, those of investing bones in italic capitals, other references in
small italics. a. c. hy . anterior cornu of hyoid ; actb. acetabulum; AST. astragalus; b. hy . basi-
hyal; C. calcar; CAL. calcaneum; EX.OC. exoccipital; FE. femur ; jon. jon'. fontanelles;
FR.PA. fronto-parietal; HU. humerus; IL. ilium; MX. maxilla ; olf. cp. olfactory capsule;
ot. pr. otic process; p. c. hy. posterior cornu of hyoid; PMX. premaxilla; PROT. pro-otic;
QU. ]U. quadrato-jugal; RA.UL. radio-ulna; SP.ETH. sphenethmoid; SQ. squamosal;
S.SCP. supra-scapula; sus. suspensorium; TI.FI. tibio-fibula; tr. pr. transverse process; UST.
urostyle; V.r, cervical vertebra; V.g, sacral vertebra; VO. vomer; 1!-V, digits of hand; I,
the prepollex, a sesamoid bone; 1- V, digits of hind foot. B, the fourth vertebra, anterior face.
a. zyg. a nterior zygapophysis; en. centrum; lm. lamina; n. sp. neural spine; pd. pedicle; tr. pr.
transverse process. (After Howes, slightly a ltered.)
 PHYLUM CHORDATA 393
are the lower jaw and a small plate of mingled bone and cartilage, the hyoid
apparatus. This lies in the floor of the mouth and is the sole representative in
the skull of the entire hyobranchial or gill-bearing skeleton of fishes.
As in teleosts, a number of investing bones can be removed from the skull
without injury to the underlying chondrocranium. The latter, however, is
not, as in the Trout (p. 298), the primary cranium alone, but, as in the Dipnoi
(p. 361), the primary cranium plus the palatoquadrate or primary upper jaw.
The cranium in the strict sense includes the brain-case and the auditory and
olfactory capsules. The palatoquadrate (pal. qtt.) is not a solid mass fused

FrG. 265.-Rana: Skull. A . from beneath, with the investing bones removed on the right
side (left of figure); B. from the left side, with mandib le and hyoid; C. from behind, the investing
bones removed at sus. a. c. hy. anterior cornu of hyoid; aud. cp. auditory capsule; b. hy. body of
hyoid ; COL. columella; DNT. dentary ; EX.OC. exoccipital; for . mag. foramen magnum;
f. ov. fenestra ovalis; FR.PA . fronto-parietal ; M.MCK. mento-meckelian ; MX. maxilla; NA .
n asal; Nv. 2, optic foramen; N v. 5, 7. foramen for fifth and seventh nerves ; Nv . 9 , 10, foramina
for ninth and tenth nerves; oc. en. occipital condyle; olf. cp. olfactory capsule ; ot. pr. otic process;
PAL. palatine; pal. qtt. palata-quadrate; PA.SPH. parasphenoid; p. c. hy. posterior cornu of
hyoid; ped. pedicle; PMX. premaxilla; PROT. pro-otic; PTG. pterygoid; QU.]U. quadrate-
jugal; SP.ETH. sphenethmoid; SQ. squamosal; stp. stapes; sus. (quad.) suspensorium
(quadrate); VO. vomer. A minute investing bone, the septomaxillary, which is present above
the maxilla, close t o the nostril , is not h ere shown. (Sec p. 424.) (After Howes, slightly altered.)

throughout its length with the cranium, as in the Dipnoi. Instead, it is a

slender rod attached to the cranium at either end, but free in the middle. It
is divisible into three regions : a posterior quadrate region or suspensorium
(sus.), an intermediate pterygoid region, and an anterior palatine region. The
suspensorium extends backwards, outwards, and downwards from the auditory
region of the cranium. It is immovably united to the cranium by its forked
proximal end. One branch of the fork-the otic process (Fig. 266, ot. pr.)-is
fused with the auditory capsule. The other-the p edicle (ped.)-is fused with
the trabecular region immediately anterior to the auditory capsule. Ventrally
the suspensorium furnishes an articular facet for the mandible, and is connected
with the delicate rod-like pterygoid region. This passes forwards and joins the
VOL. II. BB
394 ZOOLOGY
palatine region, which is fused with a transverse bar, the antorbital process, the
inner end of which is fused with the olfactory capsule.
The occipital region of the cranium contains only two bones. These are the
exoccipitals (EX. OC.), which lie one on each side of the foramen magnum (for.
mag.) and meet above and below it. There is no trace of supra- or basi-occipital.
Below the foramen magnum are a pair of oval projections, the occipital condyles
(oc. en.), furnished by the exoccipitals and articulating with the cervical
vertebra.
Each auditory capsule is ossified by a single bone, the pro-otic (PR. OT.).
There are no other ossifications of the auditory region (p. 395). In the adult
the pro-otic fuses with the exoccipital. It
presents on its outer surface, behind the
otic process of the suspensorium, a small
aperture, the fenestra ovalis. This is
closed by a membrane and cartilage. Be-
yond is the cavity of the auditory capsule,
containing the membranous labyrinth.
In front of the auditory capsules a
FIG. 266.- llana: Cartilaginous skull of considerable part of the cranial wall is
tadpole. In mid-metamorphosis. au. cp. formed of cartilage, and presents above
auditory capsule; br. 1-4. branchial arches;
c. hy. ceratohyal; col. columella; mck. Mec- a single large and a pair of small
kel's cartilage; olf. cp. olfactory capsule; opt.
for, optic foramen; or. pr. orbital or muscular fontanelles (Fig. 264, Jon., Jon.'). Ante-
process of suspensorium; ot. pr. otic process; riorly it is ossified by the sphenethmoid,
pal. ptg. palata-pterygoid bar ; qu. quadrate;
sip. operculum'. (After A. 1\I. l\farshall, or girdle-bone (SP. ETH.), a short
slightly altered.)'
bony tube divided by a transverse
partition into an anterior compartment which lodges the hinder ends of the
olfactory sacs, and a posterior compartment which contains the olfactory
bulbs.
The anterior compartment is again divided by a vertical partition which
separates the olfactory sacs from one another, and the transverse partition is
perforated for the transmission of the olfactory nerves.
The olfactory capsules (Figs. 265, 266, olf. cp.) have a delicate cartilaginous
roof and floor produced into irregular processes which help to support the
olfactory sac. They are separated from one another by a vertical plate of
cartilage, continuous behind with the girdle-bone and representing the unossi-
fied part of the internal septum. The anterior wall of each is produced into a
small, curved, rod-like rhinal process. The whole of the primary palatoquadrate
arch is unossified in Rana temporaria; in certain other anurans, however, the
quadrate appears as a distinct ossification.
To this partly ossified chondrocranium the usual investing bones are
applied above and below. Covering the roof of the brain-case is a single pair
of bones, thefronto-parietals (FR. PA.). In the majority of frogs no distinct
 PHYLUM CHORDATA 395

trontal or parietal centres of development are visible at any stage. In the few
where separate ossifications do occur, their independent existence is extremely
transient and, at the most, lasts only about a day. Appearance and fusion
have been demonstrated before the limb-paddle stage. (A supra-temporal
bone exists in the skull of bufonid forms.) Over the olfactory capsules
are paired triangular nasals (NA.), and applied to their ventral surfaces
are small paired vomers (VO.). On the ventral surface of the skull is a
large T-shaped parasphenoid (PA. SPH.). The stem of this structure under-
lies the basis cranii. Its two arms extend outwards beneath the auditory
capsules.
In bony fishes, it will be remembered, the palatine and pterygoid are
replacing bones, formed as ossifications of the palatoquadrate cartilage. In
the frog this cartilage is, as we have seen, unossified, but to its ventral face two
investing bones are applied. These are a small rod-like palatine (PAL.), and
a three-rayed pterygoid (PTG.). The pterygoid has an anterior arm extending
forwards to the palatine, an inner arm applied to the pedicle of the suspen-
sorium, and an outer arm extending along the whole inner face of the suspen-
sorium. It will thus be seen that bones originally preformed in cartilage may
give place to investing bones, developed in corresponding situations, but alto-
gether independent of the cartilage The latter remains unossified.
The suspensorium, as we have seen, is strengthened on its inner face by the
outer arm of the pterygoid. Externally it is similarly supported by a hammer-
shaped investing bone, the squamosal (SQ.). The upper jaw is formed by three
investing bones: the small premaxilla (PMX.) in front, then the long, narrow
maxilla (MX.), and finally the short quadrato-jugal (QU. ]U.), which is con-
nected posteriorly with the quadrate cartilage and squamosal (see alsop. 467).
The mandible contains a persistent Meckel's cartilage, as a sort of core,
outside which are formed two bones: a long angulo-splenial on its inner face,
and a short dentary (DNT.) on the outer face of its distal half. The actual distal
end of Meckel's cartilage is ossified as a small replacing bone, the mento-
meckelian (M. MCK.), present in osteolepid fish (Jarvik).
The hyoid apparatus consists of a shield-shaped plate of cartilage, the
body of the hyoid (b. hy.), This is produced at its anterior angles into slender
rods, the anterior cornua (a. c. hy.), which curve upwards and are fused with the
auditory capsules, and at its posterior angles into partly ossified rods, the
posterior cornua (p. c. hy.), which extend backwards, embracing the glottis.
Two other cranial structures are noteworthy. External to the squamosal
is a ring of cartilage, the annulus tympani (Fig. 93, p. 144), unique to Amphibia,
which supports the tympanic membrane as the frame of a tambourine supports
the parchment. Inserted into the fenestra ovalis is a nodule of cartilage, the
columella, the middle of whose length is hammer-shaped and ossified, while
its cartilaginous head, or extra-columella, is fixed to the inner surface of the
 ZOOLOGY
tympanic n1embrane (see footnote, p. 414). Behind the columella, three
quarters of the area of the fenestra ovalis is filled with membrane in which
lies a large cartilaginous ' operculum' plate.
The comparison of the frog's skull with those of fishes is facilitated by a
study of its development. In the tadpole, or larval frog, there is a cartilaginous
cranium (Fig. 266) connected on each side with a stout inverted arch, like the
subocular arch of the Lamprey or the palatoquadrate of Chimcera or Neocera-
todus, and, like them, developed from the dorsal region of the mandibular arch.
The quadrate region (qu). of this primary upper jaw is well in front of the eye,
the axis of the suspensorium being inclined forwards and the mandible very
short, in correspondence with the small size of the tadpole's mouth. The
quadrate is fused by an ascending process with the trabecular region. An otic
process (ot. pr.), unites it with the auditory capsule. Behind the suspensorium
are distinct hyoid (c. hy.) and branchial (br. 1-4) arches supporting the gills by
which the tadpole breathes. As development proceeds, the axis of the suspen-
sorium is rotated backwards, producing the wide gape of the adult, and the stout
palatopterygoid region of the subocular arch (pal. ptg.) gradually assumes the
slender proportion it has in the adult. The greater part of the hyoid arch gives
rise to the anterior cornua of the adult hyoid apparatus. The body of this is
formed from the basihyal and basibranchials, and its posterior cornua is pro-
bably derived from the fourth branchial arch. The columella is developed in-
dependently, but may perhaps represent an epihyal or dorsal segment of the
hyoid arch. Thus, with the assumption of aerial respiration, the complex
branchial skeleton is reduced to a simple structure for the support of the
tongue, larynx, and the muscles activating the floor of the mouth and throat.
The pectoral girdle has essentially the structure already described (p. ror) in
general terms as characteristic of the pentadactyle Craniata, but it reflects the
fact that the fore-limbs do relatively light work such as bracing the animal after
each jump and occasionally transferring food to the buccal cavity (p. 400).
To its various components are attached voluntary muscles which operate the
forelegs and (a feature carried down from piscine ancestryy others which depress
and elevate the lower jaw and pharyngeal floor. The depression of the pharyngeal
floor causes an inrush of air through the nostril into the buccal cavity, after
which the nostrils are closed and an elevation of the buccal floor forces the air
through the glottis and into the paired bronchi to the lungs (see also Respiration,
p. 403). The scapula (Fig. 267) is ossified, and is connected by its dorsal edge
with a suprascapula (Fig. 264, S. SCP.) formed partly of bone, partly of calcified
cartilage, and developed from the dorsal region of the embryonic shoulder-
girdle. The coracoid is also ossified, but the procoracoid is represented by a
bar of cartilage and has an investing bone, the clavicle, closely applied to
it. These bones serve as struts to take the strains imposed on each side by
·the mobile humerus, the rounded head of which articulates with the glenoid
 PHYLUM CHORDATA 397

cavity (Fig. 267). The suprascapula overlaps the anterior vertebrre. The
coracoid and procoracoid are connected ventrally by a cartilage, the epicoracoid.
This is in close contact with its fellow of the opposite side, so that the entire
shoulder girdle, like that of a dogfish, forms a single inverted arch.
Passing forwards from the anterior ends of the united epicoracoids is a rod
of bone, the omosternum, tipped by a rounded plate of cartilage, the episternum,
and passing backwards from their posterior ends is a similar but larger bony
rod, the mesosternum. This is also tipped by a cartilaginous plate, the xiphister-
num. These two structures are the first indication of a sternum we have yet
encountered, with the possible exception of the median ventral element of the
shoulder-girdle of H eptranchias. The omosternum is developed as paired

1plsternum

F rG. 267.- Rana: Pec-

toral girdle. The cleithrum
(Fig. 263, p. 382) is retained
in anurans, alone among ex-
tant tetrapods. The procora-
coid cartilage (in associa-
tion with the clavicle) is not
d istinguished in t he above
drawing. The cartilagin-
ous elem ent anterior to the
epicoracoid at the p roximal
end of the clavicle is the
precoracoid (not arrowed).
(From ventral aspect.)

%iphlsternum

forward extensions of the epicoracoids which undergo fusion. The meso-

sternum and xiphisternum arise as two pairs of rods lying posterior to the
epicoracoids, and subsequently one of each pair unites with the other. This
sternal apparatus of the frog (and of the Amphibia in general) differs develop-
mentally from the structures in the higher vertebrates to which the same name
is applied. In the latter, the sternum is formed from detached portions of
embryonic ribs (costal sternum).
The fore-limbs deviate from the typical structure (p. roo) chiefly in the
fusion of the radius and ulna into a single radio-ulna (Fig. 264, RA. UL) , and
in the presence of only four complete digits with a vestigial one, the prepollex,
on the radial side. The last is well-developed as a copulatory aid in males. It
represents a sesamoid bone. The complet e digits are the second to the fifth of
the typical hand. Six carpals only are present, the third, fourth, and fifth
digits articulating with a single bone which has apparently risen by the fusion
of the third, fourth, and fifth distalia and of at least one centrale.
 ZOOLOGY
The pelvic girdle is peculiarly modified. Although it contains the usual
three principal units of tetrapods in general, these are in detail unlike those
of any other group. The girdle consists of two long, curved bars articulating
in front with the transverse processes of the sacral vertebra (Fig. 264) and unites
posteriorly in an irregular vertical disc of mingled bone and cartilage which
bears on each side a deep, hemispherical acetabulum (Fig. 268, G). Into this
articulates the femur or thigh bone. The curved rods are the ilia (Il.
P.). They expand posteriorly and unite with one another in the median plane
to form the dorsal portion of the disc and about one-half of the acetabulum.
The posterior portions of the disc and acetabulum are furnished by the ischia
(Is.), fused with one another in the sagittal plane, their ventral portions by
the similarly united pubes (Kn.). The ilium and ischium are formed of true
bone, the pubis of calcified cartilage: the union
of the elements in the median plane is the
G
pubic symphysis.
In the larva the ilium is vertical, but during
development it becomes lengthened and at the
I same time rotated backwards, thus bringing
Kn the articulation of the hind-limbs as far back
FIG. 268.-Rana: Pelvic girdle.
From right. G. acetabulum; fl. P. as possible.
ilium; Is. ischium; Kn. pubis. In the adult the median pubic union, along
(After W iedersheim.)
with the bilateral iliac junction with the sacral
vertebra (Fig. 264, V. 9), gives a rigidity to the apparatus as a whole. The
ilia together form long jointed levers especially adapted for jumping. The
hind-legs are flexed when each leaping movement begins. As the animal leaps,
the legs simultaneously elongate with a backward transmission of force to the
feet, which momentarily remain braced on the ground. The frog lacks a
stabilising tail, and its centre of gravity is situated just behind the sacrum
(Fig. 264). This latter fact, together with the position of each acetabulum and
the length of the ilia, enables the relatively awkward animal to retain its
equilibrium during forward propulsion. Frogs of several species are able to
leap 3 or 4 feet under normal conditions. In a place called Angel's Camp, in
Calaveras County, California, some thirty thousand gentlemen assemble each
spring to bet at The Jumping Frog Jubilee. In May 1948 a frog called Helio-
trope (of Rana catesbiana) leapt I I feet 5 inches aided, doubtless, by not
entirely natural stimuli. In 1953 South Africans entered the Californian con-
test with a frog named Leaping Lena (species not stated), which had reputedly
leaped 24 feet 3l inches. We are unable to report the result of the homeric
international and interspecific contest that ensued. 1
The muscles of the tetrapod limb are numerous and complex. Each seg-
1 At a South African gathering termed the Frogolympics 'a single leap of fourteen feet [was]
recorded for one of the species' (Rose). The species in the picture is R.fasciata.
 PHYLUM CHORDATA 399
ment has its own set by which the various movements are performed. There
are muscles passing from the trunk to the limb-girdles ; from the trunk or the
limb-girdles to the humerus and femur ; from the humerus and femur to the
radio-ulna and tibio-fibula ; from the fore-arm or shank to the digits; and from

FIG. 269.-Rat~a: Musculature.

Ventral aspect. On the left side (right
of figure) many of the superficial
muscles have been cut and reflected to
show the deep layer. add. brev.
adductor brevis; add. long. adductor
longus; add. mag. adductor magnus;
del. deltoid ; ext. cr. extensor cruris;
ext. trs. extensor tarsi; FE. femur; gn.
hy. genio-hyoid; gstr. gastrocnemius;
fly. gl. hyoglossus; ins. ten. inscriptio
tendinea ; /. alb. linea alba; my. hy.
mylo-hyoid; obl. int. obliquus internus;
obl. e.-rt. obliquus extern us; o. st.
omosternum; p. c. hy. posterior cornu
of hyoid; pet. pectoralis; petn. pec-
tineus; per. peronams; ret. abd. rectus
abdominis; rect. int. maj. rectus
internus major; rect. int. min. rectus
internus minor; sar. sartorius; sb. mt.
sub-mentalis; sem. ten. semi-tendino-
sus; lib. ant. tibialis anticus; tib. post.
tibialis posticus; TI. Fl. tibiofibula;
vast. int. vastus internus; x. st.
xiphisternum.

one segment of a digit to another. For the most part, the limb-muscles are
elongated and more or less spindle-shaped, presenting a muscular portion or
belly which passes at either end into a tendon of strong fibrous tissue serving to
fix the muscle to the bones upon which it acts (p. 74).
In the second segment of the hind-limb the tibia and fibula are fused to
form a single tibio-fibula (Fig. 264, TI. Fl.). The two bones in the proximal
row of the tarsus-the tibiale or astragalus (AST.) and the fibulare or calcanettm
400 ZOOLOGY
(CAL.)-are greatly elongated and provide the lever-like leg with an additional
segment. There are three tarsals in the distal row, one of which appears to
represent the centrale, another the first distale, and the third the fused second
and third distalia. There are five well-developed digits. On the tibial side
of the first there is an additional spur-like structure or calcar (C.). This is
formed of three bones, a metatarsal and two phalanges. Such an additional
digit is a prehallux.
All the bones of the limbs consist of the shaft, formed of true bone and of
extremities of calcified cartilage (p. 73). Both in the freshly-prepared and in
the dried skeleton the distinction is a very obvious one.
The musculature, as we have seen, has undergone great modifications in
correspondence with the complex movements performed by the limbs. Like-
wise the abandonment (in the Anura but not in Urodela) of the sinuous mode
of swimming is associated with considerable alterations. The dorsal muscles of
the trunk are no longer divisible into myomeres (p. 86), but take the form of
longitudinal or oblique bands lying partly above the vertebrre, partly between
the transverse processes, and partly between the ilia and the urostyle. The
longissimus dorsi, for example, extends from the head to the urostyle. The
segmental origin of this muscle is suggested by numbers of narrow fibrous
cross-bands. The urostyle is braced on the pelvic girdle by paired coccygeo-
sacralis and coccygeo-iliacus muscles. The movement from water to land
placed additional weight on the ventral musculature and differentiated a
system of slings to support the abdominal viscera. There are a paired median
band, the rectus abdominus (Fig. 269 ret. abd.) with longitudinal fibres and
a double layer of oblique fibres-obliquus externus (obl. ext.) and internus
(obl. int.)-extending from the vertebral column to the recti. Both the
extensor dorsi and the rectus abdominis are traversed at intervals by trans-
verse bands of fibrous tissue, the inscriptiones tendinece (ins. ten.). The right
and left recti are united by a longitudinal band of tendon, the linea alba
(l. alb.).
Alimentary Canal and Associated Structures.-The wide mouth leads into a
capacious buccal cavity which has in its roof the internal or posterior nares (Fig. 270,
p. na.); these appear to be homologous with the oro-nasal grooves of fishes.
A pair of projections, due to the downward bulging of the large eyes, invest
the cavity in addition to the openings of the Eustachian tubes (eus. t., see below).
On its floor is the large tongue (tng.), attached in front and free posteriorly,
where it ends in a double point. The tongue can be suddenly projected,
point foremost, to capture and engulf insects. It is kept sticky by inter-
maxillary glands. Neither these, nor other mucous glands of the buccal cavity,
produce digestive enzymes. A system of cilia also exists, and appears to circu-
late oral fluids over the surfaces of the buccal cavity. Immediately behind the
tongue is a slit-like glottis (Fig. 270, gl.). Teeth are arranged in a single series
 PHYLUM CHORDATA 401

round the edge of the upper jaw, attached to the premaxillre and maxillre.
There is also a small patch of teeth (vo. t.) on each vomer just internal to the
posterior nostril. The teeth are without pulp or nerve-tissue. They are small
conical bodies, their bases ankylosed to the bones. They prevent smooth or
slimy prey (millipedes, slugs, insects, and worms) from writhing out of the
mouth. There are no teeth on the lower jaw.
The buccal cavity narrows towards the pharynx, which leads by a short
cesophagus or gullet (gul.) into a stomach (st.) consisting of a wide cardiac and a

FIG. 270.-Rana: Visceral relationships. From left; the viscera somewhat displaced.
an. anus; b. d . bile.duct; b. hy. body of hyoid; bl. urinary bladder; bl'. its opening into the
cloaca; c. art. conus arteriosus; cblm. cerebellum; cl. cloaca; en. 3, centrum of third vertebra;
cp. ad. corpus adiposum; crb. h. cerebral hemisphere; d.ly. s. dorsal lymph sinus ; du. duodenum;
ep. cor. epicoracoid; eus. t. Eustachian tube; FR. PA. fronto-parietal; gl. glottis; gul. gullet ;
I L. ilium; IS. ischium ; kd. kidney; l. au. left auricle; l. lng. left lung; lr. liver; M. MCK. m ento-
meckelian; n . a 1, neural arch of first vertebra; olf. l. olfact ory bulb; opt. l . optic lobe; o. ST.
omosternum; pcd. pericardium ; PMX. premaxilla; pn. pancreas; p. na. posterior naris; pu.
pubis; ret. rectum ; r. lng. right lung ; s. int. ileum; sp. cd. spinal cord ; SPH. E>H. sphenethmoid;
spl. spleen ; st. stomach; s. v. sinus venosus; tng. tongue; ts. testis; ur. mesonephric or urinary
duct; ur' . its aperture into the cloaca; UST. urosty le ; v. ventricle; v. ly. s. ventral lymph sinus ;
vo. t. vomerine teeth; vs. sem. vesicula seminalis.

short, narrow pyloric division. The stomach has a highly convoluted epi-
thelium pitted with mucous cells, and, in the crypts, possesses tubular secretory
glands which produce pepsin and hydrochloric acid. A pyloric sphincter
guards the entrance to the duodenum (du.). This, the first portion of the small
intestine, passes forwards parallel with the stomach. This part of the gut is
richly supplied with goblet-cells which discharge mucus. Here digested food is
absorbed into the hepatic portal system (p. nz). The rest of the small intestine
(ileum) is coiled. The large intestine or rectum (ret.) is very wide and short, and
passes without change of diameter into the cloaca, which possesses a cloacal
aperture.
The liver (lr.) is composed of two large lateral, and one small median, lobes.
The left lobe is itself partly divided. Between the right and left lobes lies a
402 ZOOLOGY
large gall-bladder (Fig. 271, G). The bile constantly secreted by the liver passes
into the gall-bladder via cystic ducts (D.cy.) as well as directly into the bile-duct
by way of minute hepatic ducts (Dh1). The bile-duct runs from the gall-
bladder through the pancreas (P), a diffuse pale-coloured compound gland of
exocrine and endocrine function (p. 152). The pancreas is held by mesentery
between stomach and duodenum. Digestive juices elaborated by the exocrine
(acinous) cells of the pancreas flow down the pancreatic duct which is bound to
the bile duct within a common
sheath, and they empty into the
duodenum through a common
opening.
Morphologically associated with
the alimentary tract is the spleen
(Fig. 270, spl.), a small red globular
blood reservoir attached by mesen-
tery to the anterior end of the rec-
tum. In the Salientia it is ha:mato-
pretic in function (see alsop. 447).
In addition to the pancreas cer-
tain other glands can be examined
during dissection of this area.
The paired thyroids are small in-
conspicuous endocrine organs lying
below the floor of the mouth in
front of the glottis and lateral to
the hyoid apparatus. Removal
Py of the thyroids in tadpoles pre-
FrG. 271.-Rana: Abdominal viscera. De., vents full metamorphosis, although
Dc.l common bile-duct; De.• its opening into the lungs and reproductive organs can
duodenum; D . cy. cystic ducts; Dh., Dlt. 1 hepatic
ducts; Du. duodenum; G. gall-bladder; L , L 1 , ()2, still develop and growth may not
L", lobes of liver, turned forwards; Lhp. duodena-
hepatic omentum, a sheet of peritoneum connect - be impaired. The administration
ing the liver with the duodenum; M . stomach; P . of thyroid extracts will cause
pancreas; P 1 , pancreatic duct; Py. pylorus.
(After Wicdersheim.) retardation of growth and sudden
metamorphosis in various anurans.
The periodic ecdysis, or moult, of the keratinous epidermal layers of the skin is
under the control of secretions of the anterior pituitary (Fig. g6, p. rso), and
thyroid. If either gland is removed the old cornified layers remain unshed
as a dark thick covering. Thyroid administration re-establishes moulting.
Parathyroids (p. 152), which do not seem to have been demonstrated in fishes,
occur in the Anura as paired ovoid bodies. There is evidence that, as in -higher
forms, they are concerned with calcium metabolism. The thymus gland is
also paired. Situated behind and below the tympanic membrane, these bodies
 PHYLUM CHORDATA

are of doubtful function. Certainly there is, up to the present, no acceptable

evidence that they are endocrine organs.
On the ventral face of each kidney (Figs. 270 and 273) there occurs an elon-
gated, yellow compound adrenal or suprarenal gland (p. 152). We have seen
that in cylostomes and fishes the homologues of medullary and cortical tissue of
the Mammalia occupy different situations (p. 152). In the Amphibia these
dissimilar tissues have come together and interdigitate as in reptiles and birds.
Respiratory System.-The lungs (Fig. 270, lng.) are paired elastic sacs lying
in the anterior part of the crelom above the heart and liver. Their size and
appearance vary greatly, according to their state of distension. Each contains
a spacious cavity and has its walls raised into a complex network of ridges
between which are alveoli (Fig. 74, p. 107). The septa between these are abun-
dantly supplied with blood-vessels. The two lungs open anteriorly into a small
laryngo-tracheal chamber, which communicates with the mouth by the narrow,
slit-like glottis. The glottis and walls of the laryngo-tracheal chamber are
supported by a framework provided by the arytenoid and cricoid cartilages.
The mucous membrane of the laryngeal chambers is raised into a pair of hori-
zontal folds, the vocal cords. These vibrate as air is forced back and forth
between lungs and vocal sacs, which are buccal diverticula connected with the
mouth by small slit-like apertures. The vocal sacs are resonators. Vocal
chords occur in both sexes, but are much better developed in the male, which
alone possesses vocal sacs (Fig. 298) and a voice. In breathing, the frog keeps
its mouth closed, and by depressing the floor of the mouth, draws air into the
buccal cavity through the nostrils. The floor of the mouth is then raised, the
nostrils, which are valvular, are closed by a system of cartilaginous levers
activated by the tight closure of the lower jaw, and the air is forced through
the glottis into the lungs. Thus, inspiration is achieved by the activity of
muscles arranged, and operating, essentially like those of fishes, from which the
amphibian mechanism is clearly derived.
Within the lungs, the oxygen is taken up by the hcemoglobin of the erythro-
cytes which are coursing through the capillaries in the septa between the alveoli,
and here, too, occurs the dissociation and excretion of carbon dioxide. In addi-
tion, frogs are capable of both cutaneous and buccal respiration. Gaseous
exchanges cannot take place through a dry membrane: both skin and buccal
surfaces are kept moist by special means. Buccopharyngeal respiration takes
place at the same time as lung respiration: after the glottis is closed and traps
air in the alveoli (p. 107), the nostrils are opened and a repeated raising and
lowering of the buccal floor sucks air into, and forces it from the buccal cavity,
the roof of which is invested with a respiratory capillary system.
Buccopharyngeal respiration is possibly less important than has been
generally thought. Measurement of the capillary surface in the mouth of Rana
and of Bufo has shown that they account for only 0·9 per cent., whereas those of
 ZOOLOGY
the lung and skin account for almost 65 per cent. and 35 per cent. respectively
of the total respiratory surfaces. Cutaneous breathing is probably continuous,
and may be particularly important in species that periodically submerge in
mud.
Blood-vascular System.-Fig. 272 shows some of the complexity of the
anuran heart and ventral vessels, including devices which appear to keep the
de-oxygenated and oxygenated blood-streams apart. There is no complete
agreement as to how much mixing occurs. Certainly in the musculo-cutaneous
vein there must be a mixing of the red cells oxygenated at the skin with (see
below) those partly saturated with carbon dioxide at the muscles. A further
mixing takes place in the right auricle.
A traditional view is that fairly complete separation of the main streams is
achieved. It was later claimed that injected X-ray opaque material and
cinephotography showed that, despite the trabeculre and the differentiated
pressures within the adjacent vessels, complete (or at least considerable) mixing
of the two streams occurs in the ventricle. This view, too, has been challenged,
and it now appears that the oxygenated and venous streams of blood are kept
tolerably separate and selectively distributed to the arterial arches, although
by a mechanism different from that envisaged in the traditional account (p. 406).
The heart lies in the pericardia! cavity ventral to the <:esophagus and in front
of a septum transversum that completely separates the pericardia! and crelomic
cavities. The heart is ensheathed by the pericardium, a double-walled sac. The
inner wall (epicardium) is applied to the heart surface. The two walls unite
at the base of the arterial arches but around the heart they are separated by
a pericardia! space containing serous fluid.
The heart consists of five chambers-a triangular sinus venosus (Figs. 272
and 273), right and left auricles (r. au., l. au.), a muscular ventricle (v., ventr.)
and a conus arteriosus (con. art.). The thin-walled sinus venosus receives
deoxygenated blood from the general circulation via three large vessels, the
right and left pre-cavals (anterior venre cavre) and the post-caval (posterior
vena cava) veins. From the sinus venosus the blood is delivered into the
right auricle through sinu-auricular valves. (Here we see a striking advance
on the Dipnoi in the greatly increased size of the left auricle, which is, however,
only about half the volume of the right.) The two auricles are separated by
the inter-auricular septum (Fig. 272), and into the left auricle runs a common
pulmonary vein which has received oxygenated blood brought from each lung
in the right and left pulmonary veins. The junction between pulmonary vein
and left auricle is unguarded by valves. We see that one auricle contains
deoxygenated blood (mixed with some oxygenated in the skin and buccal
cavity), and the other auricle freshly oxygenated red cells. The auricles con-
tract almost synchronously, and each squeezes its cargo of blood through a
common auriculo-ventricular aperture, guarded by two pairs of valves (one pair
 PHYLUM CHORDATA

int. car. a.

tr.
rt. syst. tr.

Lt. aur.

ch. tend.

J.M .

F 1c . 272.- Rflnn: Cardiac anatomy and circulation. Ventra l view. Simplified by the exclu·
sion of the pylangial (ventriculo-conal) valves from n ear the base of the spiral valve (sp. valve).
To the right of the spira l valve is (unmarked) the cavum aorticum, and to its left the cavum
pulmo-cutaneum. The labelled structures are: alr-v. valve. atrioventricular va lve; car. lab.
carotid labyrinth; ch. tend. chord<e tendine<E ; con. art. conus a rteriosus ; ext. car. a. external
carotid a rtery ; inter-aur. sept. inter-auric ular septum ; int. car. a. interna l carotid a rtery; lt. aur.
left auricle ; lt. car. tr. left carotid trunk; lt. syst. tr. left systemic trunk ; pul. v. pulmonary vein;
rt. aur. right auricle; rt. car. tr. right carotid trunk; rt. syst. tr. right system ic trunk; s. atr. ap.
sinu-a trial ap erture ; sept. med. septum media le; sept. pr. septum principale; sp. valve. sp iral
valve ; ventr. v entricle. Xl: Common entra nce of pulmo-cutan eous arteries (below spiral valve);
X 2 : Right pulmo-cutaneous trunk; X 3 : L eft pulmo-cut aneous trunk. (Data from J. R. Simons.)
 ZOOLOGY
large and the other pair small) into the single ventricle. The latter has a
transversely elongated cavity. Its dorsal and ventral walls are raised up
into muscular ridges or carnece with
interstices between them. It is gener-
ally held that these help to prevent
the mixing of the chemically dissimilar
streams of blood. The contraction of
the muscular ventricle follows auricular
contraction immediately. Blood is
driven into the thick-walled conus
arteriosus, which springs from the right
side of the base of the ventricle, and is
expelled into the arterial arches. The
conus arteriosus is incompletely divided
by the spiral valve into two passages,
the cavum aorficum leading to the caro-
apm. tid and systemic arches, and the cavum
pulmo-cutaneum leading to the more
dorsally placed entrance of the pulmo-
cutancous arches.
Traditionally it has been held that
the blood first entering the conus from
the ventricle passed into the cavum
pulmo-cutaneum and its associated
arches. It was believed that when the
resistance to flow in these arches had
risen, the blood next flowed into the
systemic arches, and finally because the
FIG. 273.-Raua: Arterialsystem. Rela- carotid labyrinths (see below) caused the
tionship with the heart, lungs, kidneys, and
left testis (from the ventral aspect). car. carotid arches to exert the highest re-
internal carotid artery; car. gl. carotid laby- sistance the last blood to leave the ven-
rinth; c. art. conus arteriosus; car. tr. caro-
tid trunk; ccel. mes. creliaco-mesenteric tricle flowed into the last-named vessels.
artery; cu. cutaneous artery; d. ao. dorsal
aorta; du. duodenal artery; gs. gastric It has now been shown that while
artery; hp. hepatic artery; il. iliac artery; a selective distribution of the blood un-
int. intestinal arteries; kd. kidney; l. au.
left auricle; tg. external carotid artery; t11g. doubtedly occurs, the mechanism which
lung; ces. resophageal artery; put. pul-
monary artery; put. cu. tr. pulmo-cutaneous brings it about cannot operate as out-
trunk; r. au. right auricle; r11. renal arteries; lined above. In Rana, Bufo, Hyla, and
set. subclavian artery ; spl. splenic artery;
spm. spermatic artery; ts. testis; syst. ts. Xenopus the movement of the blood in
systemic trunk; v. ventricle; vert. verte- the three pairs of arches has been found
bral artery.
to occur simultaneously. Furthermore,
traces of the pressure waves recorded from the arches by optical and electronic
manometers show that the patterns from all arches are similar and synchronous.
 PHYLUM CHORDATA

Despite this, a selective distribution does indeed take place, although this is
rather less perfect than earlier authors believed. In general, more than half the
blood ejected from the left auricle enters the carotid and systemic arches,
while about the same proportion of that discharged from the right auricle
enters the pulmo-cutaneous arches. The precise means by which this is
achieved is still a matter of speculation.
The paired branches of the short ventral aortre are each divided by two
longitudinal partitions into three vessels. These are an inner or anterior (the
carotid trunk (car. tr.)), a middle one (the systemic trunk or aortic arch), and an
outer or posterior (the pulmo-cutaneous trunk (pul. cu. tr.)). The systemic
trunks communicate separately with the conus at its distal end (sometimes
described as the bulbus arteriosus) and the carotid trunks open slightly to the
right of the junction of the two systemic trunks. The opening of the carotid
trunks is guarded by a valve.
After being bound together in the way described for a short distance, the
carotid, systemic, and pulmo-cutaneous trunks separate from one another.
The carotid trunk divides into an internal (Figs. 272 and 273, car.) and external
carotid (lg.) artery which supply the head. The former has at its base a small
swelling, the carotid labyrinth (car. gl.), which has a spongy interior containing
numerous cavities and channels. This structure was sometimes called the
carotid 'gland', but it is non-secretory. There is, however, evidence that the
carotid labyrinth acts as a chemo- and baroceptor.
The systemic trunks curve round the resophagus towards the dorsal midline.
The right arch becomes the dorsal aorta as it passes posteriorly in the median
position. The left arch touches the dorsal aorta briefly and communicates
with it by a small aperture and is then continued as the cceliaco-mesenteric
artery.
Apart from the arteries to the head, the lungs, and the skin, those supplying
all parts of the body arise either from the dorsal aorta or from one of the
systemic arches. The pulmo-cutaneous trunk divides into two: a pulmonary
artery (pul.) to the lungs, and a cutaneous artery (cu.) to the skin.
In the tadpole there are four aortic arches, each consisting of an afferent
and an efferent branchial artery connected by the capillaries of the gills. As
the water-breathing larva undergoes metamorphosis into the air-breathing
adult the gills disappear. The first aortic arch loses its connection with the
dorsal aorta and becomes the carotid trunk. The second enlarges, retains its
connection with the dorsal aorta, and becomes the systemic trunk. The third
disappears. The fourth sends off branches to the lungs and skin, loses its
connection with the dorsal aorta, and becomes the pulmo-cutaneous trunk.
Venous return. The blood from each side of the head is returned by internal
(Fig. 274, int. ju.) and external (ext. ju.) jugular veins into the precaval vein (pr.
v.), which also receives the brachial z·ein (br.) from the fore-limb, and the
 ZOOLOGY
musculo-cutaneous vein (ms. cu.) from the skin and muscles of the side and back,
and part of the head: the two prccavals open separately into the sinus venosus.
The course of the blood from the posterior part of the body is very different
from that seen in fishes-the differences being due partly to the absence of a
tail, partly to a peculiar modification of the lateral veins, and partly to the
replacement of the cardinals by a postcaval vein, found among fishes only in the
Dipnoi (p. 361).

FrG. 274.-Rana: Venous

system. Relationship with the
heart, lungs, liver, kidneys, and
right testis (from the dorsal
aspect). ant. abd. anterior ab-
dominal vein; br. brachial vein;
br cd. cardiac vein; ds. lmb. dorso-
lumbar vein; du. duodenal vein;
ext. ju . external jugular vein; jm.
femoral v ein; gs. gastric vein; hp.
hepatic vein; hp. pt. hepatic
portal vein; int. intestinal veins;
int. ju. internal jugular vein; kd.
kidney; !. au. left auricle; lng.
lung; lvr. liver; ms. cu. musculo-
cutaneous vein; pr. cv. precaval
vein; pt. cv. postcaval vein; put.
pulmonary vein; pv. pelvic vein;
r. au. right auricle; rn. renal
veins; rn . pt. renal portal vein;
sc. sciatic v ein; spl. splenic vein;
spm. spermatic vein; s. v. sinus
venosus; ts. testis; ves. vesical
veins.

Two portal systems occur. The blood from the front part of the hind-leg is
brought back by a femoral vein (fm.), which, on reaching the ccelom, divides
into two branches, a dorsal and a ventral. The dorsal branch is the renal
portal vein (rn. pt.). This receives the sciatic vein (sc.) from the back of the leg
and passes to the kidney, in which it breaks up into capillaries. (It should be
remembered, of course, that the minute vessels that form the kidney glomeruli
(seep. 155) come from the renal artery.) The ventral branch is the pelvic vein
(pv.). This unites with its fellow on the opposite side to form the anterior
 PHYLUM CHORDATA

abdominal vein (ant. abd.) which passes forwards in the ventral body-wall,
between the linea alba and the peritoneum, to the level of the sternum, where
it turns inwards and divides into two branches, both breaking up into capillaries
in the liver.
Just as the anterior abdominal vein enters the liver it is joined by the
hepatic portal vein (hp. pt.), bringing the blood from the stomach, intestine,
spleen, and pancreas. The spleen (Fig. 270, spl.) is a small, red, globular body
near the head of the rectum. The abdominal vein also receives vesical veins
(ves.) from the urinary bladder, and a small cardiac vein (cd.) from the heart.
It represents the lateral abdominal veins of Chondrichthyes united in the
middle ventral line : the pelvic veins represent their posterior free portions.
The blood is collected from the kidneys by the renal veins (rn.), which unite
to form the large unpaired postcaval vein (pt. cv.). This passes forward through
the liver, receives the hepatic veins (hp.) from that organ, and finally opens
into the sinus venosus. Thus the blood from the hind-limbs has to pass
through one of the two portal systems on its way back to the heart. Part of it
goes by the renal portal veins to the kidneys, and thence by the renal veins to
the postcaval. The rest goes by the pelvic and abdominal veins to the liver,
and thence by the hepatic veins to the postcaval. From here it enters the
sinus venosus for re-distribution via the right auricle. Lastly, the blood which
has been oxygenated in the lungs is returned by the pulmonary veins (pul.)
directly to the left auricle.
It will be seen that there is no trace of cardinal veins in the frog. In the
larva, however, both anterior and posterior cardinal veins are present. During
metamorphosis, the ductus Cuvieri unite (as in fishes) and become converted
to the precavals. The posterior portions of the posterior cardinals contribute
to the formation of the postcaval. The anterior portions disappear.
Blood. As in other vertebrates, oxygen is transported by unstable associa-
tion with the red respiratory pigment, hremoglobin, carried in the erythocytes.
Compared with that of the Mammalia, however, anuran hremoglobin has a
relatively low affinity for oxygen even when differential temperatures are
taken into consideration. (There is evidence, too, that carbon dioxide trans-
port is somewhat less efficient in the Amphibia.) As in fishes, birds, and reptiles,
the erythocytes are nucleated throughout their life. They are relatively large
oval discs, measuring about 20 X I5p.. as compared with about 7ft in, for ex-
ample, the rabbit. Erythrocytes are formed in the kidneys, and hrematopetic
tissue occurs also in the spleen and bone marrow, as in higher vertebrates.
Destruction of effete cells occurs in the spleen and liver; some undergo frag-
mentation in the blood-stream. White cells consist of large phagocytic macro-
phages, monocytes, phagocytic polymorpho-nuclear granulocytes (of various kinds),
and lymphocytes (p. 70). If a fine cannula filled with pathogenic bacteria is in-
serted into the peritoneal cavity of a frog and withdrawn after a few hours, it is
VOL. II. CC
4IO ZOOLOGY

found to be covered with 'wandering' phagocytic granulocytes, which have

migrated to the source of infection to act in approximately the same manner as
in higher tetrapods. Thrombocytes, probably essentially equal in function to
blood-clotting platelets in the Mammalia, also occur.
Lymphatic System.-Vertebrates have a closed system of vessels, so that
except in parts of special organs (such as liver, adrenal, and spleen) the materials
carried in the capillaries do not come into direct contact with the general body-
cells. The capillaries are separated from other body-tissues by :fluid-filled
tissue-spaces. These are filled with tissue-fluid, which is essentially blood
plasma that has exuded through the walls of the capillaries. Across the :fluid-
filled tissue-spaces must pass oxygen and food substances brought by the blood.
The passage of :fluid from capillaries to tissue-spaces is carried out partly by
pressure exerted by the heart-beat and partly by osmotic pressure of the blood
and the adjacent tissue :fluids.
As well as :fluids and solutes, leucocytes can also pass from the blood-stream
to the tissue-spaces. The tissue-spaces communicate with minute lymph-
vessels which continue into larger vessels to make up a highly developed lym-
phatic system (p. n5). This, in the frog, is remarkable for the dilatation of
many of its vessels into large lymph-sinuses. Between skin and muscle are
spacious subcutaneous sinuses (Fig. 270, v.ly.s.), separated from one another by
fibrous partitions. The presence of these sinuses allows the skin of the frog to
slide back and forth across the underlying structures. The dorsal aorta of the
frog is surrounded by a capacious subvertebral sinus. Lymph is driven through
the above diffuse system of vessels into the venous system by means of anterior
and posterior pairs of lymph-hearts. The anterior pair is situated beneath the
supra-scapulre. The other pair can be seen beside the posterior end of the
urostyle. These lymph-hearts open into veins, and so the lymph :fluid and its
inclusions become once more mixed with the general circulation (see also
crelomic fluid, p. 415).
Nervous System.-The brain (Fig. 275) resembles, in pattern, that of the
Dipnoi. There are large optic lobes, a well-developed diencephalon, and large
fully evaginated hemispheres and olfactory bulbs, the last-named fused in the
median plane. The corpora striata, or basal ganglia of the cerebral hemi-
spheres, are connected together, as in all vertebrates, by an anterior commissure
(Fig. 275, D, Com., lower line), above which is another commissure (upper
line) partly representing the hippocampal commissure of the brain of reptiles
and mammals. The third ventricle is covered by a thick choroid plexus.
The aqueduct of the mid-brain extends dorsolaterally into the optic lobes
of the tectum. The lateral ventricles are large ~avities in the cerebral
hemispheres which extend into the olfactory bulbs. The hypothalamus is well
developed, and to the infundibular stalk is attached the hypophysis or pituitary
gland. This gland is of compound structure and great functional complexity
 PHYLUM CHORDATA

and is connected to the hypothalamus by a portal system (p. 149). The pineal
body is vestigial in the adult, being represented only by the stalk. The
anterior choroid plexus (of the third ventricle) is found a little in front of the
pineal. In the larva the pineal body occurs outside the skull and immediately

FrG. 275.-Rana: Brain,

pituitary, and cranial nerves. D-
A. dorsal; B, ventral; C. opt.cl£
lateral; D. in longitudinal - il!f
vertical, but not quite mid-
line, section. Cb. cerebellum; f'pt.l n1 H~>-4":1ill!r.~- Cr.C
Cer. H. cerebral hemispheres; - opl.l
IV
ch. plxl. anterior, and ch. plx2 • v pd
posterior choroid plexus (re- (///
VIII
moved in A); Com. commis-
sures. the two in front the
anterior and liippocampa!. the
two above the superior or
habenular and the posterior;
Cr. C. crura cerebri; Di.
diencephalon; f or. M . fora-
m en of Monro; i . iter, or
aqueduct of Sylvius; inj. in·
fundibulum; J'Jed. obl. me-
dulla oblongata; Olf. l. olfac-
tory bulb; opt. ch . optic
chiasma; Opt. l. optic lobe ;
opt. v. optic ventricle; pin.
stalk of pineal body; pit.
pituitary body; Sp. cd. spinal
cord; v3 • third ventricle ; v'.
J
fourth ventricle; I - X, cere-
bral nerves; 1Sp. zSp. spinal
nerves. (After Parker.
A-C, after Gaupp; D, from
\Viedersheim, after Osborn.)

beneath the skin. The cerebellum is extremely small, as might be expected

in a relatively inactive animal that moves principally in a single plane.
The cranial nerves are on much the same pattern as in the fishes, but the
loss of gills in the adult has led to alterations in the course and significance of
the ninth and tenth. The spinal nerves, too, have become modified in their
arrangement and function along with the evolution of limbs. Correspond-
ing with the number of vertebrre, there are only ten pairs of spinal nerves,
412 ZOOLOGY

of which the second and third unite to form a brachial plexus giving off the nerves
to the fore-limb, while the seventh to the ninth and a branch of the tenth join
to form a lumbo-sacral or sciat£c plexus giving off the nerves to the hind-limb.
Of the remainder, the first spinal nerve performs the function of the hypo-
glossal (Fig. 275, rSp.), supplying the muscles of the tongue: it passes out be-
tween the first and second vertebrre. The minute fourth, fifth, and sixth spinal
nerves carry fibres to the muscles and skin of the body. The cloaca and bladder
are supplied by a second branch of the tenth. This nerve emerges from an
aperture near the anterior end of the urostyle. The short spinal cord ends in
a delicate filament, the filum terminate.
Autonomic Nervous System.- The sympathetic system in Rana is represented
by a pair of sympathetic trunks which lie ventral to the vertebral column.
Each trunk consists of ten ganglia joined longitudinally by connectives; in
addition, each ganglion is connected to the corresponding spinal nerve by rami
communicantes. Minute fibres may extend from ganglia of one chain to those of
the opposite side. From the anterior cranial ganglion of the chain on each side,
nerve-fibres continue into the head to communicate with the vagus and other
cranial nerves. The whole system, metamerically arranged, is divisible into
four regions-the cephalic, cervico-branchial, abdominal, and sacro-coccygeal.
Sympathetic fibres travel widely to innervate the dilator muscles of the iris
and muscles of the heart, aorta, arteries, arterioles, and the skin ; and other
organs, such as the lungs, stomach, intestine, liver, pancreas, spleen, kidneys,
adrenal tissues, gonads, bladder, cloaca, and rectum. In addition, parasympa-
thetic fibres from various cranial nerves, mainly the vagus, and from the
hinder part of the spinal cord (representing the sacral parasympathetic outflow)
supply most of the above structures. The heart-beat is augmented by
sympathetic stimulation and depressed by vagal fibres. The sympathetic has
a constrictor effect on arteries and arterioles. Vaso-dilatation is also under
nervous control. Although the evidence is often conflicting in detail, it is clear
that in the frog there is an autonomic nervous system essentially similar in basic
pattern to those found in homreothermous tetrapods.
Organs of Special Sense.-The olfactory organs show profound modifications
in accordance with assumption of terrestrial life and aerial respiration. Each
olfactory chamber has two openings : the anterior naris or external nostril and
the posterior naris (Fig. 89, p. 138) or internal nostril. The latter opens into
the mouth immediately lateral to the vomer. It is probable that the internal
nostrils were formed by the modification of the oronasal groove of ancestral
forms. The twin passages are separated by a nasal septum. The olfactory
chambers within are lined with sensory epithelium, the fibres from which form
the olfactory nerves. Across this epithelium passes air as it travels to and from
the lungs. Further, a stream of water is circulated across the epithelium when
the frog is swimming, and so the animal can smell while submerged. In addi-
 PHYLUM CHORDATA

tion, the vomeronasal (Jacobson's) organ has arisen in the Amphibia. This is
formed by means of a naso-pharyngeal evagination. It is lined with sensory
epithelium supplied by fibres from the olfactory and trigeminal cranial nerves.
Jacobson's organ communicates with the olfactory chamber and the buccal
cavity and enables the animal to smell substances taken into the mouth (p. 479).
Chemoreceptors for the appreciation of taste occur in the tongue and palate.
The eyes, too, show adaptations to the terrestrial existence. Lachrymal glands
produce a watery fluid (tears) which bathes the anterior eye surface and washes
away injurious foreign bodies. Eyelids are developed: the upper lid is im-
mobile; from the lower has developed the transparent nictitating membrane

FIG. 276.-Rana: Eye and optic

tract. During metamorphosis from
the tadpole there develop •terres-
trial' features, such as the eyelids,
nictitating membrane (sec below),
Harderian gland, and nasolachrymal
(tear) duct. The six typical verte-
brate eye muscles (p. 142) occur,
together with a powerful retractor
bulbi and a levator bulbi. The former
(derived from part of the external
rectus) is involved in swallowing as
well as in the protection o f the eye-
ball. T he levator bulbi is derived
from masticatory muscles. ac. area
centralis (with local concentration of
visual cells); io. inferior oblique; ir.
inferior rectus ; ll. lower lid; lm.
lens muscles (protractors); n. optic
nerve; nm. nictitating membrane (a
transparent, independently movable
part of the lower lid said not to be
homologous with that of higher tetra-
pods); pn. pupillary nodules; s.c.
scleral cartilage; so. superior oblique;
sr. superior rectus; ul. upper lid ; z.
zonule (fibres are embedded in
vitreous). (Modified after Walls. )

which flicks across the eyeball and assists in the cleansing process (Fig. 276).
(In Man the remnants of a functionally similar reptilian structure are retained as
a small pink fold, the plica semilunaris, between the medial ends of the eyelids.)
The arrangements within the nearly spherical eyeball also reflect the changed
visual circumstances of tetrapods. The cornea is rounded, the lens is more
flattened than in fishes, and the two structures are widely separated. This
enables the protractor lentis muscles to move the whole lens forward (as homo-
logous muscles move it slightly backwards in modern fishes). These muscles
run in close association with the fibres that support the lens in its fluid bath;
and radial muscle processes, the musculus tensor chorioidea, probably related
to the ciliary muscles in higher forms, also occur. The above mechanism
enables the t errestrial amphibian to focus on distant objects to an extent that
would be impossible in animals living submerged in water. The iris is under
 ZOOLOGY
the control of circular sphincter muscles and radial dilator muscles, which,
although under nervous control (p. II9), are said also to be susceptible to
response by direct illumination. Both rods and cones occur in the retina.
In the ear, the organ of equilibration presents the same basic pattern from
fishes to the Mammalia, but in the Amphibia we find accessory structures
related to the reception, amplification, and conduction of vibrations from the
external environment. Thus, a middle ear, equipped with a tympanic membrane
or ear-drum, appears. Between this and the outer wall of the auditory capsule
is a considerable space, the tympanic cavity (Fig. 93, p. 144). This com-
municates with the pharynx by the short Eustachian tube already described
(Fig. 270, eus. t.), so that a probe thrust through the tympanic membrane
from outside passes directly into the pharynx. In the roof of the tympanic
cavity lies the columella (Fig. 265), its head, or extra-columella, attached to the
inner surface of the tympanic membrane, its foot-plate or 'stapes' 1 fixed in
the membrane of the fenestra ovalis. Vibrations striking the tympanic
membrane are communicated by the columella and stapes to the fenestra
ovalis, thence to the perilymph, and next to the membranous labyrinth of the
inner ear. This is composed of the utriculus and semi-circular canals, and the
sacculus with its diverticular lagena (Fig. 94,). The endolymphatic ducts
do not communicate with the exterior, but run dorsa-anteriorly and fuse at
the anterior end of the medulla oblongata. Dissection of this region will
reveal large aggregations of white calcareous particles. In some Anura these
extend far down on each side of the vertebral column. These occur in paired
sac-like organs which are connected with the inner ear; nevertheless their
function is unknown. The connection of the tympanic cavity with the
pharynx (via the Eustachian tube) helps to maintain equality of pressure on
each side of the tympanic membrane. There seems little doubt that the tym-
pana-eustachian passage is homologous with the first or hyomandibular gill-cleft,
although, in anurans, it is formed independently of the clefts and never opens to
the exterior. From the consideration of their habits there can be no doubt that
hearing in frogs is exceedingly important, more especially during the breeding
season. It is probable, too, that hearing is important in hunting and escape
activity. It has been experimentally shown that some anurans can appreciate
auditory stimuli in the range between 50 and Io,ooo vibrations per second.
Lateral line organs occur in the tadpole. The lateral line cells are ciliated
and are receptive to water-vibrations. The organs along the body are inner-
vated, as in the fishes, by the ramus lateralis from the vagus. Those of the
cephalic region are served by the vagus and, to a lesser degree, by branches
from the facial and glossopharyngeal. The whole system, in ranids, disappears
during metamorphosis (see, however, Fig. 302, p. 449).
1 The terminology of the structures of the columella is unsatisfactory. The whole is homo-
logous with the stapes of the Mammalia.
 PHYLUM CHORDATA 415
Endocrine Glands.-The compound pituitary gland is at the base of the
hypothalamus (p. 127). Its various parts differ in their relationships from those
in the higher tetrapods. Thus in the Anura t:he anterior lobe (pars distalis) is
not anteriorly placed. The intermediate and neural parts and also the pars
tuberalis of higher groups are represented, and the organ as a whole appears
to show a considerable advance over those of fishes so far investigated. For
the role of anterior and posterior lobes in colour change in Xenopus, seep. 450.
The anterior pituitary has powerful partial control of the primary sex
organs. There is little doubt that its activity is considerably influenced by
environmental changes through the intermediacy of the central nervous system,
including the hypothalamus. Injections of macerated anterior pituitary
material will cause unseasonal ovulation. The testes of frogs, too, are under
partial control of the pars distalis. Other endocrine glands are the thyroid,
paired parathyroids (p. 152), and paired adrenals and gonads (see below).
Urinogenital Organs.-The kidneys (Figs. 277 and 278, N.) are flat, some-
what oval bodies, of a dark red colour, lying in the posterior region of the
crelom. On the ventral face of each kidney is an elongated, yellow adrenal
gland (p. 152). Considerable numbers of minute nephrostomes occur on the
ventral kidney-surface and propel cmlomic fluid (almost identical with lymph)
from the crelom into the renal veins. It will be recalled that blood comes to
the kidney from two entirely separate sources (p. II2). Branches of the renal
artery run generally to the numerous glomeruli (p. 157). To the associated
uriniferous or renal tubules run branches of the renal portal veins. Each
glomerulus is composed of an arteriole (from the renal artery) which ends in a
knot of capillaries whose outlet is a vessel connecting with the renal vein.
The capillary knot is almost surrounded by an invagination of the end of a renal
tubule which becomes a double-walled, delicate Bowman's capsule.
The vessel leaving the glomerulus is usually smaller than that entering it,
and there is evidence that both blood and osmotic pressures are higher in the
glomerulus than in the contiguous excretory capsule. A protein-free filtrate,
containing waste nitrogenous products, passes over into the capsule and,
helped by the beat of cilia, down the uriniferous tubule. At the same time
there occurs a selective reabsorption of fluids and dissolved substances valuable
to the animal, and these are transferred back into the general circulation. The
urinary residue, including waste metabolites, passes down the tubule, and with
the cargo from numerous others, is taken up by one of many collecting tubules
which open into the mesonephric or Wolffian duct. Each of these (Fig. 277, Ur.)
passes backwards from the outer border of the kidney and opens into the
dorsal wall of the cloaca (Cl.). The kidney is developed from the mesonephros
of the embryo, the urinary duct from the mesonephric duct. In the larva,
a large pronephros is present and is, for a time, the functional kidney.
Opening into the cloaca on its ventral side is the urinary bladder (Fig. 270,
 ZOOLOGY
bl.) (p. 401), an organ now encountered for the first time. It is a bilobed, thin-
walled, and very delicate sac into which the urine passes by gravitation from
the cloaca when the vent is closed. This sac is a urinary bladder and is quite
different morphologically from the organ of the same name in fishes (which
is a dilatation of the mesonephric duct) and is distinguished as an allantoic
bladder. From this urinary bladder, in the frog, further fluid can be reabsorbed
into the circulatory system.
Because of its semi-permeable surface, the frog is in constant danger of
being either waterlogged or dehydrated in ex-
tremes of climate. It overcomes the former
FK danger by the extraordinary efficiency of its
glomerular system, which enables it to eliminate,
when necessary, up to one-third of its body-
weight in fluid per day. However, as we have
seen (p. 285), not even the auxiliary extra-renal
water-absorbing mechanism of the bladder can
enable the frog to withstand high tempera-
tures (see alsop. 385).
Reproduction.-In the male the testes (HO) are
white ovoid bodies lying ventral to the anterior
ends of the kidneys. They are attached to the
kidneys by folds of mesorchium, which are con-
tinuous with the peritoneal epithelium that
cloaks the ventral face of each kidney and lines
the entire body cavity.
The testes contain long, convoluted semin-
FIG. 277.- Rana: Urino- iferous (spermatogenetic) tubules, and in the
genital system in male. A o.
dorsal aorta; Cl. cloaca; Cv. interstices between are held numbers of Leydig
postcaval vein; FK, fat-bodies; or interstitial cells which produce male sex
HO, testes; N, kidneys; S, S',
apertures of mesonephric ducts hormone under the stimulation of a pituitary
into cloaca; Ur. mesonephric
ducts. (After Wiedersheim.) gonadotrophin (p. rso). The seasonal liberation
of male testosterone in turn modifies the second-
ary sexual organs, including the nu,ptial pad (p. 390), and leads to appro-
priate changes in behaviour. Seminiferous activity, too, is under the control
of the anterior pituitary. The spermatozoa are shed into collecting tubules,
which lead to a number of delicate vasa efferentia. These leave the inner edge
of each testis, pass between the mesorchia! folds, and enter the nearby kidney
and become connected with the urinary tubules. The spermatic fluid is thus
passed into the urinary tubules and carried off by the W olffian duct, which in
the male frog is therefore a urinogenital duct. This duct is attached to the
lateral edge of the kidney. The spermatozoa are carried down this towards a
vesicula semina/is (Fig. 270, vs. sem.) or seminal vesicle, which is a dilation, of
 PHYLUM CHORDATA

glandular appearance, near the cloaca. This vesicle receives spermatozoa

through numerous small ducts in the outer side of the Wolffian duct. Storage
occurs (in at least some species) before amplexus (see below). This mechanism
has remained at a very primitive level, and is another indication that the
parental amphibian stock separated very early in the evolutionary history of
vertebrates.
Attached to each testis are lobed
bodies of a bright yellow colour-the
fat-bodies (FK) or corpora adiposa, which
are food reserves during restivation.
The ovaries (Fig. 278, Ov.) are large, oa....
folded, multi-lobed sacs, each of which
is held in position by a peritoneal fold,
the mesovarium, which also covers the
ovary as a delicate membrane, on the
surface of which the black-and-white
ova project. A fat-body is attached to
each ovary.
The ovary is under the control of
the anterior pituitary, and in turn pro-
duces a female sex hormone, or hormones, Ut ...... .
which influences the secondary sexual
organs and behaviour. It varies con-
siderably in size according to season
and reproductive condition. The ovaries
of some of the Ranidre may contain as
many as 2o,ooo eggs, each measuring
about r·75 mm. in diameter. The Giant
FIG. 278.-Rana: Urinogenital system in
Toad (Btifo marinus) can produce r6,ooo female. N, kidneys ; Od. oviduct; Ot. its
eggs in a single day. ' Ripe' ovaries cwlomic aperture; Ov. left ovary (the right is
removed); P, cloacal aperture of oviduct; S,
usually give the whole body a puffed S ' , cloacal apertures of mesonephric ducts;
U t. posterior dilatation of oviduct. (After
appearance. The ova are extruded Wiedersheim.)
through the ovarian wall directly into
the crelom. There is experimental evidence that, in R. pipiens at least, the
eggs cannot yet be fertilised. Each oviduct (od.) is a greatly convoluted tube,
the narrow anterior end of which opens into the crelom by a slit-like aperture,
the ostium tubce (ot.). The paired oviducts open close to the roots of the
lungs. Ova are driven into the oviducts by the action of peritoneal cilia.
(Some species have cilia even on the surface of the lungs and pericardium.) In-
side the oviducts, the eggs take on an albuminous covering and can thereafter
be fertilised. As they are propelled down the tube by oviducal cilia they
become progressively more gelatinous. Finally, they reach the wide, thin-
 ZOOLOGY
walled posterior parts of the tubes (ut.), which become immensely dilated until
the eggs are shed.
Fertilisation is external. In amplexus the male applies himself to the
dorsal surface of the female, aided by a modified additional paired digit, the
prehallux, and the nuptial-pad (pp. 390, 416). So tenacious is the grip that a
male's fore-legs may sometimes be broken before it will release its hold. There
is experimental evidence that this activity, once under way, involves essentially
a simple reflex spinal arc, since it may continue after decapitation.
The male of R. temporaria has been known to clasp a fish or floating stick
at the height of the sexual season, in the absence of a female. The swollen
female may carry the male around for several days .until the eggs are shed.
As they are voided, the male pours spermatic fluid over them, achieving
fertilisation. The process in the majority of species occurs in water.
Development.-The eggs are laid in water in gelatinous aggregations. The.
gelatinous material swells in water and protects the conspicuous eggs from
small aquatic predators and buffers them against injury. Each egg has a black
and a white hemisphere. The former is always directed upwards and surrounded
by a sphere of jelly. The egg is telolecithal, the protoplasm being mainly
accumulated in the pigmented hemisphere, while the white hemisphere is
loaded with yolk. After the male sheds spermatic fluid over the eggs, the
spermatozoa penetrate the albuminous jelly and vitelline membrane. In a
short time the jelly swells and becomes thereafter impermeable to spermatozoa.
Segmentation begins by a vertical division of the fertilised egg into two
cells (Fig. 279, A), and is soon followed by a second vertical furrow at right angles
to the first (B), and then by an equatorial furrow placed nearer the black than
the white pole (C). Thus the eight-celled embryo consists of four smaller
black cells and four larger white cells. Further divisions take place (D), the
black cells dividing rapidly into micromeres (mi.), the white, more slowly,
into megameres (mg.). As in previous cases, the presence of yolk hinders the
process of segmentation. The pigmented micromeres (D-F, mi.) give rise
to the ectoderm, which is many-layered: the megameres (mg.) contribute to
all three germ layers and are commonly called yolk-cells. During the process
of segmentation a blastocr:ele (E, bl. cr:el.) or segmentation-cavity appears in the
upper hemisphere.
The black now begins to encroach on the white hemisphere until it covers the
whole embryo except for a small patch, known as the yolk-plug (G, H,yk. pl.), at
what will become the posterior end. This process is one of epiboly : the margin
of cells surrounding the yolk-plug forms the lips of the blastopore.
The archenteron (I, ent.) arises by invagination of cells at the blastopore.
The archenteron is at first a very narrow cleft, but soon widens considerably.
For a long time it does not actually communicate with the exterior, since the
blastopore is filled with the yolk-plug. As the archenteron extends forwards,
 PHYLUM CHORDATA

A B c

FIG. 279.-Rana: Development. A - F, segmentation; G, overgrowth of ectoderm; H,

I, establishment of germinal layers; ], K, assumption of tadpole-form and establishment of
nervous system and the notochord. The disputed enteric canal (see text) is also shown. L,
newly-hatched tadpole. bl. crol. blastocrele; blp. blp'. blastopore; br. I, br. 2, gills; br. cl. branchial
arches; e. eye; ect. ectoderm; end. endoderm; ent. enteron; f. br. fore-brain; h. br. hind-brain;
m. br. mid-brain; md.f. medullary fold; md.gr. medullary groove; nzes. mesoderm; mg. megameres;
mi. micromeres; nch. notochord; n. e. c. neurenteric canal; pcdm. proctod<eum; pty. pituitary
invagination; ret. commencement of rectum; sk. sucker; sp. cd. spmalcord; stdm. stomod<eum;
t. tail; yk. yolk-cells; yk. pl. yolk-plug. (A-D, F-H, and] from Ziegler's models; E, I, K,
and L after A. M. Marshall.)
420 ZOOLOGY
the blastocrele gradually disappears. The walls of the archenteron are formed
by endodermal cells, except for a strip of chordamesoderm lying along the
antero-posterior axis of the archenteron roof. This separates off to form a
notochord, whilst the endodermal cells form gut walls, the archenteron provid-
ing the lumen of the gut. Meanwhile mesodermal cells have invaginated
ventrally and laterally to the yolk-plug and extended forwards and upwards
between the ectoderm and endoderm layers. The mesoderm subsequently
forms somites and lateral plate mesoderm.
The edges of the lower margin of the blastopore now begin to approach one
another, and, uniting in the median plane, give rise to a vertical groove. In
the meantime medullary folds (H, md. f.) appear and mark the dorsal surface.
They are at first widely separated, but gradually approach one another and
close over the medullary groove (md. gr.), thus giving rise to the central nervous
system. Posteriorly they are continuous with the lips of the blastopore. The
neurenteric groove becomes closed in behind. It has been claimed in the past
that the archenteron communicates with the neurocrele by a neurenteric canal,
but most recent workers doubt whether this actually exists, or if it does, that it
persists for more than an hour.
The embryo soon begins to elongate. One end is broad, and, becoming
separated by a slight constriction, is marked out as the head. The other end
is bluntly pointed and is the rudiment of the tail (t.). On the ventral surface of
the root of the tail a proctodceum (pcdm.) appears and communicates with the
archenteron. The head and tail become more distinct. A pit-the stomodceum
(], R, L, stdm.)-appears on the antero-ventral surface of the head, and, imme-
diately behind it (in R. temporaria), a semilunar area with raised edges, the
sucker (sk.). The anatomical relationships of these structures vary among
species. At each side of the head two branched processes appear. These
are the external gills (br. r, br. 2), and the regions from which they arise mark
the positions of the first and second branchial arches.
The embryos are now hatched as tadpoles. They swim freely in the water or
adhere to weeds by means of the sucker, which is really a glandular apparatus.
They are still blind and mouthless; the stomodreum is not yet in communica-
tion with the archenteron. Soon a third pair of external gills appears on the
third branchial arch, and the first two pairs increase greatly in size. The
stomodreum joins the archenteron. Gill-slits (branchial clefts) are formed
between the branchial arches, and eyes appear. The mouth is small, bounded
by lips beset with horny papillre and provided with a pair of horny jaws. This
buccal rasp fragments the browsed food into minute particles which, in suspen-
sion, are gulped in by the action of the masticatory muscles and, entangled in
mucus, are carried into the resophagus by pharyngeal cilia. Strings of food-
impregnated mucus thus reach the stomach. The food is passed into the
intestine, which grows to a relatively great length and is coiled.
 PHYLUM CHORDATA 421

Soon the external gills show signs of shrivelling. At the same time internal
gills not unlike those of fishes are developed in the branchial clefts. A fold
of skin, the operettlttm, appears in front of the gills on each side. It grows from
the region of the hyoid arch, and extends backwards until the gill-slits and
external gills are covered. In Rana, the operculum on the right side soon
unites with the body-wall so as to close the branchial aperture, but on the left
side an opening (the spiracle) remains for a considerable time as the sole means
of exit of water from the opercular chamber and therefore all visceral clefts.
In other genera the opening is on the right side; and in yet others it is
bilateral. At this stage the tadpole is strikingly fish-like. The lungs now
appear, and the larva for a time becomes truly amphibious : it rises periodically
to the surface to breathe air.
Meantime, limbs are developed. The hind-limbs appear as little rounded
buds, one on each side of the root of the tail. The fore-limbs arise beneath
the operculum and are therefore hidden at first. Soon, however, they emerge
by forcing their way through the operculum on the right and the spiracle on
the left. As the limbs increase in size the tail undergoes a progressive shrinking.
The mouth widens by the backward rotation of the suspensorium. The
intestine undergoes a relative diminution in length as vegetable is exchanged
for animal diet. The small, tailed frog can now leave the water and hop about
upon land. Its tail is soon completely absorbed. Metamorphosis is complete.
Partial neoteny has been recorded several times in R. esculenta. In this
rare developmental abnormality metamorphosis is halted at the bud-stage in
the hind-limb, yet the animal increases in overall size, becoming a bloated and
gigantic tadpole 70 or 8o mm. long. Removal of the thyroid gland (p. 151)
produces a very similar condition in the laboratory (see also pp. 2, 3).

INTERRELATIONSHIP OF AMPHIBIAN GROUPS

The classification of the Amphibia is uncertain. There are few known links
between the various orders found in the late Palreozoic and early Mesozoic rocks
(p. 3) and the threewidelydivergent tribes-frogs, salamanders, andcrecilians-
that survive to-day (p. 388). The principal extinct groups are as follows:

SUB-CLASS APSIDOSPONDYLI
This great group is characterised by the possession of vertebrre which
exhibit considerable variation, but which are probably a direct inheritance from
crossopterygian ancestry. They have centra formed from cartilage blocks in
units of two which are ossified, in varying degree, as anterior intercentra and
posterior pleurocentra. We will see that such a basic pattern persists through-
out the vertebrate series. The apsidospondylous amphibians fall into two super-
orders, the Labyrinthodontia and the Salientia (frogs, p. 428).
422 ZOOLOGY

SUPER-ORDER LABYRINTHODONTIA
The name Labyrinthodontia is derived from the characteristic pattern of
the teeth (Fig. 280). In cross-section the teeth show a prominent radiating
infolding of the enamel surface. Laby-
rinthodonts, arising from a fish ancestry
in the Devonian, appear to exhibit at
least two distinct evolutionary trends.
From primitive aquatic forms there
developed the extremely abundant
Rhachitomi which were to become, in
the Permian, markedly terrestrial.
These in turn gave rise to the Stereo-
spondyli, which became degenerate in
many respects, and secondarily aquatic.
Accompanying this general trend were
progressive skeletal changes which were
FIG. 28o. - Labyrinthodontia: Tooth particularly impressive in skull and
structure. The labyrinthine infolding is
here shown in the stereospondy lid Mastodon- vertebrre (Fig. 28r). This trend did not
saurus. (From B.M. (N.H.) Catalogue.) survive the Triassic (p. 3).

w
Embolomerl /chthyostegalla Rhachltomi Stereospondyli

gJ,'P p.c.,.__
~ ~ ~
(fJ
ic.

FIG. 281.- Labyrinthodontia: Skulls and vertebrre. The top series shows vertebrre in lateral
view. i. c. intercentrum; sf>. spinous process. Each pleurocentrum (p. c.) is black. The
middle series shows skulls in palatal view, illustrating trends in vacuolation and variation in the
parasphenoid (cross-hatched). The bottom series illustrates the proportions of height to widtb in
each skull. The arrows indicate trends in labyrinthodont evolution according to Colbert. The
skulls, drawn to the same length, are not to scale. (After Colbert.)
 PHYLUM CHORDATA

Probably stemming from primitive labyrinthodonts in another direction

were the essentially aquatic Embolomeri. Some have suggested that these may
have shared their ancestry with the Carboniferous stem-reptiles (pp. 458, 486).

ORDER lCHTHYOSTEGALIA
These are the earliest known Amphibia (Fig. 282). One, embedded in the
Upper Devonian of Canada, was the remarkable Elpistostege, perhaps the
nearest approach to a true prototetrapod yet described (p. 382). This animal

FrG. 282.-Super-order Labyrinthodontia, Order Ichthyostegalia, Family Ichthyostegidse.

Ichthyostega. Reconstruction. These late Devonian and Carboniferous amphibians existed
in Greenland when the climate was rather different from now. They p erhaps retained traces of
their osteichthian ancestry in the possession of a tail supported b y true fin-rays. The skulls
were often more than 6 inches long. (See also Figs. 281 and 283.) (From B.M. (N.H.) Catalogue.)

was perhaps very close to the rhipidistian crossopterygians in its cranial struc-
ture, which in fact seemed to be intermediate between the crossopterygians and
Ichthyostega to be dealt with below. It is unfortunate that few elements have
been discovered. It is not known whether Elpistostege had fins or legs. Only
a part of its dermal cranial roof is known from a single specimen.
Ichthyostega and I chthystegopsis, which are found in Greenland fresh-water
deposits of either Upper Devonian or Lower Carboniferous age, are now much
better known. The skull of I chthyostega was about 8 inches long and possessed
three interesting features (Fig. 283). A pre-opercular and possibly a sub-oper-
cular persisted on the back of the squamosal and quadratojugalbones as a relic
 ZOOLOGY
of the fish opercular series, although no actual operculum has been shown. The
septomaxilla formed part of the dermal covering of the skull. The nose con-
sisted of a pit on the under side of the skull, which was bridged by a process of
the maxilla and divided into anterior and posterior parts. This last condition
is reminiscent of that found in the Dipnoi and some of the early crossopterygians,

FIG. 283.-Icllthyostega: Cranium. bs. basisphenoid; do. supraoccipital; ec. ectopterygoid

en. external nares; f. frontal; in. internal nares; j . jugal; /. lachrymal ; m . maxilla; n . nasal
p. parietal; pf. postfrontal; pl. palatine; pm. premaxilla; po. postorbital ; pop. preopercular
prj. prefrontal; ps. parasphenoid; pt. pterygoid; pv. prevomer ; q. quadrate; qj . quadrate-jugal
r. rostral ; sm. septomaxillary ; sq. squamosal; st. suprat emporal; t. t a bular. (After Romer.)

while in all tetrapods the anterior opening of the nasal pit has migrated to the
upper surface of the skull and become completely closed off from the posterior
opening, so as to form true internal and external nares (Fig. 89, p. 138).

ORDER RHACHITOMI

These (e.g. Eryops) were among the most plentiful and perhaps typical
labyrinthodonts. They flourished during the Permian and Triassic, but may
have arisen much earlier. Their rhachitomous vertebrre had a semi-lunar,
wedge-like intercentrum and one or two (posterior) pleurocentra in addition to
a vertical neural arch (Fig. 28r). Eryops, about 5 feet long, had short powerful
legs and five toes. The large depressed skull had large interpterygoid cavities.
 PHYLUM CHORDATA

An otic notch occurred. Cranial and lower lateral elements were united. In
general, we have the impression of a superficially crocodilian animal which
spent much time out of the water.

ORDER STEREOSPONDYLI

These (e.g. Capitosaurus, Cyclotosaurus, Paracyclotosattrus (Fig. 284), arose

from rhachitome stock. Transitional types lived in the early Triassic.

FIG. 284.- Super-order Labyrinthodontia, Order Stereospondyli, Family Capitosauridre. Para-

cyclotosaurus. This Upper Triassic form (P. davidi) was probably the largest labyrinthodont
known. It reached a length of about I I feet and its skull, heavily a rmoured with sharp teeth, was
some 3 feet in length. This p erhaps represents almost the end-phase of a labyrinthodont trend
that led to the abandonment of land for a secondarily aquatic life. The limbs were degenerat e,
and the great weight rested principally on the plate-like clavicles and interclavicle. T h e upward-
looking eyes reveal its bottom-hunting existence. The fore-feet of many stereospondyls (unlike
Cyclotosaurus) p ossessed only four t oes. P aracyclotosaurus was found in the Wianamatta fish-bed of
St. Peter's, Sydney, alongside fishes closely related to Palmoniscus (p. 286) and Semionotus (p. 292) .

These still labyrinthodont animals were undergoing a general degeneration

and were returning to the water, and there is evidence that, at the same
time, some had specialised to a degree that allowed the colonisation of the sea
(e.g. the probably piscivorous, long-snouted Trematosaurus). If this is indeed
so, there is posed the fascinating problem of their larval development (p. 385).
The Stereospondyli had vertebrce in which the pleurocentra had vanished or
VOL. II. DD
 ZOOLOGY
almost so, the intercentra joining almost the whole strU<.~ture. Skull ossifica-
tion was much reduced except in the exoccipitals,. which formed a double
condyle. In the brain-floor region the basi-occipitals and basisphenoid had
become unimportant compared with the hyper-development of the para-
sphenoid, which had apparently firm unions with the pterygoids. Large
interpterygoid vacuities occurred.
These animals were large and heavy-headed. The skull of M astodonsaurus
was more than three feet long, though some forms (e.g. Brachyops) had short
(though heavy) heads. The flattened skulls, dorsally placed eyes, and massive
supporting pectoral plate (formed from clavicles and interclavicles) suggest a
shallow-water, bottom-dwelling existence. It is certain that the weak limbs
would have been inadequate efficiently to carry such a heavy, short body
unsupported by water.
These curious creatures disappeared towards the end of the Triassic,
possibly as a result of competition from the many aquatic reptiles which had
then arisen.
ORDER EMBOLOMERI
Appearing first in the Carboniferous, the Embolomeri became abundant
during the Permian, but did not survive beyond the Trias. Some of the early
forms are very important, as they show features which are fish-like, yet at the
same time they provided possible ancestors of the reptiles. Superficially some
of the Carboniferous labyrinthodonts were rather like modem crocodiles, and
some of them reached a length of more than 3 metres. Palceogyrinus (Fig. 285 B)
had a long, cylindrical body with a powerful tail and poorly developed limbs.
It has been claimed that the shoulder-girdle was attached to the back of the
skull by a post-temporal bone, as in fishes. The build of the skull of these
Carboniferous forms was very similar in general structure to that of the osteo-
lepid fishes (Fig. 285 A), except that the specialised hinge (between the parietals
and post-parietals) was missing. An important feature is seen in the structure
of the teeth, which had the dentine infolded at the base into labyrinthine
grooves, as in osteolepids. The vertebrre were embolomerous, consisting of a
neural arch resting on two notochordal centra, an anterior intercentrum and a
posterior pleurocentrum. The shoulder-girdle recalls that of the osteolepid
fishes and the scapulocoracoid bone has increased in size. The pelvis was
peculiar in being the only one known among tetrapods not directly attached to
the vertebral column. It was held in place by ligaments.
The remarkable Eogyrinus, which lived during the Lower Carboniferous, and
which is found particularly in coal-m~asure deposits, had double centra in
which both (anterior) inter-centra and (posterior) pleuracentra formed complete
discs. One or both of these were united with the neural arch. Both skull,
roof, and palate were well-ossified and primitive in arrangement. The latter
had vacuities between the pterygoids and the parasphenoid. There was on
 PHYLUM CHORDATA 427
each side a mobile articulation between cranium and palate, a condition
resembling that of bony fishes. A well-developed stapes (arising from the
piscine hyomandibular) and an otic notch occurred. The body and tail were
long and superficially crocodilian. The limbs were short with, possibly,
fish-like girdles.

FIG. 285.-Fish-amphibian relationships: Cranial bones. A. Osteolepis (see p. 355). a

Devonian fish from the Red Sandstone of Scotland, and B. Pala;ogyrinus, a Carboniferous am.
phibian (Embolomeri) from the Middle Coal Measures of Europe. The skull architecture of both
animals shows considerable similarity. In the amphibian the remarkable hinge between parietals
and post-parietals is missing. Fr. frontal; I. Par. interparietal; ju. jugal; L. lachrymal; Na.
nasal; Par. parietal; P. par. post-parietal; P.O. post-orbital; Pr. pre-frontal; 5. orb. supra-
orbital; Sq. Squamosal; 5. T. Supra-temporal; T. tabular; T. T. inter-temporal (modified after
Watson).

Some of the Embolomeri were primitively aquatic, others apparently ter-

restrial and, in addition, others seem to have become re-adapted to lake-life.
At the same time 'the fundamental morphology of the skeleton is strikingly
uniform throughout the group' (Watson).

ORDER SEYMOURIAMORPHA

The Seymouriamorpha (e.g. Seymouria) (Fig. 286) is one of the most interest-
ing known groups because it represents a stem apparently developed astride
the transition line between the Amphibia and Reptilia. Seymouria exhibited,
as Watson has shown, the phenomenon of mosaic evolution, which we shall see
displayed with equal beauty in Archceopteryx, on the borderline between
reptiles and birds (Fig. 414, p. 599), and in the ictidosaurs (p. 517), which likewise
possess both typically reptilian and mammalian features.
Seymo1tria was about 2 feet long and was Lower Permian in age. It had a
relatively small, pointed head, dorsal nostrils, a comparatively thick body, and
 ZOOLOGY
short tail. All its limbs were pentadactyle, short, muscular, and thrust from
the mid-line in an apparently reptilian manner. Further evidence of its
affinity with the stem-reptiles (p. 458) is as follows: the atlas and axis were
formed from anterior cervicals, and the basiventrals were reduced to form
intercentra. There were a long interclavicle, and a ventral coracoid ossifica-
tion in the shoulder girdles. The pelvic girdles were anchored by means of
two sacral vertebrre. The ilium was dorsally expanded for the attachment of
strong walking muscles. Had only fragmentary material, with merely the
above characters apparent, been available for study there would be small
argument against the inclusion of Seymouria among the Reptilia. But most
of the remaining pieces of the mosaic, so to speak, could readily come from
an embolomerous labyrinthodont amphibian. The skull was strongly ossified;
and temporal fossre were absent. An intertemporal bone, absent in reptiles,
was retained. The palate, too, was primitive, and bore labyrinthodont-
pattern teeth in pairs on vomers and palatines, as well as in pairs on the jaw.
The tympanic membrane, bounded above by the supratemporal and below by
the squamosal, stretched across an otic notch, which extended forward below
the tabular and supratemporal so that the quadrate sloped forward. The
fenestra ovalis was below the basal level of the brain. There was practically
no neck; the shoulder-girdle lay beneath the jaw. The vertebrre showed
little differentiation and the cervical elements bore ribs. Intercentra were
present, and the ribs were large and double-headed, the knobs for attachment (to
the intercentra) being retained. Seymouria perhaps lacked a cleithrum (present
in most cotylosaurian reptiles); and some fossils show evidence of lateral line
canals in the head region. If such canals did in fact exist they make certain
its amphibian relationship.
It has been suggested that the Upper Permian Kotlassia exhibits erroneously
changes typical of Amphibia that have reverted to the aquatic life. Kotlassia,
incidentally, was erroneously reported to possess cosmine tissue (p. 83).
Despite their obvious labyrinthodont affinity, it would seem that the
Seymouriamorpha appeared too late to constitute the basis for the main
reptilian radiations. The earliest reptiles had already appeared in the Car-
boniferous. It would seem likely that both Seymouriamorpha and Cotylo-
sauria shared a common labyrinthodont ancestry.

SUPER-ORDER SALIENTA (ANURA, BATRACHIA)

Frogs very like those of to-day occurred at least as early as the Jurassic
(p. 3) and, like the Urodela (p. 432), are both degenerate and highly specialised
in their anatomy. There is some fossil evidence that the Anura are descended
from aberrant labyrinthodonts (p. 422). Protobatrachus (Order Proanura), an
indubitable frog ancestor from the Triassic of Madagascar, had a skull not
unlike that of extant forms. The trunk, however, was longer than that of
 PHYLUM CHORDATA

modern frogs, and although the ilia were elongated, the post-cranial skeleton,
including the caudal vertebrre, was not otherwise predominantly anuran.
Other, more ancient, frog-like amphibians (e.g. the Upper Carboniferous
Amphibamus and Miobatrachus) show cranial, vertebral, and other characters
that suggest a labyrinthodont affinity. For these the Order Eoanura ('dawn
frogs') has been erected. It should be remembered, nevertheless, that these
animals have not been unquestionably proved to be ancestral to modern
batrachians.

FIG. 286.-0rder Seymouriamorpha, Family Seymouridre, Seyrnouria: Mosaic evolution.

Seymouria (Lower Permian of North America) possessed both amphibian and reptilian characters.
but its sideline flourished after the stem-reptiles (p. 458) had arisen and S eymouria lived contem-
poraneously with them. It was about 2 feet long and is often placed with the reptiles. but it
probably p ossessed lateralis organs (Fig. 302, p. 449). If so it is by definition an amphibian.
(After Williston.)

All the modern frogs and toads are, in the adult, four-legged, squat-bodied,
and without tails. They fall into the following families: Liopelmidre (primitive
frogs, with amphiccelous vertebrre, restricted to the New Zealand genus Liopelma
and the North American A scaphus; a remarkable example of discontinuous
distribution. Ascaphus has a cloacal extension which apparently acts as an
intromittent organ during internal insemination) ; Discoglossidre (primitive
Eurasian and Philippine frogs with opisthoccelus vertebrre, free sacral vertebrre
with biconvex centre, and three pairs of ribs. Known from Upper Oligocene
and Miocene and related to next family); Pipidre (African and South American
anurans lacking tongue and eyelids- except for Pseudohymenochirus, possessing
lower eyelids. This group includes Xenopus); Pelobatidre (ribless anurans from
430 ZOOLOGY

Eurasia, East Indies, and Seychelles) ; Bufonidre (widely distributed true toads
with all vertebrre proccelus and a double condyle to the coccyx, and including
both toothed and toothless genera which are sometimes placed in separate
families; common from the Miocene onwards); Brachycephalidre (a poly-
phyletic group of Neotropical forms, allied to the Bufonidre; with proccelus
vertebrre and the pectoral girdles in partial or complete mid-line fusion) ;
Hylidre (widely distributed bufonids with an intercalary cartilage or bone
between the last two phalanges of each digit : tree-frogs, usually with claw-
shaped phalanges ; but some are terrestrial or fossorial ; a Miocene fossil is
known); Ranidre (true frogs of wide distribution except in South America
and Australia, in each of which occurs only Rana; Miocene); Rhacophoridre
(tropical Eurasian and African, Madagascan, East Indian ranid tree-frogs with
cylindrical diapophyses and intercalary cartilages between the last two
phalanges of each digit); Brevicipitidre (narrow-mouthed toads of ranid
affinity and cosmopolitan distribution which differ skeletally from ranids
chiefly in their more dilated sacral diapophyses; some advanced forms have
become toothless and have lost all the ventral elements of the pectoral girdle
except the coracoids).

SUB-CLASS LEPOSPONDYLI
The members of this side-line (which includes extant salamanders and
crecilians) are essentially characterised by relatively simple lepospondylous
vertebrre. It is held that the centrum of this type of vertebra was, unlike that
of the Apsidospondyli, not pre-formed in cartilage, but instead arose by
direct ossification around the embryonic notochord. Each centrum sometimes
retained an aperture housing part of a persistent notochord.
The Lepospondyli were small, late Palreozoic amphibians, ranging from a
few inches to a couple of feet in size. They flourished in the Carboniferous and
disappeared in the Permian (p. 3). They (e.g. Diplocaulus) had the neural
arches and the centra of the vertebrre co-ossified. Although the order includes
forms of a generalised build, they were for the most part bizarre animals, with
the posterior corners of the skull-roof enormously enlarged (Fig. 287). A few
were legless and superficially snake-like (Fig. 288).

ORDER AISTOPODA
These small amphibians had long, snake-like bodies with as many as one
hundred vertebrre and, often, curiously forked ribs (e.g. Ophiderpeton, Fig. 288).
They are known only from Carboniferous deposits.

ORDER NECTRIDIA
These bore symmetrically opposed neural and hremal spines on their caudal
vertebrre. At least two distinct groups occurred in the Upper Carboniferous.
 PHYLUM CHORDATA 431
One type was superficially snake-like and almost, or entirely, limbless (e.g.
Sauropleura), whilst the other pos-
sessed small limbs and a flattened
skull with grotesque protuberances.
Thus Diplocaulus (Fig. 287) was
about 2 feet long and had a huge
triangular head and small limbs.
Its eyes looked upwards. The skull
bore an extraordinary armament
formed by the outgrowth of the

s
tabulars. The cranium was largely
unossified, and other degenerative
changes were manifest. If we dis-
miss the all-important (from the
diagnostic viewpoint) vertebrre, (B)
(C)
these could be mistaken for stem
labyrinthodonts (p. 422). FrG. 287.-Lepospondylid radiation: Skull of
nectridian (with a vertebra characteristic of sub-
ORDER MICROSAURIA
class). A. 'Horned' skull of Lower Permian Diplo-
caulus in dorsal view (a little less than ! natural
(ADELOSPONDYLI) size). B. Growth stages from very young (r) to
large adult (5). C. Lepospondyl vertebra in right
Only a few microsaurians are lateral view (~ths natural size) (cf. Fig. 28r, p . 422).
known. In them the neural arches f. frontal; j. jugal; l. lachrymal; p. parietal ; pj.
prefrontal; pm. premaxillary ; p. o. postorbital;
were loosely attached to single pp. post-parietal; prj. prefrontal; sq. squamosal;
st. supratemporal. (After Colbert.)
centra, which were excavated on
each side. Lysorophus from the late Permian is known in some detail. It had
an incompletely roofed skull, and showed
much other evidence of cranial reduction,
including the loss of circumorbital bones,
which left the orbit open below. Its large
branchial arches were ossified. This
can lead only to the conclusion that gill-
breathing was continued during adult life.
These animals have been called Permian
urodeles, but they retain too many laby-
rinthodont characters to be closely re-
lated to the Caudata, the earliest true
FIG. 288.-Lepospondylid radiation: examples of which were Cretaceous (p. 3).
Skeleton of aistopod. Snake-like form of the
Carboniferous Ophiderpeton. Certain nectri-
dians (Fig. 287) became similarly elongated. ' ORDER PHYLLOSPONDYLI '
(From Romer, after Fritschi.)
The amphibians believed to consti-
tute the 'Order Phyllospondyli' may, m fact, have been small-gilled larval
labyrinthodonts of rhachitome affinity.
432 ZOOLOGY

ORDER URODELA (CAUDATA)

Although attempts have been made to derive the Urodela (newts and
salamanders) from the Dipnoi, it is generally held that, however widely divergent
are the three surviving lines, all amphibians come from the same stock (p. 282).
Salamanders and newts (Figs. 290, 291) are degenerate animals with a
determinable ancestry from the Cretaceous (e.g.
Hemitrypus). They show many similarities to the
early Lepospondyli, but no Jurassic or Triassic links
have been discovered. It is not without interest that
one of the most celebrated of all fossils is that of a
urodele. This is an Upper Miocene example of the
cryptobranchid Andrias (Fig. 289) unearthed by
the Swiss, Scheuchzer, in the early 18th century and
illustrated and described as Homo diluvii testis ('Man,
witness to the Deluge ' ) in a contemporary treatise.
Appended too was the following homily (tr.):
'Oh, sad remains of bone, frame of poor Man of sin,
Soften the heart and mind of r ecent sinful kin.'

It was left for Cuvier to show that the sinner was a

salamander.
+J. Modern urodeles (Figs. 290, 291) probably fall into
\ ~ eight families as follows: Hynobiidre (Asiatic land
salamanders); Cryptobranchidre (large semi-larval
Amphibia derived from the above which range through
Asia and North America- known from the Miocene
onwards); Amblystomidre (American salamanders
differing from the above in their fused angulars, long
premaxillary spines and internal fertilisation and from
FIG. 289.-A witness of
the next family in their short prevomers and amphi-
the Deluge (see adjacent crelous vertebrre); Salamandridre (Eurasian and Ameri-
text). (From B.M. (N.H .)
Catalogue.) can Amphibia, including newts and terrestrial species
-e.g. Salamandra-which have vomerine t eeth on
each side of the parasphenoid, and opisthocrelus vertebrre; Oligocene fossils are
known); Amphiumidre (Genus Amphiuma of North America, including the
' Conger-eel', a semi-larval creature of salamandrid derivation possessing
lungs, a bony pterygoid, posterior prevomeral processes, and premaxillary
spines separating the nasals); Plethodontidre (American and European lung-less
aquatic and t errestrial salamanders of salamandrid (see above) stock with a
permanently cartilaginous pterygoid, vomerine teeth forming dentigerous
patches over the parasphenoid, and a naso-labial groove); Proteidre (per-
manently larval salamanders, with lungs and a distinctive long, anteriorly
 PHYLUM CHORDATA 433

FIG. 290.-Urodela: Permanent larvm. I. Typhlomolge rathbuni, a blind plethodontid of

North American caves (Sub-order Salamandroidea); 2. Amphiumameans, the Conger of North
America (Salamandroidea) ; 3· Necturus macttlosus, North American Mud-puppy (Proteida), of
the same family as the unpigmented European Olm; 4· Siren lacertina (Meantes); 5· Crypto-
branchus alleganiensis (Cryptobranchoidea). (After Noble.)
Although all extant sub-orders except one (Ambystomoidea) are represented above, it should
be remembered it is only in the sub-orders Proteida and Meantes that all forms are permanently
larval. The genic and dependent physiological factors controlling this varying condition (which
represents further evidence of a retreat to the aquatic medium) have arisen independently several
times.
434 ZOOLOGY
directed pubo-ischium, including only the North American Necturus and the
blind, unpigmented, cave-dwelling Proteus of Europe); Sirenidre (American
amphibians, e.g. Siren and Pseudobranchus of dubious affinities which, as adult
forms, resemble the young of other families in possessing only forelimbs and an
apparently ' juvenile' skull. At the same time they possess reduced pterygoids,
a separately ossified coracoid, and a prominent vomeronasal organ).

F IG. 29r.-Sub-orde r Salamandroidea, Family Salamandridre,

So lomandra sal aTnalldra. E uropean Salamander. (After
Cuvier.)

F 1G. 292.-0rde r Apoda, Family Crecilidre:

Cephalic and genital structures. Left: H ead o f
C<Bcilia lmtacula, showing site of protrusible ten .
tacle between nostril and indist inct eye. Right:
Eversible introm itte nt organ, armed with copu-
latory grapples (see also pp. 550, 8go) . (After
oble.)

ORDER APODA (GYMNOPHIONA , CECILIA)

This little-known group consists of aberrant blind, worm-like, limbless,

tropical Amphibia with a very short tail and an intromittent organ in the male
(Fig. 292). The body is usually grooved transversely, and in the grooves are
often series of minute scales, a legacy from Carboniferous ancestors. The eyes
are lidless and sometimes covered by cranial bones. A protrusible sensory
tentacle occurs between nostril and orbit. The vent is almost terminal.
Cranial elements and scales seem to be lost with progressive specialisation. The
aquatic genus Typhlonectes has a flattened tail. Development of eggs may be
internal (e.g. Typhlonectes) or external (from large-yolked eggs laid in burrows
 PHYLUM CHORDATA 435

as in Rhinatrema). All crecilians belong to a single family-the Crecilidre-

which contains nineteen genera and about fifty-five species. Fossils are
unknown, and the living animal is rarely seen.
Crecilians are to-day restricted to damp, tropical areas, excluding Mada-
gascar and Australasia. They are often extremely rare in regions that at
first sight appear to provide for them eminently suitable habitats. Neverthe-
less they are unusually common in the Seychelles.
It has been suggested that the Apoda may be descended from the Carboni-
ferous microsaurian Lepospondyli. These, as their name suggests, were once
thought to be of reptilian affinity, but their amphibian relationship now seems
well-established (p. 431).

GENERAL ORGANISATION OF THE AMPHIBIA

External Characters.-An example of Urodela with persistent gills and
lateral line is afforded by the North American Water-newt, or Mud-puppy,
Necturus maculosus (Fig. 290). The animal is sometimes more than a foot
long. The elongated trunk is separated by a slight constriction from the
depressed head, and passes imperceptibly into the compressed tail, which is
bordered by a continuous median fin unsupported by fin-rays. The limbs are
small and weak in proportion to the body, and in the ordinary swimming
attitude are directed backwards, more or less parallel to the sagittal plane.
The upper arm and thigh take a direction backwards and slightly upwards.
The fore-arm, hand, and the shank and foot extend backwards and down-
wards. Each limb thus presents an external or dorsal and an internal or
ventral surface, an anterior or pre-axial border (which terminates in the first
digit) and a posterior or postaxial border (which terminates in the last digit).
The eyes are small and have no eyelids. There is no tympanic membrane.
The mouth is wide and bordered by thick lips. On each side of the neck are
two gill-slits leading into the pharynx, the first between the first and
second branchial arches, the other between the second and third. From the
dorsal end of each of the three branchial arches springs a branched external
gill. Very similar in its external characters is the blind, cave-dwelling, Proteus
of Europe, which is, however, colourless. Siren (Fig. 290) differs mainly in its
elongated eel-like body and in the absence of hind-limbs. All three genera are
perennibranchiate or persistent-gilled.
The remaining Urodela are often called caducibranchiate o:r deciduous-gilled,
and furnish a complete series of transitions from derotrematous forms which,
while losing the gills, retain the gill-clefts, to salamandrine forms in which all
trace of branchiate organisation disappears in the adult. In the American
Amphittma (Fig. 290) the body is eel-like and the limbs are extremely small.
There are no gills in the adult, but two pairs of gill-openings are retained
 ZOOLOGY
throughout life. In Cryptobranchus there is a single branchial aperture, some-
times present on the left side only; but, as in the previously mentioned genera,
four branchial arches are retained. In Megalobatrachus, the Giant Salamander
of Japan and China, all trace of gill-slits disappears, but two branchial arches
persist. This animal reaches a length of 5 feet. Lastly, in such species as the
Spotted Salamander (Salamandra maculosa, Fig. 291) of Europe, and the com-
mon British newts (Triturus), the adult has no trace either of gills or gill-slits,
and the branchial arches are much reduced. The limbs in terrestrial sala-
manders stand out from the trunk, and are plantigrade. All trace of a median
fin disappears, the tail becoming nearly cylindrical.
In the Anura the body is always characteristic in shape. The head is
large and depressed with a wide mouth and large tympanic membranes in most
genera. The snout in burrowing forms is often unusually sharp. It may be
equipped with dermal ossifications, or it may remain blunt and be subcutane-
ously invaded with bone. The trunk is short and the tail absent, and the
hind- much larger than the fore-limbs. In many species of Hyla an extra-
ordinary arboreality is made possible by plate-like adhesive discs at the
termination of the digits of all four legs. These discs are not suctorial in
operation, but possess a moist, fibrous, corrugated anti-skid external layer
which engages the leaf or stem. Capillary attraction is involved, as well as
adhesion by means of mucus. The mucus is expressed by the action of collagen
fibres which operate on the glands during the motions of climbing. A unique
intercalcary cartilage developed between the terminal and penultimate joints
facilitates adjustments of the adhesive disc in relation to the toe-hold. Some
tree-frogs (e.g. Chiromantis) have opposable digits in the manner of reptilian
chamreleons (p. 5I9} and many arboreal mammals.
In some toads (e.g. Bufo bufo), and most tree-frogs, the webs between the
hind-toes are reduced or absent. In Rhacophorus, a genus of East African
tree-frog, however, a web-surface greater than that of the ventral expanse of
the body has been developed. Ventral surface and webbing of the elongated
digits together produce a volplane mechanism functionally comparable with
that of the reptile Draco (p. 518) and certain marsupials (p. 717) and rodents.
Thus R. malabaricus can glide more than 30 feet from tree to ground. Hyla
venulosa (of the Lower Amazon) can glide safely from a height of no less than
140 feet, even though its digits are unwebbed and not considerably enlarged
(Cott).
In the Apoda (Fig. 292), the body is elongated and snake-like, the head is
small and not depressed, and limbs are absent. There is hardly any tail.
The vent (an.) occurs at the posterior end of the body on the ventral surface.
These tropical animals mostly live in burrows. They are blind, or almost
blind, but possess sensory tentacles lodged in pits.
The skin of Amphibia is soft and usually slimy, owing to the secretion of
 PHYLUM CHORDATA 437

the cutaneous glands, which is sometimes poisonous. Thus when the European
Natterjack (toad) (Bufo calamita) is frightened it exudes a protective white
secretion of pungent odour. Many toads (and salamanders) have small
poison-glands aggregated into a prominent swelling, the parotoid glands, on
each side of the head. The Natterjack has an analogous gland on each hind-
leg. The large and conspicuous dorsal warts of toads are each perforated by
a pore that leads to a poison gland beneath. On gentle handling by Man the
toad does not bring the glands into operation, but when seized by a predator
it ejects poison from all over its body. The exudate contains two active
toxins, bufotalin and bufogin, which, when swallowed, cause nausea, respira-
tory and muscular. impairment, as well as a digitalis-like action on the heart.
Large experimental doses of toad-venom cause the mammalian heart to slow
and finally stop in systole. If attacked, the large Crested or Warty Newt
(Triturus cristatus) of Europe discharges a secretion from its dorsal glands
sufficiently venomous to cause a cat to foam at the mouth.
The colour of the skin is often brilliant : the Spotted Salamander is
yellow and black, and many frogs are green and gold, scarlet and black, and
so on. The green colour of tree-frogs is protective, serving to conceal them
among the foliage of the plants on which they live. The beautiful and strongly
contrasted hues of the Spotted Salamander and of some frogs may be instances
of warning colours ; the animals are inedible owing to the acrid secretion of
their cutaneous glands, and their conspicuous colours serve to warn off the
birds and other animals which would otherwise devour them. Some tree-
frogs are said to show no sign of fear of frog-eating birds, while the edible and
more plainly coloured species are in constant danger.
The secretion of the conspicuous black and blue Dendrobates tinctorius of
Central America is sufficiently venomous to be used by the ' Indians ' as an
arrow poison. Phyllomedusa has a peculiar adaptation involving brilliant
' flashes ' that are visible only when the animal leaps. The sudden flash and
sudden disappearance of colour may have confusion value. In many tree-
frogs the brightness of the coloration varies with changes in light intensity in
the forest. Quite apart from camouflage or warning, some colours are no doubt
of sexual significance, but few experimental data are available on this, or any
other point concerning the actual use, as apart from the physiological mechan-
isms, of colour.
In many toads the skin is dry, and covered with warts which assist in
harmonising the animals with their surroundings. The development of a dry
skin by anurans was a considerable step towards homreothermy, but as long
as the anamniote egg was retained they had no chance of initiating a develop-
ment similar to that which led to the emergence of the Reptilia (seep. 387).
Exoskeleton.-An exoskeleton is present in many burrowing Apoda in the
form of small dermal scales. In some Anura bony exoskeletal plates occur
 ZOOLOGY
beneath the skin of the back. In an Asiatic urodele, Onychodactylus, and in the
African toad, Xenopus, small, horny claws are present on the digits. The South
American frog Leptodactylus pentadactylus has horny chest grapples. With
these and a few other exceptions the skin is devoid of hard parts. In the
Australian Limnodynastes the endoskeletal long bone of the thumb perforates
the surface and is an exoskeletal grappling aid during amplexus.
Endoskeleton.-The vertebral column is usually divisible into a cervical region,
containing a single vertebra devoid of transverse processes ; an abdominal or
thoracolumbar region, containing a variable
number of vertebrre with transverse processes
and often with ribs ; a sacral region, con-
taining usually a single vertebra, the large
transverse processes-or the ribs-of which
give attachment to the ilia ; and a caudal
region, forming the skeleton of the tail. In
the Apoda the caudal region is very short,
and there is no sacrum. In the Anura the
caudal region is represented by a single rod-
shaped bone, the urostyle. The total number
of vertebrre may reach 250 in Urodela and
Apoda. In Anura there are commonly only
nine vertebrre and a urostyle although ten
occur in the primitive family Ascophidre and
nine in some advanced genera.
FIG. 293.- Urodela: Chondrocran-
In the 'lower' Urodela (e.g. Amblystoma)
ium. Proteus anguinus in dorsal view. the centra are biconcave, as in fishes. They
ant antorbital process ; Ex.OC. exoc-
cipital and epiotic; hy. md. hyomandi- are shaped like a dice-box and are lined at
bular; i. n. internasal plate ; nch. noto-
chord; ot. pr. otic process; ped. either end by cartilage which is continuous
pedicle; PR.OT. pro-otic; QU. quad- between adjacent vertebrre. The bony shell
rate; SP.ETH. sphenethmoid. (After
W. K. Parker.) is developed before the cartilage appears, so
that the vertebrre are, in fact, investing bones.
The neural arches, on the other hand, are far better developed than in any fish,
and have well-formed zygapophyses. These articulate by synovial joints.
The Apoda also have biconcave vertebrre, but in the' higher ' Urodela (e.g.
Salamandra (Fig. 291)) and the Anura absorption of cartilage takes place be-
tween adjacent centra in such a way that the convex end of one fits into the
concave end of the next, forming a cup-and-ball joint. In the ' higher '
Urodela the convexity is on the anterior, the concavity on the posterior face of
each centrum, and the vertebrre are said to be opisthoccelous. In the Anura
they are usually, as in Rana, procrelous.
The first or cervical vertebra bears paired articular surfaces for the condyles
of the skull, and between them the anterior face of the centrum gives off, in
 PHYLUM CHORDATA 439
Urodela, a projection called the odontoid process. The Urodela, moreover,
have ribs articulating with the transverse processes of the abdominal and
sacral vertebrre. These are short bones, forked proximally ; and their com-
pressed transverse processes are correspondingly divided. The sacral ribs of
urodeles give attachment to the ilia, and the caudal vertebrre bear hremal arches.
In at least two genera the sharp free end of ribs penetrates the integument,
apparently as a protective device (Pleurodeles, Tylototriton) (see alsop. 739).
The skull of Urodela differs from that of the frog in many important

FIG. 294.-Urodela: Skull. Salamandra atra. Lejt,of.rom above; Right, from below. In both
the investing bones are removed on the right side of the figure . Af, antorbital process; As,
alisphenoid region; Bp, basal. plate ; Can, nasal cavity; Ch, posterior nares; Ci, process of
internasal plate; Cocc. occipital condyles; F. frontal; Fl. olfactory foramen; Fov. fenestra
ovalis; IN. internasal plate; Lgt. ligament connecting stapes with suspensorium; M. maxilla;
N . nasal; Na. nasal aperture; NK . olfactory capsule; OB . auditory capsule; os. sphenethmoid
(orbitosphenoid) ; Osp. supraoccipital region; P. parietal; Pa. ascending process of suspen-
sorium; ped. pedicle; Pf. prefrontal; Pmx. premaxilla; Pot. otic process of suspensorium;
Pp. palatine process of maxilla; Ps. parasphenoid; Pt. pterygoid bones; Pte. pterygoid cartilage;
Rt. foramen for ophthalmic branch of trigeminal; Qtt. quadrate; Sqtt. squamosal; St. stapes;
Vo. vomer; Vop. vomero-palatine ; Z . process of internasal plate; I I. optic foramen; V. trigeminal
foramen; VII. facial foramen. (After Wiedersheim.)

respects. Most strikingly, the trabeculre do not meet either below the brain
to form a basis cranii, or above it to form a cranial roof. Thus, when investing
bones are removed, the cranium (Fig. 293) is completed above and below in
the parachordal or occipital region only. Anterior to this it has side walls,
but no roof or floor. There is, above, a huge superior cranial fontanelle, and
below an equally large basicranial fontanelle. The former is covered, in the
complete skull, by the parietals and frontals, the latter by the parasphenoid.
In the perennibranchiate forms N ectttrtts and Proteus the trabeculre remain,
even in the adult, as narrow cartilaginous bars. The chondrocranium is
actually of a lower or more embryonic type than that of any other Craniata,
with the possible exception of Petromyzontia and Myxinoidea (p. 175).
440 ZOOLOGY
In the Urodela, moreover, the parietals (Fig. 294, P.) and frontals (F.)
are separate. The parasphenoid (Ps.) is not T-shaped. The palatine and
vomer are sometimes represented by a single bone, the vomeropalatine (Vop.),
bearing teeth. The suspensorium is inclined forwards, as in the tadpole, not
backwards, as in the adult frog. The hyoid arch is large, and its dorsal end
may be separated as a hyorrw.ndibular. There are three or four branchial
arches. These are large in the perennibranchiate forms, but undergo more
or less reduction in caducibranch species, although they never form such a
simple structure as that seen in the frog. The stapes has no extra-columella
attached to it, and, in correspondence with this, there is no tympanic cavity
or membrane.

FIG. 295.-Apoda: Skull. I ch-

tltyophis glutinosa. X 3· A,
Lateral; B, Ventral; C, Dorsal
view. A. posterior process of the
A os articulare; Ca. carotid foramen;
Pm
Ch. choana or posterior nasal
opening; F. frontal; ] . jugal ;
LO. exoccipital; Mx. maxilla ; N .
nasal; No. nostril; 0. orbit; P.
parietal; Pa. palatine; Pm. pre-
maxilla; Pof. postfrontal; Prj.
prefrontal; Pt. pterygoid; Q.
quadrate; 5. squamosal; St.
stapes; T. tentacular groove ; Vo.
vomer; X. exit of vagus nerve.
(After Sarasin.)

In the Anura there is a very wide range of variation in the skull. Among
the most important points are the presence, in a few species, of small supra-
and basi-occipitals. In others the roofing investing bones are curiously
sculptured and so strongly developed as to give the skull a singularly robust
appearance.
In the Apoda (Fig. 295) very little of the original cartilage remains in the
adult state, but the investing bones are very large and form an extremely
complete and substantial structure, especially remarkable for the way in
which the small orbit (0.) is completely surrounded by bones.
The shoulder-girdle of Urodela (Fig. 296) is chiefly remarkable for the great
size of the unossified coracoids (A, Co., B, C) which overlap one another on
the ventral body-wall. The procoracoid (Cl.) is also large, and there is no
clavicle. The sternum (St.) is usually a more or less rhomboid plate of cartilage
between the posterior ends of the coracoids, and there is no omosternum.
 PHYLUM CHORDATA 441
In N ecturus, however, the sternum presents a very interesting structure : it
is a narrow, irregular, median bar, sending off branches right and left into the
myocommas. This condition suggests an origin by the fusion of abdominal
ribs, or supporting structures
A
developed between the ventral
portions of the myomeres, just as
the true ribs are formed between
their dorsal 'portions. In the
Anura the epicoracoids either
simply meet one another in the
middle ventral line, as in Rana,
or overlap, as in the Fire-toad
(Bombinator) and the tree-frogs
(Hyla). The overlapping of the
coracoids, in Anura as in Urodela,
is sometimes correlated with the
absence of an omosternum. In B
the Labyrinthodontia there was
a median ventral investing bone,
the inter-clavicle, connected on
each side with the clavicle and
extended backwards to the ster-
num. There was also, on each
side, a cleithrum, connected with
the adjacent clavicle. This is re-
tained in the extant Ascaphidre.
In the pelvic girdle of the
Urodela the combined pubic and
ischiatdic regions (Fig. 297, P.,
S't
Is.) of the right and left sides FIG . zg6.-Urodela: Shoulder-girdle and sternum,
are united to form an elongated A , right side of girdle of Salamandra ; B, shoulder-
and sternum of Amblystoma (Axolotl) from the
cartilaginous plate which gives girdlev entral aspect. a, b, processes of scapula; C . (in
off on each side, above the ace- B), coracoid; Cl. procoracoid; Co. (in A), coracoid ;
G. (in A), glenoid cavity; L. its cartilaginous edge ;
tabulum (G.), a slender vertical Pj. (in B), glenoid cavity ; S. scapula; SS. supra-
scapula ; st. sternum; •, t- nerve foramina. (After
rod, the ilium (Il.). Ossifications Wiedersheim.)
are formed in the iliac and
ischiatdic regions, but the pubic region remains cartilaginous. The resemblance
of the pelvis of the lower Urodela, and especially of Necturus, to that of Polyp-
terus (p. 290) and of the Dipnoi (p. 361) is noteworthy.
Attached to the anterior border of the pubic region there occurs in many
Urodela and in X-enopus a rod of cartilage, forked in front , the epipubis (Ep.).
It is developed independently of the pelvis and its relations to that structure
VOL. II, E E
442 ZOOLOGY

are very similar to those of the sternum to the shoulder-girdle ; it has, in fact,
been proposed to call it a pelvi-sternum.
The limbs of Urodela differ from the typical structure already described
only in detail. There are usually four digits in the fore-limb and five in the
hind-limb. In Anura the limbs are modified by the fusion of the radius and
ulna and of the tibia and fibula, and by the
u
great elongation of the two proximal tarsals.
A prehallux is frequently present.
Musculature. In the 'lower' Urodela the
muscles of the trunk and tail occur in the
form of typical myomeres like those of fishes.
Again, a newt (Triturus) swims essentially by
the serial contraction of its myotomes, as
do fishes. When it wriggles quickly on dry
land it employs much the same swimming
mechanism. When it moves quietly along it
raises itself on its weak legs, which operate
FIG. 297.-Urodela: Pelvic girdle. like those of Anura. In the ' higher '
Salamandra. a. b. processes of epi·
pubis; Ep. epipubis; Fo. obturator urodeles the myomeres become converted
foramen; G. acetabulum; Il. ilium; into longitudinal dorsal bands (the extensors
Is . ischium; P. pubis; Sy. pubo-
ischiatic symphysis; t, process of pubis of the back), paired ventral bands (the recti
present in some urodeles. (After
Wiedersheim.) abdominis), and a double layer of oblique
muscles, covering the flanks.
Alimentary Canal and Associated Structures. A few Amphibia (Ceratophrys,
possibly Amphignathodon) bite, but in most the teeth are used to hold struggling
prey before swallowing. The teeth are always small and ankylosed to the
bones: they may be singly or doubly pointed. They occur most commonly
on,the premaxillre, maxillre, and vomers, but may also be developed on the
dentaries, palatines, and, in one instance, on the parasphenoid. In many
Anura, such as the Common Toad, teeth are altogether absent. A deciduous
fee tal dentition occurs in ovoviviparous, but probably not in oviparous crecilians.
Although the teeth are somewhat rasp-like, they are apparently without func-
tion and are replaced by adult teeth at about the time of parturition (Parker).
The tongue in many urodeles is fixed and immovable, like that of a fish. In
most Anura it is free behind, as in the frog, but in Xenopus and Pipa it is
absent.
In some anurans the tongue is capable of swift and prolonged expansion for
the capture of prey. Its adhesive power in many frogs and some urodeles is
enhanced by secretions from lingual glands (in the tongue itself) and from the
intermaxillary (internasal) gland lying between the premaxillre and the nasal
capsule. This opens into the buccal cavity. The latter gland is less prominent
in urodeles and is absent in crecilians. Anurans are additionally equipped with
 PHYLUM CHORDATA 443
a pharyngeal gland that discharges into the internal nares. The buccal cavity
and cesophagus of the Anura possess mucus-producing goblet cells. The
cesophagus may be ciliated. Stomach-glands of anurans secrete both pepsin
and hydrochloric acid. The stomach is enormously distensible, and together
with the duodenum forms a U-shaped loop in which lies the pancreas. The
small intestine is in all Amphibia a tube of almost uniform width. It is almost
straight in Apoda, and is extremely long only in certain tadpoles (p. 420). A
spiral valve does not occur: an increased absorption surface is obtained by
great increase in length. Some tadpoles with a specialised protein diet lack
the ' watch-spring ' intestine of common herbivorous forms. There is some

Frc. 298.-Anura: Vocal apparatus. Left: Rana

lemporaria (Brown or 'Common' Frog) croaking with dis·
tended internal vocal sacs. R ight: Rana ridibunda (Marsh
Frog) croaking with distended external sacs. (Redrawn
after Malcolm Smith.) Inset: Buccal cavity of S caphiopus
showing entrance to left vocal sac (e. v. s.). The entrance
to the Eustachian tube is marked E. t. (After Noble.)

evidence that the liver is of unusual importance in fat storage (in addition to
the fat bodies). Certainly there is much evidence that amphibians can live
for long periods without food. Axolotls, for example, have remained alive
for 650 days under conditions of starvation, losing about So per cent. of the
former body-weight.
Respiratory Organs and Voice.- Those of amphibians were the first true
voices evolved, for, unlike fishes, frogs possess a larynx and trachere supported
by a cartilaginous skeleton. In frogs the laryngeal chamber is divided into
two parts by the vocal organs, which are lip-like continuations of the epithelium
of the arytenoids. They call with the mouth closed. In many species air
is forced from lungs across the larynx to inflate grotesquely the vocal sacs
which, developed as buccal diverticula, serve as resonators (Fig. 298). Among
the most characteristic sounds of tropical continental rain-forests are the calls
of tree-frogs-the clear, bell-like note of Hyla gratiosa (the 'Smith'), which
444 ZOOLOGY
sounds like a blow on an anvil, the bird-like whistle of H. avivoca, and the
cicada-like stridulation of H. Jemoralis: these are only a few frog sounds from
the New World alone.
With very few exceptions Amphibia possess external gills in the larval state,
but of course such delicate structures are not retained in adults of species which
spend much time on dry land. In the perennibranchiate Urodela external
gill organs are retained throughout life. They are branched, are abundantly
supplied with blood, and spring from the dorsal ends of the first three branchial
arches. The epithelium covering them is ectodermal, so that they are
cutaneous and not pharyngeal gills. Therefore they may be of a different
nature from the so-called external gills of the embryos of Elasmobranchii
and Holocephali, which are the filaments of internal gills prolonged through
the branchial apertures.
Internal gills are developed only in the larvre of Anura. They appear as
papillre on the outer borders of the branchial arches below the external gills.
They closely resemble the internal gills of fishes and appear to be homologofls
with them, although it seems probable that their epithelium is ectodermal.
In most adult Amphibia lttngs are formed as outgrowths of the ventral wall
of the pharynx. The right and left lungs communicate with a common
laryngo-tracheal chamber, supported by the cartilages of the larynx and opening
into the mouth by a longitudinal slit, the glottis. In the more elongated forms,
such as Siren, Amphiuma, and the Apoda, the laryngo-tracheal chamber is
prolonged into a distinct trachea or wind-pipe, supported by cartilages. In the
Apoda, a tracheal lung may occur as in some snakes (p. 540); and the left lung
is usually rudimentary. In some salamanders (e.g. all plethodontids) lungs
are absent, and respiration is exclusively cutaneous and pharyngeal.
The salamander lung, when present, acts also as a hydrostatic organ.
Species living in well-rerated mountain streams, where they must often lurk
under rocks to avoid fast currents, have lungs that are reduced by comparison
with those of pond-dwelling forms. A co~parable reduction in the swim-
bladder (p. 339) is seen in mountain teleost fishes.
The Amphibia display interesting supplementary adaptations to cutaneous
respiration (p. 403). In Cryptobranchus there occur highly vascular folds in
which capillaries penetrate almost to the epidermal surface ; these folds are
mobile in a manner that suggests the gill motion of otherurodeles (e.g. Necturus).
The so-called African Hairy Frog, Astylosternus, has extensive tracts of vascular
papillre investing the flanks and hind legs of the male during the breeding
season. These probably compensate for its reduced lungs and the greater
need for oxygen during periods of reproduction. The aquatic Typhlonectes
(Apoda) has a richly va,scular skin through which it respires.
Blood-vascular system.-The amphibian heart always consists of a sinus
venosus, right and left auricles, a single ventricle, and a conus arteriosus. The
 PHYLUM CHORDATA 445
sinus venosus opens into the right auricle. The pulmonary veins enter the
left (p. 405). In Rana the interauricular septum is complete; in Salamandra
it is perforated, and in some lungless urodeles it is fenestrated in a curious
arrangement of muscular strands with intervening spaces.
The conus arteriosus of the anuran heart always possesses a well-developed
longitudinal spiral valve, which incompletely divides the conus into two
passages. One of these leads to the carotid and systemic arches ; the other
gives access to pulmo-cutaneous arches. In many urodeles the spiral valve is
either greatly reduced or absent; as in Apoda. In all amphibians two sets of
valves (which prevent back-flow of blood) are present. One such set is situated

inl.ca.r B

FIG. zgg.-Urodela: Circulation and respiration. Sala:nandra. A, larva; B, adult. aj. br.
a. r-4, afferent branchial arteries; b. ao. bulbus aortre; car. gl. carotid labyrinth; c. art. conus
arteriosus; d. ao. dorsal aorta; d. bot. ductus Botalli; ex. br. 1-3, external gills; ext. car. external
carotid; int. car. internal carotid; l. au. left auricle; lng. lung; pl. plexus, giving rise to carotid
labyrinth; put. a. pulmonary artery; r. au. right auricle; v. ventricle. (Altered from Boas.)

at the junction of the ventricle and conus ; the other set lies between the conus
arteriosus and the bulbns aorta.
There is never any trace of an inter-ventricular partition such as has arisen
in the Reptilia (p. 474). In the Dipnoi, too, we saw an almost completely
divided ventricle (p. 370). Although there can be no suggestion that the
tetrapods descended from lung-fishes, it is perhaps not impossible that the
ventricles of ancestral tetrapods, too, were divided, and that there has been a
secondary loss in the highly specialised, yet degenerate, Amphibia of to-day.
In the perennibranchiate Urodela, and in the larvre of air-breathing forms,
the circulation is essentially like that of a fish. The bulbus aortre (Fig. 299.
A, b. ao.), which represents an abbreviated ventral aorta, gives off four afferent
branchial arteries (af. br. a. I-4), three to the external gills, and a fourth
which curves round the gullet and joins the dorsal aorta directly. From
each gill an efferent branchial artery brings back the oxygenated blood, and the
 ZOOLOGY

efferent arteries unite, in a somewhat irregular way, to form the dorsal aorta
(d. ao.). Each afferent with the corresponding efferent artery constitutes an
aortic arch. Short connecting branches unite the afferent and efferent arteries

FIG. 3oo.-Urodela: Venous

system. Salamandra (from ven.
,· tral aspect). A bd. V. abdominal
Carcl.~osf. vein; Card. post. (Az.), azygos
(lh!Jyos.) vein; Gaud. V. caudal vein; Cut.
m. left musculo.cutaneous vein;
C11t. mt. the same on the right
side (partly removed); D . in-
testine; Duct. Cuv. precaval
vein; H. heart; Jug. ext. exter-
V. Carai1if· ·· · nal jugular; jug. int. internal
71M·s rwtetl jugular; Lg. V. mesenteric vein;
L. Pf. h epatic portal system;
L. V. hepatic vein; N, kidney;
Nier. Pft. Kr. renal portal
system; Sin. ven. sinus venosus;
Subcl. subclavian vein; V. adv.
branches of renal portal vein;
V. Cava inf. postcaval; V. iliaca,
iliac vein; V. port. hepatic portal
vein; V. rev. renal veins; *,cloacal
veins; t. branch of iliac to renal
portal vein; t t, latera l vein.
(After W iedersheim.)

of each gill. Carotids (ext. car., int. car.) arise from the first efferent artery,
and, when the lungs appear, a pulmonary artery (pul. a.) is given off from the
dorsal portion of the fourth aortic arch of each side. When the gills atrophy
(B) (in those Urodela which lack gills in the adult), the first aortic arch loses
its connection with the dorsal aorta and becomes the carotid trunk. The
 PHYLUM CHORDATA 447
second increases in size, forming the main factor of the dorsal aorta, and
becomes the systemic trunk. The third undergoes varied reduction, and the
fourth becomes the pulmonary artery, its dorsal portion retaining its connec-
tion with the systemic trunk in the form of a small connecting branch, the
ductus Botalli (d. bot.). In the Anura, as we have seen, the third arch vanishes
completely, and there is no ductus Botalli.
In the venous system, the Urodela exhibit very clearly the transition from
the fish-type to the condition described in the frog (p. 272). The blood from
the tail is returned by a caudal vein (Fig. 300, Caud. V.), which, on reaching the
ccelom, divides into two renal portal veins, one going to each kidney. From the
kidney the blood is taken, in the larva, into paired cardinal veins, each of
which joins with the corresponding jugular to form a ductus Cuvieri. In the
adult the anterior portions of the cardinals undergo partial atrophy, becoming
reduced to two small azygos veins (Card. post.) which receive the blood from
the region of the back. Their posterior portions unite and are continued
forwards by a new unpaired vein, the postcaval (V. Cava. inf.). This, joined
by the hepatic veins, pours its blood into the sinus venosus. The iliac vein
from the hind-leg divides into two branches. One joins the renal portal.
The other, representing the lateral vein of elasmobranchs, unites with its fellow
in the middle ventral line to form the anterior abdominal vein (Abd. V.) and
joins the hepatic portal (V. port.). Its blood, after traversing the capillaries
of the liver, is returned by the hepatic vein into the postcaval.
The erythrocytes are oval and nucleated, and some are remarkable for
their unusually large size. Those of the urodele A mphiuma are the largest
erythrocytes known, being almost Sop,. in diameter. Hyla arborea has 700,000
red cells per cubic millimetre, whilst aquatic urodeles have a relatively low count
(e.g. 36,ooo in Proteus). Red cells seem to arise from a cell indistinguishable
from lymphocytes in the kidney of some tadpoles and in the spleen of the
adults of certain frogs. In other species they arise in the bone-marrow, as in
the Mammalia. Thrombocytes of blood-clotting function occur.
Nervous System and Organs of Special Sense.-The brain of Urodela differs
from that of the frog in its more elongated and slender form, in the com-
paratively small size of the optic lobes, and in the non-union of the olfactory
bulbs. The organ of Jacobson is present except in Proteus and Necturus.
The olfactory sacs always open into the mouth by posterior nares situated
behind or external to the vomers. Apart from its importance in food gather-
ing, the olfactory sense is probably vital in reproduction and other behaviour.
Many frogs, as everybody knows, have a characteristic odour. Some (e.g. the
Mink Frog, Rana septentrionalis) are said to smell like mink, others (e.g.
Pelodytes) rather like onions and others (e.g. Bufo vulgaris) like vanilla. These
odours, and those of certain salamanders (e.g. Hydromantes) come from granular
or mucous glands on the skin.
 ZOOLOGY
The eye, consequent upon its emergence from the water and the necessity
for a longer-sighted vision, exhibits important modifications (p. 413), and in
the Urodela, which have retreated to the water, the lachrymal (tear) ducts
are generally still retained. In at least some Apoda a single hypertrophied
lachrymal gland has migrated and is found lodged in the sightless eye-socket ;
its tears probably lubricate the adjacent sensory tentacle (p. 434). The eye-
lids (including the nictitating membrane) were another response to the pro-
tection needed in terrestrial animals. In certain anuran tree-frogs the lids
are transparent. In a few primitive forms lids are absent.
Urodeles. Apoda, and some Anura have no tympanic cavity or membrane,
and no extra-columella. Occasionally (e.g. Ascaphus) even the 'stapes' (see
p. 144) may be lost. In some genera one end of the 'stapes' (footplate

FIG. JOI.-Urodela: Exteroception. Ground vibrations are transmitted via forelimbs and
opercular muscles to the perilymph in the adult terrestrial salamander (right). This channel of
communication remains incomplete in aquatic larval salamanders (left). Col. columella; F. V.
fenestra vestibuli; H. A. hyoid arch; L. S-C. ligamentum squamoso-columellare; lvl. L. skeleton
of the lower jaw; llf. Op. musculus opercularis; Op. operculum; Pq. palatoquadratum; Sq. os
squamosum; S. S. suprascapula; St. C. stilus columell<e. (From Noble, after Kingsbury and
Reed.)

of columella) abuts on to the squamosal, from which it probably transmits

vibrations to the jaw region. In some urodeles and anurans the stapes is
greatly reduced and is associated with a nodule of cartilage inserted in the
fenrestra ovalis. It is probable that this structure, the operculum, occurs only
in amphibians and has no homologue in other vertebrates. Amphibia which
have dispensed with a tympanic membrane have the operculum connected
to the scapula by a minute opercular muscle. It is probable that vibrations
from the forelimbs are conveyed on the tensed opercular muscle to the oper-
culum and finally to the inner ear (Fig. 301).
In the perennibranchiate urodeles and in the larvre of air-breathing forms
lateral line organs occur. Among Anura these organs are generally lost during
metamorphosis, but in a few strongly aquatic species they are found in the
adults. Those of Xenopus, for example (Fig. 302), are superficial, being
restricted to the epidermis. Groups of neuromast organs are arranged in
small ridges or plaques distributed in rows that probably correspond with
 PHYLUM CHORDATA 449

those of fishes (pp. 136, 137). Innervation is by two sets of cranial nerves. Of
these, the anterior lateralis originates in a nucleus in the fourth ventricle. Its

stratum lateralis tactile mucou:;

spongiosum organ organ glanQ

100)-1
stratum two laterolis cutaneous
com pactum nerve fibres nerve
sensory
hairs

10J1

cell of
Malpighian
layer /
basement/
membrane

supporting
cell
FIG. 302.-Xenopus: Lateral line and tactile organs. An elaborate system of organs occurs
on the head, back, sides, throat, chest, and belly. Above: A lateralis plaque, showing arrangement
of lateralis and tactile organs and their nerve-supply. Below: a single lateralis organ, showing its
situation in the epidermis and the relationships of the sensory and supporting cells. (After
Murray.)

ganglion fuses with that of the Vth and Vllth nerves, but most of its fibres
separate to continue as the tractus supraorbitalis to the organs in head and
snout. The mixed ramus of the Vllth nerve carries some fibres ventrally.
450 ZOOLOGY
The posterior lateralis originates in a nucleus situated immediately behind that

.
of the anterior lateralis and forms a mixed ganglion with the Xth, later again
to become distinct and to run superficially to branch and supply the more

:,. .!
posterior rows of neuromast organs. Each
/,..,...............)..,_' group of such organs is innervated by two big
/ ' and one or two smaller fibres. The larger fibres
/e •\·,
branch extensively at the base of each in-
i .
! •
i
dividual organ, the smaller ones less so. The
~ i
\ •... // nerve-endings lie be-
........................····· low or between the
FIG. 303.-Xenopus: Colour- sensory cells which
change. Left: Melanophores in terminate in hair-
wholly contracted condition, making
for pallor. Right: Opposite con- like processes that
dition . (Redrawn after Hog ben and
Slome.) are in contact with
the surrounding
water. The lateralis organs are probably sufficiently
sensitive to detect disturbances made by minute Ov.---
aquatic organisms.
Endocrine System.-The general tetrapod pattern
is fully established.
The relative non-specificity of hormones has led to
the clinical use of Xenoptts as a pregnancy indicator:
its ovary responds rapidly to chorionic gonadotrophin
(p. 905). Its utility in the laboratory is enhanced
by the absence of any terrestrial phase in its life-
cycle.
Colour change in anurans appears to be almost
wholly under pituitary control and not directly in-
fluenced by the nervous system (cf. some fishes,
reptiles). A posterior pituitary hormone (intermedin
or' B-substance ')causes the expansion of contracted
melanophores (Fig. 303) and a general darkening
of the integument. Pituitary extracts from other
tetrapods produce a similar effect. Total hypo- F1c. 304.-Urodela: Urino-
physectomy causes partial contraction and relative genital system in female. lg.
(Ur.) \Volffian duct (and col-
pallor. If the posterior lobe is ablated and the lecting tubules); mg. (Od.) ovi-
duct; N. kidney; GN. non-
anterior part left intact, full contraction and paling renal part of -kidney; Ot .
result. Thus it has been suggested that a special oviducal funn el; Ou. ovary ;
t, longitudinal canal. (From
W-substance, causing pallor, is elaborated by the Wiedersheim, after Spengel.)
adenohypophysis (see Hogben, Waring).
Urinogenital System and Osmoregulation.-Notwithstanding the many
changes that accompanied the colonisation of land, the internal apparatus of
 PHYLUM CHORDATA 451
the kidneys of extant amphibians retains a typical piscine appearance. Water
loss ashore, however, is retarded by the reduction of glomerular filtration by
means of a hormone from the pars neuralis (Fig. g6, p. rso) which constricts
glomerular arterioles (cf. mammals, p. 88g).
A second means of water conservation involves the changes in the permea-
bility of the skin. Water, when available, seems to be passively absorbed
through the skin of most Anura, but there is evidence
that permeability, and therefore the absorption
rate, is increased by a second pituitary principle.
Sodium chloride, too, enters through the in-
tegument. During periodic moults the intake of
both water and salts are considerably increased. It
is not yet known whether a means of reducing skin
permeability exists. Certain desert-dwelling frogs,
which survive droughts by restivating in dry sub-
surface mud, can absorb water with startling
rapidity (p. 385). The kidneys of such frogs
(Cyclorana, Notaden, andHeleioporus) possess venous
sinuses into which drain peritoneal funnels. During
restivation, water stored in the peritoneal cavity is
drawn through these channels into the general cir-
culation. No amphibian, as far as is known, is
aglomerular (cf. fishes, p. 347), but in Cyclorana the
glomeruli are reduced in size and vascularity.
In the Urodela the kidneys (Figs. 304, 305) are
much elongated and are clearly divided into two
portions : a broad posterior part, the functional
kidney (N), and a narrow anterior sexual part (GN.) FrG. 305.-Urodela: Urino-
genitalsystem in male. a. col-
connected in the male with the efferent ducts of the lecting tubules (see also Fig.
testis. Numerous ducts leave the kidney and open 304); G. N. non-renal part of
kidney; H o.testis ; lg.Wolffian
into the Wolffian (mesonephric) duct (lg. (Ur.)), duct ; mg. vestigial (in male)
Miillerian duct; N . kidney;
which thus acts as a ' ureter ' in the female, and as V e. vasa efferentia; f, longi-
a urinogenital duct in the male. The oviduct (mg. tudinal canal. (From Wieder-
sh eim, after Spengel.)
(Od.)) is developed from the Mullerian duct, a rudi-
ment of which (mg., mg1 .) occurs in the male. In the Apoda the kidneys extend
the whole length of the ccelom, and in the young condition are formed of
segmentally arranged portions, each with a nephrostome and a glomerulus, as in
myxinoids (p. 197). A pronephros is present in the larva, but disappears in
the adult. A urinary bladder is almost always present, opening into the cloaca
and having no connection with the mesonephric ducts. In some Apoda the
cloaca can be protruded and acts as an intromittent organ.
The testes of Amphibia show a great deal of variation. In the elongated
452 ZOOLOGY
Apoda they are long bodies which look not unlike a string of beads.
The expansions contain numerous seminiferous tubules. These discharge
through paired ducts. Crecilians with shorter bodies possess less diffuse organs.
In Taricha = Triturus Leydig cells are lacking. These animals depart from
the typical vertebrate interstitial pattern in that connective tissue cells of the
tubule walls are seasonally converted to an endocrine function as in certain
fishes (Fig. 97, p. 152).
The Bufonidre and Atelopondinre are perhaps unique in the possession of
Bidder's organ. This structure occurs in males of all such species, and addition-
ally in the females of some at the anterior end of tb,e gonads. It is better
developed in the male, but it is not always well defined even in adults (e.g.
Bufo carens). In the females of some species it disappears with maturity.
It would appear that Bidder's organ is a mesogonadic derivative which in
certain circumstances (and after castration) can develop in either sex into a
functional ovary. Broods of motherless toads have been raised from feminised
males. It has been suggested, but not yet proved, that Bidder's organ is an
endocrine gland.
Reproduction and Development-External fertilisation takes place in
most Amphibia. N ectophyrnoides (p. 386) produces well-yolked eggs, but the
larvre are retained in the oviduct until the completion of metamorphosis.
Each surviving group contains such ovoviviparous individuals, i.e. those in
which eggs are retained and development proceeds to sollle degree in the
oviduct. The larvre of at least one such ovoviviparous urodele, Salamandra
atra, remain in the parent tract after the exhaustion of the egg-yolk. Only
a few larvre survive the ensuing competitive, intra-uterine cannibalism; these
also ingest non-viable eggs, as well as red cells extruded by the hremorrhaging
oviducal wall. The larvre of this urodele possess long, plume-like externa:l
gills during its oviducal existence; these are shed before birth. If, however,
the unborn young are removed from the oviduct and placed in water, they
swim about like ordinary aquatic larvre until they lose the long gills and
develop a shorter set.
In many Urodela the spermatozoa are aggregated into spermatophores by
glands in the wall of the cloaca. These are deposited on the body of the female.
They are then taken into her cloaca and so internal fertilisation occurs. Newts
of the genus Triturus undergo elaborate courtship displays. A male follows a
chosen female, stimulating her mouth-parts, flanks, and cloaca with pressure
from his muzzle. He will impede her progress, and display with tremulously
vibrating tail, and by other means. Finally he deposits one or more spermato-
phores, each containing hundreds of spermatozoa. The latter struggle vio-
lently, their heads projecting from the gelatinous secretion (from the male
cloacal glands) that encloses them. If the female has been sufficiently stimul-
ated she will press her cloaca over a spermatophore and enclose it. Once in the
 PHYLUM CHORDATA 453

female tract, the sperms break free, and ascend to the receptaculum seminis or
spermatheca, where they remain until the eggs are ready to be fertilised. Mean-
while, the gelatinous discarded spermatophore sheath is shed and the female will
pick up no further spermatophores until the current batch of eggs are shed.
The eggs, which in some species are believed to be fertilised in the cloaca,
are laid singly. They, too, are enclosed in a gelatinous material (secreted by

FIG. 306.- Anura: Amplexus and parental care. Marsupial Frog (Gastrotheca marsupiata)
(X3). As the eggs (not shown in situ) are extruded from the female cloaca they are fertilised
and swiftly thrust by the male's hind-legs into the damp environment of the incubation pouch on
her dorsal surface. The period of development is from 2t (summer) to 3! months. The right top
inset shows relative egg-size. (From photographs by Alan Goffin.)

oviducal glands), which enables them to be attached to leaves (often wrapped

up by the female), floating community of algre, or, where aquatic vegetation
is lacking, to rotting leaves and other objects. It will be seen, therefore, that
some amphibians (as with many bony fishes, pp. 297, 350) exhibit reflex
behaviour patterns as complex and intriguing as those of the birds, excepting
perhaps only the Ptilonorhynchidre (p. 562).
Several remarkable instances of parental care are known among the
Amphibia. A number of different species of frogs and toads construct nests
454 ZOOLOGY

or shelters of leaves or other materials in which the eggs are deposited and
the young are developed. Phyllomedusa, a South American tree-frog, glues its
eggs to foliage hanging over water, and, after hatching, the tadpoles drop
straight into the appropriate environment. In the Obstetric Toad (Alytes
obstetricans) of Europe, the male winds the strings of eggs-formed by adhesion
of their gelatinous investment-round his body and hind-legs. 1 Here they are
retained until the tadpoles are ready to be hatched. In Rhinoderma darwini,
a small South American frog, the eggs are transferred by the male to the
relatively immense vocal sacs that extend over its ventral surface: there they
develop. The eggs of this species are much fewer and larger than in others.
Another anuran, Pseudis para-
doxa, is remarkable in that the
tadpole is many times larger
than the adult. Still other
fascinating larval-and paren-
tal-adaptations occur in the
Anura. In Gastrotheca there
occurs a special dorsal pouch in
the female integument. This
pouch, in which are carried
fertilised eggs, opens a little
(Fig. 305). In G. marsupiata
anterior to the cloacal aperture
the young are hatched as tad-
FIG. 307.-Anura: Ovoviviparity. In the Surinam poles, but in G. ovijerum emerg-
Toad (Pipa dorsige1·a ) the eggs are lodged in lidded pits
in the dorsal surface of the female. (After Mivart. ) ence is delayed until meta-
morphosis is complete.
In the Surinam Toad (Pipa americana) an even more astonishing arrange-
ment is found (Figs. 307, 308). The dorsal skin of the female becomes soft and
gelatinous, and here the male places and spaces the eggs. Each sinks into a
small pouch, over which develops an operculum, which, it has been suggested,
comes from a remnant of the egg envelope, reinforced by integumental secre-
tions. Thus, the young develop moist and safe in maternal tissue. Between
the invaginated pits arises a rich vascularisation. In each larva there develops
1 The 'midwife Toad' figured in one of the most unhappy frauds in scientific history. Kam-
merer showed that this animal, which norma lly m ates ashore (and which docs not possess horny,
pigmented nuptial pads, p. 390), can be induced experimentally to mate in relatively warm wat er.
He asserted, with apparent sincerity, that his aquatic-mating ma les developed nuptial pads, a nd
further, that subsequent generations of laboratory-bred toads inherited such acquired, pigmented
pads in progressive degree. These conclusions were accepted by some, but disputed by Bateson
a nd others. K ammerer's r esults could not be duplicated in repeat experiments. In 1926, when
Noble examined histologically a museum specimen that had been previously exhibited by Kam-
merer, no nuptial pads were found, but, instead, a subcutaneous substance possessing properties
indistinguishable from those of Indian ink. A few weeks later Kammerer shot himself on the
Hochschneeberg. He left a lett er admitting fraud but, it should be added, no confession of its
authorship. He was aged 46.
 PHYLUM CHORDATA 455
a curiously broad and vascular tail : it is suspected that metabolic exchanges
take place between maternal and embryonic tissues in the manner of a primitive
placenta. The larva does not develop gills, and has been reported to be born
as a tadpole about eighty days after egg-deposition.
Crecilians are either oviparous and lay in burrows (e.g. Ichthy ophis (Fig. 309),
Hypogeophis) or ovoviparous (e.g. Typhlonectes, Geotrypetes, Schistometopum,
Chthonerpeton, Gymnopis) . In Geotrypetes, the large (about 4 mm.), yolky
eggs migrate to the posterior end of the oviducts where they rest one closely
behind another. When the yolk has almost gone, and the plumose external

Muscle
FIG. 308.-Anura: Pseudo-placentation. Gaseous exchanges occur between the maternal
tissues and the expanded, leaf-like t ail of the pouch larva of Pipa dorsigera (Fig. 307). (After
Amoroso.)

gills are disappearing, t he embryos (now about 25 mm. long) hatch and are
next spaced throughout the length of the oviducts where they develop until
they are about 75 mm. in length. Nutrition is now carried out by the oral
absorption of uterine ' milk ' (see also p. 278). Both oviducal wall and fretal
epithelium are highly vascular, no doubt allowing gaseous exchanges. The
neonatus appears to slough its corneal layer and to develop a relatively tough
stratum corneum better adapted for its subsequent burrowing mode of existence.
In some species the larvre for a time lead an aquatic life. During this period
they possess, like tadpoles, a tail with a tail-fin which is later absorbed. The
larvre of most Apoda have long external gills (Fig. 309).
Salamanders show many fascinating examples of physiological adaptation,
456 ZOOLOGY
including predogenesis (p. 2). Most of them undergo complete metamorphosis.
Some come ashore as adults to live in damp situations, returning to water
periodically to breed. Others retain gills as well as lungs, and remain perma-
nently in the water (e.g. the Northern American N ectums, 'Mud-puppy' or
'Water-dog'). Others have lost their lungs altogether. Such creatures have
lost one of the principal attributes that made the initial amphibian emergence
possible and have returned to a life very roughly approximating that of their
early ancestors. A classical case of predogenesis is furnished by the remarkable
Mexican Axolotl, which frequently breeds in the gilled or larval state. The
experimental administration of thyroxin (p. 151), especially when the animal
is very young, causes it to lose its gills, develop lungs, and emerge from the
water in an adult form very like, if not identical with, the black-and-orange

FIG. 309.-Apoda: Reproduction and development. I chthyop!tis glutinosa (I) . ( x 1), a nearly
ripe embryo, with gills, tail-fin, and still with a considerable amount of yolk; 2, female guarding
her eggs, coiled up in a hole underground; J, a bunch of newly-laid eggs; 4, a single egg, enlarged,
schematised to show the twisted albuminous strings or chalaz<e within the outer membrane,
which surrounds the white of the egg. (After P. and F . Sarasin.)

Tiger Salamander (Amblystoma tigrinum) of North America. Metamorphosis

can be induced also by reducing the water level in which the axolotl lives and
thus making gill respiration difficult, whilst at the same time facilitating
pulmonary respiration. This was how metamorphosis in the animal was first
discovered. It is to the branchiate stage that the name Axolotl properly
applies.
The Alpine Newt (Tritums alpestris) is of especial interest in that in the
French and I tali an lowlands complete metamorphosis occurs, whereas in the
colder Lombardy lakes there has evolved a race that is often neotenous.
Among the perennially larval forms no known experimental manipulation will
induce metamorphosis.
Segmentation of the egg in the Anura and Urodela is always complete, but
unequal. In Pipe. and Alytes there is a large quantity of food-yolk, and the
developing embryo lies on the surface of a large yolk-sac. In the Apoda the
eggs are of large size, and segmentation is partial. The formation of segments
 PHYLUM CHORDATA 457

at the pole of the egg opposite that at which the formation of the embryo begins
takes place only at the stage of gastrulation. The embryo is coiled over the
surface of the yolk, as in teleosts (p. 293).

CLASS REPTILIA
INTRODUCTION
It is difficult satisfactorily to define the Reptilia. They form a hetero-
geneous group and can be loosely described as amniotes which lack the diag-
nostic characters of birds or mammals. They (and consequently and ulti-
mately birds and mammals) arose from a Carboniferous (p. 3) amphibian
stock which had probably developed the shelled amniote egg. By virtue of its
complicated membrane structures, such an internally fertilised egg enabled
the complete escape from the water that was impossible in allied amphibian
groups. Within the protective, yet permeable, horny or occasionally cal-
careous egg-shell was developed the fluid-containing amnion which provided
an aquatic environment for the developing land animal. Respiration was
carried out by means of the richly vascular allantois ; and metabolites were
excreted into, and stored in, the allantoic cavity. The proteinous and other
contents of the yolk sac allowed nourishment for the developing embryo
during a prolonged embryonic life. In some reptiles at least the albuminous
' egg-white ', accumulated during the journey down the oviduct, protected the
yolk and other contents, while its watery nature allowed the dilution of yolk
that would be used to feed the developing embryo. (Possibly it provides vital
mineral salts as well.) Reptiles needed none of the bizarre reproductive
devices evolved by the Anura (pp. 386, 452) in their attempts to colonise dry
land.
In addition to the amnion and allantois, to be described presently (p. 649),
reptiles have gastrocentrous vertebrre (p. 463) and a metanephric kidney (p.
153). Although keratinous, horny scales are said to be absent in some reptiles,
their occurrence as a complete covering of ectodermal origin is a character-
istic of the group and almost peculiar to it. In the Mesozoic these scales were
often modified to form spectacular means of protection and adornment in the
form of enormous horns (Fig. 342). Although rarely petrified in themselves,
they often left unmistakable impressions in the matrix, and so their form and
extent can be determined.
The reptilian epidermis is always hardened and cornified, and sometimes
forms substantial plates of horny material such as the scutes of 'tortoise-shell'
in turtles which protect the soft parts from injury and desiccation. Underlying
bony plates-osteoderms-frequently reinforce the exoskeleton, sometimes
fused with the endoskeleton. As the outer layer of scales and claws wear
away, the horny tissue is replaced by proliferations from the epidermis below.
VOL. II. FF
 ZOOLOGY
The term Amniota is used for the group formed by reptiles, birds, and
mammals, the three 'highest' classes of vertebrates. Fishes and amphibians
are referred to as the Anamniota. The division of the Amniota into the three
classes mentioned above is kept because it is convenient.
Some of the earliest known reptiles, embedded in late Carboniferous deposits
about 250 million years old, can be separated only with difficulty from the
Amphibia on the examination of surviving parts. Permian representatives,
however, were numerous, and admit of no such uncertainty. By the late
Palreozoic (p. 3) the first great reptilian radiation was well under way with
distinctive anapsid and synapsid forms predominating. These early reptiles
were relatively short-legged, heavy, slow, and were rarely more than ten feet
long (Fig. 310).

FIG. Jio.- Sub-class Anapsida, Order Cotylosauria, Sub-order Captorhinomorpha, Family

Captorhinidse. LabidoBaUrull. This lower Permian American 'stem reptile' was a little over 2 feet
long. The contemporary Limnoscelis (p. 406) was about 5 feet long, but most coltylosaurs were
smaller. (From Romer, after Williston.)

The cotylosaurs ('stem reptiles') had disappeared by the late Triassic, but
from them had arisen the material for a further reptilian radiation which
produced, in the Mesozoic, one of the most spectacular and fascinating faunal
groups that ever lived. By mid-Mesozoic, ancestors of each of to-day's reptilian
orders had appeared, together with the forerunners of the birds (p. 507) and
mammals (p. 513). More than fifteen orders, and dozens of families, arose.
The individual animals ranged in size from swift, lizard-like creatures a few
inches long up to lumbering semi-aquatic swamp-dwellers weighing more
than so tons. The sea was likewise a centre of reptilian diversification ; and
a few forms began to volplane through the air. It is as true as it is trite to
say that the Mesozoic was the ' age of reptiles' and that then they ' dominated
the earth '.
Most of these animals do not seem to have survived the Cretaceous or, at
the latest, the early Crenozoic. His not known what caused their disappear-
ance. Certainly there is believed ro have been a widespread elevation of the
earth's surface together with changes in temperature and vegetation. This
would, no doubt, affect a poikilothermous population. But most of the
marine reptiles vanished as well. There is evidence of an increased aridity,
 PHYLUM CHORDATA 459
which would certainly embarrass the gigantic swamp-dwelling dinosaurs.
Doubtless climatic changes affected many successful reptiles by extinguishing
the food upon which they were irrevocably adapted to feed. It has been sug-
gested, too, that many of the small-brained (e.g. Fig. 342, p. sn) Mesozoic
reptiles perhaps disappeared because they evolved no means of defending their
eggs from predaceous ancestral mammals. This may have been a factor. Vague
(in the current state of knowledge) notions concerning genetic instability leading
to over-specialisation and phylogenetic senescence have also been put forward.
The smaller, swifter, more adaptable, warm-blooded ancestral mammals
arose about the beginning of the Jurassic or even earlier and the marsupials
and the eutherians became clearly diversified in the Cretaceous (p. 3). The
best that can be said at present is that a combination of climatic and other
factors, possibly including competition by the arising Mammalia, and perhaps
by the birds as well, now caused the relatively sudden extinction of numerous
hitherto highly successful types.
Excluding birds and mammals, the only animals of reptilian stock that have
survived in considerable numbers are lizards and snakes. Lizards (Lacertilia)
arose in the Triassic and have been common ever since. Snakes (Ophidia =
Serpentes), the most recent reptilian development, arose from lizards (possibly
related to the Platynota, p. 497) at some time in the Mesozoic, but did not
become abundant until the Eocene. The more typical serpents did not appear
in numbers until the Oligocene. Venomous snakes first appeared in well-
established families as recently as the Miocene.
Of the multitude of reptiles that dominated the earth during the Mesozoic
(p. 3), representatives of only four widely divergent orders are alive to-day.
These are:
I. Lizards and snakes (Order Squamata), an example of which, the lizard
Lacerta, will be dealt with in c:!etail (p. 462).
2.The lizard-like Tuatara (Sphenodon = Hatteria) 1 of New Zealand
(Order Rhynchocephalia).
3· Turtles and tortoises (Order Chelonia).
4· Crocodiles, gharials, alligators, and caimans (Order Crocodilia).
1 There exists considerable confusion concerning the correct generic name of the Tuatara.
In I8JI, J. E. Gray described the skull of what he thought to be a new species of lizard. He
conferred the name Sphamodon in allusion to its wedge-shaped teeth. Eleven years later he
described the external appearance of an apparently new reptile in a collection from New Zealand
and, not realising that it was, in fact, Sphcenodon, called it Hatteria punctata, placing it in the
agamid group of lizards. Quoting from the notes which accompanied the collection, he added
that 'the natives called it "Tuatara "'. Little further attention was paid to the creature until
in 1867 Albert Gunther at the British Museum carefully studied the anatomy of several specimens
and concluded that it was not a lizard at all. He referred it to a new order, the Rhynchocephalia
(' beakheads '), which, it is now known, go back at least to early Mesozoic times. Animals
almost identical with the Tuatara were in existence by the Jurassic (p. 3).
Strictly speaking, the Tuatara should be called neither Sphenodon nor Hatteria, but, in fact,
Sphcenodon. An application to the International Commission on Zoological Nomenclature has
recently been made to validate the emendation to Sphenodon from Sphcenodon and in anticipa-
tion of its success the more familiar spelling will be used hereafter in this volume.
 ZOOLOGY
Despite the retention of many primitive characteristics, and the drastic
reduction in reptilian numbers and species after the Mesozoic, many families
remain successful. The Squamata are particularly diversified and numerous.
Modern lizards number nearly 3,ooo species (of some twenty families), many of
which are common. There are more than 2,500 species of snakes distributed
among twelve families. The Tuatara (Sphenodon), on the other hand, is the
sole living representative of an order of lepidosaurian reptiles (p. 495) that was
widespread during the Mesozoic. It still survives on small islands off the coasts
of islands (New Zealand), and probably only because its habitat was not
colonised by terrestrial mammals, including Man, until relatively recently.
The turtles and tortoises-slow, heavily-armoured, primitive creatures-still
number more than 200 species. Many forms, even the largest, remain plentiful
in places where they lire not too constantly molested by Man. The Crocodilia,
another ancient and once widespread group, now contains fewer than twenty-
five species. Many of these remain common where their numbers are not
reduced by skin-hunters but very large examples are now extremely rare in
most parts of the world.
Excluding only the circumpolar regions, land reptiles are spread all over
the globe. A few lizards extend southwards into the tip of South America
and one lizard (Lacerta vivipara), and one snake (Vipera berus), cross the limit
of the Arctic Circle. The exclusion of reptiles from very cold regions is conse-
quent on their failure to become homoiothermous, i.e. to regulate and retain
heat generated by muscular activity. The poikilothermy (p. 386) of reptiles
probably caused them to disappear, for example, from what later became the
British Isles, during the glacial epochs. Ireland failed to get any snakes back
(as distinct from the lizard, L. vivipara) because it was separated from the
European Continent before Scotland and England, good St. Patrick notwith-
standing. Many of the smaller reptiles inhabiting temperate countries hiber-
nate during the winter. It is a sad and historical fact that the famous tortoise
(Testudo grceca) belonging to successive bishops of Peterborough, and reputedly
220 years old (not an historical fact, see alsop. 489), died when left exposed in
winter after being dug up by a gardener for' a trifling wager' in the r8th century.
Yet reptiles flourish exceedingly in many temperate countries. Thus, the
mildly poisonous Common Adder or Viper (Vipera bents) is probably as com-
mon on the Kintyre Peninsula of Scotland as is any single species of snake in
tropical Africa or New Guinea. Although Reptilia are in general unquestion-
ably more successful in hot countries, their inability to regulate their tempera-
ture causes some of them (e.g. Lacerta vivipara) to die if experimentally exposed
to prolonged summer sunshine, even in northern latitudes. Many desert
reptiles lie in the shade of rocks, or in burrows, to escape the heat during the
hottest time of day, and by constantly changing their position in relation to
sources of external heat manage to keep their body temperature fairly stable.
 PHYLUM CHORDATA

It will have become obvious that Lacerta, the example chosen, is only
broadly typical of the class as a whole of which an abbreviated, and provisional,
classification is as follows :

CLASS REPTILIA

Sub-class Anapsida
Order Cotylosauria
Sub-orders Captorhinomorpha (Upper Carboniferous-Permian)
Diadectomorpha (Permian-Upper Triassic)
Order Chelonia (Testudinata) (Middle Permian-Recent)
Sub-orders Eunotosauria (Permian)
Amphichelydia (Upper Triassic-Lower Cretaceous)
Pleurodira (Upper Cretaceous-Recent)
Cryptodira (Upper Jurassic-Recent)
Sub-class Ichthyopterygia
Orders Mesosauria (Lower Permian)
Ichthyosauria (Middle Triassic-Upper Cretaceous)
Sub-class Synaptosauria
Orders Protorosauria (Permian-Triassic)
Sauropterygia
Sub-orders Nothosauria (Triassic)
Plesiosauria (Triassic-Cretaceous)
Placodontia (Triassic)
Sub-class Lepidosauria
Order Eosuchia
Sub-orders Younginiformes (Upper Permian-Lower Triassic)
Choristodera (Upper Cretaceous-Eocene)
Acrosauria (Upper Jurassic)
Orders Rhynchocephalia (Lower Triassic-Recent)
Squamata
Sub-orders Lacertilia (Sauria) (Triassic-Recent)
Ophidia (Serpentes) (Lower Cretaceous-Recent)
Sub-class Archosauria
Order Thecodontia
Sub-orders Pseudosuchia (Triassic)
Phytosauria (Triassic)
Order Crocodilia (Loricata)
Sub-orders Protosuchia (Upper Triassic-Jurassic)
Sebecosuchia (Cretaceous-Eocene)
Mesosuchia (Jurassic-Eocene)
Eusuchia (Cretaceous-Recent)
 ZOOLOGY
Order pterosaurla
Sub-orders Rhamphorhynehoidea (Jurassic)
pterodaetyloidea (Upper Jurassic-Upper Cretaceous)
Order Saurischia
Sub-orders Theropoda (Middle Triassic-Upper Cretaceous)
Sauropoda (Lower Jurassic-Upper Cretaceous)
Order Ornithisehia
Sub-orders Ornithopoda (Upper Jurassic-Upper Cretaceous)
Stegosauria (Lower Jurassic-Lower Cretaceous)
Ankylosauria (Cretaceous)
Ceratopsia (Upper Cretaceous)
Sub-class Synapsida
Order Pelycosauria
Sub-orders Ophiaeodontla (Permian)
Sphenaeodontia (Lower Permian-Lower Triassic)
Edaphosauria (Permian)
1 Order Therapsida

Sub-orders Dinocephalia (Middle Permian)

Dieynodontia (Permian-Triassic)
Theriodontia (Permian-Triassic)
Order Ictidosauria (Upper Triassic-Middle Jurassic)

EXAMPLE OF THE CLASS.-A UZARD (LACERTA)

The most striking external differences between lizards (p. 389) and frogs
are the covering of scales, the comparative smallness of the head, and the
presence of a distinct neck, the great length of the caudal region, the shortness
of the limbs, and the approximate equality in length of the anterior and pos-
terior pairs. The anterior limbs are situated just behind the neck, springing
from the trunk towards the ventral surface. The fore-limb, like that of anurans,
is divided into three parts : the upper-arm or brachium, the fore-arm or
ante-brachium, and the hand or manus. There are five digits provided with
horny claws, the first digit or pollex being the smallest. The hind-limbs arise
from the posterior end of the trunk towards the ventral aspect. Each, again
like that of frogs, consists of three divisions-thigh or femur, shank or crus,
and foot or pes. The pes, like the manus, terminates in five clawed digits, of
which the first or hallux is the smallest. The head is somewhat pyramidal,
slightly depressed: the openings of the external nares are situated above the
anterior extremity. The mouth is a wide, slit-like aperture running round
the anterior border of the head. At the sides are the eyes, each provided
with upper and lower opaque, movable eyelids and with a transparent thir.d
1 A new and somewhat different classification has been recently suggested by Watson and
Romer. Sec also Crompton for recent work on ictidosaurs.
 PHYLUM CHORDATA

eyelid or nictitating membrane, which, when withdrawn, lies in the anterior angle
of the orbit. Behind the eye is a circular patch of skin-the tympanic mem-
brane-corresponding closely to that of the frog, but somewhat sunk below
the general level of the skin. The trunk is elongated, strongly convex dorsally,
flatter at the sides and ventrally. At the root of the tail on the ventral surface
is a slit-like transverse aperture-the cloacal aperture. The tail is cylindrical,
thick in front, gradually tapering to a narrow posterior extremity ; it is nearly
twice as long as the head and trunk together.
There is an exoskeleton of horny scales covering all parts. These are
formed from folds of the dermis, each covered with a thick horny epidermal
layer. In size they differ in different
positions. On the dorsal surface of the
trunk they are small, hexagonal, and
indistinctly keeled. On the ventral sur-
face they are larger and are arranged in
eight longitudinal rows. Immediately
in front of the cloacal aperture is a large
pre-anal plate. A collar-like ridge of
larger scales surrounds the throat. On
the tail the scales are elongated, keeled, FIG. 31 I.-Lacerta: Vertebrre. A,
anterior; B . posterior view of a thoracic
and arranged in regular transverse vertebra; C, lateral, D, anterior view of
vertebra; E , lateral view of axis.
(annular) rows, giving the tail a ringed atlas
cent. centrum; ltyp. hypapophysis of axis;
appearance. On the surface of the limbs lat. lateral piece of atlas; lig. ligamentous
band dividing the ring of the atlas into
the scales of the pre-axial (radial or two; neur. neural arch of atlas; od.
tibial) side are larger than those of the pt.odontoid process; pr. zy. pre-zygapophysis;
zy. post.zygapophysis; rb. rib ; sp.
postaxial (ulnar or fibular). The scales spine; vent. ventral piece of atlas.
on the upper surface of the head (head-
shields) are large, partly bony in structure, and have regular and characteristic
arrangement. The skin is periodically shed. The outer layers disintegrate
into fragments instead of being sloughed off in toto as in many of the snakes and
some lizards.
Endoskeleton.-The vertebral column is of great length and composed of a
large number of vertebrre. It is distinctly marked out into regions, a cervical
of eight vertebrre, a thoraco-lumbar of twenty-two, a sacral of two, and a
caudal of a considerable but indefinite number. A vertebra from the anterior
thoracic region (Fig. 3II, A, B) presents the following leading features. The
centrum (cent.) is elongated and strongly proccelous, i.e. the anterior surface is
concave, the posterior convex. The neural arch bears a short neural spine
(sp.) . There are pre- and post-zygapophyses (pr. zy., pt. zy.), the former with
their articular surfaces directed upwards, the latter downwards. On each side
at the junction of centrum and neural arch is a facet-the capitular facet-for
the articulation of a rib. The cervical vertebrre in general are similar in
 ZOOLOGY
essential respects to those of the trunk, but are somewhat shorter. The first
two, however, differ greatly from the others. The first is the atlas (C, D). It
has no distinct centrum, but is in the form of a ring. Ventrally on its anterior
face it bears a smooth articular facet for the occipital condyle of the skull.
It consists of three distinct ossifications, one ventral, the others dorso-lateral.
The latter do not quite meet dorsally, being separated by a space bridged
over by membrane. The second vertebra or axis (E) has a short conical
process-the odontoid process (od.)-projecting forwards from its centrum.
In the natural position of the parts the odontoid process (which is a part of the
centrum of the atlas, and is not actually fused with, though firmly fixed to,
the axis) lies in the lower or ventral part of the opening of the atlas. It is
separated by a ligamentous band from the upper portion which corresponds to
the neural arch, and lodges the anterior end of the spinal cord. On the ventral
surface of the axis, and of each of the following five or six vertebrce, is a distinct
bony nodule, sometimes termed the intercentrum or hypapophysis (hyp.). The
sacral vertebrce have short centra and strong expanded processes-the trans-
verse processes-which abut against the ilia. These are probably sacral ribs.
The anterior caudal vertebrce are like the sacral, but have the centra longer,
the transverse processes more slender, and the neural spines longer.
The posterior caudal vertebrce become gradually much smaller posteriorly,
and the various processes reduced in prominence, until, at the end of the tail,
the whole vertebra is represented merely by a rod-like centrum. Attached to
the ventral faces of the centra of a number of the anterior caudal vertebrce
are Y-shaped chevron bones-the upper limbs of the Y articulating with the
vertebra, while the lower limb extends downwards and backwards. In nearly
all the caudal vertebrce the centrum is crossed by a narrow transverse un-
ossified zone through which the vertebra readily breaks. This singular adapta-
tion, also found in Sphenodon and a few fossil reptiles, is widespread among
lizards, and is a protective mechanism which enables the lizard to twist off
its tail when the appendage is seized by an enemy. The severed tail jumps
about by reflex action and presumably occupies the attention of the hunter
while the lizard escapes. Such self-mutilation or autotomy is controlled by
segmentally arranged caudal muscles which part near the point where the tail
is grasped. The proximal part of the tail, near the reproduction organs, has
no such power. Further, the organ can be regenerated. Occasionally two
new tails develop, forking grotesquely.
The ribs are slender, curved rods, the vertebral ends of which articulate
only with the capitular facets of the corresponding vertebrce. There is no
direct articulation with the transverse processes. The ribs of the five anterior
thoracic vertebrce are connected by means of cartilaginous sternal ribs with the
sternum. The posterior thoracic ribs do not reach the sternum, the sternal
ribs being very short, and free at their ventral ends. The cervical ribs, which
 PHYLUM CHORDATA

are present on all the cervical vertebrre with the exception of the first three, are
all shorter than the thoracic ribs, and none of them is connected with the
sternum. Thus, as regards the structure of the vertebrre themselves, there is
little to distinguish the posterior cervical from the anterior thoracic; but,
for convenience of description, the first thoracic is defined as the first vertebra
having ribs connected with the sternum.
The sternum (Fig. 312, st.) is a rhomboidal plate of cartilage with a small
central space, or f ontanelle, completed by membrane. Posteriorly it is pro-
duced into two slender flattened
processes. On its antero-lateral
borders are articular surfaces for
the bones of the pectoral arch, and
on its postero-lateral borders and
the processes are small facets for
the sternal ribs.
In the skull (Fig. 313) the
chondrocranium, though persistent,
is replaced by bones t o a greater
extent than in frogs, and the num-
ber of investing bones is more con-
siderable. On the dorsal and lateral
surface are a large number of dermal
roofing bones. At the posterior end
F IG. 3 r 2.- Lacerta: Pectoral arch and
the rounded aperture of the foramen sternum. cl. clavicle; cor. coracoid; ep . cor.
; epist. episternum; glen. glenoid
magnum (for. mag.) is surrounded by epicoracoid
cavity for h ead o f humerus; pr. cor. procora·
four bones- a basioccipital (bas. oc.) coid; r . r- r . 4 first to fourth sternal ribs; sc .
scapula; st. sternum; supra. sc. suprascapula.
below, exoccipitals (ex. oc.) at the (After Hoffmann.)
sides, and a supraoccipital (supr. oc.)
above. The basioccipital forms the floor of the most posterior portion of the
cranial cavity. Posteriorly it bears a rounded prominence, the occipital
condyle (oc. cond.). In front of it, forming the middle portion of the floor of
the cranial cavity, is the basisphenoid (bas. sph.), not represented in frogs. In
front of this is an investing bone, the p arasphenoid (para.), corresponding to
the bone of the same name in the frog and trout, but here much reduced in
size and importance and ankylosed with the basisphenoid. Compared with
the skulls of other amniotes, it can be seen that those of reptiles in general
retain relatively large areas of unossified cartilage in the adult, particularly in
the anterior region. This arrangement allows considerable elasticity and
aids the swallowing of large prey. Further, when a lizard opens its mouth it
raises the upper as well as lower jaw. To enable this upper movement a
special and remarkable hinge exists in the roof of the cranium. Here the
supraoccipital and parietal elements are separated by an extensive cartilaginous
 ZOOLOGY

process, which, attached to the occipital element, articulates loosely into a slot
in the parietal and allows considerable play between the two areas and, in
some reptiles, an astonishing widening of the gape (Fig. 332, p. 498). Skulls of

FIG. 313. -Lacerta: Skull. A, dorsal; B, ventral; C, lateral views. ang. angular; art.
articular; bas. oc. basioccipital; bas. ptg. basipterygoid processes; bas. sph. basisphenoid; col.
epipterygoid; cor. coronoid; dent. dentary; eth. ethmoid; ex. oc. exoccipital; ext. nar. external
nares; for. mag. foramen magnum; Jr. frontal ; int. nar. internal nares ; ju. jugal; lcr . lachrymal;
max. maxilla; nas. nasal; oc. cond. occipital condyle; olf. olfactory capsule; opi. ot. opisthotic;
opt. n. optic nerve; pal. palatine; par. parietal; para. parasphenoid; par. f. parietal foramen;
p. mx. premaxillre; pr. Jr. pre-frontal; ptg. pterygoid ; pt. orb. postorbital or lateral postfrontal;
qu. quadrate; s. ang. supra-angular; s. orb. supraorbitals; sq. squamosal; supra. t. 1. supratem-
poral I; supr. oc. supraoccipital; trans. transverse or ectopterygoid; vom. vomer. The unlettered
bone internal to pt. orb. in A is the postfrontal. The transverse line behindjr. is a superficial mark,
not a suture. (After W. K. Parker.)

this type, whenever movement between the upper jaw and brain-case occurs,
are termed ' kinetic '.
In the wall of the auditory capsule are three ossifications-pro-otic, epiotic,
and opisthotic (opi. ot.). The first remains distinct, the second becomes merged
 PHYLUM CHORDATA

in the supraoccipital, and the third in the exoccipital. The exoccipital and
opisthotic are produced outwards as a pair of horizontal parotic processes.
The large orbits are closely approximated, being separated only by a thin
vertical interorbital septum. The cranial cavity is roofed over by the parietals
(par.) and frontals (Jr.). The former are united together. In the middle is a
small rounded aperture-the parietal foramen (par. f.). The frontals remain
separated from one another by a median frontal suture. Between them and
the united parietals is a transverse coronal suture. The nasal cavities are
roofed over by a pair of nasals (nas.). A small prefrontal (pr. Jr.) lies in the
front of the frontal, and helps to bound the orbit anteriorly. Another small
bone-the lachrymal (lcr.)-perforated by an aperture for the lachrymal duct,
lies at the anterior extremity of the orbit, just within its border. A row of
small bones-the supraorbitals (s. orb.)-bounds the orbit above. Behind is
a postorbital or lateral postjrontal (pt. orb.) articulating with the frontal.
Just behind the postorbital is a supra-temporal bone (supra t. I), in close rela-
tion to which is the squamosal (sq.), 1 which bends forwards and upwards to form
with the post-orbital the superior temporal arch. The superior temporal vacuity
is obliterated in Lacerta. At the anterior extremity of the snout is a median
bone formed by the coalescence of the two premaxillce (p. mx.). This bears the
four anterior teeth of each side. On each side behind the premaxilla is the
maxilla (max.), consisting of two portions, an alveolar bearing all the rest of the
teeth, and a palatine extending inwards on the roof of the mouth, together with
an ascending process articulating with the nasal and pre-frontal above. Articu-
lating behind with each maxilla is a jugal (jtt.), which forms the posterior half
of the ventral boundary of the orbit. The quadrate (qu.) articulates movably
with the parotic process, and bears at its distal end the articular surface for
the mandible.
In the anterior portion of the roof of the mouth, articulating in front with
the premaxillre and maxillre, are the vomers (vom.). Behind and embracing
them posteriorly are the flat palatines (pal.). The elongated pterygoids (ptg.)
articulate in front with the posterior extremities of the palatines. Behind,
each articulates with the corresponding basipterygoid process (bas. ptg.) of the
basisphenoid, and sends back a process which becomes applied to the inner
face of the quadrate. A stout bone, the transverse bone or ectopterygoid
(trans.), extends between the maxilla externally and the pterygoid internally.
Extending nearly vertically downwards from the pro-otic to the pterygoid is
a slender rod of bone, the epipterygoid (col.).
The columella is a small rod partly composed of cartilage and partly of bone
the outer end of which is fixed into the inner surface of the tympanic membrane.

1 Tn early reptiles (e.g. Limnoscelis, p. 458; see also Seymouria, p. 429) the supra-temporal,
squamosal and tabular may be borne on the posterior lateral wall of the skull. In extant reptiles
these are generally reduced to one or two and their identities are not clear.
 ZOOLOGY
The inner end is attached to a small aperture, the fenestra ovalis, in the outer
wall of the auditory capsule between the pro-otic and the opisthotic.
Certain depressions or fossre and apertures or foramina are to be observed
in the skull. The foramen magnum, the parietal foramen, and the orbits have
already been mentioned. The posterior temporal fossa is situated on either
side of and above the foramen magnum, bounded above and externally by
the roofing bones, and on the inner side by the bones of the occipital region.
The inferior temporal fossa is bounded internally by the pterygoid, and is
separated from the palatine foramen by the transverse bone. The lateral
temporal fossa is the wide space in the side wall of the skull behind the orbit.
The bony bar which limits it above is the superior temporal arch ; a bony
inferior temporal or quadratojugal arch is here absent. The tympana-eustachian
fossa, situated in the auditory region, is bounded by the bones of that region
together with the quadrate. The posterior or internal nares are bounded
posteriorly by the palatines. The anterior or external nasal aperture is situated
at the anterior extremity of the skull bounded by the nasals and premaxillre.
Each ramus of the mandible consists of six bony elements in addition to
the slender persistent Meckel's cartilage. The proximal element, the articular
(art.), bears the articular surface for the quadrate and is produced backwards
into the retro-articular process. The angular (ang.) is a splint-like bone
covering the ventral edge and the lower half of the outer surface of the articular.
The supra-angular (s. ang.) overlies the dorsal edge and upper half of the outer
surface of the same bone. The dentary (dent.) forms the main part of the distal
portion of the mandible, and bears all the mandibular teeth. The splenial is
a flat splint applied to the inner face of the dentary. The coronoid (cor.), a
small, somewhat conical bone, forms the upwardly directed coronoid process
immediately behind the last tooth. All these, with the exception of the articu-
lar, are investing bones.
The hyoid apparattts (see Fig. 318, b. hy., p. 473; and p. 92) consists r. of
a median cartilaginous rod, the basihyal; 2. of the anterior cornua, elongated
cartilaginous rods which, connected ventrally with the basihyal, curve round
the resophagus and end in close relation with the ventral surface of the auditory
capsule; J. of the middle cornua, rods of cartilage ossified at their proximal ends,
and 4· of the posterior cornua, cartilaginous rods arising from the posterior edge
of the basihyal and passing backwards and outwards. The middle cornua are
vestiges of the first, the posterior of the second branchial arch.
In the pectoral arch (Fig. 312) the coracoids are flat bones articulating with
the antero-lateral border of the sternum, and bearing the ventral half of the
glenoid cavity (glen.) for the head of the humerus. A cartilaginous epicoracoid
(ep. cor.) element lies on the inner side of the procoracoid and coracoid. A
large gap or fenestra divides each coracoid into a narrow anterior portion-the
procoracoid (pr. cor.), and a broader posterior portion, the coracoid proper (cor.).
 PHYLUM CHORDATA

The scapulte (sc.) articulate with the outer ends of the coracoids, and each bears
the dorsal half of the glenoid cavity. Dorsally, the scapulre become expanded,
and each has connected with it a thin plate of partly calcified cartilage-the
suprascapula (supra. sc.). This extends inwards towards the spinal column on
the dorsal aspect of the body. An element not hitherto met with, except in
the Labyrinthodontia (p. 422), is the interclavicle or episternum (epist.). This is
a cross-shaped investing bone, the stern of which is longitudinal. In the pos-
terior portion of its extent it is closely applied to the ventral surface of the
anterior part of the sternum, while the cross-piece is situated a little in front
of the scapula. The clavicles (cl.) are fiat curved bones articulating with one
another in the middle line and also with the anterior end of the interclavicle.
The bones of the fore-limb consist of a proximal bone or humerus, a middle
division composed of two bones-the radius and ulna-and a distal division
or manus. In the natural position of the parts the humerus is directed, from
the glenoid cavity with which it articulates, backwards, upwards, and outwards.
The radius and ulna pass from their articulation with the humerus downwards
and slightly forwards. The manus has the digits directed forwards and out-
wards. When the limb is extended at right angles to the long axis of the
trunk, it presents, like that of the frog, dorsal and ventral surfaces, and pre-
axial and postaxial borders. In this the radius is seen to be pre-axial, the ulna
post-axial. In the natural position the pre-axial border of the humerus is
external. The distal end of the fore-arm is rotated
in such a way that, while the pre-axial border faces flT

forwards and outwards at the proximal end, it '

faces directly inwards at its distal end. The manus is
rotated so that its pre-axial border faces inwards.
The humerus is a long bone consisting of a shaft
and two extremities. Each extremity is formed of an
epiphysis of calcified cartilage. The proximal ex-
tremity is rounded, the distal (trochlea) pulley-like
with two articular surfaces, one for the radius and the
other for the ulna. The radius is a slender bone con- F IG. 3!4·- Lacerta:
sisting, like the humerus, of a shaft and two epiphyses. Carpus. Left, dorsal v iew.
R . radius; U . ulna; c
The distal extremity has a concave articular surface for centrale ; i . intermedium ;
u . ulnare; 1-5,
the carpus, and is produced pre-axially into a radial r.theradiale; five distal carpals; t,
styloid process. The proximal end of the ulna is pisiform; I - V, the five
metacarpals. (After Wied-
produced into an upwardly directed process-the ersheim.)
olecranon : the distal end bears a convex articular sur-
face for the carpus. The carpus (Fig. 314) is composed of ten small polyhedral
or rounded carpal bones. These consist of a proximal row containing three
bones-the radiale (r.), ulnare (u.), and intermedium (i.). of a centrale (c.), and of
a distal row of five (I- 5); with an accessory or pisiform {"f) bone attached to the
470 ZOOLOGY

distal epiphysis of the ulna on its postaxial side. The first digit or pollex con-
sists of a metacarpal and two phalanges, the second of a metacarpal and three
phalanges. The third digit consists of a metacarpal and four phalanges, the
fourth of a metacarpal and five phalanges, and the fifth of a metacarpal and
three phalanges. The number of phalanges in the first four digits is, there-
fore, one more than the serial number of the digit.
The pelvic arch (Fig. 315) consists of two triradiate bones, the ossa inno-
minata, each ray being a separate bone. On the outer side at the point from
which the rays diverge is a concave articular surface-the acetabulum (Ac.)-
for the head of the femur. From the region of the acetabulum one of the rays,
the ilium (I.), a compressed rod,
Cep passes upwards and backwards to
PP
articulate with the sacral region of
the spinal column. A second ray
-the pubis (p.)-passes down-
wards and forwards to meet its
fellow in the middle line. The
articulation is termed the pztbic
I symphysis. In the middle line in
front, between the anterior ends
of the pubes, is a small nodule of
calcified cartilage, the epipubis
(Cep.). The third ray or ischium
FIG . JIS.-IAcerta: Pelvis. Ventral view. Ac.
acetabulum; Cep. epipubis. Fo'. foramen for (Is.) runs downwards and back-
obturator nerve; Hp. Is. hypoischium; I. ilium; wards, and articulates with its
tt, process representing the pre-acetabular part of the
ilium; Lg. ligament; Is. ischium; p . pubis; pp. fellow in the ischiatic symphysis.
prepubis; 5. Is. ischiatic symphysis. (AfterWieder-
sheim.)
The ventral ends of the two bones
are separated by a plate of cal-
cified cartilage (5. Is.). Between the pubes and ischia is a wide space, pulso-
ischiac, divided by a median ligament (lg.) into a pair of apertures, the pulso-
ischiac foramena. A smaller aperture in each pubis, the obturator foramen (F o.),
transmits the obturator nerve. A small rod of bone, the os cloacce, or hypo-
ischium (Hp. Is.), passes backwards from the ischiatic symphysis and supports
the ventral wall of the cloaca.
The hind-limb consists, like the fore-limb, of three divisions. These are
termed respectively the proximal or femur, the middle or crus, and the distal
or pes. The proximal division consists of one bone, the femur; the middle
division of two, the tibia and fibula; the distal of the tarsal and metatarsal
bones and the phalanges. When the limb is extended at right angles with the
trunk, the tibia is pre-axial and the fibula postaxial. In the natural position
of the parts the pre-axial border is internal in all three divisions of the limb.
The femur is a stout bone consisting of a shaft and two epiphyses. The
 PHYLUM CHORDATA 471

proximal epiphysis develops a rounded head which fits into the acetabulum.
Near it on the pre-axial side is a prominence, the lesser trochanter. A nearly
obsolete prominence on the postaxial side represents the greater trochanter.
The distal extremity is pulley-shaped, with internal and external prominences
or condyles for articulation with the tibia. Immediately above the external
condyle is a prominence or tuberosity for articulation with the fibula. The
tibia is a stout, curved bone, along the anterior (dorsal) edge of which runs a
longitudinal cnemial ridge. The proximal extremity presents two articular
surfaces for the condyles of the femur. The fibula is slender, the proximal end
articulating with the external tuberosity of the femur, the distal with the tarsus.
The tarsits comprises only three bones in the adult, one large proximal tibio-
fibulare (tb. fb.), and two smaller distal (tars. dist.). Each digit consists of a
metatarsal and phalanges. The number of the
latter is respectively, two, three, four, five, and three.
The first and second metatarsals articulate with the
tibial side of the tibiofibulare, the rest with the
distal tarsals. In lizards the proximal head of
the fifth metatarsal is hooked forwards and the fifth
toe is peculiarly off-set backwards.
Alimentary Canal and Associated Structures.-
The upper and lower jaws, forming the boundary of FIG. 31 6.- Lacerta: Tarsus.
the aperture of the mouth, are each provided with a jb. fibula; tb. tibia; tb. fb .
tibio-fibulare; tars. dist. distal
single row of small solid conical teeth, and there is tarsals. (After Gegenbaur.)
in some species a patch of similar teeth on the
pterygoid. The teeth are adapted to catching and holding, and not for chewing.
On the floor of the mouth-cavity is the tongue, a narrow elongated fleshy organ,
bifid in front. It is believed that, as with snakes, the bifid tongue of lizards is
concerned with smell as well as taste. There is evidence that scent particles
are picked up by the tongue tips and introduced into the Organs of]acobson,
which are accessory organs of smell (p. 481). Mucous and salivary glands occur.
In the single genus of truly poisonous 1 lizards (Heloderma) salivary glands, as
in many snakes, are specialised as venom-producing organs. The poison gland,
however, is probably a modified infra-labial and therefore not homologous with
that of snakes (p. 535).
The cesophagus leads into the stomach (Fig. 317, stom.), a muscular-walled
cylindrical organ which, though expansible, is little wider than the resophagus.
Digestion begins in the stomach and is probably completed after the material
passes through the pyloric sphincter into the small intestine, which is relatively
abbreviated, as in most carnivorous animals. There is a hepatic portal sy stem.
At the point where the small intestine joins the wider large intestine or rectum,
1 It is sometimes erroneously claimed that the Australian goannas (Varanus spp.) are
venomous, but the poisoning from their bites is merely the result of bact erial infection such as
might occur after attack by a dog.
 4>-
'-l
ovd N

ap.co

I.

za.;:-1-.---
inz.na tr h.JI

N
0
0
t""
0
CJ
><

FIG. 317.-Lacerta: Visceral relationships. Lateral dissection of female (semi-diagrammatic). au. auricles; b. del. bile-duct;
car. art. carotid artery; caud. art. caudal artery; cbl. cerebellum; cer. cerebral hemispheres; clo. cloaca; ccec. c<ecum; ccel. a. creliac
artery; dors. aort. dorsal aorta; ext. na. external nares; eye, eye; g. bl. gall-bladder; hy. basi-hyal; int. na. internal nares;
kd. kidney; lar. larynx; l. br. left bronchus; liv. liver; l. lng. left lung, opened to show internal structure; l. ov. left ovary ;
tned. medulla oblongata; mes. art. mesenteric artery; ces. resophagus; olf. olfactory bulb; opt. optic lobe; o~·d. oviduct (turned
aside); parov. parovarium; pnc. pancreas; sp. co. spinal cord; spl. spleen; stom. stomach ; tnge. tongue; lr. trachea; ur. bl. urinary
bladder; vent. ventricle.
 PHYLUM CHORDATA 473
the latter is produced into a short ccecum (Fig. 317, cac.). The liver (lr.) is
divided into right and left lobes, and a gall-bladder (g.bl.) lies in a cleft at the
lower margin of the right lobe. The whitish pancreas (pn.), of compound
structure and function (p. 152), is situated in the loop between the stomach and
the duodenum, the anterior part of the small intestine. The bile-duct from the

FIG. 318.-Lacerla: Visceral relationships.

Ventral dissection of male showing the alimen-
tary, circulatory, respiratory, and urinogenital
organs. The liver (lr.) is divided longitudinally
and its two halves are displaced outwards; the
alimentary canal is drawn out to the animal's
left; the cloaca with the urinary bladder and
posterior ends of the vasa deferentia is removed, :t.cl.v
as also is the right adipose body. a. co. anterior
cornu of hyoid; az. azygos or cardinal vein; b.
Tty. body of hyoid; c. caudal vein; c. ad. adipose
body; c. m. creliaco-mesenteric artery; ca.
c<ecum; cr. carotid a rtery ; d. ao. dorsal aorta;
du. duodenum; e. ju. external jugular vein; ep.
epididymis ; ep. g. epigastric vein ; f. a. femoral

,,.
art ery; f. v. femoral vein; g. b. gall-bladder;
i. ju. interna l jugular vein; i l . ileum; i. m.
posterior mesenteric arteries; k. kidney; Ia. o.
left aortic arch; l. au. left auricle; lg. lungs;
lr. liver; m. co. middle cornu of hyoid; p. a .
pulmonary artery; pc. pericardium; p. co.
posterior cornu of hyoid; pl. pelvic vein; pn.
pancreas; pt. c. postcaval v ein; pt. v. hepatic
portal vein; p. v. pulmonary vein; r. rectum;
r . au. right auricle; r. h. a. right h epatic artery;
sc. sciatic vein; scl. a. subclavian artery; scl. v.
subclavian vein; spl. spleen ; st. stomach; s. v.
sinus venosus; th. thyroid gland; tr. traGhea;
t. testis; v. ventricle. (After T . J. Parker.)

gall-bladder runs to, and through, the pancreatic tissue, where it receives
pancreatic juice from several pancreatic ducts. Then it reaches the duodenum
as a hepato-pancreatic duct. The stomach is attached to the body-wall by a
fold of peritoneum, the mesogaster ; the small intestine by a mesentery ; and
the rectum by a mesorectum. From the dorsal surface of the liver to the stomach
there extends a thin fold, the gastro-hepatic omentum. This is continued
backwards as the duodena-hepatic omentum, connecting the liver with the first
portion of the small intestine. The rectum leads into an extensive cloacal
VOL. II. GG
474 ZOOLOGY
chamber which is subdivided into a coprodeeum and urodeeum and finally a
proctodeeum which terminates in a sphincter. The posterior, but not the
anterior, peritoneum of the body cavity is pigmented black.
During the present dissection the following structures can be found :
r. The thyroid gland, a whitish, crescentic endocrine body on the ventral
wall of the trachea a short distance in front of the heart.
2. Two pairs of thymus bodies (anterior and posterior) in close relationship
with the origin of the paired carotid arteries. There is no evidence that they
are of endocrine function.
3· The spleen, a sac of blood-storage and other functions, is held by
mesentery near the pylorus (Fig. 317).
4· The paired endocrine adrenal glands (suprarenals), lying near the gonads
(p. 152). The cortical and medullary homologues are interdigitated as in
amphibians.
5. The adipose bodies (Fig. 318, c. ad), which are two masses of fat of some-
what semilunar shape in the posterior part of the abdominal cavity between the
peritoneum and the muscles of the body-wall.
Blood-vascular System.-The heart shows an advance on that of the
Amphibia even though the arterial and venous streams are probably not com-
pletely separate. The organ is enclosed in a thin transparent membrane, the
pericardium, and is lined with endocardium. It consists of a sinus venosus,
right and left auricles, and an incompletely divided ventricle. Fig. 319
shows the ventral and dorsal surfaces of the heart after removal of the
pericardium.
The reptiles show a gradual absorption of the sinus venosus. Chelonians
retain a large one, but in many it has disappeared, having been incorporated
into the right atrium, where its remnants are identifiable as valve-like struc-
tures at the entrances of the great veins. In Lacerta three venre cavre empty
independently into the sinus venosus. There are no valves. The sinus
venosus lies dorsal to the right auricle, into which it empties through a slit-
like sinu-auricular aperture (Fig. 320, a. c. v.) guarded by folds of the auricular
wall. The left auricle is the smaller of the two and receives the single pul-
monary vein on its dorsal wall near the interauricular septum. There is no
valve. Although thin, the interauricular septum is complete. It runs on to
a flap of the atrioventricular valve (a. v. v.), so that the blood passes from the
two auricles to the ventricle through two openings (Foxon et al.). The auricles
have their inner surfaces raised into a network of muscular ridges-the musculi
pectinati. Removal of the ventral wall of the ventricle reveals an almost
complete oblique interventricular septum (i. v. s.). Almost horizontal, this
separates the right ventral part of the ventricle into a cavum ventrale (c. v.).
Associated with the base of the pulmonary artery, this is sometimes termed the
 PHYLUM CHORDATA 475
cavum pulmonale. The rest of the ventricle constitutes the cavum dorsale,
which is in tum partly subdivided by myocardial trabeculce. Some of these are
fused, giving rise to a prominent ridge, or secondary septum, which partially
divides the cavum dorsale as well. The resultant left cavity is the cavum
arteriosum; the right the cavum venosum. The ventricle therefore consists
of three incompletely separated cavities.
From this tripartite ventricle spring the three principal arteries (aortce).
From the right side (cavum ventrale) originates the pulmonary artery (Fig. 3I9,
p. a.). Immediately dorsal arises the left systemic arch. Further dorsally,
I.e o.

r.av.c.
+-::r-r.a.
l.ov.c.

FrG. 319.-Lacerta: Heart and large vessels. Ventral (left) and dorsal views. /. a. left
auricle; l. a. v. c. left anterior vena cava; l. c. a. left carotid arch; l. p. a. left pulmonary artery;
l. s. a. left systemic arch; p. a. pulmonary artery; p. a. p. a. position of aperture of pulmonary
anery; p. i. a. s. position of interauricular septum; p. v. pulmonary vein; p. v. c. posterior vena
cava; r. a. right auricle; r. a. v. c. right anterior vena cava; r. c. a. right carotid arch; r. p. a.
right pulmonary artery; r. s. a. right systemic arch; s. v. sinus venosus. (After Foxon, Griffith,
and Price.)

and to the left, the right systemic arch takes origin. From this both carotid
arteries arise. Each principal artery is guarded basally by a valve.
The course of blood distribution may be as follows : venous blood is emptied
into the right auricle. Oxygenated blood from the lungs enters the left auricle.
During ventricular diastole (relaxation) each auricle empties into the cavum
dorsale of the ventricle. The blood which enters the cavum ventrale 'has to
do so from the cavum dorsale over the edge of the incomplete septum' (arrow
in Fig. 320). On ventricular systole (contraction) this blood will presum-
ably pass into the pulmonary arteries ; for the septum is so aligned that
when the ventricle contracts its free edge is brought against the opposite wall
of the ventricle. Thus functional division of the ventricle is brought about
in such a way as to connect the cavum ventrale with the base of the pulmonary
arch. The opening into the left systemic arch remains connected with the
476 ZOOLOGY
cavum dorsale' (Foxon et al.). In the Chelonia, on the other hand, the left
systemic arch opens to the right of the septum.
The substantial difference in size of the two auricles (see above) and the
differential capacity of the cavum ventrale and the cavum dorsale make it
impossible for equal quantities of blood to be sent to the lung and to the
systemic circulation during each heart-beat, as occurs in the double circulation
that has arisen in birds (p. 589) and mammals (p. 671). Some mixture in the

I.a . FIG . 320. -l..acerta:

Cardiac structure and cir-
o .p.v. culation. The ventral wall
has been removed from the
a .v.v. two auricles and the greater
part of the ventricle to
show cavum ventrale and
origin of the pulmonary
artery. a. c. v. aperture of
sinus venosus; a. p. v. aper-
ture of pulmonary vein;
a. v. v. auricular ventricular
valve; c. d. cavum dorsale ;
c. v. cavum ventrale; i . v. s.
interventricula r septum ;
l . a. left auricle; l. c. a. left
carotid arch; l . s. a . left
systemic arch; p. a. pul-
m onary artery; 1'. a. right
a uricle ; r. c. a. right carotid
arch; 1'. s. a. right systemic
arch ; v . p. valve at base of
pulmonary artery. (After
Foxon, Griffith, and Price.)

Reptilia of arterial and venous blood must inevitably occur. The most recent
work suggests that oxygenated blood from the left auricle, entering the left
part of the cavum dorsale, goes principally through both carotid arches, and
the right systemic arch. Some of the right systemic blood may be added via
the ductus caroticus to that in the internal carotid. De-oxygenated blood
from the right auricle is emptied into the right portion of the cavum ventrale
and passes into the pulmonary arteries for re-oxygenation at the lungs. The
blood in the right part of the cavum dorsale goes principally into the left
systemic arch and probably passes through the ductus caroticus on that side
into the carotid.
 PHYLUM CHORDATA 477
As regards circulation in general, the two aortic arches curve backwards
round the <:esophagus, one to the right and the other to the left. The two arches
are non-equivalent and the right carries more highly oxygenated blood than the
left. The right arch gives off two subclavian arteries as it passes to the mid-
dorsal position, and then runs posteriorly as the median dorsal aorta. The
left arch also passes to the mid-dorsal position, but it communicates with the
dorsal aorta by a small aperture only and is continued mainly as the cmliaco-
mesenteric artery (c. m.). This shortly divides into two trunks: a cmliac
(Fig. 317, cal. a.) supplying the stomach, spleen, pancreas, duodenum, and
left lobe of the liver, and an anterior mesenteric supplying the posterior part of
the small intestine. Three small posterior mesenteric arteries given off farther
back supply the large intestine. Posteriorly, after giving off renal and genital
branches, and a pair of large iliacs to the hind-limb, the dorsal aorta is con-
tinued along the tail as the caudal artery (Fig. 317, caud. art.). Throughout
its length, in addition to the larger branches mentioned, the dorsal aorta
gives origin to a regularly-arranged series of pairs of small vessels, the inter-
costal and lztmbar arteries. These give off branches that enter the neural canal,
and others that supply the muscles and integument.
The venous blood from the tail is brought back by means of a caudal
vein (Fig. 318, c.). This bifurcates at the base of the tail to form the two
renal portal veins each of which carries blood to an adjacent kidney. Before
entering the kidney each gives off a pelvic (lateral) vein (pl.) These are
entered by the femoral and sciatic veins from the hind-limbs before they unite
to form the median epigastric or abdominal (ep. g.), which eventually enters
the left lobe of the liver. The efferent renal veins, carrying the blood from the
kidneys, combine to form a pair of large trunks, which soon unite to form the
median postcaval. The postcaval runs forwards towards the heart, and, after
receiving the wide hepatic vein from the liver, enters the sinus venosus.
Two precavals, right and left, carry the blood from the anterior extremities
and the head to the sinus venosus. The right precaval is formed by the union
of the internal and external jugular and the subclavian. On the left side the
precaval is formed by the union of internal jugular and subclavian, the left
external jugular being absent. The liver is supplied, as in other vertebrates,
by a hepatic portal system of vessels, blood being carried to it by a portal vein.
This is formed by the union of gastric, pancreatic, splenic, and mesenteric
veins. The spleen (Fig. 317 and 318, spl.) is a small red body lying in the
mesogaster, near the posterior end of the stomach. The reptilian erythrocyte
is nucleated and leucocytes of various kinds occur. The lymphatic system is
highly developed. The main trunk divides and enters the precavals. Lymph-
hearts occur.
Respiratory System.-Lizards, unlike frogs, have no auxiliary means of
respiration. Although the lungs of Sphenodon and others are little more
478 ZOOLOGY
elaborate than those of anurans, those of most lizards, including Lacerta,
show a considerable advance. Each organ (Fig. 317, lg.) is a fusiform sac,
the inner lining of which is raised up into a network of delicate ridges giving
the appearance of a honeycomb. These ridges are much closer and more
numerous towards the anterior than towards the posterior end of the lung.
In this arrangement we see what is probably a hint of the highly complex
system later to be observed in birds in which dead-space air is almost eliminated
by the circulation of the air-current across the anterior respiratory surface
during both inspiration and expiration.
Air is drawn in from, and driven back to, the nostrils by means of a mechan-
ism quite unlike that of fishes and Amphibia. Air is sucked into the elastic
lungs as the trunk region in general is expanded by muscles which elevate the
ribs. We shall see that this method of respiration is characteristic of the
truly terrestrial vertebrates. In the Mammalia it is further complicated by
the presence of the muscular diaphragm (Fig. 464, p. 676).
The structures lying between the nostrils and the lungs are a slit-like aper-
ture, the glottis (situated behind the tongue), into which it leads, and a short
chamber, the larynx. The larynx is less prominently developed than in many
of the Amphibia. Its walls are supported by cricoid and arytenoid cartilages.
From the larynx an elongated cylindrical tube, the trachea, passes backwards
on the ventral side of the neck. Its wall is supported by a large number of
small rings of cartilage, the tracheal rings. Posteriorly the trachea bifurcates
to form two similar but narrower tubes, the bronchi, one of which enters each
lung.
Brain.-The reptilian brain (Figs. 321, 322) lies quite loosely within the
cranial cavity, surrounded by a tough fibrous membrane, the dura mater.
It does not fill the cranium to nearly the same extent as does the brain of a
mammal, a fact to be considered in the interpretation of endocranial casts of
fossil forms. The hinder part of the brain is well protected by the bones of
the back and base of the skull as well as those of the roof. The front of the
brain, however, is more lightly covered, for, as we have seen, the side walls of
the cranial cavity in the region between, and in front of, the eyes are often
mainly cartilaginous or membranous.
The brain is small, both absolutely and relatively. In Lacerta it makes up
only about 0·5 per cent of the body weight. In certain dinosaurs, whose bulk
can be estimated at about 20 tons, the brain was only a few inches long. In
general structure it falls into the typical vertebrate pattern ofjore-, mid-, and
hind-brain, the last-named being continuous with the spinal cord. In the
fore-brain are paired ovoid cerebral hemispheres (relatively bigger than those of
Amphibia) and their contained lateral ventricles. Anteriorly each hemisphere
continues into an olfactory stalk that terminates bluntly in an olfactory bulb
which received fibres from the olfactory epithelium of the nasal cavity. A
 PHYLUM CHORDATA 479
vomeronasal nerve from the organs of jacobson (p. 481 and Fig. 374, p. 544);
entering an accessory olfactory bulb
is prominent in many reptiles.
The extensive neopallium of the
mammalian cerebral cortex (p. 678) A B
probably has its homologue in a
small dorsal area in each cerebral
hemisphere which appears to be con-
ck.p
nected with the thalamus. Little
information, however, is available
concerning the significance of the
reptilian cortex.
The most prominent part of the
hemisphere is the corpus striatum,
which occludes much of the lower
and lateral aspects of each ventricle.
t:.h o.t
This massive organ reaches an even
('
greater development in birds (p. 593)
in which it appears to be concerned
with innate ('instinctive ') patterns
of behaviour. Behind and between
the cerebral hemisphere and in close
association with the corpora striata c.h o.l

lies the small rounded diencephalon

and enclosed third ventricle (Fig.
322). With the latter the lateral
ventricle of each side communicates
by means of the interventricular
foramina of Monro. Each lateral
m.o
ventricle contains a vascular, epi- FIG. 321.-Lacerta: Brain and cranial nerves.
thelial choroid plexus which secretes A, dorsal, with the left h emisphere (c. h.) and
optic lobe (o. l.) opened. B, ventral. C, from
cerebrospinal fluid. A small thala- the left . D, in longitudinal vertical section.
mus, which appears to receive sen- a. c. anterior commissure; aq.c. s.crura aqueduct of
Sylvius; cb. cerebellum; c. cerebri;
sory impulses from the brain-stem c. h. cerebral hemispheres ; ch. p. choroid plexus;
corpus striatum; f . m. foramen of Monro;
and cord, occurs in the diencephalon. c.inf.s.infundibulum; m. o. m edulla oblongata; o. c.
Below is the large unpaired hypo- optic chiasma; o.l. opticlobes; oif. olfactory bulbs
with their peduncles or tracts; o. t. optic tracts ;
thalamus, which is probably con- o. v. aperture between aqueduct of Sylvius and
ventricle ; p. c. posterior commissure ; pn.
cerned with the integration of optic pineal apparatus; pty . pituitary body; v. 3, 3rd
metabolic and visceral activities. ventricle (diacrele); v. 4, 4th ventricle (metacrele) ;
I - XII, cranial nerves. (After Parker.)
The brain-floor is produced ventrally
into a tubular process, the infundibulum, which ends in a compound pituitary
gland or hypophysis of diverse endocrine function (see p. 149). There is a
480 ZOOLOGY
connecting hypophysial portal system (Fig. 95) between brain and gland. The
roof of the diencephalon is produced into a median outgrowth, the p ineal
apparatus (Fig. 321, D, pn.; Fig. 322, Z) , which is divided into two parts.
One of these has connected with its distal extremity an eye-like structure, the
parietal organ or pineal eye (Fig. 322, pa.), lying in the parietal foramen, while
the other is the pineal organ or epiphysis (see p. 128).

FIG. 322 .- Lacerta: Brain and hypophysis. Longitudinal section in an embryo ( x 26) . ao.
olfactory area; bl. blood-sinus; bo. olfactory bulb; c. cerebellar commissures ; ca. anterior com-
missure; ch. superior (habenular) commissure; ck . central canal of spinal cord; cp. posterior com-
missure; cpa. anterior pallial (hippocampal) commissure; cpa. posterior optic commissure; cpp.
posterior pallial (aberrant) commissure; cs. spinal commissure; cw. swelling of optic chiasma; e1 •
paraphysis; hm . cerebral hemisphere ; hy. hypophysis; in. cartilaginous inter-orbital septum;
J (and s). ventricle of infundibulum ; opt. optic chiasma ; pa. parietal organ ; pch. choroid plexus
on medulla oblongata; R . 4th ventricle; ro. optic recess; sl. sulcus intraencephalicus posterior;
tp. tuberculum posterior superius ; tr. lamina t erminalis; vb. ventral flexure of medulla oblongata;
vc. valvula cere belli; vp. posterior medullary velum ; vt. v elum tra nsversum; Z . epiphysis.
(From Wiedersh eim, T. J. Parker, after K. von Kupffer.)

The mid-brain roof (tectum) is relatively elaborate. It has been established

that its large paired optic lobes receive most of the fibres from the optic nerves.
It is probable that the mid-brain is an important co-ordinating centre function-
ing in a manner somewhat comparable with the mammalian cerebral cortex
(p. 678). The cerebellum (of the hind-brain) is small and lacks lateral lobes and
is concerned primarily with equilibrium. It surmounts the medulla oblongata
on the ventral surface of which there has not arisen any structure corresponding
to the pons found in the Mammalia. Of the twelve cranial nerves that
 PHYLUM CHORDATA

arise mainly from the mid- and hind-brains, the vagus (Xth) and the cranial
root of the accessory (Xlth) may be fused. From the medulla, the spinal cord
is continued backwards throughout the length of the neural canal, becoming
slightly dilated opposite the origins of the two pairs of limbs and tapering
greatly towards the posterior end of the tail. For an outline of the autonomic
nervous system see pp. ng, 412.
Endocrine glands.-These fall into the general vertebrate pattern (p. 147).
The thyroid is single but may be paired in some species, the two organs some-
times being connected by a narrow band of tissue. Near the arterial arches,
and some distance from the thyroid, occur bodies that are probably homologous
with the mammalian parathyroids. The
adrenals are situated close to the gonads;
their medullary and cortical homologues
are interdigitated. For the pituitary, see mn
p. I49·
Organs of Special Sense.-The olfactory
organs are more complex than those of the sc
anura. The nasal cavities (Fig. 8g, p. 138)
open at the end of the snout by the exter-
nal nares, and into the cavity of the mouth
FIG. 323. -Lacerta: Audioequilibra-
by a pair of slit-like internal nares situated tion. Lateral view of right m embranous
near the middle line of the palate. The labyrinth. aa. anterior ampulla; ac. audi-
tory nerve; ade. opening of the ductus
external aperture opens into a vestibule endolymphaticus; ae. external ampulla;
ap. posterior ampulla; br. basilar branch
through which the air passes to the sensory of nerve; ca. anterior semicircular canal;
epithelium of the nasal or olfactory cavity ce. external semicircular cana l; cp. pos-
terior semicircular cana l; cus. canal con-
proper. This contains a convolut ed turbi- n ecting utriculus a nd sacculus; de. ductus
nat bone over which the mucous membrane endoly mphaticus ; l. lagena ; mb . basilar
membrane ; raa, rae, rap. rl. branches of
extends. Below each nasal cavity, and auditory nerve; s . sacculus; 55. common
cana l of communicat ion between a nterior
separated from them in the adult, is an and posterior semic ircular canals and
organ of Jacobson or vomero-nasal organ after utricle; 11. utriculus. (From \Vicdersheim,
I<etzius.)
(].].), a small sac with strongly pigmented
walls supported by cartilage. Each opens into the mouth by a duct on either
side at the front of the palate and is supplied by a separate vomeronasal branch
of the olfactory nerve known as the vomero-nasal nerve. This sense-organ is
important in most lizards and snakes and appreciates scent particles introduced
into it by the tongue tips. There is experimental evidence that in some reptiles
it plays a part in activities such as trailing prey and locating members of the
opposite sex. Paired ducts communicate with the buccal cavity, probably
enabling the olfactory appreciation of substances held in the mouth.
The eye (Fig. 375, p. 545) has a cartilaginous sclerotic with a ring of small
bones supporting it externally. There is a cushion-like pecten, a vascular pig-
mented process similar to that occurring in birds (p. 596). This projects into
 ZOOLOGY
the inner or vitreous chamber of the eye. Cones are proportionally very
numerous in the retina. Accommodation is brought about by the ciliary
muscles, which press the ciliary body against the peri-
phery of the lens and deform into it a more rounded
shape suitable for close vision. Two glands, the lachrymal
and the Harderian, lie in the orbit and produce moisten-
ing and cleansing secretions. The eyes are protected
also by lids. In addition, there is a third lid, the
nictitating membrane, which sli.des across the moist cornea.
The ear consists of two principal parts, the internal
ear or membranous labyrinth, and the middle ear or
tympanic cavity. The latter is closed externally by the
tympanic membrane (Fig. 93, p. 144). It communicates
with the cavity of the mouth by the Eustachian tube,
which is narrower and longer than in frogs. The inner
wall of the tympanic cavity is formed by the bony wall
of the auditory region of the skull, in which there are
two fenestrre-the fenestra ovalis and the fenestra rotunda.
The columella stretches across the cavity from the
FIG. 324.- Lacerta: tympanic membrane, and is fixed internally into the
Male urinogenital sy-
stem. The ventral wall membrane covering over the fenestra ovalis.
of the cloaca is removed,
the bladder turned to The parts of the membranous labyrinth (Fig. 323) are
the right, and the peri- enclosed by the bones of the auditory region. Between
toneal covering of the
left testis and epidi- the membranous wall of the labyrinth and the surround-
dymis is dissected away.
bl. urinary bladder ; ing bone is a small space containing fluid, the perilymph.
b. lg. fold of peritoneum The labyrinth itself consists of the utriculus with the three
supporting epididymis;
ell. anterior and cl 2 • semicircular canals and the sacculus with the lagena. The
posterior divisions of utriculus (u.) is a cylindrical tube, bent round at a sharp
the cloaca; ep. epidi-
dymis; k . kidney; mso. angle: the semicircular canals (ca., ce., cp.) are arranged
mesorchium; p. copu-
latory organs, of which as in vertebrates in general (p. 145). A narrow tube, the
the right is shown re- ductus endolymphaticus, leads upwards towards the roof
tracted (p') and the left
everted (p); r . m. r e- of the skull and ends blindly in the dura mater. The
tractor muscle of latter ;
r. ridge separating an-
sacculus is large and rounded. The lagena (l.) forms a
terior and posterior flattened, not very prominent, lobe, and is of simple
divisions of cloaca; ret.
rectum; ret'. its opening form. It contains what is probably a special organ of
into the cloaca; t. testis; hearing, the papilla basilaris.
u . g. urinogenital papilla
and aperture; v. d. vas Urinogenital System.- The kidneys (Figs. 324 and
deferens. (After T. J.
Parker.) 325, k.) are a pair of irregularly shaped, dark red
bodies, each consisting of two lobes, anterior and
posterior, situated in close contact with the dorsal wall of the posterior por-
tion of the abdominal cavity, and covered with peritoneum on their ventral
faces only. Their posterior portions, which are tapering, are in close contact
 PHYLUM CHORDATA

with one another. Each has a delicate duct, the ureter, opening posteriorly
into the urodamm of the cloaca. From here water is reabsorbed. A urinary
(allantoic) bladder (bl.) (see alsop. r5g), a thin-walled sac, opens into the cloaca
on its ventral side. To reach the bladder, from which fluid is probably
reabsorbed, the urine must first pass into the
cloaca and remain undischarged.
In the male the testes (Fig. 324, t.) are two
ocl'
oval white bodies. The right testis is situated
just posterior to the right lobe of the liver; the
left organ IS somewhat farther back. Their in-
ternal structure, and functions, follow the general
craniate pattern (p. 152). Each testis is attached
to the body-wall by a fold of the peritoneum, the
mesorchium (mso.). The epididymis (ep.) extends
backwards from the inner side of each testis,
and passes behind into a narrow convoluted tube,
the vas deferens or spermiduct (v. d.), which opens
into the terminal part of the corresponding
ureter. Grooves guide the spermatozoa from the
urodreum to twin penial structures (' hemipenes ')
(Fig. 324, p. 1 ), which arise from the proctodreum
as eversible folds. Vascular and erectile, these
assume a cylindrical form when everted (Fig. 378,
p. 550). Each has a dilated and bifid apex and
FIG. 325.-Lacerta: Female
is used singly and independently of the other for urinogenital system. The ven-
the discharge of spermatozoa into the female tral wall of the cloaca, the
urinary bladder, the posterior
tract. end of the left oviduct, and the
Fertilisation must, of course, be internal in ovary peritoneal investment of the left
and oviduct :rre removed.
truly terrestrial vertebrates and occurs in the b. lg. broad ligament; c/ 1 • anter.
anterior part of the oviduct before the enclosure ior and cl 2 • posterior divisions of
the cloaca; k. kidney; mso.
of the egg by its protective coverings. m esova rium ; od. left oviduct;
od'. its peritoneal aperture; od" .
The ovaries (Fig. 325, ov.) are a pair of irregu- aperture of r ight oviduct into the
larly oval bodies with their surfaces raised up cloaca; ov. ovary; ur. aperture
of ureter. (After T. J. P arker.)
into rounded elevations which mark the position
of the oocytes. They are situated a little farther back than are the testes,
and each is attached to the body-wall by a fold of the peritoneum, the m eso-
varium (mso.). The eggs are extruded into the body cavity (od'.) and pass
into the anterior ends of the oviducts (od.). These are thin-walled, wide
tubes which communicate with the urodreum. Their opening (od.") is distinct
from, and a little in front of, those of the ureters. A fold of the peritoneum,
the broad ligament (b. lg.), attaches the oviduct to the body-wall.
 ZOOLOGY

INTERRELATIONSHIP OF THE REPTILIAN GROUPS

One of the most noteworthy features of the reptiles is the evolution of the
temporal region of their skulls. The structure of this region has led to their
classification into five principal groups, but it should be borne in mind that
a considerable difference of opinion exists concerning the true affinities of
many animals that are still known only by fragments of endo- or exoskeleton.
The basic classification of the Reptilia on temporal openings (Fig. 326)
illustrates certain early evolutionary trends, but it is now generally believed

supratemporal poslfronlal

poJiorbillll
SYNAPSID
PARAPSID

squ•moJ.ol poslorbillll
ANAPS/0
squamosal poslorbilal

EURYAPSID
DIAPS/0
FIG. 326.-Reptilia: Basic skull structure. For explanation, see Table 3, p. 485. (After
Colbert.)

that such a classification can also be very misleading. For example, the
ichthyopterygians (p. 490) and sauropterygians (p. 492), long classified
together in the ' Parapsida ', are now thought to be only very distantly
related. The two immense assemblies, Lepidosauria (p. 495) and Archosauria
(p. 498), both often believed to show the diapsid condition, have also diverged
separately from the primitive reptilian stock. An outline of the basic grouping
is given in Table IV.
The most primitive forms, the Cotylosauria ('stem reptiles'), had a complete
bony roofing in the temporal region-the anapsid condition-as did the early
 PHYLUM CHORDATA

Amphibia from which reptiles originated (see pp. 458, 486). According to
most authorities, this is still found in living Chelonia (sometimes modified
by emargination), which, together with the cotylosaurs, form the sub-class
Anapsida (Fig. 484).
From this primitive skull may have evolved two principal types. The
first of these has a single vacuity in each side, and is known as synapsid. The
vacuity was bounded at first by the postorbital and squamosal bones above,
and the squamosal and jugal bones below, but later the parietal appeared
within the opening owing to the enlargement of the vacuity and reduction in

TABLE IV. Temporal Openings of Reptiles.

Condition. I Number and Arrangement. Principal Groups.

Anapsid None. Solid roof to skull Cotylosaurs, chelonians *(turtles)

Synapsid One. Low behind eye with postorbital Pelycosaurians and mammal-like
and squamosal meeting above reptiles

Parapsid One. High behind eye usually with post- Mesosaurs and ichthyosaurs *
frontal and supratemporal meeting
below

Euryapsid One. Behind eye bordered below by Protorosaurs and sauropterygians

postorbital and squamosal

Diapsid Two. Postorbital and squamosal usu- Eosuchians, Rhynchocephalia

ally meet between them (Sphenodon), dinosaurs, pterosaurs,
Crocodilia Squamata (lizards and
snakes)
I
* Condition sometimes or always shown in modified form.

size of the postorbital. Reptiles having this type of skull-the Synapsida-

are mammal-like and certainly include the ancestors of mammals which them-
selves possess the synapsid skull further modified by the loss of the post-
orbital and quadratojugal bones, and by the inclusion of the quadrate within
the ear.
Another modification of the primitive type is known as diapsid. Here
there are two vacuities in the temporal region, with the postorbital and
squamosal meeting between them. This condition is found in many reptiles
(e.g. Crocodilia, 'dinosaurs', pterosaurs). In a modified form it occurs in
lizards, snakes, and persists also in birds. The lizards lost the quadratojugal,
thus opening the lower vacuity below. The snakes, the most modern group
of reptiles, carried this modification still further by reducing or losing the post-
orbital and losing the jugal. The birds lost the postorbital, with the result
that the two vacuities and the orbital opening are confluent.
A few small reptiles (e.g. Arceoscelis) have a single temporal vacuity differ-
ing from the synapsid type in that the postorbital and squamosal meet below
the opening. Some authorities believe this vacuity is homologous with the
 ZOOLOGY

upper vacuity of the diapsid skull, and that the vacuity of the synapsid skull
is homologous with the lower vacuity of the diapsid skull, and so have given it
another name-parapsid. Often included in this sub-class are two aquatic
groups, the M esosa~tria and Ichthyosauria, which have single temporal vacuities
in the skull. Their skulls, however, are peculiar, and there is not a great deal
of evidence in the skeletons of these reptiles to ally them to the other orders.
It appears, therefore, that the primitive reptiles with a completely roofed,
or anapsid, skull gave rise to the Synapsida, and to the various diapsid and
parapsid groups.
SUB-CLASS ANAPSIDA
ORDER COTYLOSAURIA
The oldest reptiles, the cotylosaurs, range from the Upper Carboniferous
to the Upper Trias when they became extinct. Many of them show little
advance on their ancestors, the labyrinthodont Amphibia. The group is
probably mixed in origin. The members composing it are held together
chiefly by two characters : the complete roofing of the skull, and the flattened,
plate-like pelvis. These reptiles showed few specialisations, and varied in
size from about a foot to 10 feet in the case of the parasaurs. Two sub-orders
are recognised as follows :

SUB-ORDER CAPTORHINOMORPHA
These (e.g. Limnoscelis, Captorhinus, Labidosaurus (Fig. 310, p. 358)) were
primitive Lower Permian cotylosaurians which varied considerably in structure.
They had high, narrow skulls contrasting with the typically flattened amphibian
form. At the same time many of the characteristic cranial features of the
early Amphibia were retained. The skull was completely roofed and strongly
ossified. The amphibian otic notch was lost. The position of the tympanic
membrane has not yet been certainly established. These animals seem to
have been the forerunners of the Synapsida.

SUB-ORDER DIADECTOMORPHA
These (e.g. Diadectes, Procolophon) survived into the Triassic. These
probably carried themselves clear of the ground, and some possessed remark-
able teeth that suggest a herbivorous diet. They developed a peculiar otic
notch and a short and heavy skull. Parasaurus had a large skull studded
with protuberances and plate-like bones along its dorsal surface. The sub-
order includes the majority of the cotylosaurs and has some features in common
with the Diapsida.
ORDER CHELONIA
The principal trend in amniote evolution involved, of course, adaptation to
terrestrial conditions but in all classes from Amphibia to Mammalia various
 PHYLUM CHORDATA

groups retreated to, or reconquered, the water. The sea in particular, with its
rich, unending harvest of food, has been repeatedly invaded by reptilian, avian,
and mammalian lines.
The turtles (Fig. 327) including tortoises, probably represent an early off-
shoot from the primitive cotylosaurs. They retain certain unmistakable anapsid
characteristics. No convincing explanation has been forthcoming to account
for the development of, and retreat into, their extraordinary, box-like exo-

FIG. 327.-Sub.class Anapsida, Order Chelonia, Sub-order Cryptodira, Farniiy Chelonidre.

Chelonia. The Green (Edible) Turtle (C. mydas) re-entering sea after nocturnal egg-laying on a
tropical beach. (Partly after flash-light photograph: Sarawak Museum journal.)

skeleton; and it is of high interest that, having established this peculiarly

bizarre armour, they have, in many respects, remained stable and relatively
unaltered since the Triassic some r6o million years ago (p. 3).
Living turtles-they are generally called tortoises when essentially terres-
trial-range from a few inches to about 6 feet in length. One turtle (Dermo-
chelys) weighs considerably more than half a ton. Both herbivorous and
carnivorous forms exist. There are more than two hundred living species,
some of which have returned to an almost completely aquatic life.
Their early development is still obscure. A good series of specimens is
available from the Triassic onwards, but their pre-Mesozoic history is but
 ZOOLOGY
sketchily discernible. Permian specimens would tell palreontologists a great
deal-but no undoubted chelonian has been found in these deposits. Possibly
Eunotosaurus, of mid-Permian age, is close to the main line of chelonian ances-
try, but the skull roof of the specimen is damaged; it is not known if it had
a temporal aperture. Eunotosaurus still possessed teeth (see below) ; and
although the limb-bones were badly preserved, the smaller number of vertebrre
and broad ribs are suggestive, even though the animal had neither true carapace
nor plastron (p. 527). The plates of Eunotosaurus were almost spatulate
rib-expansions, and it is now generally accepted that the characteristic plates
of chelonians develop not from ribs, but from separate bony dermal plates
which unite with each other and with supporting ribs and vertebrre. The
more restricted plastron of the turtles appears to have arisen from abdominal,
and, anteriorly, from clavicle and interclavicle elements of the ancient dermal
pectoral girdle.
In embryonic development the girdles, more or less inside the ribs in modern
adults, do not change their position. Instead, the adult condition arises by
the radial expansion of the carapace (which becomes fused with the ribs early
in development) and plastron until the margins of the developing armour
extend over, enclose, and protect the girdles (Ruckes). At the same time,
however, there is no proof that phylogeny has duplicated ontogeny.
Once established, the turtles flourished both in numbers and, often, in in-
dividual size. The Cretaceous Archelon was about 12 feet long with a similar
approximate width across the flippers. Protected by their grotesque armour
and secure from too drastic competition in specialised ecological niches, some
lines remained relatively abundant in the Crenozoic after most of their Mesozoic
relatives had vanished. Of the larger forms, a horned turtle Meiolania (a
giant with a skull about 2 feet wide) survived in Australia until the Pleistocene.
The only considerable, but still relatively superficial, changes undergone
by chelonians have been various reductions in armour and the modification of
land-limbs into oar-like flippers. In sea-going turtles the proximal limb-
elements are compacted ; the distal ones are lengthened, and support the
paddles with which the animal skulls swiftly alopg. The retreat from the
land and terrestrial predators has enabled certain marine forms (e.g. the
modern Dermochelys) to reduce their weight by an enormous reduction in
exoskeleton. Most people believe all chelonians to be excessively slow-
moving, but marine turtles, and Dermochelys in particular, are by far the
fastest reptiles and, surprisingly, rival many large land tetrapods for speed.
Terrestrial chelonians, though always slow, have often improved their agility
by reduction of armour, particularly on islands free from predators. Others
have developed hinged armour (e.g. Cinixys); and the African Testudo tornieri
has reduced its dermal armour to be sufficiently soft-bodied to squeeze between
crevices in the rocky areas in which it lives.
 PHYLUM CHORDATA

Chelonians are said to live to a great age, but few authenticated data are
available. The famous turtle with 'G. Washington, I75I' carved on its
carapace is believed to have been a fraud, so to speak; and the Napoleonic
giant of St. Helena, reputed to have lived more than rzo years, was probably
'composition of two animals whose period of residence ... overlapped'
(Loveridge, cited by Carr). Gilbert White is said to have kept a tortoise
aged forty when acquired, for another fifteen years; and there is a famous
tortoise, believed to have been left by Captain Cook in 1777. that still lives in
Tonga. This scorched (by a bush fire) and battered veteran resides in the
Palace grounds and bears the title of Tui (High Chief) M alila and is 'the
greatest character, outside the Royal Family', in the islands (Snow). The
animal is said to have been actually branded by Cook. In 1923 it was reported
to be blind and that, when walking, it 'creaked like an ox-cart' (Flower).
It was still alive in 1956. If, as seems probable, this venerable reptile (which is
traditionally given a token portion of State banquets) is in fact one of the
original pair left by Cook, it must be at least r8o years old.
The modern chelonian skull is without temporal vacuities (p. 484). The
modern body is enclosed in a shell of bony plates, consisting of a dorsal carapace
and a ventral plastron, partly of dermal and partly of endoskeletal origin.
There is usually on the surface an epidermal exoskeleton of horny plates.
The vertebrre and ribs of the thoracic region are firmly fused with the body
carapace, into the composition of which they enter.
The limbs are sometimes terminated by clawed digits adapted for terrestrial
locomotion, sometimes modified into the shape of flippers. There are no
teeth, and the jaws have a horny investment. The lungs are compound sacs.
In essentials the heart and brain resemble those of the Squamata (p. 496).
There is a median penis differing in structure from that in Squamata, but
essentially like that of crocodiles (p. 499).
The following modern groups are recognised :
Sub-order Pleurodira ('side-necked' turtles), of southern countries. These
date from the Mesozoic, and generally curve the neck and draw the head side-
ways under the shell. Families: Pelomedusidre, which are freshwater animals
of Africa, Madagascar, and southern Australia. In these the skull is covered
with horny shields, and the limbs are not paddle-shaped, and the neck is
completely retractible. Chelidre ('snake-neck' turtles) of South America
and Australia. These are rather like the above, but the neck is long and not
completely hidden when retracted. Kinosternidre (musk terrapins of North and
Central America).
Sub-order Cryptodira, the dominant and almost cosmopolitan chelonians
of to-day and which date from the Cretaceous. These retract the head by means
of an $-curvature of the neck. Families: Dermatemydidre (Central American
terrapins allied to the previous group). Platysternidre (freshwater-dwellers of
VOL. II. HH
490 ZOOLOGY
Asia). Emydidre (almost cosmopolitan freshwater reptiles, but absent from
Australia; they exhibit striking parallelism with the next family). Testu-
dinidre (including most chelonians; cosmopolitan except for Australasia).
These show gradations between terrestrial and aquatic extremes. They include
the diamond black terrapins (Malaclemy s) of the epicure, the famous Giant
Tortoises (Testudo) of the Galapagos, African and American gophers and many
others). Chelydridre (snapping turtles of North and Central American fresh-
waters with large heads, long 'alligator-like ' tails, rough shells, and projecting
folds of skin on neck and limbs). Chelonidre (true sea-going turtles of which the

FIG. 328.- Sub-class Ichthyopterygia, Order Ichthyosauria, F a mily Ophthalmosaunare.

Ophtltabnosaurus. The Mesozoic ichthyosaurs, bearing convergent resembla nces to extant
dolphins, seem to have been the most perfectly adapted of all aquatic reptiles. They could not
lay ashore (cf. Chelonia and Crocodilia). Fossilised young have been found in the cloacal region
of adults, suggesting a n ovoviv iparous mode of reproduction. (From B.M. (N.H .) catalogue.)

three best known are the essentially herbivorous Green or 'Soup ' turtle (Chelo-
nia), Hawksbill (Eremochelys) and Loggerhead (Caretta)). Trionychidre ('soft-
shelled' turtles with reduced carapace and plastron covered with skin, fleshy lips
and soft proboscis, and a flat and rounded form, from Africa, Asia, and North
America). Dermochelidre (Dermochelys = Sphargis, the' Leathery Turtle ' with
dermal skeleton consisting of a mosaic of bony plates embedded in the dorsal
skin).
SUB-CLASS ICHTHYOPTERYGIA
ORDER MESOSAURIA
The true position of the mesosaurs (e.g. M esosaurus) has not been established
with certainty. They occur in late Carboniferous or early Permian beds and
 PHYLUM CHORDATA 49I
were unquestionably reptiles that had become aquatic. They were freshwater
lake-dwellers with slender bodies some 3 feet long. Their hind-legs were much
the more powerful pair, and they had a long, laterally compressed tail. Their
vertebrre had features not unlike those of the cotylosaurs from which they were
possibly derived as an exceedingly ancient branch.

ORDER ICHTHYOSAURIA
These are fully marine, superficially fish-like, reptiles with a skull with a
single lateral temporal vacuity (e.g. Ichthyosaurus, Ophthalmosaurus (Fig. 328)).
Some were of very large size (30 or
40 feet long), with a somewhat fish-
like body and a large head produced
into an elongated snout or beak.
They had no neck but possessed an
elongated tail and limbs in the form
of swimming-paddles. The vertebrre
were amphiccelous. A sacrum was
absent, so that only pre-caudal and
caudal regions are distinguishable.
The ribs in the cervical region had
two heads for articulation with the
vertebrre. A sternum was absent,
but there was a highly developed
system of abdominal ribs. The skull
was produced into an elongated
rostrum, formed chiefly of the pre-
maxillre, and with small nostrils
situated far back. The orbits were
large and contained a ring of bones FIG. 329.-Ichthyosauria: Locomotory speci·
lisations. The tetrapod limbs became paddles,
developed in the sclerotic. A colu- and the tail scull-like. The more primitive
mella was present and articulated forms (e.g. Mixosauridre) of the T riassic still
possessed tails of a fairly ' typical' reptilian form
with the quadrate, and there was a (top left figure) . Advanced forms (e.g. Ichthyo-
sauridre of the Jurassic a nd Cretaceous) had
large parietal foramen. The quad- evolved a homoceral tail superficially similar to
rate was immovably fixed to the With that of many modern teleosts (bottom left ).
this caudal development there was a re-
skull. The pterygoids met in the duct ion in the size of the pelvic paddles. At
the same t ime there occurred a marked increase
middle line and extended forwards in the size of the pectoral paddles (bottom right)
to the vomers, and so separated which became stabilisers functionally similar to
those of modern sharks. (Redrawn from Colbert,
the palatines, as in Sphenodon. The after various authors.)
pectoral arch contained only cora-
coid, scapula, and clavicle. There was no precoracoid. The coracoids were
broad bones which met ventrally for a short distance without overlapping.
The bones of the pelvis were not strongly developed. There was no sacrum,
492 ZOOLOGY

i.e. the ilia had lost their connections with the spinal column. The pubes
and ischia of opposite sides met in ventral symphyses. There was no obturator
foramen. Humerus and femur were both short, and the rest of the bones of
the limb were disc-like or polyhedral. The phalanges were numerous, and
usually arranged in more, but sometimes in fewer, than the usual five series.
The teeth were not in separate sockets, but set in a continuous groove.
Some forms were toothless. The tail vertebrre in the middle forms had a
characteristic downward bend which in later forms became almost a right
angle. The body thus extended into the lower lobe of a large vertical caudal
fin (Fig. 329).
The ichthyosaurs had become so fish-like that they would probably be
helpless ashore. It is almost certain that ichthyosaurs were unable to lay
their eggs ashore, and the presence of smaller, probably juvenile individuals
within the ribs of adults suggests, but does not prove, that they were vivi-
parous. The ancestry of the group is still in dispute. They appeared in the
Triassic, flourished exceedingly in the Jurassic, and disappeared mysteriously
in the Upper Cretaceous. Their distribution was almost world-wide.

SUB-CLASS SYNAPTOSAURIA
The synaptosaurians were late Palreozoic and Mesozoic aquatic reptiles
possessing a single temporal (' parapsid ') vacuity high in the skull. Many of
them, however, had little else in common, and the present arrangement may
not reflect their true affinities.
ORDER PROTOROSAURIA
This terrestrial group may represent the earliest synaptosaurians. The
order was one of the few such found in the Lower Permian. Arceoscelis may
have been an example, although recent evidence suggests that it was a cotylo-
saur (p. 486), and that the order Protorosauria may be more appropriately
placed near the eosuchians (p. 495). Arceoscelis was an agile lizard-like reptile
about a foot long with amphicrelous vertebrre and many other primitive features
in skull and girdles. Protorosaurians ranged to several yards in length.
Their affinities have not been established with certainty.
ORDER SAUROPTERYGIA
These were aquatic reptiles with a single temporal vacuity in the skull,
which is bounded below by a postorbital-squamosal arch. The coracoids are
single. There are three sub-orders :
SUB-ORDER NOTHOSAURIA
These were small and somewhat primitive Middle and Upper Triassic
amphibious reptiles with webbed feet. They were probably close to the
ancestry of the two following groups.
 PHYLUM CHORDATA 493

SUB-ORDER PLACODONTIA
The placodonts had long limb-bones and a specialised palate armed with
massive crushing teeth on the palatine bones which were probably adapted
to crushing molluscs. The vertebrre are deeply amphiccelous. The body
was covered with heavy bony scutes. They were confined to the Middle
Trias, e.g. Placodus, Cyamodus.

SUB-ORDER PLESIOSAURIA
These were aquatic reptiles, sometimes more than 40 feet long. They had
flattened bodies. Their appearance was quite distinctive, and Dean Buckland,
the celebrated rgth-century geologist (and divine), likened a plesiosaur to a

~.._. ,.}.(~­

----

FIG. 330.-Sub-class Synaptosauria, Order Sauropterygia, Sub-order Plesiosauria. Elasmo-

.'laurus. Plesiosaurs sculled on, or near, the surface with paddles supported by numerous finger·
bones that remained elongated and were never compressed like those of ichthyosaurs (Fig. 329).
Hip and shoulder musculature was powerful. The teeth were many, sharp, and long; and were
adapted to holding and swallowing struggling fish. Some plesiosaurs approached a length of 50
feet. The large flying reptile is Pteranodon (seep. 503).

snake threaded through a turtle's shell. This apt description, of course, refers
only to the general shape, for the body was not heavily armoured (Fig. 330).
The limbs were modified to form swimming-paddles. The vertebrre of plesio-
saurs were usually amphiccelous. The sacrum consisted of one to five vertebrre.
494 ZOOLOGY
There was no sternum. In the skull there were large premaxillre. A bony
secondary palate was absent and an ectopterygoid present. There was a well-
marked parietal foramen. The ring of bony plates (developed in the sclerotic
found in the orbit of some fossil reptiles) was not developed. The pectoral
arch presented some remarkable features. The coracoids always met in a
ventral symphysis, and the ventral portions (acromial processes) of the scapulre
often met. In front there was, in most cases, an arch of bone, consisting of a
median and two lateral portions, which probably represent the interclavicle
and the clavicles: in some forms this arch was reduced or absent. In the
pelvis the broad pubes and ischia met in the mid-line. The two symphyses
remained separate, or united and divided the space into two separate obturator
foramina. The teeth were implanted in distinct sockets.
The ventral elements of each limb-girdle (scapulo-coracoid and pubo-
ischium) were expanded and flattened and gave attachment to the large muscles
operating the paddles which sculled the reptile through the water.
A divergent stem, represented by Elasmosaurus, had an enormously long
neck containing nearly eighty vertebrre. At the other extreme was Kronosaurus
with a short neck and a skull about eight feet long. The Plesiosauria dated
from the Trias and extended to the Cretaceous.
The various reports of huge sea-serpents made from time to time have
led some to believe that plesiosaurs may still exist, as do crelacanth fishes
(p. 356), in remote seas .. For example, in r8og an animal with a 'snake-like
body' allegedly 56 feet long was washed ashore in the Orkneys and reported
as a sea-serpent. Home identified the only parts preserved as belonging to the
'Great Basking Shark' (Cetorhinus) (p. 227). D' Arcy Thompson, however,
believed the animal almost certainly to be a huge oar-fish (Regalecus (p. 297)).
Again, in 1848 the captain and officers of H.M.S. Dcedalus claimed that they
saw a 'sea-serpent' about 6o feet long with a head 'without doubt that of a
snake' off the African coast. As recently as 1917 officers and men of H. M.S.
Hilary thought they saw a 'sea-serpent' with a black and glossy head near
Iceland. It was sunk by anti-submarine fire, without surviving parts. Further,
the numerous accounts of the Loch Ness 'monster' will be recalled.
In view of the discovery of living crelacanths it would not be difficult to
imagine that a plesiosaur or related form might still exist in remote waters
except for one important fact-that sea-serpents must be lung-breathers and
so spend much of their time on the surface. Therefore, they could not remain
unobserved in these days of constant voyaging in all oceans, even including
southern waters right to the Antarctic pack-ice. Porpoises swimming in line
ahead are said to look very like a big undulating sea-serpent. ' Sea' -serpents
have been reported from the Victoria Nyanza where the undulatory movements
of three or four otters swimming in line conform closely to classic descriptions.
Undulating flights of migratory sea-birds, large fishes moving in line ahead,
 PHYLUM CHORDATA 495
attenuated masses of drifting seaweed and giant squids with trailing arms all
assume shapes likely to influence the credulous. Certain eels reach a length of
about 6 feet while stilllarvre (p. 352). This has provoked the suggestion that
some sea-serpent sightings may have referred to the occasional surface appear-
ances of the (as yet undescribed) adult.

SUB-CLASS LEPIDOSAURIA
Representatives of this sub-class typically have two temporal vacuities
(p. 484), though these have become reduced in the more specialised forms.
The surviving lepidosaurs belong to the Orders Rhynchocephalia (e.g. the
surviving Sphenodon) and Squamata (lizards and snakes).

ORDER EosucHIA

The eosuchians appeared in the Upper Permian. Youngina, the best-

known specimen, had an interparietal, and tabular bones, in the skull and
retained a parietal foramen. It had no antorbital foramen. There are
adequate reasons for believing it was extremely primitive and sufficiently
unspecialised possibly to have been ancestral to the Squamata (lizards, p. 496)
and therefore ultimately also to the serpents. (Thalattosaurs, once thought to
be eosuchians, are now believed to be aberrant marine lizards.)

ORDER RHYNCHOCEPHALIA

Members of this ancient order have lost the tabulars and interparietal of
the Thecodontia while retaining the large parietal foramen, in which, in the
living form, the non-functional median eye (p. 546) can still be made out.
There is no antorbital foramen. The group, although apparently never very
large, was once widespread. It is known from the Middle Trias (e.g. Hypero-
dapedon, Rhynchosaurus) of Africa, Europe, Asia, and the Americas. The sole
living representative of this group, Sphenodon (Fig. 331), is the lizard-like
Tuatara, which grows to about 2 feet long and has well-developed pentadactyle
limbs adapted for walking. A nearly identical- animal, Homceosaurus, has
been found in Jurassic rocks. The survival of Sphenodon in New Zealand is
probably due to the absence of terrestrial mammals (including Man) until
comparatively recent times. Today Sphenodon is restricted to coastal islands
where it burrows into the soft soil, emerging particularly at night. It is
rigidly protected by law. The anal opening of Sphenodon is transverse.
There are no' hemipenes' (p. 550). The vertebrre are amphicrelous, and inter-
centra are present. The ribs are single-headed and have uncinate processes.
There is a sternum and abdominal ribs. The teeth are acrodont. The lungs,
heart, and brain resemble those of Squamata.
 ZOOLOGY

ORDER SQUAMATA
The lizards (Lacertilia = Sauria) and snakes (Ophidia= Serpentes) are
the most successful living reptiles. In addition, the order contains numerous
extinct families. In general, Squamata are reptiles in which the skull has
secondarily lost either one (Lacertilia) or both temporal vacuities (Ophidia).

FIG. 331 .--Sub-class Lepidosauria, Order Rhynchocephalia, Family Sphenodontidre. Spl1eno-

do11. The single surviving species of the entire order is the Tuatara, now confined to coastal
islands of New Zealand. It grows to about 2 feet long. The skeleton retains features clearly
suggestive of an cosuchian ancestry.

The surface is covered with horny epidermal scales, sometimes with the addi-
tion of de!'mal ossifications. The cloacal aperture is transverse. There is a
pair of eversible penes in the male (Fig. 378, p. 550). The vertebrre are nearly
always proccelous. When present, the sacrum usually consists of two vertebrre.
The ribs have simple vertebral extremities. The quadrate is movably articu-
lated with the skull, and there is no inferior temporal arch. The limbs, when
present, are sometimes adapted for terrestrial locomotion and sometimes for
swimming (mosasaurs). The teeth are acrodont or pleurodont (see below).

SUB-ORDER LACERTILIA
Modern opmwn regards the lizards as derivatives of primitive Eosuchia
(p. 495), an ancestry they probably share with Sphenodon (Fig. 331). The
 PHYLUM CHORDATA 497

Lacertilia are Squamata in which the limbs are often well developed. The
mouth is capable of being opened to only a moderate extent. There are
usually eyelids and a tympanum. A sternum and an episternum generally
occur (Fig. 312).
Lizards appeared in the Triassic, and have been abundant ever since,
They include the Gekkota (geckos); Iguania (not to be confused with Aus-
tralian members of the Varanidre, the goannas), which includes the true iguanas,
agamas, and chameleons; Scincomorpha (a huge group, including our selected
type Lacerta); Amphisbrenia (burrowing, worm-like creatures superficially
resembling the amphibian Apoda, and of broadly similar habits); Anguimorpha
(e.g. slow-worms, Anguis, which possess, as do some other lizards, bony scales
or osteoderms); and the Platynota including the living Varanidre (of which
the East Indian Komodo Dragon is about 12 feet long and the extinct Australian
Pleistocene M egalania attained a length of more than 20 feet), and the Gila
Monster and Mexican Bearded Lizard (Heloderma).
Of the above, the Iguania and Platynota are known from the Cretaceous,
and the Ardeosauridre, possibly allied to the geckos, occurred in the Upper
Jurassic. It should be emphasised (as usual) that there is as yet no general
agreement as to the best method of classifying the Lacertilia.

SUB-ORDER OPHIDIA (SERPENTES)

Snakes appear to have been derived from a lizard ancestry in Mesozoic
times. They are Squamata with long, narrow bodies, typically devoid of
limbs. The mouth is usually capable of forming a relatively wide gape by
the divarication of the jaws (Fig. 332). The maxillre, palatines, and pterygoids
are freely movable. The rami of the mandible are connected together only
by elastic fibres at the symphysis, and so they can be widely separated. There
is no separate supra-temporal ossification. Sternum and episternum are
absent. As in lizards, a lachrymal duct carries away the tears and opens into
the mouth in close relationship with the duct of Jacobson's organ (p. 543).
The middle ear is degenerate and the tympanum is absent (see alsop. 144). As
in some lizards, eyelids are replaced by transparent spectacles (p. 545).
Reptiles that were possibly snakes occurred in the Lower Cretaceous, but
the Ophidia did not become abundant until the beginning of the Tertiary.
Some apparently marine forms (e.g. Palceophis) occurred in the Eocene, and
large constricting snakes like the modern boas and pythons are also found in
deposits of this age. The Colubridre (see below), containing the majority of
smaller, harmless types (e.g. grass-snakes), did not expand until the Oligocene.
The fanged, venomous types are only certainly identified from the Miocene
onwards. The venomous Elapidre and Viperidre (including the rattlesnakes)
are first seen as recently as the Miocene-Pleistocene.
As regards modern serpents, the sub-order Ophidia includes the Boidre
 ZOOLOGY
(often large, non-venomous, pythons and boas); Typhlopidre (small, blind,
non-venomous, worm-like burrowing reptiles); Leptotyphlopidre (possibly
allied to the above); Ilysiidre (harmless, burrowing snakes) ; Uropeltidre
(harmless, burrowing snakes allied to the foregoing). (Most of the above have
vestigial hind-limbs.) Xenopeltidre (containing a single genus, Xenopeltis,
from south-east Asia) ; Colubridre (the great majority of living snakes divided
into two groups-the harmless Aglypha, without grooved teeth, and the

FIG. 332.-Sub-class Lepidosauria, Order Squamata, Sub-order Ophidia, Family Viperidre.

Crotalus. Rattlesnake, showing wide divarication of jaws during a stabbing strike. Rapidity
of attack (like their speed over the ground (p. 521) is grossly over-rated: the present species (C.
viridis) was timed to strike at 7! feet per second. Note terminal caudal 'rattle' (see also Fig. 369,
p. 534). (From a flashlight photograph by Van Riper.)

venomous Opisthoglypha, with one or two enlarged grooved poison fangs);

Micruridre = Elapidre (sometimes thought to be Colubridre and including the
venomous cobras, true coral snakes, mambas, kraits, and the Australian black
and tiger snakes, death-' adder', and many others); Hydrophidre (venomous
sea-snakes with valvular nostrils and vertically compressed, rudder-like tails) ;
and Viperidre (venomous vipers or true adders, and also rattlesnakes, with
poison fangs in mobile maxillary bones which allow the teeth to swing, and lie
parallel with the roof of the buccal cavity when not in use).
The affinities of the different families of snakes are not sufficiently well
known to justify segregating them in larger groups of super-familial rank.
The rattlesnakes are sometimes accorded sub-family rank (Crotalinre), and the
pythons are often separated from the boas as the Pythoninre.

SUB-CLASS ARCHOSAURIA
The members of this group are diapsids (p. 484) without inter-parietal,
tabular bones, or a parietal foramen. They are probably all descendants of
Thecondontia (see below). The archosaurians form an assembly of reptiles
which differ widely from the lepidosaurian Eosuchia, Rhynchocephalia, and
Squamata already considered (p. 495). Some were bipedal and exhibited
 PHYLUM CHORDATA 499
correspondingly striking modifications in the limb girdles. The hind legs were
longer, and more powerful, than the others and were slung vertically below
the body. The palatal teeth were lost in all but a few primitive forms. Some
forms became toothless with the adoption of an almost certainly horny beak.
There was a vacuity on the outside of the lower jaw between dentary, subangular
and angular, and almost always an antorbital vacuity in front of the orbit.
Of this great and widely varying assembly of reptiles only the Crocodilia
and the birds survive today. The osteology of modem crocodiles, together
with Jurassic bones, leaves no doubt as to their relationships (p. 461).

ORDER THECODONTIA
The thecodonts of the sub-order Pseudosuchia (e.g. Ornithosuchus,
Euparkeria) arose in the Triassic. They were small carnivorous reptiles
possessed of long, slim skulls and sharp teeth in sockets along the jaw-edges.
The hind-legs were the longer pair, and possibly indicated the dawn of bipedal
gait. Many pseudosuchians possessed a dorsal protection consisting of rows of
curious bony plates (e.g. Aetosaurus).
The Triassic thecodonts of the sub-order Phytosauria (e.g. Phytosaurtts,
Mystriosuchus) are of special interest in that they had become aquatic, and
developed, before the true crocodiles evolved, many features characteristic of
that group to-day. This convergent evolution is illustrated by the elongated
skull, the formidable array of stabbing and holding teeth, and the general
' crocodilian ' form. The external nostrils were sometimes set above the skull
level, enabling the animal to breathe with nearly all of the body submerged,
but, unlike in Crocodilia, they were posteriorly placed near the eyes.
After flourishing briefly, the phytosaurs were exterminated before the
beginning of the Jurassic coincident with, and possibly in consequence of, the
rise of the Crocodilia, a strikingly successful collateral amphibious line.

ORDER CROCODILIA (LORICATA)

The amphibious crocodiles, alligators, and their allies are the largest living
reptiles and the only remnants of a once-widespread archosaurian stock
that survived the Mesozoic. They seemed to be descended from Triassic
thecodonts and, in fact, preserve a few primitive features relatively unchanged.
Reptiles that were probably crocodiles have been found in Triassic deposits
(Protosuchia) and unmistakable crocodiles, representing various side-lines,
were common in the Cretaceous. The sub-order Eusuchia, in which modem
forms (Fig. 333) are placed, dates from this period (p. 3).
The general features of modem Crocodilia are as follows : The dorsal
surface, or both dorsal and ventral surfaces, are covered with rows of sculp-
tured bony scutes. Epidermal scales are also present. The vertebral centra
are either amphiccelous, flat at each end, or proccelous. The first caudal
soo ZOOLOGY
vertebra is unique in being convex at each end. The anterior thoracic vertebrre
have elongated and bifid transverse processes, and to this transverse process
the capitulum and tuberculum of the rib are both attached. The sacrum
consists of two vertebrre. The ribs are bifid at their vertebral ends. The
quadrate is immovable. There is a substantial lagena. The tympanic
membrane is protected by two scaly flaps which are operated by special
muscles and close the external meatus when the reptile dives. There is a

FrG. 333.-Sub-class Archosauria, Order Crocodilia, Sub-order Eusuchia, Family Crocodilidre.

Crocodyltcs. The Asiatic Marsh or Mugger Crocodile (C. palustris) in characteristic basking pose
with jaws agape. Essentially freshwater, it a lso inhabits salt lagoons (in Ceylon). Growing
to a length of 13 feet, it lives on fish and occasiona lly birds a nd small animals, only rarely attacking
Man. It is said t o be capable of .est ivation . From fifteen t o twenty eggs a re buried in sandbanks
and h at ch in about fifty days. The newly emergent young are about ro inch es long a nd a re at
first relatively defenceless. (F rom a photograph b y L a nsler.)

complicated pressure-equalising Eustachian system. A sternum and ab-

dominal ribs are present. The pelvis has the peculiarity of having the pubis
excluded from the acetabulum. The limbs are adapted for walking and
swimming. The teeth are lodged in sockets. The external nostrils occur on
eminences at the end of the snout. These are opened by a longitudinal dilator
muscle and closed by a constrictor muscle which completely encircles the
dilator and squeezes it against the back of the nostril, closing the aperture
during submersion. The fibres of both muscles are unstriped and innervated
from the sympathetic system (Bellairs and Shute).
 PHYLUM CHORDATA SOI

A horizontal partition, the secondary palate, is developed by shelves which

grow out from the maxillre, palatines, and, in the later crocodiles, the pterygoids.
Thus the internal nares open into the throat (Fig. 138, p. 8g). In life they are
covered by a fleshy valve formed by two elements. The first is a muscular
flap of tissue which projects downwards from the palate immediately anterior
to the internal nostrils. The second is a flap-like extension of the tongue which
is supported by a hyoid process. When in apposition, the two elements form
a valve which seals the internal nares and enables the animal to breathe, with
jaws open beneath the surface, so long as the tip of the nose is exposed to the
air. This power to breathe through the relatively restricted surfaced part of
the animal enables it to lurk almost completely hidden and to seize unsuspect-
ing prey. Further, a crocodile can submerge with its jaws holding struggling
prey without being in danger of flooding its nose or lungs.
The Salt-water or Estuarine Crocodile (Crocodylus porosus), common in the
north-west Pacific, India, and Southern China, is especially deadly to Man.
It grows to a length of 25 feet, and like other species, has a remarkable facility
for suddenly thrusting the anterior third of its body swiftly up out of the
water to seize an incautious man or beast.
The lungs of crocodiles are relatively complex. Further, the ventricle of the
heart is completely divided in recent forms, but there are only two aortic arches.
The pulmonary artery and left aortic arch come from the r£ght ventricle into
which venous blood flows from the cavals and right auricle. The right aortic arch
arises from the left ventricle. The two aortre are applied together and twisted ;
but no mixing of blood occurs through a joint opening, the foramen of Panizzi
(White). (We will see later that the pulmonary and systemic systems of birds
have become completely separated by the abortion of the left arch (p. 589).)
The opening of the cloaca is elongated in the direction of the long axis
of the body. There is a median erectile penis grooved for the passage of
semen. A clitoris occurs in the female.
This order includes among living forms the true crocodiles, the gharials, as
well as the alligators and caimans. Many extinct families have been recognised.
In some countries there is considerable, and often acrimonious, dispute as
to whether the local animals are crocodiles (Fig. 333) or alligators. A key for
generic identification (after Watson), based on characters of the head and skull,
is given below.
I. Snout very long and slender ; the halves of the lower jaw fused in front
as far back as the 15th tooth.
(a) 27-29 teeth on each side of the upper jaw. Gavialis (the gharials of
N. India).
(b) 20-21 teeth on each side of the upper jaw. Tomistoma (the false
gharials of Borneo and Sumatra).
soz ZOOLOGY
II. Snout triangular or rounded; the halves of the lower jaw not fused
farther back than the eighth tooth.
(a) The fourth tooth of the lower jaw fits into a notch in the side of the
upper jaw. Crocodiles.
(r) Nasal bones dividing the nasal aperture into two. Crocodylus
(Africa to south China, northern Australia, New Guinea, and
western Pacific; southern United States to Venezuela and
Ecuador).
(2) Nasal bones not dividing the nasal aperture; snout turned up
in front. Osteola:mus (W. Africa).
(3) Nasal bones not dividing the nasal aperture; snout not turned
up. Osteoblepharon (Congo).

(b) The fourth tooth of the lower jaw fits into a pit in the upper jaw.
Alligators.
(r) Nasal bones dividing the nasal aperture. Alligator (southern
United States and south China).
(2) Nasal bones not dividing the nasal aperture. Caiman (tropical
South America).
Along with the tendency to assert that any dangerous crocodilian is a
'gator', there is an equally ridiculous belief among 'practical men' that in
crocodiles the upper, and not the lower, jaw is hinged on the cranium.
As regards size, only the Salt-water (see above) American (C. aculzts), Orinoco
(C. intermedius), and Indian Gavial (Gavialis gangeticus) have been proved to
exceed 20 feet. The biggest Alligator (Alligator mississippiensis) is about 20
feet long, yet is harmless. The Nilotic Crocodile (C. niloticus), like C. porosus,
is known to kill Man. The biggest scientifically recorded Nilotic Crocodile was
only r6 feet long but more massive examples have undoubtedly been shot by
hunters. An American Alligator has lived to the age of fifty-six years.

ORDER PTEROSAURIA (PTERODACTYLA)

The Pterosauria were toothed and tailed flying reptiles in which the skeleton
was modified in a manner analogous to that of the birds. In some later
forms the teeth were absent and seem to have been replaced by a beak.
The bones were thin, light, pneumatic and cross-strutted like those of many
birds (see Fig. 387, p. 572). The fore-limb became a wing in which the fourth
finger was greatly elongated and supported a membranous patagium. The
first three fingers were small and clawed, and the fifth was absent. The carpus
was reduced, but had a spur-like sesamoid bone added to it. Though the
sternum was large, keeled, and well ossified, clavicles and interclavicle were
absent. The pelvis had pubic bones which joined one another. The hind-
 PHYLUM CHORDATA 503
limbs were pentadactyle and rather weak, the fibula being reduced or
absent. The skull was pointed and elongated, sometimes to an exaggerated
degree. Brain-casts take a generally similar form to those of birds : the
cerebellum was well-developed and the paired anterior parts associated with
olfaction seem to have been reduced as in flying animals in general. The
external nares were placed far back. The antorbital vacuity was large, and
the eyes bordered with sclerotic plates. There was no parietal foramen. The
head was supported at a right angle on a long neck of eight or nine proccelous
vertebrre.

FIG. 334.- Sub.class Archosauria, Order Pterosauria, Sub.order Pterodactyloidea, Family

Pteranodontidre. Pteranodon. A toothless giant of the Cretaceous, with a wingspan of up to 27
feet (see also Fig. 330, p. 493) . On the other hand, Pterodactylus and certain other genera were
little bigger than sparrows. For a member of the second sub.order (Rhamphorhynchoidea) see
Fig. 414, p. 599· (From Romer, after Eaton.)

There has been no general agreement concerning the origin of flight (p. 6oz).
It has been suggested that these animals probably first volplaned from emi-
nences or, on the other hand, may have made gliding leaps while running at
speed on their powerful hind-legs. The spade-like sternum and pectoral girdles,
though substantial, certainly do not convey the suggestion of a musculature
sufficiently powerful to support flight, as developed in birds, or even the bats.
It is probable that the pterosaurs were essentially gliders, and it is not im-
possible that many were marine animals which used air-currents to skim over
the waves in a manner similar to that of the petrels. Their general structure
makes it appear that they would have the greatest difficulty in becoming air-
borne after landing, but that they could hang efficiently to cliffs or trees by
means of their fingers (Fig. 334), or perhaps upside down, like bats, by their toes.
 ZOOLOGY
The wing-membranes were supported essentially by one hugely elongated
finger (Fig. 330). This suggests that they could not have existed efficiently
among trees, for, if its poorly supported membrane were badly torn, a pterosaur
would be unable to fly. A bird, on the other hand, can be severely battered and
lose many feathers without being grounded; and the membranes of a bat, sup-
ported by several fingers, can be holed or even torn and yet the animal still
can fly. Some pterosaurs (e.g. Pteranodon) had a wing-span of nearly 27 feet;
others (e.g. Pterodactylus) were little larger than canaries. It is difficult to
believe that these flying reptiles were not homa:othermous to a fairly consider-
able degree, but in any case they faded, possibly as a result of competition
with the emergent true birds, at the end of the Mesozoic.

FIG. 335--Rhamphor-
hynchoidea: Skull and den-
tition. Scaphognathus. D.
pre-orbital aperture; Fr.
frontal; ]u. jugal; Mx.
maxilla ; N. nasal opening;
Pm:r. premaxilla; Qu.
quadrate. (After Zittel.)

The group flourished during the Jurassic and Cretaceous. There were
two sub-orders : Rhamphorhynchoidea and Pterodactyloidea.
The Rhamphorhynchoidea were characterised by a long tail which was,
in some species at least, terminally dilated by free caudal ribs; and by the
possession of a fifth toe on the hind foot (examples: Rhamphorhynchus (Fig.
414, p. 599), Dimorphodon).
The Pterodactyloidea had an extremely reduced tail, no cervical ribs, and
an incomplete fifth toe. Some forms were edentulous. Examples were:
Pterodactylus, Scaphognathus (Fig. 335); Pteranodon, a toothless form with a
curious balancing (or steering) backward prolongation of the head (Figs. 330,
334) and Ornithodesmus.

DINOSAURS
Among the archosaurians were also the fantastic Mesozoic creatures some-
times referred to as the 'Order' Dinosauria. These are readily divisible by a
series of clearly marked characters into two groups, each of which ranks as an
order. These are the Saurischia and the Ornithischia (Fig. 336).
Dinosaurs are often thought exclusively as immense animals, but some
were no bigger than a hen. In each order existed both large and small
reptiles. The biggest were the most massive animals ever to exist on land :
the bulk of the largest dinosaurs has been exceeded only by the biggest
 PHYLUM CHORDATA

modern whales. Both dinosaur groups showed great variation and extreme
specialisation in many respects.
ORDER SAURISCHIA
Saurischians possessed a triradiate pelvis (broadly similar to that of the
ancestral thecodonts) which was basically distinct from the bird-like arrange-
ment (F ig. 337) that arose in the Ornithischia (p. 507). In the skull there

FJG. 33r..---S ub-class Archosauria: (Or ders Saurischia a nd Ornithischia). L eft: Tyrannosa urus.
a carnivorous sa urischian fro m t h e North America Cr et a ceo u s whic h was about r8 feet high a n d
som e 50 fe et lon g. R ight: Iguanodon. a h er b ivor o u s ornithisch ian o f the E u ropean Cre taceou s
whic h r each ed a h eight of a b o u t 14 feet. T h e two p rimary d in osau r st ocks a re w idely divergen t .
a f u n damental differentiating ch aracter bein g t h e stru cture of t h e pelv is (Fig. 337). Saurischia n s
were p r imitively carnivorou s , a lthou gh som e became q u a d r u pedal and her bivorous (Fig. 339).
Ornithisch ians were primarily h erbivorous , a n d remained so u n t il their extinction . S ome
became a rmour ed quadrupeds in the Cretaceo u s . Oth er divergen t arch osaur ia n stocks a r e t h e
p t erosaurs (Fig . 334), crocodiles (Fig . 333), a nd b irds (Fig. 414 , p. 599). (Partly after vario u s
a uthors.)

were two, or one, antorbital vacuities, which were often very large. Teeth
were present on t he premaxillre. There existed two saurischian sub-orders-
t he Theropoda and Sauropoda.
SUB-ORDER THEROPODA
Included in t his group were the earliest true dinosaurs. Appearing in the
Triassic, these were relatively small, slender, hollow-boned reptiles which
VOL. II. II
so6 ZOOLOGY
(including the elongated tail) reached a length of not more than 10 feet.
Ancestral theropods (e.g. Ornitholestes) were long-necked and bipedal, with the
tail counterbalancing the body. The hands and small fore-limbs were used in

FIG. 337.-Dinosaurs: Pelvic

structure. Left: Typical repti-
lian triradiate hip girdle of Sauri-
schia. R ight: Typical tetra.
radiate, bird-like, girdle of Orni-
thischia. ac. acetabulum; il.
ilium; isc. ischium; p. pubis.

p.

food-gathering: they were carnivorous, with gaping jaws and many sharp
teeth.
From the above may have developed the ornithominid ostrich-dinosaurs
(e.g. Struthiomimus) which appeared in the late Cretaceous. These swift,
bipedal reptiles retained the hollow bone structure. They had enlarged fore-

FIG. 338.--8aurischia: Axial skeleton. Vertebral column and pelvis of Tyrannosaurus. The
massive vertebral column, pivoted at the open acetabula, was slung between the opposite halves
of the pelvis. In the sitting pose the main weight {total up to 20 tons) was probably transmitted
through the strong pubic column and its expanded 'shoe'. The cervical vertebr::e are shortened,
crowded, and curved upward, and so shorten the adverse leverage against the pelvis in carrying
the huge head {Fig. 336). {From Gregory, after Osborn.)

limbs, but the reduction in both number and size of teeth, and the development
of both jaws into a superficially beak-like structure, suggest a change to a less
restricted diet.
Meanwhile there arose collaterally the great carnosaurian dinosaurs-big-
headed, with wide gaping jaws equipped with long, stabbing, and tearing teeth
which were sometimes recurved. These (e.g. Tyrannosaurus) (Fig. 338) finally
reached a length of about so feet, stood perhaps 20 feet high, and fed on con-
 PHYLUM CHORDATA 507
temporary herbivorous dinosaurs. The disproportionally great size of the
head (more than 4 feet long) was offset by the development in the skull of a
remarkable fenestration which reduced weight and yet still provided anchorage
for the powerful masticatory musculature essential to the animals' feeding
habits. The carnosaurian neck was short, and the fore-limbs abbreviated and
relatively minute. Those of Gorgosaurus (a reptile about 30 feet long) were
unable to reach the mouth and were reduced to two digits that were probably
useless for dealing with prey. The body was bulky and the tail elongated and
of many vertebrce. The bones of the hind-legs were massive and obviously able
to support several tons, and supplementary sacral vertebrce were attached to
the pelvic girdle. These animals were extinguished at the end of the Cretaceous,
possibly owing to the disappearance of their large herbivorous prey.
Meanwhile, yet another therapod group arose during the Triassic (e.g.
Yaleosaurus, Plateosaurus) and ultimately, it is believed, gave rise to the next
group.
SUB-ORDER SAUROPODA
These reptiles (e.g. Apatosa~trtts, Diplodocus, Brachiosaurus (Fig. 339)) were
probably the largest and heaviest of all land animals. They grew to a length
of more than So feet. and weighed probably as much as 50 tons. The stock
arose in the Jurassic. The sauropods were quadrupedal and herbivorous.
They had small skulls and teeth, weak jaws, and elongated necks and tails.
The brain was inordinately small ; it would seem that the biggest dinosaur
had a brain only 6 inches long. Spinal enlargements for the relay of impulses
occurred often in the brachial region (between the shoulders) and always in the
sacral region. The latter centre was many times bigger than the brain (see
also Fig. 342). In these dinosaurs, too, temporal fenestrce lightened the cranium.
In addition, the huge dorsal vertebrce were made less heavy by cavernous
hollows in both centra and arches. These vertebrce were also produced into
long spines to which supporting ligaments and tendons were presumably
attached. The limb-bones were stout and non-pneumatic, but despite their
strength there is much reason to believe that the biggest sauropods could not
support their tremendous bulk on dry land.
It seems probable that the sauropods were swamp-feeders (Fig. 339).
Little is known of their precise mode of grazing, but it is obvious that they
needed a constant and tremendous harvest of lush pasture to support their
prodigious bulk. They must have been highly vulnerable to any widespread
climatic change towards aridity.

ORDER 0RNITHISCHIA
The' bird-like' dinosaurs possessed a pelvis (essentially like that retained in
modern birds (p. 578)) in which the pubis lay parallel with an extended ischium
soS ZOOLOGY
(Fig. 337). In the skull the antorbital vacuities were always small, and some-
times obliterated. One of their most noticeable characters was the possession
of a' predentary 'bone in the mandible that may have supported a horny beak.
As a result, the teeth on the premaxilla= and on the front of the lower law

FIG. 339.- Dinosaurs: Respiratory adaptations in unrelated bottom-feeders. L eft: The

ornithopod Parasauroloplws (Family Hadrosaurid;:e = Trachodontid<e) of the North American
Cretaceous. The grotesque crest, formed by an extension of nasal and premaxillary bones, con-
tained an air-chamber of considerable capacity, perhaps enabling the animal to keep its head
submerged for prolonged periods of bottom grazing. Right: The sauropod Brachiosaurus
(Family Brachiosaurid::e) of the African and North American Jurassic. An ally of Apatosaurus
(= Brontosaurus). this is perhaps the biggest terrestrial animal that ever lived, reaching a prob-
able bulk of some 50 tons. The nostrils had shifted to an eminence at the top of the h ead,
enabling the animal to breathe with only a small portion of its bulk exposed. (Partly after
Colbert.)

became suppressed. All the Ornithischia were herbivorous and they fall into
the four following sub-orders:

SUB-ORDER ORNITHOPODA
These were bipedal, digitigrade forms with a complete and slender post-
pubis, and a small antorbital vacuity. Abundant in the Upper Jurassic and
Lower Cretaceous (e.g. Camptosaurus, Iguanodon (Figs. 336, 340)) they grew to
at least 30 feet long. An edentulous beak and broad (sometimes spatulate)
teeth occurred in the very early types, suggesting that the ornithopods were
herbivorous almost from their beginnings (Fig. 341). Despite their name,
 PHYLUM CHORDATA 509
the early ornithopod foot was not nearly as bird-like (in the passerine sense)
as that of the theropods (p. sos)-their feet had four toes, anteriorly directed.
Another relatively primitive ornithopod was the Upper Cretac~us Hypsilo-
phodon, a small, possibly partly arboreal, dinosaur about as big as a wallaby
and superficially not unlike one in certain of its features.
"i'he primitive ornithopods gave rise to two far more specialised types-the
large hadrosaurs, or 'duck-billed dinosaurs', were one of these. The hadro-
saurs became exceedingly common in the Upper Cretaceous by means of
efficient aquatic specialisation. They are well known : many excellently

FJG. 340.-0mithischia: Endoskeleton.

Igua?JOdou (Fig. 336) was a bipedal
vegetarian that probably roam ed E u rope
in herds du ring the Cretaceous. Associated
skeletons of nearly thirty of t hese a nimals
which see m t o have fa llen into a crevice
ha ve b een disco vered in Belgium. The
termi nal phalanx o f the thu m b is special-
ised into a defensive bony spike. Note
arra ngement of pelvic elements (see Fig.
337). co. coracoid; is. ischium; p. p re-
pubis ; pp. post-pubic process (pu bis);
sc. scapu la; I - IV, 1- V, dig its. (From
Zittel , after Dollo.)

preserved remains have been found, including even ' mummies ' in which
petrified skin and foot-webbing is preserved. The hadrosaurs (e.g. Anato-
saurtts = Trachodon, Edmontosaurus) had long, toothless beaks, posteriorly
placed nostrils, and crowded in the jaws, as many as z,ooo small, grinding
teeth arranged in parallel rows. Certain genera (e.g. Corythosaurus, Krito-
saurtts) developed peculiar bony crests by alteration of the premaxillary and
nasal elements above the nasal region.
The great spatulate beak and the enormous battery of grinding teeth
suggest that the hadrosaurs obtained their food by grovelling in the mud for
vegetable material. In several species it has been possible to follow the internal
air passages from the external nostrils, and there are indications that they
passed in a loop through the grotesque crest before reaching the internal nares.
The by no means unplausible suggestion has been made that the dome-like
osseous 'combs' may have been pneumatic in function and allowed their
sro ZOOLOGY
possessors to grovel underwater, and so exploit a special niche-the swamp-
bottom-more fully than would be otherwise possible.
The troodonts, another specialised and aberrant, ornithopod stem, were,
in wide distinction from the hadrosaurs, probably exclusively dry-land forms
(e.g. TroOdon of the Upper Cretaceous). They had an axial skeleton very
similar to that of the camptosaurs (p. 508), but their skulls were extremely
specialised in the possession of a solid dome of bone (much of it derived from
the frontals and parietals) often produced into knobs and spikes. The most
likely explanation for this bizarre armoury so far offered is that it may have
been used as defensive, armoured battering ram against the often far larger,

A 13 c
FIG. 341.- Dinosaurs: Dentition of unrelated herbivores. A, inner aspect of a crown from
right lower jaw and B , hinder aspect of tooth from left lower jaw, of Iguanodon, an ornithopod
(see Fig. 336). C, Tooth of Cetiosaurus, a sauropod (see Fig. 339). All natural size. (From
B. M. (N.H.) Catalogue.)

contemporary, predatory dinosaurs with which the non-aquatic troodons had

to contend. Perhaps it deterred aggressors with a sudden, intimidating charge
such as is employed by the modern rhinoceroses.

SUB-ORDER STEGOSAURIA
The stegosaurs (e.g. Stegosaurus (Fig. 342)) were slow-moving, essentially
Jurassic (including Liassic), reptiles which grew to a length of about 25 feet.
They were quadrupedal, but the fore-limbs were the shorter pair. The skull
was small, and the brain tiny and considerably smaller (as with the Sauropoda,
p. 507), than the lumbar spinal enlargement. The back, neck, body, and tail
were protected by a double series of huge, flattened, generally triangular and
vertical spines and plates, particularly above the lumbar and pelvic regions.
The tails of these extraordinary monsters were armed with two pairs of formid-
 PHYLUM CHORDATA srr
able spikes, serially and sub-terminally placed, apparently converting the
appendage into a damaging, lashing weapon which may have protected the
unarmoured flanks.
Other stegosaurs were Scelidosaurus of the early Jurassic, and Kentruro-
saurus which flourished just before the Cretaceous.
SUB-ORDER ANKYLOSAURIA
These, too, were heavily armoured but quite differently and more exten-
sively so ; in fact, only the chelonians (p. 486) have developed a more complete
protection. They (e.g. Ankylosaurus, Nodosaurus) lived in the Cretaceous,

FIG. 342.- Dinosaurs: Brain, spinal enlargements and hypophysis. In the herbivorous orni-
thopod Stegosaurus the brain (arrowed) was small compared with the brachial enlargement of the
cord and a mere r j2oth of the bulk of the sacral enlargement. The latter was involved in the
control of the hind-limbs and the powerful armoured tail, which probably lashed defensively at
contemporary carnivorous Sauropoda. The inset shows the relatively large size of the pituitary
body (see p. 149) in relation to the brain of a troodont (Ornithopoda) . Although Stegosaurus
and similar types could exist with a brain about the size of that of a kitten, t he pituitary, concerned
with manifold body functions including growth, was large. (Modified after Colbert.)

and were slow-moving quadrupeds with short, broad skulls (with closed tem-
poral regions) in which the basic elements were reinforced with an additional
plating of polygonal bones. The dorsal surface of the body was protected by
a mosaic of small osseous plates and the short, stout legs were guarded by
projecting shoulder spines. The tail, too, was surrounded by bony rings, and
sometimes armed with elongated spines. The analogy between these animals
and the early, heavy, tanks of the First World War is inescapable.
SUB-ORDE R CERATOPSIA

By the end of the Cretaceous, the so-called dinosaurian age was drawing
to a close with the development of one of the most interesting groups of all, the
srz ZOOLOGY
ceratopsians or horned dinosaurs. These were quadrupedal forms with huge
skulls, bony horns, and a great protective, sometimes solid, sometimes fenes-
trated, mantle of bone formed by grotesque prolongations of the parietals and
squamosals over the neck. Among these was Triceratops, which grew to a
length of about 20 feet. The skull may have accounted for almost one-third

FIG. 343.-Sub-class Synapsida, Order Pelycosauria, F a mily Sphenacodontidre. Dimetr·odoJI.

Pelycosaurs represent a very early stage in the development of the mammal-like reptilian trend.
Dimetrodon was about I I feet long, a nd was perhaps one of the commonest Permian carnivores.
The function of the grotesque ' sail' (supported by elongated vertebral spines, Fig. 344) is obscure.
It probably occurred in both sexes. There have b een suggestions that it was a thermoregulator;
others have variously considered it to be 'protective ', or a warning, device or merely 'a h eredity
maladjustment'. The persistence of the sail in two divergent, successful stocks argues in favour
of function. The dagger-like anterior teeth were perhaps adapted t o a fish diet. Edaphosaurus,
about the same size, had a n armoury of stubby teeth b oth in the jaws and on the fla t upper and
lower masticatory plates that lay within its buccal cavity.

of the total length of the animal. It was sometimes surmounted by paired

horns (of a character curiously akin to that of to-day's ungulates, p. 84r) and,
in addition, a median nasal horn.
These animals were abundant about 6o million years ago when, excluding
only the stegosaurs, each major dinosaur group still survived. All, however,
apparently disappeared with a startling abruptness, geologically speaking, by
the beginning of the succeeding Tertiary (p. 3).
 PHYLUM CHORDATA 5I3

SUB-CLASS SYNAPSIDA
The Synapsida were widespread, often mammal-like reptiles with a single
lateral temporal vacuity primitively lying below the
postorbital and squamosal. The brain-case was high and,
as a result of the broad supraoccipital, the inner ear was
placed low down. The teeth in the more advanced forms
were usually heterodont. The lower jaw was flattened
from side to side instead of being rounded in section, and,
except in the most primitive members, the dentary was
relatively large. There were always a coracoid and pre-
coracoid in the shoulder-girdle. This condition has per-
sisted in monotremes (p. 699), but in other extant mam-
mals only the posterior of the two coracoids remains.
The synapsids dominated the land fauna of the
Permian and much of the Trias. They are found abund-
antly in some deposits in North America and South
Africa, and are known also from South America, East
Africa, India, Central Asia, Russia, and Europe. Re-
mains of the earlier types (pelycosaurs) (Fig. 343) occur
mainly in America, and those of the more advanced
forms (therapsids) in South Africa and Russia.
The group is especially interesting because it shows
the various st ages by which the mammals evolved from
the earliest reptiles.

ORDER PEL YCOSAURIA

The pelycosaurs, the most primitive members of the

sub-class, occurred in the Upper Carboniferous, and
especially in the Lower Permian. Some members, such
as the small, long-tailed Varanosaums , differed from the
captorhinomorph cotylosaurs only in having the t emporal FIG. 344.- Pelyco-
vacuity. Others had become specialised. Dimetrodon sauria: Vertebral spine.
One of a series of ' sail'
had enormously elongated neural spines to its vertebrre supports in Eda-
phosaurus. The short
(Fig. 344). These are believed to have been connected transverse 'yards' (of
by skin and to have formed a huge 'sail'. Dimetrodon which only dorsal rem-
nants remain) were
was more than ro feet long and had dagger-shaped stab- lacking in Dimetrodon
(Fig. 343). About one
bing and holding teeth. Other pelycosaurs were adapted quarter na tural size.
to a vegetarian diet (e.g. Edaphosaurus). The pely- (From B .M. (N.H.)
Catalogue.)
cosaurs, like the cotylosaurs, had relatively short limbs
and a sprawling gait, but the remaining orders carried the body higher and
were probably much more active.
 ZOOLOGY

ORDER THERAPSIDA

The therapsids were a varied and successful group of reptiles which have
been claimed almost to bridge the 'entire evolutionary gap between a primitive
reptile and a mammal' (Romer). The pelycosaurs showed relatively little
advancement from the cotylosaurs (see above), but among the present group
many mammalian diagnostic features (as far as hard parts are concerned) are
present, even though the elements of the lower jaw had not been reduced to
a typical dentary (p. 652). The teeth, however, were diversified into incisors,
canines, and, in the later types, multi-cusped cheek teeth. In advanced types

FrG. 345.-Sub-class Synapsida, Order Therapsida, Sub-order Dicynodontia, Family Dicyno-

dontidre. Kannenteyeria. A Lower Jurassic survivor of the dicynodont trend that appeared in
the Middle Permian and became extremely widespread and plentiful at the end of the Pabeozoic.
In general, these creatures may have been herbivorous swamp-dwellers. Kannemeyeria was
about 6 feet long. Others were much bigger (skull 3 feet long) and many were very small (skulls
few inches big). This group (and the Pelycosaurs) represent two of the several mammal-like
trends that appeared early, yet were extinguished. (After Pearson.)

the occipital condyle had become double. The pineal eye had disappeared,
the temporal opening had grown bigger, and the quadrate and quadratojugal
had been reduced. A secondary palate was sometimes present. The limbs
in the higher forms were already suggestive of the mammalian pattern of the
future. As will be seen below, there were other features that suggest that the
mammals were derived from carnivorous reptiles of the therapsid stem.
Included among the order are the sub-orders Dicynodontia and Therio-
dontia. The former were for a time the most successful synapsid order, being
found in extraordinary abundance from the Middle Permian to the Lower
Trias and surviving until the Middle Trias. Typically they were of a heavy
build, varying from I to 7 or 8 feet in length (Fig. 345). Their skull was very
specialised, having huge temporal fossre, T-shaped squamosals, and small
 PHYLUM CHORDATA 515
quadrates. In the majority the dentition was reduced to a pair of canines
(which were sometimes confined to the males) and the jaws were covered by a
horny beak. The post-cranial skeleton was very mammal-like though clumsy.
There was an acromion process on the scapula, the precoracoid was excluded
from the glenoid cavity (Fig. 346), the ilium was considerably elongated, and
the number of phalanges had been reduced from the reptilian number of
2 3 4 5 3 to the mammalian 2 3 3 3 3. Dicynodon was the commonest genus
with about seventy species. Lystrosaurus, which occurred abundantly in the
Lower Trias, had the snout elongated and turned down and was probably
aquatic. Several genera, such as Endothiodon and Esoterodon, kept post-
canine teeth and one form, Eumantellia, had incisors.

FIG. 346.-Dicynodontia: Pec-

toral girdle. Ventral aspect. CL.
clavicle; CO. coracoid; ICL. in-
terclavicle; PCO. precoracoid;
SC. scapula; ST. sternum.

The third sub-order, the Theriodontia, were almost certainly ancestral to

the mammals, those of the cynodont group, in particular, approaching the
mammalian organisation. They are found from the Middle Permian to the
Upper Trias, and are known from Africa, South America, and Russia. The
group showed considerable variation in size. Some forms were less than a foot
in length; others were the size of a lion. The majority were carnivorous or
insectivorous, but some of the later members may have been herbivorous.
The evolutionary changes of the theriodonts show the gradual appearance
of those characters which are typical of the mammals. In the skull (Figs. 347
and 348) the temporal vacuity was enlarged by the reduction and loss of the
postfrontals. A secondary palate was formed by the outgrowth of a shelf
from the maxillre and palatines. The basioccipital was reduced and paired
exoccipital condyles were formed. Also, the epipterygoid (columella cranii)
expanded to form the mammalian alisphenoid. In the lower jaw the dentary
increased steadily in size until the post-dentary bones were very small and
almost hidden in outer view. At the same time, a distinctly mammal-like
coronoid process was developed. The teeth of the early members were simple,
516 ZOOLOGY

though there were enlarged canines, which in some of the big gorgonopsians
resembled the tusks of sabre-tooths (Fig. 568, p. 796). The post-canine teeth of
the later members became specialised in a variety of ways, and were often very

Q Q J
FIG. 347.-Cynodontia: Skull. Thrinaxodon, a Lower Triassic cynodont. L eft: ventral,
Right: dorsal view. ECPT. ectopterygoid; EPT. epipterygoid; F. frontal; ]. jugal; J•.
lachrymal; MX. maxilla; N. nasal ; P. parietal ; PAL . palatine; PAR. parasphenoid; P iHX.
premaxilla; PO. postorbital; PRF. pre-frontal; PRO. pro-otic; PT. pterygoid; PV. vomer;
Q. quadrate ; Q]. quadrat o-jugal; ST. st apes; SQ. squamosal. (After Farrington.)

FIG. 348.-Gorgon-
opsia: Skull. Lateral
view in a Permian
gorgonopsid. } U . ju-
gal; LA. lachrymal;
NA . nasal; MAX.
maxilla; P F. pre-
frontal; PMX. pre-
maxilla; PO. postor-
bital ; SMX. septa-
maxillary; SQ. squa-
mosal.

mammal-like. The limb-girdles approached the mammalian condition in such

characters as the formation of an acromion process on the scapula, the great
expansion of the ilium, and the enlargement of the primitive pubic foramen
to form the huge obturator foramen. The form of the limb-bones, too, ap-
proached the mammalian condition, and the number of phalanges in the feet
 PHYLUM CHORDATA 5I7
was reduced to the mammalian number of 2 3 3 3 3 by the degeneration, and
finally the loss, of one phalange in the third toe and two in the fourth.
FIG. 349.-Sub-class Sy-
napsida, Order Therapsida,
Sub -orde r Theriodontia,
Infra-order Gorgonopsia,
Family Gorgonopsidre.
Lycrenops. Gorgonopsids
were primitive carnivorous
theriodonts that flourished as
early as the Middle and Upper
Permian and were a lready
relatively mammal-like in
appearance. (After Colbert. )

There were a number of different lines of evolution within the group, but
those are not yet fully understood. The Permian Gorgonopsia (e.g. Lyccenops
(Fig. 349)) and Therocephalia (e.g. Lyco-
saurus) were on the whole more primitive
anatomically than the (mainly) Triassic
Cynodontia (Fig. 347) and bauriamorphs
and may have been respectively ancestral
to them.
The Upper Permian and Triassic
Cyodontia (of the Theriodontia) were
perhaps the most mammal-like of all, and
interesting hypotheses, suggesting that
they were already warm-blooded, hairy,
and milk-producing, have been based on
recently revealed structural details (see
Watson). Cynognathus was a predaceous
wolf-sized animal with large canines and
cheek-teeth that bore accessory cusps:
this dentition showed that the animal cut
its food before swallowing. There was a FrG. 35o.- Sub-class Synapsida, Order
Ictidosauria, F a mily Tritylodontidre.
secondary palate and an advancement to- Hienothm·iurn. This Upper Triassic
wards the mammalian condition in ante- Chinese reptile h a d an unmistakably
mammal-like cranium, jaws, a nd cusped
rior and posterior girdles and elsewhere. dentition. T he ictidosaurs, ranging from
Triassic to the Middle Jurassic , appear t o
have been as intermediate between reptiles
ORDER IcTIDOSAURIA and mammals as were Seymouria (Fig .
286, p. 429), and A rclueeopteryx (Fig. 414,
The ictidosaurs (e.g. Tritylodon, Bieno- p . 599) between amphibians and reptiles,
therium (Fig. 350)) as a group ' closed the and reptiles a nd birds, resp ectively. Y et ,
like the a bove t wo ' mosaic' anima ls, the
gap between mammals and reptiles ' ictidosaurs could not possibly have b een
ancestral to the high er group whose
(Colbert). They showed examples of the characters they sh are. (After Colbert .)
mosaic evolution already discussed in re-
lation to Seymouria (p. 427) and Archceeopteryx (p. 6oo), but at the same time
represented a specialised branch off the principal mammalian stem. They
518 ZOOLOGY
retained, though in a reduced condition, the typical reptilian quadrate and
articular: the mammalian middle ear had not arisen. However, the temporal
opening was large and confluent with the orbit, and in general the cranium,
jaws, and dentition revealed an unmistakable trend towards a mammalian
condition.
The ictidosaurs ranged from the Upper Triassic to the Middle Jurassic.
The possibility has been suggested that the prototherians (p. 687) on the one
hand, and the marsupials (p. 704) and the eutherians on the other, may have
descended from separate, though collateral reptile stems.

GENERAL ORGAN/SA TION OF RECENT REPTILIA

External Features.-In external form, as in some other respects, certain of
the Lacertilia exhibit the least specialised condition to be observed among the
living Reptilia. Lacerta is such a central type that the general account of that
lizard (p. 462) applies in all the points of cardinal importance to a large pro-
portion of the group. Modifications take place, however, in a variety of
directions. The gape is generally wide and able to accommodate relatively
large prey, but some lizards of specialised feeding habits have a small and
atypical mouth. One such lizard is Moloch horridus, the Thorny Devil of
Central Australia, which feeds on ants and has a small mouth placed beneath
its remarkably ' thorny ' head. Some lizards possess erectile frills, and dorsal
protuberances are common. The tail region is usually, as in the example,
extremely long and tapering; but in some groups of lizards it is comparatively
short and thick; and in others it is depressed and expanded into a leaf-like
form. In the aboreal cham;eleons (Fig. 351) the long and tapering tail is used
as a prehensile organ, which is coiled around branches of trees and aids the
animals in climbing. The flying-lizards (Draco) of the East Indies have an
astonishing skeletal specialisation which enables them to make jumping glides
from branch to branch. Ribs are laterally developed and adapted to the
support of paired expansible membranes. Although Draco is called a' flying-
lizard', it does not of course progress in the manner of a bird, bat, nor even a
pterodactyl.
Among limbs, those of Lacerta are typical. In the cham;eleons (Fig.
351) both fore- and hind-limbs become prehensile by a special modification in
the arrangement, and mode of articulation, of the digits. The three innermost
digits of the manus are joined together throughout their length by a web of
skin, and the two outer members are similarly united. The two sets of digits
are so articulated that they can be brought against one another with a grasping
movement analogous to that of the foot of a parrot, or of the hand of Man.
A similar arrangement prevails in the pes, the only difference being that the
two innermost and three outermost digits are united. The lizards Chlamydo-
 PHYLUM CHORDATA 5I9
saurus and Amphibolurtts have hind-limbs which enable them to run on their
hind-feet with the forelegs entirely elevated from the ground. The tropical
and sub-tropical geckos (Gekkota) are enabled by sucker-like discs on the ends
of their toes to run readily over vertical or even overhanging smooth surfaces.
These harmless, and indeed useful, little creatures can actually leap several

Fie. Jsr.- Lacertilia: Abororealand hunting specilisa.-

tions. In Chama~/eo and its allies (Cham<eleonid.e) manus,
pes, and tail are adapted for holding. The eyes are set
in independently movable turrets and enable the lizard
(which changes colour to harmonise with altered sur-
roundings) to look in all directions without body move-
ment (p. 546). T he sticky tongue is protrusible to a
much greater length t han shown. The illust rated animal
is the African C. xenorhinus. (From Johnston, flash·light
photos and B.M. (N.H .) specimens.)

inches upside down, capture an insect on a ceiling, and still rarely fall to the
floor.
On the other hand, in some groups of Lacertilia, such as the blind-' worm'
(Anguis), limbs are entirely absent, or are represented merely by vestiges.
Numerous intermediate gradations exist between these and forms (e.g
Lacerta) with well-developed limbs. Limbless lizards (Fig. 352) bear a very
close adaptive resemblance to snakes, not only in the absence of the limbs,
but also in the general form of the body and the mode of locomotion. The
largest living lizard, the Komodo Dragon (Varanus), reaches a length of 12
feet.
520 ZOOLOGY
The body of a snake is elongated, narrow, and cylindrical, usually tapering
towards the posterior end, and sometimes with, but more usually without, a
constriction behind the head. In the absence of limbs, the beginning of the
short caudal region is indicated only by the position of the cloacal opening.
The fore-limbs are never represented even by vestiges, but in pythons and
other snakes there are inconspicuous vestiges of hind-limbs in the form of
small claw-like processes.
Lacking functional limbs, snakes move on land or in water by the move-
ment of spinal column and muscles. This movement pushes the sinuous body
horizontally against the resistant surrounding medium. Thus, most snakes
can glide through a twisted glass tube, but not through a straight one of about
their own diameter. A few thick-bodied snakes (e.g. boas, pythons, vipers)

FIG. 352.-Lacertilia: Fossorial specialisations. Pygopus and its a llies (Pygopodid.e: the
snake- or legless-lizards) live either in burrows or under rocks and vegetation. The body is
serpentine. Fore-limbs have been lost and the hind-limbs reduced to sca le-like vestiges. Some
have los t the ear aperture a nd movable eyelids. The pupil is vertical (see Fig. 377. p. 547) . About
eighteen species, ranging up to 2 feet in length, occur in Austra lia a nd New Guinea. (After Brehm.)

sometimes progress slowly in a straight line essentially by means of contraction

waves which pass along the costocutaneous musculature from head to tail.
These specialised muscles are attached to the ventral scales which engage pro-
jections in the ground and so draw the animal along. The North American
side-winders (Crotalus) and the Egyptian sand-vipers (Cerastes) move over loose
sand by means of a series of horizontal double loops which engage the earth and
propel the animal along sideways. One of the most interesting forms of
reptilian progression is that of the various tropical and subtropical tree-snakes.
These are thin and elongated, making for greater distribution of weight as
they glide across leaves and twigs from one tree to another. The Asiatic
Chrysopelea can ascend vertical walls and tree-trunks aided by laterally-
keeled ventral scales which engage very tiny projections.
The thrusting serpentine movement of a frightened or angry snake gives
a much greater impression of speed than is actually achieved. Few travel at
 PHYLUM CHORDATA 521

more than four miles per hour. Meinertzhagen, however, reported that he
'timed' a speeding Black Mamba (Dendroaspis angztsticeps) at 7 m.p.h.
The mouth of a snake is capable of being very widely opened by the free
articulation of the lower jaw, and it is this which mainly distinguishes it from
the snake-like lizard. Other points of distinction are the absence of movable
eyelids in the snake and also the absence of a tympanum.
The largest living snakes are probably the Reticulated Python (Python
reticulattts) of India and Malaya and the Anaconda (Eunectes murinus) of
South America. The above python has been recorded to reach a length of
33 feet, but specimens more than 25 feet long are rare. Remarkable stories
have been told of the alleged length of the Anaconda. This bulky boa is the
heaviest of all snakes, but the biggest specimen yet recorded was 25 feet long,
and weighed more than 300 lb. The biggest venomous snake is the Indo-
Malayan King Cobra or Hamadryad (Naia hannah), which may grow to 18
feet. The Taipan (Oxyuranus scutellatus) of North Australia and Southern
New Guinea is sometimes more than 10 feet long. Likewise, the giant
South American pit-viper, the Bushmaster (Lachesis muta (Fig. 377)), and the
green phase of the slender, semi-arboreal African Black or Green Mamba, both
grow to about the same size. The longest sea-snakes (Hydrophidre) grow to
a length of 8 feet, which is scarcely long enough to have given rise to the
repeated stories of sea-serpents (p. 494). Sea-snakes, exceedingly common in
some tropical waters, are sometimes extremely poisonous. Some come ashore
in sea-weed or bask on rocks but they cannot move efficiently out of water.
Sphenodon ( = Hatteria), theN ew Zealand Tuatara (Fig. 331), the only living
representative of the Rhynchocephalia, is a lizard-like reptile with a well-
developed laterally-compressed tail, and pentadactyle extremities, very similar
to those of a typical lizard. The upper surface is covered with small granular
scales, and a crest of compressed spine-like scales runs along the middle of the
dorsal surface. The lower surface is covered with transverse rows of large
squarish plates (see alsop 496.).
In the Chelonia (Fig. 327) the body is short and broad, enclosed in a hard
armoured dorsal carapace and a ventral plastron. These are in most cases
firmly united, apertures being left between them for the head and neck, the
tail and the limbs. This armoury is formed by two components, firstly
external horny scutes ('tortoise-shell' of turtles) which are modified scales
and secondly curved underlying bony plates. The horny and bony elements
do not coincide at their junctions, making for added strength. The neck is
long and mobile; the tail short. The limbs are fully developed though short.
In some (land and fresh-water tortoises) they are provided each with five free
digits terminating in curved horny claws. In the turtles the digits are closely
united together, and the limb assumes the character of a 'flipper' or swimming-
paddle. The cloacal aperture is longitudinal.
VOL. II KK
 522 ZOOLOGY
The Crocodilia (Fig. 333), the largest of living reptiles, have the trunk
elongated and somewhat depressed, so that its breadth is much greater than its
height. The snout is prolonged, the neck short, the tail longer than the body
and compressed laterally. The limbs are relatively short and powerful, with
five digits in the manus and four in the pes, those of the latter being partly or
completely united by webs of skin. The eyes are dorsally placed and the nostrils
are situated near the end of the snout and capable of being closed by a sphincter
muscle. The cloacal aperture is a longitudinal slit. The dorsal and ventral
surfaces are covered with thick, squarish horny scales, often pitted or ridged,
those of the dorsal surface of the tail developed into a longitudinal crest.
Integument and Exoskeleton.-Reptiles have no sweat-glands. In the
Chelonia there occur axillary inguinal scent-glands, but little is known of the
function of their emanations. The Kinosternidre (musk-turtles) of North and
Central America produce an especially pervasive odour and one species is
known as the Stinkpot Terrapin (Sternotherus odoratus). Crocodiles possess
anal glands that give off a musky odour (into the cloaca) which is said to be
significant in courtship. Some reptiles are able to absorb water through their
skin to some extent. The scales of Squamata differ considerably in form and
arrangement in different groups. Sometimes they are smooth, sometimes sculp-
tured or keeled. Sometimes they are similar in character over all parts of the
surface but usually there are specially developed scales-the head-shields-
covering the upper surface of the head. In the majority of snakes the ventral
surface is covered with a row of large transversely elongated scales, the ventral
shields. In certain lizards (particularly the geckos) the scales are reduced and
modified into the form of minute tubercles or granules. In some lizards special
developments of the scales occur in the form of large tubercles or spines. Under-
lying the horny epidermal scales in some lizards (scincoids) are series of dermal
bony plates. In the integument of the geckos are numerous minute hard
bodies which seem to be intermediate in character between cartilage and bone.
The outer layers of the horny integument are non-nervous and non-vascular:
they are dead, and gradually worn away (as are, for that matter, the outer
cells of the human skin). Keratin is continuously produced by cells in the
deep epidermal layers just above the vascular dermis. The keratinous cells
may fall off piecemeal (as occurs whenever we wash our hands) or may slough off
almost wholly or in segments, as when a snake discards its old skin. The caudal
rattle (Fig. 332) of the various American rattlesnakes (Sistrurus, Crotalus)
arises as a succession of up to a dozen loose, unshed dermal constrictions: new-
born snakes possess no rattle. Its vibration, heard in some species at a distance
of about 20 yards, may be a threat and warning device (see alsop. 437). In
the snake-like amphisbrenid lizards the scales are arranged in annular rings
round the body and tail.
In addition to modification of the scales, the integument of the chamreleons
 PHYLUM CHORDATA 523

is remarkable for the changes of colour which it undergoes. These changes are
due to the presence in the dermis of pigment-cells which contract or expand
under various influences. In chamreleons, the relatively quick changes in the
chromatophores seem to be exclusively under the control of the autonomic
nervous system. Nerve transection leads to a rapid darkening of the area
previously innervated. The response in Anolis (which can change from bright
green to dark brown) is more complex. In an illuminated bright container
Anolis tends to become a vivid green, but goes brown in illuminated black
surroundings. It can change from green to brown in from five to ten minutes ;
but the reverse change takes more than twice as long. Such quick changes
do not occur in blinded animals, though there is evidence that a primary
response operates extra-optically. There is experimental evidence that in
several species hormones from the pituitary gland form an important inter-
mediate link between exteroceptor organs and chromatophores. Hypo-
physectomised bright green Anolis cannot be changed to brown except by
injections of pituitary extract. Transection of appropriate nerves, or even
of the cord, does not inhibit colour responses. Further, skin-grafts quickly
change in colour and likewise, the experimental exclusion of the blood supply
from a part quickly results in its paling. Less conspicuous and rapid changes
of colour take place in other lizards and snakes.
In the Chelonia, scales are confined to the head and neck, the limbs, and
the tail. With the exception of the soft tortoises, and the Leathery Turtle
(Dermocltelys), both dorsal and ventral surfaces are covered by a system of
large horny plates. A series of horny head-shields usually cover the dorsal
surface of the head. Beneath the horny plates of the dorsal and ventral
surfaces are the bony carapace and plastron, largely composed of dermal
bones, but so intimately united with elements derived from the endoskeleton
that the entire structure is best described in connection with the latter (p. 526).
The curious arrangement of the bony plates, with their trinodal and equi-
angular seam junctions, is an interesting biological expression of the physical
principal of conservation of border. Other biological examples are the shells
of extinct eurypterids (Vol. I) and the comb-structure of honey-bees. The
same principle is manifest in the directions taken by mud-cracks, and the
hexagonal columns seen in basaltic lavas and other kinds of rocks, both volcanic
and plutonic (D'Arcy Thompson).
In the Crocodilia the whole surface is covered with horny plates or scales,
each usually marked with a pit-like depression about the centre, those on the
dorsal surface being ridged longitudinally. Underlying each of these, which
are of epidermal derivation, is a thick pad of dermal connective tissue which,
in the case of the dorsal scales, is replaced by a bony scute. In the caimans (p.
502) thin scutes also occur under the ventral scales.
A periodical ecdysis or casting and renewal of the outer layers of the horny
524 ZOOLOGY
epidermis takes place in all the reptilia with the exception of the crocodiles.
Sometimes this occurs in a fragmentary manner; but in snakes and many
lizards the whole comes away as a continuous slough (see, however, p. 522).
Endoskeleton.-The vertebrre are always fully ossified. Among recent
forms, the geckos and Sphenodon (Fig. 353) are exceptional in having the centra
amphicrelous, with remnants of the notochord in the intercentral spaces. The
rest of the recent groups for the most part have the centra procrelous. In
many extinct forms, the neural arches were not directly attached to the centra
by bone (temnospondyly) : in recent forms there is a bony union (Stereospondyli)
either through a suture or by fusion. Intercentra may be represented by
intervertebral disks of fibrocartilage (Crocodilia) or by bony elements formed
by ossification of the ventral portions of the disks (geckos, Sphenodon). In
lizards in general, and in crocodiles, there are inferior processes (hypapophyses) ,

FIG. 353·-
Sphenodon:
Vertebra. The FIG. 354.-Pytlwn: Vertebrre. Anterior and posterior
centrum is views. n . s. neural spine; p. z. pre-zygapophysis; pt. z.
amphicw lou s . post-zygapophysis; t. p. transverse processes ; z. a.
(After Headley.) zygantrum; z. s. zygosphene. (After Huxley.)

perhaps representing intercentra, situated below the centra in the anterior

cervical region. Chevron bones (inferior arches) occur in the caudal region of
many reptiles (Sphenodon, Lacertilia, Crocodilia).
In snakes and iguanas, in addition to the ordinary articulating processes
or zygapophyses, there are peculiar articular surfaces termed zygosphenes and
zygantra (Fig. 354). The zygosphene is a wedge-like process projecting for-
wards from the anterior face of the neural arch of the vertebra. It fits, when
the vertebrre are in their natural positions, into a depression of corresponding
form-the zygantrum-on the posterior face of the neural arch of the vertebra
in front. To this arrangement, as well as to the deeply concavo-convex centra,
the extraordinary flexibility and strength of a snake's backbone are due.
The various regions of the spinal column are well marked in most of the
lizards, in the Rhynchocephalia, in the Chelonia, and in the Crocodilia (Figs.
355, 356). In the snakes (some of which may have several hundred vertebrre)
and many of the snake-like lizards only two regions are distinguishable-pre-
caudal and caudal. In the others there is a sacral region comprising usually
 '""1 cr''"O Ill ~ ......
~ 0 '""1 :::!. ..... ~
..... ::sg!ll:;-0
0 .("[) (I) (fl <
::s ...... (fl Ill ...... ("[)
0 ::r !-" ::s t:r' '""1
-(I) 0... ("[) (t
...... '"~::).
::r :::..:0"'
;><: .... . '""1
.... ..... Ill
(}) I
([) 0 ::s VJ ..... ?J ?a .,.
:-;-~&·· cr'
>= ~fl.) ..... 1---J 0
:=:"'s::s::r:;.
~ftl.-..(1)
'Tl,_.t:r':::::no
-· (t) (i) ~ ~
C!q 0 t-< ~ ~
fl.) t:r'
Ul·
w 5l ()Pl..-.
'-..} ([)i:IC"l
FIG. 355.-Crocodylus: Skeleton. C. caudal region of spinal column; D. thoracic region; F . f1bula; Fe. femur; • (/) ;.::;. 0... t:r'
'"0
H. humerus; }. ischium; L. lumbar region; R. radius; Ri. ribs; 5. sacrum; Sc. scapula; Sta. abdominal ribs; T. tibia; a~:::..:(fl ::r
'::"-""'~ill"([) fl.)
~
U. ulna. (After Zittel.) ><:
..... . .,.
::s 0 fl.)
8::s ("[)< t-<
~~iS..o...~ c
C"l.~O<d s:::
e:.. ::r ("[) ::s (")
fl.)PJC":>;.t(]q
rt::so([)'1:) ~
0... 0... ::s cr' '""1 0
:;d
cr' So p;· ~ g
t:1
~ ("[) So fl.) ~ >
~ () ("[) (il ~ ...,
(!) 1-1 - Ul
§ g ~~til
>
.... 0 .... <: fl.)
t:r' 0... ~ ~!]
(t) -· "'-l .•
fl.)~~(fle:.
:::!. (fl (fl ::t. s
fi;:;.Plofl
Ill (I) 9' &. .__..
::s(il~ft>O'
FrG. 356.-Sp/ienodon: Skeleton. AT. atlas; AX. axis; C. carpal bones; CL. clavicle; CO. coracoid; D. r, 2, 3, 4, 5, 0... ..... " '""1
..... ::s-
digits ; FE . femur; F . fibula; HU. humerus; HY. hyoid; IL. ilium; IS. ischium; l\lC. metacarpals; MT. metatarsals; ....
t:r' (fl () .... Ill
PB. pubis; PR. pro-atlas; Q. quadrate; R. radius; SC. scapula; SP. spinal column (points to the spinous process of (I) Ill .... 0 ;.::;.
one of the thoracic vertebr.e); SSC. supra-scapula; T. tibia; TA. tarsals; U. ulna; UP. uncinate process. (After 0
0 .....
() ~ o...O'()
Headley.) ()
;:\ >=
(I)
0::s p; s
~: 0....-.. .... (Jl
.... fl.)
-· .....
0 -·
Ill 0 IV
e:. ::s 0... ::s ::s U•
526 ZOOLOGY
Ribs are developed in connection with all the vertebrce of the pre-sacral
or pre-caudal region. In the caudal region they are usually replaced by in-
ferior arches; Sphenodon, Chelonians, Crocodilians, and snakes have caudal ribs
which become fused with the vertebrce. In the Lacertilia only a small number
(three or four) of the most anterior of the thoracic ribs are connected with the
sternum by cc;.rtilaginous sternal ribs. The rest are free, or are connected
together into continuous hoops across the middle line. In the so-called
flying lizards (Draco) a number of the ribs are greatly produced, and support
a pair of wide flaps of skin at the sides of the body, enabling the animals to
glide. The widely expansible hood that is part of the intimidatory mechanism
of the cobras (Naia) is controlled by
the lateral spread of elongated ante-
rior ribs. In Sphenodon (Fig. 356)
and Crocodilia (Fig. 355) each rib
has connected with it posteriorly a
flattened curved cartilage, the un-
cinateprocess (see also Fig. 388, p. 572).
In Chelonia (Fig. 358) the total
number of vertebrce is always smaller
than in members of other orders.
Frc. 357.- Ct·ocodylus: Vertebrre. Anterior The cervical ribs are small and fused
elements in young animal. A. atlas; Ep. axis;
h. articulation of atlas with axis; IS. inter- with the vertebrce. The cervical and
vertebral discs; o. pro-atlas; Ob. neural arches; the caudal are the only regions in
Po. odontoid bone ; Ps. neural spines; Pt.
tra nsverse processes; R, R 1 , R 2, ribs; s. arch which the vertebrce are movable upon
of atlas; u. median piece of atlas; WK. centra.
(Afte r Wiedershie m .) one another. The vertebrce of the
trunk, usually ten in number, are im-
movably united with one another by means of fibrocartilaginous intervertebral
discs. Each of the neural spines, from the second to the ninth inclusively, is
flattened and fused with a flat plate of dermal origin, the ne·ural plate (Fig. 359),
and the row of plates thus formed constitutes the median portion of the
carapace. The ribs are likewise immovable. A short distance from its origin
each passes into a large bony dermal costal plate, and the series of costal plates
uniting by their edges form a large part of the carapace on either side of the row
of neural plates. The carapace is made up of the neural and costal plates
supplemented by a row of marginal plates (Figs. 358 and 359) running along
the edge, and nuchal and pygal plates situated respectively in front of and
behind the row of neural plates. In some cases the neural plates (Chelodina)
and even the costal plates and ribs (Testudo loveridgei) are absent.
The bony elements of the plastron of the Chelonia are an anterior and
median plate (entoplastron) and four pairs of plates which, in their order from
before backwards, are termed epiplastra, hyoplastra, hypoplastra, and xiphi-
plastra. The median element probably corresponds to the interclavicle or
 PHYLUM CHORDATA 527
episternum of other reptiles. The first pair (epiplastra) probably correspond
to the clavicles. The others seem to be of the same character as the abdominal
ribs of Crocodilia.
The carapace of the Luth or
Leathery Turtle (Dermochelys)
is distinguished from that of the
rest of the order in being com-
posed of numerous polygonal
discs of bone, and in not being
connected with the endo-
skeleton. It has been some-
times suggested that this con-
dition is the primitive one, but
fossil evidence is to the con-
trary. In the plastron the
median bone is absent. Cara-
pace and plastron are firmly
fixed together by bony union in
most species, but sometimes the
connection is ligamentous.
The sternum in the Lacertilia
is a plate of cartilage with a FIG. 358.-Chelonia: Exo- and endo-skeleton.
Cistudo, from below. The plastron has been removed
simple or bifid posterior con- and is represented on one side. C. costal plate; Co.
tinuation formed by the fusion coracoid; e. entoplastron (episternum); Ep. epiplastron
(clavicle?); F. fibula; Fe. femur ; H . humerus; H yp.
of five or six pairs of ribs. In hyoplastron; Hpp. h ypoplastron; fl. ilium; .fs.
ischium; M . marginal p lates; N u . nuchal plate; P b.
the Ophidia and Chelonia it is pubis; P ro. procoracoid or process of scapula; Py .
absent. In the Crocodilia it is pygal plates ; R. radius; Sc. scapula ; T. tibia; U.
ulna ; Xp. xiphiplastron. (After Zittel.)
a broad plate bearing the cora-
coids and two pairs of ribs with a posterior continuation which bifurcates behind.
A series of ossifications-the abdomi-
nal ribs-lies in the wall of the abdomen
in the Crocodilia (Fig. 355, Sta.), and
similar ossifications occur also in many
lizards and in Sphenodon. As already
noticed, the posterior elements of the
FIG. 359--Chelonia: Exo- and endo- plastron of the Chelonia are probably of
skeleton. In transverse section. C. costal
plate ; C1 . centrum; M. margina l plate; P. a similar character.
lateral element of plastron ; R . rib; V. In the skull, ossification is much more
expanded neural plate. (After Huxley.)
complete than in the Amphibia although
the primary chondrocranium persists to a greater extent than in mammals.
The number of bones is much greater. The parasphenoid is reduced, and its
place is taken by the large basioccipital, basisphenoid, and presphenoid.
 ZOOLOGY
A fairly typical lacertilian skull has been described in the case of Lacerta
(p. 446). Its principal characteristic features are the presence of an inter-
orbital septum, the presence of the epipterygoid, and the mobility of the
quadrate. The last of these features it shares with that of the Ophidia. The
epipterygoid is absent in some lizards, such as chamreleons and many burrow-
ing forms. The quadrate is not always movable. The skull of chamreleons
has a remarkable helmet-like appearance owing to the development of pro-
cesses of the squamosal and occipital regions, which unite above the posterior
part of the cranial roof. The skull of the amphisbrenians differs from that of
other Lacertilia and approaches that of snakes in the absence of an inter-
orbital septum.

FIG. 360. - Colubridre:

Skull. Natrix. A , from
above ; B, from below. Ag.
angular; Art. a rticular ;
B p . basioccipital; Bs. basi-
sphenoid ; Ch. internal nares;
Cocc. occipital condyle ; D t.
dentary; Eth. nasal capsule
or ethmoid cartilage ; F.
frontal; F'. postorbital;
Fov. fenestra ovalis; l'v!.
m axilla; N. nasa l; Ol. ex-
occipita l; Osp. supraoccipi-
tal; P. parietal; Pe. peri-
otic; P. f. prefrontal ; Pl.
palatine; Pmx. premaxilla ;
Pt. pterygoid; Qu. quadrate;
SA. supraangular; Squ.
squamosal ; Ts. transverse;
Vo. vomer; II, optic fora.
men. (After \Viedersheim.)

In the skull of Ophidia (Fig. 360) orbitosphenoid and pleurosphenoid or

laterosphenoid elements are absent in the adult. Their places are taken by
downward prolongations of the parietals and frontals. Along the base of the
skull are two cartilaginous rods (Fig. 360, B. T.) which are the persistent
trabeculre of embryonic life. The interorbital septum is absent. Neither
upper nor lower temporal arches are present. As in lizards, the palatines
(Pl.) are widely separated from one another, and are usually slender. They
are movably articulated behind with the pterygoids (Pt.), and the latter,
through the intermediation of the slender transverse bones (Ts.), with the
maxillre. The premaxillre are small, usually fused together and usually
toothless. The maxillre (Mx.), articulate with the conjoined lachrymal and
prefrontal (La.), which, in turn, is connected with the frontal. In Viperidre
the maxillre are short and freely movable, erecting the fangs. The long and
slender quadrate (Qu.) is freely articulated with the posterior end of the
 PHYLUM CHORDATA 529
elongated squamosal. The rami of the mandible, likewise long and slender,
are not united anteriorly in a symphysis, but are connected together merely
by elastic ligamentous tissue, so that when the mouth of the snake is opened
to allow of the entry of relatively large prey (which it swallows whole) they
are capable of being widely separated from one another. An African Rock

FIG. 36r.-Spherwdon: Skull. A, dorsal; B, ventral; C, left-sided view of skull of Sphenodon,

x -}. Col. Columella a uris; Cond. occipital condyle; E. P. ectopterygoid; F. frontal jug. jugal;
Max. maxilla; Na . nasal; Nc. anterior nasal opening; Pal. palatine; Par. parietal; Pmx. pre-
maxilla; Prj. prefrontal; Pt. f. postfrontal and postorbital; Pig. pterygoid or endopterygoid;
Q. quadrate and quadrato-jugal; Sq. squamosal; Vo. vomer. (See alsop. 467.) (From Cambridge
Natural History.)

Python (Python sebec) only I I feet 9 inches long has been known to swallow
a fully grown, horned Thomson's Gazelle (Gazella thomsoni). Post-mortem
it was found that the gazelle's spine was fractured in four places. The pelvis
and one thigh were also broken and the prey was so distorted that the ribs
on one side protruded through the other side of the body. The swallowing
process took go minutes (Pitman). Ditmars presented a freshly killed pig
weighing 8o lb. to a python measuring zo feet long and saw it swallowed whole.
530 ZOOLOGY
In primitive burrowing snakes such as the Ilysiidre, the jaw bones tend to
be solidly united with the skull. The skull of Sphenodon is on the whole
lizard-like, but there is a complete lower temporal arch. The quadrate (Q.)
is immovably fixed, wedged in by the quadrato-jugal, squamosal, and ptery-
goid. The premaxillre (Pmx.) are not fused together, but separated by a
pnr
suture. There is a broad
palate formed by the plate-
like vomers, palatines, and
pterygoids (Fig. 361).
In the Chelonia (Fig. 362)
all the bones, including the
quadrate, are solidly con-
nected together. Transverse
bones (ectopterygoids),
Tr~
lachrymals, and orbitosphe-
noids, and pleurosphenoids
are absent. The place
of the last named is taken
to a certain extent by verti-
cal downward plate-like ex-
tensions of the parietals, the
lower part of the plates per-
haps representing the epi-
pterygoids of lizards. There
may be open temporal fossre,
the inferior boundary of
which (inferior temporal arch)
may be incomplete owing to
the absence of the quadrato-
'"/ jugal). In some chelonians
(e.g. Chelonia, Fig. 362) the
FIG. 362 .-CIIelouia: Skull. bs. basi.sphcnoid; Jr . entire temporal region may
frontal ; j. jugal ; m. maxilla; ob. basi.occipital; ol.
exoccipital; op. opisthotic; os. supraoccipital; pal. pala- be covered over by a sort of
tine; par. parietal ; ph. postfrontal; prfr. prefrontal; false roof formed of expan-
pt. pterygoid; prm. premaxilla; q. quadrate; qj. quadrato·
jugal; sq. squamosal; v. vomer. (After Hoffmann.) sions of the postfrontals (ph.),
parietals (par.), and squamo-
sals (sq.) with the jugal (j.) and quadrato-jugal (qj.). The immovably fixed
quadrates Fig. 362, q.) are modified to afford a part or the whole of the rim for
the support of the tympanic membrane. The occipital condyle is sometimes
trilobed. The vomer (v.) is unpaired. The palatines (pa.) are approximated
and give off palatine plates, which for a short distance cut off a nasal
passage from the cavity of the mouth. Nasals are usually absent as separate
 PHYLUM CHORDATA 53 I

bones. The premaxillce are very small. The rami of the mandibles are stout,
and are firmly united together at the symphysis.
In the crocodiles (Figs. 363, 364), as in the
Chelonia, the quadrate (Qu.) is firmly united
with the other bones of the skull. There is
a membranous and cartilaginous interorbital
septum. There are no distinct bony orbitosphe-
noids, but pleurosphenoids are well developed.
The orbit is separated from the lateral temporal
fossa by a stout bar situated somewhat below
the surface, and formed of processes from the
postfrontal, jugal, and ectopterygoid. The
lateral temporal fossa is bounded below, as in
Sphenodon, by an inferior temporal arch com-
posed of jugal and quadrato-jugal (paraquad-
rate). The frontals are early united into one,
and the same holds
good of the parietals.
Both palatine (Pl.)
and pterygoid (Pt.),
as well as maxillce,
develop palatine FIG. 363.- C1·ocodylus: Skull.
Dorsal view in adult. Col. buttress
plates in the roof of connecting the postfrontal with the
the mouth, cutting jugal and ectopterygoid; F. frontal;
]g. juga l ; Mx. maxilla; Na. nasal;
off a nasal passage P. parietal; Pm. premaxilla; Po. f.
l ; Pr. f. prefrontal; Q.
of great length from postfronta
quadrate; Qj. quadratoju gal; R.
the cavity of the characteristic ridge on the pre-
frontal bone; Sq. squamosal; T.
mouth, the posterior p erforation in the premaxilla caused
nares (ch.) being by a p air of lower incisor teeth.
(After Gadow.)
situated far back to-
wards the posterior end of the cranial base. The
nature of the articulation between the mandible
and the quadrate is such that movement is re-
''
coec stricted to the vertical plane, and lateral displace-
FIG. 364. - Crocodylus: ment is further provided against by the develop-
Skull. V cntral view in young
animal. Ch, posterior nares; ment of a broad process of the pterygoid against
Cocc . occipital condy le ; ] g.
jugal; l'VI. maxilla (palatine which the inner surface of the mandibular ramus
process); Ob. basi-occipital ; plays, an arrangement which occurs also in most
Orb. orbit; Pl. pala tine; Pmx.
premaxill<e; Pt. pterygoid; Lacertilia.
Qj. quadratojugal; Qu. quad- In accordance with their purely aerial mode of
rate. (After Wiedersheim.)
respiration, the visceral arches are much more re-
duced in the Reptilia than in the Amphibia in general. The only well-
532 ZOOLOGY
developed post-mandibular arch is the hyoid, and even this may undergo con-
siderable reduction (Ophidia). The branchial arches are greatly reduced, or
aborted, in the adult.
There is little variation in the structure of
the limb-arches and skeleton of the limbs in the
different groups of Lacertilia. The pelvic arch
.P is distinguished in the Lacertilia in general by
its slender character. The pubes and ischia
fiif.t.c. -· . are (as is the case throughout the class) separated
from one another by wide ischio-pubic foramina.
This feature markedly distinguishes the rep-
tilian pelvis from that of the Amphibia. In
limbless forms the pectoral arch may be either
J6s.-Chelonia: Tarsus.
Emys. Right side from above. present or absent.
FIG.

F. fibula; T . tibia; (i.) f. t. c. In Sphenodon there is a foramen above the

the united tarsals of the proximal
row; Ph'. first phalanx of the fifth outer and one above the inner condyle of the
digit; 1-4, distal tarsals; I-V, humerus. There are eleven carpal elements, of
metatarsals. (After \Viedersheim.)
which there are four, including a pisiform, in
the proximal row, two centrals, and five in the distal row. The pubes are
united in a symphysis, in front of which is a cartilaginous epipubis. A large
oval foramen intervenes between the ischium and the
pubis. A cartilaginous hypoischium is attached to the
ischia behind. In the tarsus the tibial and fibular
elements are distinct, though firmly united. The inter-
medium and the centrale are firmly fixed to the tibiale.
There are three distal tarsal bones. i·
In the Chelonia (Fig. 358) the interclavicle (epister-
num) and clavicles are absent, unless, as is probable,
the former be represented by the median element of the
plastron and the latter by the first lateral pair. The
entire pectoral arch is a tri-radiate structure of which u
the most ventral and posterior ray, ending in a free ex- FIG. 366. - Alligator:
Carpus. Young animal.
tremity, is the coracoid. The other two rays are the C. centrale (?) ; R.
scapula and a process, sometimes regarded as represent- rradius; U. ulna ; r.
adiale; lt. ulnare; 1-5.
ing the procoracoid, given off on the inner side of the the five distal carpals
(not yet ossified); I and
scapula near its glenoid end. The bones of the carpus 2 united into one, and
have nearly the typical arrangement, consisting, as in also J, 4. and 5; t.
pisiform; I - V, the five
lizards, of a proximal row of three, a distal row of five, metacarpals. (After
Wiedersheim.)
and a centrale between the two. The pelvis resembles
that of Lacertilia, except that it is broader and shorter. Both pubes and
ischia meet in ventral symphyses, and epipubic and hypoischial cartilages may
be present. In the tarsus (Fig. 365) there is usually a single proximal bone
 PHYLUM CHORDATA 533

and four distalia. There are never more than two phalanges in any of the
digits.
In the Crocodilia also the clavicle is absent,
but there is an episternum. The number of
carpal elements is reduced, the largest being
two proximal bones, the radiale and the ulnare
(Fig. 366, r, u.). On the ulnar side of the
latter is a small accessory bone (pisiform;j-).
The pelvic arch (Fig. 367) differs somewhat
widely from that of other living reptiles, and
the parts have been variously interpreted.
Two bones (P.), which are usually regarded as
the pubes, exteud from the region of the
acetabula forwards and inwards, but, though
they become closely approximated anteriorly,
do not meet in a symphysis. Between and in .G
front of their anterior extremities, which are
tipped with cartilage, extends a membrane
(M.) with which are connected in front the
last pair of abdominal ribs (BR.). The pos-
terior ends of the pubes are cut off from the FIG. 367.-Alligator: Pelvis.
acetabulum by the in- Young animal in ventral view. B.
fibrous band passing between the
terposition of a pair pubic and ischiatic symphyses; BR.
last pair of abdominal ribs; F.
of bones which rnay obturator foramen ; G. acetabulum;
be parts of the ilia, Il. ilium; Is. ischium; M. fibrou s
membrane between the anterior ends
but are separately of the two innominate bones and the
ossified. The ischia Sy. last pair of abdominal ribs; P. pubis;
ischiatic symphysis; I, II, first
extend downwards and second sacral vertebrre. (After
and somewhat back- Wiedersheim.)
wards from the acetabula and are fixed together vent-
rally (at Sy.), but there is no true symphysis, as their
FIG. 368.-Crocodylus:
Tarsus. Right, from above. extremities remain cartilaginous. A hypoischium is
F. fibula; T . tibia; t.i. c.pos-
sibly the astragalus, formed not present. In the tarsus (Fig. 368) there is a probable
of the united tibiale, inter- astragalus, two proximal tarsal bones and a calcaneum
medium and centrale; f.
·fibulare (calcaneum); 1-3, -the latter having a prominent calcaneal process and
united first, second and two distal tarsal bones, together with a thin plate of
third distal tarsals; 4, fourth
tarsal; I -IV, first to cartilage supporting the first and second metatarsals.
fourth metatarsals; V?,
fifth distal tarsal and fifth The missing fifth digit is represented by a rudimentary
meta-tarsal. (From Wieder· metatarsal.
sheim.)
Alimentary Canal and Associated Structures.- Al-
though buccal digestion does not occur, terrestrial reptiles, and particularly
snakes, have active supra- and infra-labial salivary glands producing copious
534 ZOOLOGY
secretions which aid deglutition. Contrary to popular belief, however, snakes
do not 'slime' their victims before swallowing. Many snakes can disgorge prey
if frightened.
Reptilian teeth are generally thinly capped with an exceedingly hard,
highly calcified, homogeneous layer of material that is superficially like the
enamel found in mammals, but most of the tooth is made up of bone-like
dentine.
The form and arrangement of the teeth already described in the account
of Lacerta prevail in the majority of lizards. In many of them the palatal
teeth are absent. The teeth are sometimes fixed by their bases to the summit
of the ridge of the jaw (acrodont forms) and sometimes fixed by their sides to
the lateral surface of the ridge (pleurodont). They are never embedded in

FIG. 369. -Crotalus: Skull and

fangs. Rattlesnake. A. lateral
view. B. 0 . basi-occipital; B. S.
basi-sphenoid; E. 0. exoccipital;
F. 0. fenestra ovalis; La. con-
joined lachrymal and prefrontal; L.
f. articulation between lacry mal and
frontal; Mn. mandible; M .T. maxil-
la ; Na. nasal Pl. palatine; Pmp.
premaxilla; P. Sph. parasphenoid;
Pt. pterygoid; Qu. quadrate; Sq.
squamosal; II, V, foramina of exit
of the second and fifth cranial nerves.
B. transverse section at point
lettered B in Fig. A: T. trabecula!.
(After Huxley.)

sockets in any recent form and are usually shed and replaced throughout the
life of the individual. Two remarkable Mexican lizards (Heloderma) are unique in
having teeth that are grooved for the ducts of poison-glands. In snakes (Figs.
360, 369) teeth are rarely developed on the premaxillre, but are present on the
maxillre, palatines, and pterygoids, as well as the dentary of the mandible.
Sometimes they may be of the same character throughout : solid, elongated,
sharp-pointed, and usually strongly recurved, so that they have the character
of sharp hooks. The function of such teeth is to hold prey and prevent it from
struggling free while it is being swallowed. Non-venomous snakes possess
teeth only of this character. In venomous snakes some of the maxillary teeth
become poison-fangs. These are usually much larger than the ordinary teeth,
and are either grooved or perforated by a canal for the passage of the duct of
the poison-gland. In the Viperidre there is a single large, curved, poison-fang on
the maxilla with small reserve-fangs at its base. These are the only teeth
borne by the maxilla, which is very short. In the venomous colubrine snakes
the poison-fangs are either the most anterior, or the most posterior, of a
 PHYLUM CHORDATA 535
considerable range of maxillary teeth. In vipers the large poison-fang is
capable of being rotated through a considerable angle, and moved from a
nearly horizontal position (in which it lies along the roof of the mouth em-
bedded in folds of the mucous membrane) to a nearly vertical one, when the
snake opens its mouth to strike its prey. The rotation of the maxilla is
brought about by the backward or forward movement of the lower end of
the quadrate which imparts its movement to the pterygoid and thence to the
other bones of the upper jaw.
The venom is ejected by the sudden contraction of a muscle often referred
to as the temporal or masseter (Fig. 370). This muscle is probably not homo-
logous with such mammalian elements, but is, in fact, the capito-mandibularis
superficialis muscle (Colbert). The venom gland is a modified superior labial
or ' parotid'. In N aia naia it is about the shape and size of an almond kernel
and is thickly encapsulated with fibrous tissue. It is composed of a neck and
a body through the long axis of which runs a duct. The capsule supports

FrG. 370.-Crotalus: Poison ap- Afi:' ,;:: _.rm

paratus. Rattlesnake. A, eye; Ge,
I 1 'I

poison-duct entering the poison-fang

i~~·~·E·~~~~~~
at t; Km, muscles of mastication
partly cut through at *; Me. con-
strictor muscle; Me'. continuation
of the constrictor muscle to the lower
jaw; N. nasal opening; S, fibrous
poison sac; z, tongue; za, opening
of the poison-duct; zf. pouch of
mucous membrane enclosing the
poison-fangs. (After Wiedersheim.)

vascular fibrous septa which separate the glandular substance into secretory
pockets, the poison lakes of Bobeau. The roughly fan-shaped capito-mandi-
bularis superficialis muscle, originating on the post-frontal (post-orbital) bone
and parietal ridges, embraces much of the body of the venom gland. Its
contraction, which governs the biting movement, swiftly and synchronously
expresses poison from the isolated poison lakes down the central duct into the
neck of the gland and towards the adjacent fang. There is uncertainty con-
cerning the precise means by which the poison canal joins the upper intake
aperture (Fig. 371, i. ap.) of the fang, and especially how a new connection is
established after the periodic spontaneous shedding of the operative fang or
after (in some species) loss on impact with prey. In at least some species the
duct bends inwards and ends in a cavity anterior to the bases of the fangs.
Paired muscular cushions and a system of peridontal folds direct the venom
into the basal intake aperture. The venom travels down the venom canal and
is forced from the sub-terminal discharge aperture (d. ap.) at the moment of
embedding. At the time of impact the jaws of rattlesnakes are widespread to
the extent of nearly r8o degrees.
 ZOOLOGY

Snake venoms act in a variety of ways, so that it is possible to become

immunised to the venom of one species and yet die from the bite of another.
Venoms may I. act on nerve cells and cause respiratory paralysis; 2. destroy
the endothelium of smaller blood vessels and allow blood to seep into the
tissues; 3· destroy erythrocytes, and 4· cause blood coagulation. The venom
of the North Australian and New Guinea Taipan (Oxyuranus), discharged

I. lip

A 8 c
FIG. 371.- Elapidre: Dental specialisation. Spitting (A, B) compared with a non-spitting (C)
species. The venom enters the fang through an intake aperture (i. ap.), and is ejected through
the discharge aperture (d. ap.). Thick arrows indicat e direction of jet in each types. d. dentine;
en. enamel; s. suture ; v. c. venom canal. (Modified after Bogert.)

in great quantity and almost invariably fatal, acts in the first , third, and
fourth ways outlined above. The neurotoxic venom of the Indo-Malayan
King Cobra (Naia hannah) and Australian Death Adder (Acanthophis)
leads to death chiefly from respiratory paralysis. The highly destructive
venom of the Tiger Snake (Notechis scutatus) also primarily affects the nervous
system, although it is a powerful coagulant as well. The hremolysis and
hremorrhage caused by the venom of the South American Fer-de-lance (Bothrops
atrox = lanceolatus) is so pronounced that bleeding occurs from the eyes, and
 PHYLUM CHORDATA 537

renal and alimentary tract epithelium. A man bitten on one arm by a South
African Boomslang (Dispholidus typus) suffered massive extravasation on the
other as well.
The neurotoxic venom of the truculent African Black Mamba (Dendraspis
angusticeps) has killed a man within twenty minutes. The venom of the less
aggressive Gaboon Viper (Bitis gabonica) produces 'furious blood and tissue
destruction' (Ditmars) as well as respiratory paralysis. This equatorial rain-
forest reptile, of sinister beauty, 1 kills every time it bites in tropical Africa,
reputedly within an hour. Only one bitten person is known to have recovered.
This was a reptile curator at a North American menagerie who was struck on a
finger by one fang only and was in a position immediately to receive treatment
by incision, suction, intermittent torniquet, three kinds of antivenenes (none
specific for B. gabonica is yet available), hot limb-baths and packs, injections of
caffein, strychnine and novocaine, blood transfusion, and finally incisions to
allow the escape of extravasated blood and lymph. Through these incisions
tissue herniated. Voided urine consisted almost wholly of blood. Meanwhile,
respiratory paralysis intervened, and after about an hour the agonised, pulseless
patient became unconscious and appeared to be dying. He regained con-
sciousness an hour later, and left hospital after three weeks. It is claimed that
the congeneric horned West African River Jack or Rhinoceros Viper (B.
nasicornis) is equally lethal. The loud-hissing Puff Adder (B. arietans), which
often visits human habitation to feed on rodents, has a much less deadly
h::emolytic, but to some degree neurotoxic, venom which nevertheless has
occasionally been fatal within three hours.
In India, a country heavily populated by both snakes and Man, as many as
24,000 people have been killed by poisoning in a single year. This enormous
death-rate is probably matched in no other country and is perhaps largely
due to the presence of a mainly bare-footed population. The nocturnal kraits
(Bungarus spp.), Russell's Viper (Vipera russelli), and the Cobra are chiefly
responsible. In Australia and North America, on the other hand, the risk
of snake-bite is negligible: in Australia an average of five people are killed
each year. Most diurnal serpents are more wary of Man than he is of them
and their columella/quadrate attachment (p. 448) enables them to disappear
long before he comes in view. Vipers, however, are exceptional in that they
tend to lie motionless, protected by their concealing coloration. The same
is true of the viper-like Australian Death-adder (Acanthophis antarcticus),
an elapid reptile that shows convergence in both form and habits in a
country without vipers. Although apparently lethargic, vipers are capable
of lightning attack if trodden on or when near their prey. A few snakes (e.g.
1 This club-headed, thick-bodied, stub-tailed animal has a symmetrical carpet-like pattern of
greenish-yellow, mauve, black, and buff. Inflation of the body during excitement brings the
scale-edges into view. The scale colours have in life a delicate bloom, 'so that the whole design
might have been painted on velvet' (Johnson).
VOL. II. LL
 ZOOLOGY
Black or Green Mamba, King Cobra) are said to attack Man without positive
provocation during the coupling season.
Chance of survival from snake-bite depends to a great degree on the amount
of venom that the reptile habitually discharges, on its fang-length and the
depth to which they are driven into the tissues. The essentially neurotoxic,
but also coagulating, venom of the Australian Tiger Snake is drop for drop
probably more potent than that of any terrestrial species, but its bite is fatal
in only about so per cent of treated cases. The Tiger Snake has poison-glands
of limited capacity and relatively short fangs. These are a mere 3·5 mm. long.
Those of the Taipan are 12·5 mm. long and those of the full-sized (5 ft. 8 ins.)
Gaboon Viper almost 2 inches in length if measured along the curve. It has
been suggested that the neurotoxic venom of the Tiger Snake, Blue Krait of
India (Bungarus candidus), and the Formosan B. multicinctus, are toxic to the
mouse in a dosage of as little as 2-5/1,000 of a milligram and about 2 milligrams
in the case of Man.
An interesting adaptation is shown in the fangs of the' spitting' members of
the cobra family and in the South African ringhals (Hemachatus) (Fig. 371).
Here the suture (s.) line ends somewhat more proximally in a more restricted
and relatively oval discharge aperture (d. ap.) through which, when the head is
tilted at an exactly appropriate angle, the venom is explosively discharged at
the victim's eyes. The attack of the Black-necked Cobra (Naja nigricollis) has
been studied by Ditmars. It often takes the victim-Man or other animal-
completely by surprise for the reptile will rear, and eject venom accurately
into the victim's eyes at very slight provocation from a distance as great
as six feet. A six-foot cobra has been known to eject at least some venom
for a distance of 12 feet. Ditmars states that the term 'spitting' does not
correctly indicate the manner of ejection since the jaws are slightly parted
and the venom comes directly from the openings at the tips of the fangs.
'The snake rears . . . . Facing the object of anger it looks intently at one's
face . . . if it seeks to direct the poison upwards it curves its rearing pose
backwards, thus directing its head upwards. The ejection of the poison
is an instantaneous operation. The jaws are slightly opened and closed
so quickly as to appear like a snapping motion, and during this action the
poison leaves the fangs. There is no dribbling or spilling of the fluid. It
issues in twin jets.... There is every indication that, at the instant the snake
prepares to eject the poison, it contracts the temporal muscle over each
poison gland, thus producing pressure to force the toxic fluid a consider-
able distance. This flies with such force that its impact can be distinctly
heard against ordinary glass five feet away. At the instant of ejection the
snake emits a sharp hiss. This ejection of air might be an accompanying
token of anger, or it may assist the travel of the poison.' As this type of
poison is at once absorbed through the conjunctiva, the spray, if it reaches
 PHYLUM CHORDATA 539
its mark, 'throws the victim into a condition of pain and confusion, enabl-
ing the cobra to escape'. There is intense agony accompanied by acute
conjunctivitis and temporary blindness. 'Astonishing amounts of poison are
expended. After a vigorous demonstration, and the discharge of four to six
jets of poison, the glands rapidly refill . . . the new fluid is of much lighter
specific gravity than older storage.' Experimentally it has been shown that
the venom causes degeneration, and partial vascularisation of the cornea,
resulting in blindness if untreated.
Sphenodon (Fig. 361) possesses pointed, triangular, laterally-compressed
teeth arranged in two parallel rows, one along the maxilla, the other along the
palatine. The teeth of the lower jaw, which are of similar character, bite in
between these two upper rows, all the rows becoming worn down in the adult
in such a way as to form continuous ridges. Each premaxilla bears a pro-
minent, chisel-shaped incisor represented in the young animal by two pointed
teeth. In the young Sphenodon a tooth has been found on each vomer-a
condition unusual among reptiles. In the Chelonia, teeth are entirely absent.
The jaws are invested in a horny layer in such a way as to form a structure
something like a bird's beak. The Crocodilia have numerous teeth which are
confined to the premaxillre, the maxillre, and the dentary. They are large,
conical, hollow teeth which are devoid of roots, each being lodged in its socket
or alveolus (thecodont), and each becoming replaced, when worn out, by a
successor developed on its inner side.
In some lizards the tongue is short and scarcely bifid; in others (e.g.
Lacerta) the forked part is of moderate length ; and in others again, like the
monitors, the bifid tips are very long, as in snakes. The tongue of chamreleons
is an extraordinary organ. It is sub-cylindrical with an enlarged extremity,
and is so extensile that it is capable of being darted out to a distance sometimes
equalling, or even exceeding, the length of the trunk (Fig. 351). This pro-
trusion is effected "With great rapidity. In this way the animal catches insects
which constitute its food. The tongue in snakes is slender and bifid, capable
of being retracted into a basal sheath, and highly sensitive. It is used as a
tactile organ; and in conjunction with the organ of Jacobson (seep. 544). The
tongue of the Crocodilia is a thick, immobile mass extending between the rami
of the mandible. In some of the Chelonia the tongue is immobile ; in others
it is protrusible, sometimes bifid.
In the alimentary canal of the reptiles special features are the muscular
gizzard-like stomach of the Crocodilia (sometimes containing gastroliths), the
presence of a rudimentary crecum at the junction of small and large intestines
in most Lacertilia and in Ophidia, and the presence of numerous large cornified
papillre in the resophagus of the turtles.
Digestion begins in the stomach and is particularly rapid in snakes. When
a large animal is being slowly engulfed, the first part of the prey is partly
540 ZOOLOGY
digested before the hind-parts have been swallowed. After taking in an
especially large meal some reptiles (e.g. pythons) are able to do without another
for more than a year.
There is a belief, common in most countries, that in times of danger snakes
temporarily swallow their young as a protective measure. Many sober
observers, from at least the r6th century onwards, have given currency to this
belief, probably through optical illusion. When frightened, the tiny young of
several species glide quickly under the mother's jaws and out of sight beneath
her belly.
Respiratory System and Voice.-The reptiles all have an elongated trachea,
the wall of which is supported by numerous cartilaginous rings. The anterior
part of this is dilated to form the larynx, the wall of which is supported by
cricoid and arytenoid cartilages. Although reptiles are on the whole not
notable for their voices, many do communicate by sound. Geckos emit a
peculiar little yapping bark when menaced. One is known as the Barking
Lizard. It has been claimed, but not proved, that female crocodiles of some
species are made aware of the hatching of their buried eggs by the squeaking
of the young. The roar or bellow of an adult male Crocodylus porosus can be
heard a mile away. Chelonians are said to 'bark' or 'grunt', but the evidence
is conflicting. Sphenodon is said to emit a soft frog-like croak.
The reptilian trachea bifurcates posteriorly to form right and left bronchi,
one passing to each lung. The lungs of the Lacertilia and Ophidia are of the
relatively simple character already described in the lizard (p. 478). In some,
the lung is incompletely divided internally into two portions-an anterior
respiratory part with sacculated walls, and a posterior part with smooth, not
highly vascular, walls, having mainly the function of a reservoir.
In snakes the lungs usually exhibit a striking asymmetry. The right lung
is always very long and in marine species (in which it has a hydrostatic
function) it may almost reach the level of the cloaca. The left lung is often
rudimentary. In some snake-like lizards a similar asymmetry occurs, but in
amphisbrenids it is the right lung that may be reduced or aborted. In the
more primitive snakes both lungs are functional and of almost equal size. In
the Crocodilia and Chelonia the paired lungs are of a more complex character,
being divided internally by septa into a number of chambers.
Some snakes possess a tracheal 'lung'. In these the incomplete tracheal
rings allow the expansion of the tracheal membrane and the development of
alveoli which sometimes extend from the head right down to those of the right
lung proper. It has been suggested that this arrangement allows the snake to
respire tracheally when the true lung is compressed by ingested prey. In the
chamreleons (Fig. 351) a number of diverticula or air-sacs occur and are capable
of inflation, causing an increase in the bulk of the animal which doubtless has
an effect on assailants. The Australian Jew-lizard (Amphibolurt~s) is likewise
 PHYLUM CHORDATA 54 I

capable of blowing itself up with air when confronted. The increased bulk
makes it difficult for it to be swallowed by snakes and other enemies.
There has been considerable argument concerning respiration in chelonians.
The thorax is relatively restricted in movement and it has been claimed that
turtles achieve lung respiration essentially in the manner of frogs (p. 403).
It is apparently true that there is a certain amount of hyoid movement, but
it is possible that this is primarily concerned with the inflow of air for olfaction.
Current opinion is that a series of special muscles, situated ventral to the viscera
at the leg-pockets, alter ccelomic pressures and thus draw air into, and thrust
it from, the paired bronchi and lungs. In addition, two other auxiliary
respiratory devices have been reported in aquatic chelonians that spend periods
below the surface. The pharyngeal cavity of many species is highly vascular,
and it has been claimed that respiratory exchanges take place between pharyn-
geal wall and water taken in and expelled during submersion. The second
auxiliary respiratory mechanism is located in paired cloacal diverticula which
empty and fill through the vent of the submerged reptile.
Some soft-shelled turtles spend much of their time obliteratively concealed
on, and partly in, the bottom mud of waters sufficiently still and shallow to
allow the long neck to reach the surface like a periscope. The appropriately
situated nostrils slightly emerge and take in atmospheric oxygen. Such
animals, as mentioned above, can also breathe dissolved oxygen by means of
pharyngeal and cloacal exchanges when the head is submerged during feeding
and in times of danger.
Blood Vascular System.-In the reptilian heart (p. 474) the conus arteri-
osus, present as the most anterior chamber in the amphibian heart, is
completely absorbed into the ventricle, from which therefore the arterial arches
arise directly. Varying degrees of absorption of the sinus venosus into the
right auricle are also to be observed throughout the class and thus, with the
exception of Sphenodon ( = Hatteria), it is not easily distinguished externally.
Its aperture of communication with the right auricle is guarded by valves or
foldings of the wall of the auricle.
There are, as in the Amphibia, always two quite distinct auricles, the right
receiving the venous blood from the body, the left the oxygenated blood
brought from the lungs by the pulmonary veins (Fig. 320). A vital point of
difference between the heart of the reptile and that of the amphibian, however,
is that in the former the ventricle is always more or less completely divided
into right and left portions. In all the Lacertilia, Ophidia, and Chelonia
the structure is essentially as has been described in Lacerta, the ventri-
cular septum being well developed, but not completely closing off the left-
hand portion of the cavity of the ventricle from the right (cavum ventrale =
pulmonale).
This septum is more or less horizontal. The left-hand portion (which is
542 ZOOLOGY
much the larger) is further imperfectly divided into a cavum venosum to the
right, and a cavum arteriosum to the left by a second septum formed by fused
trabeculre. The secondary septum is situated on the dorsal wall of the ven-
tricle, approximately at a right angle to the primary septum. The separation
is also helped by two elongated flaps of the auriculo-ventricular valve which
project into the cavity of the cavum dorsale.
This secondary septum is probably homologous with the interventricular
septum that has arisen in the crocodiles and which persists in birds (p. sSg).
From the cavum pulmonale arises
the pulmonary artery and, from the
cavum venosum, the right and left
aortic arches. When the auricles
contract, the cavum venosum be-
comes filled with venous blood from
the right auricle, the cavum arteri-
osum with arterial blood from the
left auricle ; the cavum pulmonale
becomes filled with venous blood
which flows into it past the edges of
the incomplete septum. When the
ventricle contracts, its walls come in
FIG. 372.-Crocodylus: Heart and great contact with the edge of the septum,
vessels. Diagrammatic. The arrows show the
direction of the arterial and venous currents. and the cavum pulmonale is thus
l. aort. left aortic arch; l. aur. left auricle; cut off from the rest of the ventricle.
l. aur. vent. ap. left auriculo-ventricular aperture;
I. car. left carotid; l. sub. left subclavian; l. vent. The further contraction consequently
left ventricle; pul. art. pulmonary artery;
r. aort. right aortic arch; r. aur. right auricle; results in the venous blood of the
r. aur. vent. ap. right auriculo-ventricular aper- cavum pulmonale being driven out
ture; r. car. right carotid; r. sub. right sub-
clavian; r. vent. right ventricle. (After Hertwig.) through the pulmonary artery to the
lungs, while the blood which remains
in the ventricle (arterial and mixed) is compelled to pass out through the
aorta. But in the Crocodilia (Fig. sor) the cavity is completely divided, so
that there we may speak of distinct right and left ventricles. From the right
arise the pulmonary artery and the left aortic arch: from the left the right
aortic arch only. The right and left arches cross one another, and where their
walls are in contact is an aperture-the foramen Panizzce-placing their
cavities in communication. Recent work indicates that during systole there is
a considerable flow of blood from the right aorta to the left through this
foramen.
Nervous System.-The brain of the lizard (p. 478) is fairly representative of
that of modem Reptilia. It is in general more highly organised than the
amphibian brain. The cerebral hemispheres exhibit a distinction into super-
ficial grey layer or cortex containing pyramidal nerve-cells, and central white
 PHYLUM CHORDATA 543

medulla, not observable in lower groups. The cerebral hemispheres are well-
developed in all.
The most striking feature is the predominant development of the basal
structures, the corpus striatum, the septum, and the amygdaloid nuclei. The
cortex, dorsally situated in the pallial region of
the hemisphere, is not extensive, but is clearly
--B.o~
differentiated into three areas : medially the
hippocampal, laterally the pyriform, and be-
tween them the dorsal or general cortex. The
lateral or pyriform area is olfactory in function,
but little is known of the function of the other
two regions. There seems little doubt that from
a functional point of view, the basal structures
are by far the most important parts of the VH
reptilian hemispheres. Their close connexion
with the diencephalon (the thalamus and
hypothalamus in particular) by the basal fore-
brain bundles is evidence of this. Two commis-
sures of the hemispheres are present : a dorsal
or hippocampal and a ventral or anterior corn- lllf~~~-­
missure. These appear to correspond, at least
in part, with the commissures of the same name
in mammals, but in general the reptilian brain
shows a much closer resemblance to the brain
of birds than to those of mammals.
The mid-brain consists dorsally usually of
two closely-approximated oval optic lobes ;
rarely it is divided superficially into four. The
cerebellum is always of small size, except in the
FrG. 373.-Alligator: Brain and
Crocodilia (Fig. 373), in which it is compara- cranial nerves. From dorsal aspect.
B . ol. olfact ory bulb; G. p. epi-
tively highly developed, and consists of a median physis; HH, cerebellum ; Med,
and two lateral lobes. spinal cord; MH, optic lobes; NH,
medulla oblon gata; VH, cerebral
Organs of Special Sense.- In most Lacertilia, hemispheres ; I-XI, cranial
but not in the Ophidia, the nasal cavity con- nerves; r, 2 , first and second spinal
nerves. (After Wiedersheim.)
sists of two parts-an outer or vestibule, and
an inner or olfactory chamber- the latter having the sense-cells in its walls, and
containing a turbinal cartilage. In the turtles each nasal chamber is divided
into two passages, an upper and a lower, and the same holds good of the
hinder part of the elongated nasal chamber of the Crocodilia.
The vomero-nasal or Jacobson's organ (Fig. 374) is prominent in lizards
(p. 481) and snakes. In each, the organ is separated from the olfactory cavity
proper. It is best developed in snakes, and in lizards with a flicking, bifid
544 ZOOLOGY

tongue (e.g. Varanus). The tongue appears to pick up and convey scent
particles towards the organ. Just as sight seems to be of great importance to
most lizards (which are often sexually dimorphic), so is scent to snakes, which
are not. Jacobson's organ is probably significant in courtship, enemy-
recognition, and in tracking down prey. It has an extensive sensory area
supplied by a large vomero-nasal nerve. In lizards and snakes most of the
secretions from the lachrymal and Harderian glands find their way to the

n.s. na.

d~.
FIG. 374.-Lacertilia: Vomeronasal organ. Transverse section through snout of new-born
Anguis (Slow worm) . c.]. o. cartilage of Jacobson's organ; de . dentary; d .]. o. duct of Jacob-
son's organ; gl. salivary gland ; ] . o. Jacobson's organ; l. d. opening of lachrymal duct; M. c.
Meckel's cartilage; mx. maxilla ; na. nasal; n . cap. nasal capsule; n. cav. nasal cavity; n. s.
nasal septum ; smx. septomaxilla; t. tooth; t. b. tooth bud attached to dental lamina; to. tongue;
vo. vomer . Drawn by A. d'A . Bellairs.

lachrymal duct which ends near the outlet of Jacobson's organ. These secre-
tions may have some special property concerned with its function. In other
amniotes excess tears are shed into the nose. Adult crocodiles have lost
Jacobson's organ. It is present in Sphenodon in a rather primitive form,
where it is not unlike the condition in mammals. In chelonians it is not
separated from the nose.
The reptilian eye (Figs. 375) presents many specialisations. Lizards
(excluding Varanus), but not chelonians, rely chiefly on monocular vision.
The chamceleons, too, are exceptional and possess perhaps the most notable
 PHYLUM CHORDATA 545

FIG. 375.-Squamata: Cornea

Comparison of lacertilian Iris
and ophidian eye. It can C<Jnal oF Scl>l~mm
be argued convincingly on (,;, scler.. )
ophthalmological grounds (;ftary pr<JCt:S!et
that the ancestors of snakes I<Kk•ng
were fossorial. Walls pos- Seier~/ oss/cle
Cih~r!l mvsc-1•
tulates that in ancestral
burrowing snakes the eye
became a tiny spherical
organ from which the
scleral cartilage and ossicles
had disappeared. The
retina, apparatus of accom- [dodermal
-conus
modation, and iris muscles
(which contract and dilate
the pupil) became degene- Vitreous 6.
rate. At the same time (as
in many modern lizards). a
protective spectacle had
been evolved outside the
cornea, probably derived
from the fusion of the eye-
lids. Subsequently the
snakes re-established them.
selves above ground and Lizard
radiated successfully as a
surface-dwelling group,
though a few fossoria l and
generally rather primitive
forms survived. The eyes
of surface-dwelling snakes
became efficient but re-
tained the stamp of the
fossorial phase in ophidian
evolution. They were re-
furbished and transformed,
and substitutions were
found for many structures
that were irretrievably lost.
The spectacle remained
useful in surface, as in fos-
sorial life. The enlarged
eyeball, having lost its
stiffening scleral structures,
remained spherical. A new
method of accommodation
was achieved by the in-
vasion of the iris by the
ciliary muscles, which now
control both lens and pupil. No epichoriotdal &;mph
A new sort of yellow filter, Snake
too, was developed: the
lens itself became coloured
(in compensation for the long-lost r etinal droplets of lizards). In many diurnal forms the retina
developed 'new and unique double cones', and there were great changes in the structure of the
optic nerve and venous supply . A new type of conus papillaris (homologous with the avian pecten)
arose (seep. 640). This vascular structure probably carries nutrient substances into the vitreous
humour, whence they diffuse to the retina. The r esultant eye 'presents substitutes for all the
losses, remedies for all the defects, of the vestigial organ of the original snakes'. The dotted arrows
show the direction of application of force during accommodation. (From J. Z. Young, after
Walls.)
 ZOOLOGY

eyes of all reptiles (Fig. 351, p. 519). These bulge turret-like, and each can be
operated independen tly to a degree unknown in other animals. The eyes can
be 'turned through 180° horizontally , 90° vertically, and one eye may be made
to aim backwards while the other looks straight forward' (Walls). Thus, this
heavily camouflaged, slow-moving lizard can look for prey in all directions at
once and, although the lid-crater surrounding the small cornea restricts the
visual field, the visual axis is long and the retinal image large. The remarkable
power of convergence gives the
centrally placed fovere a common
binocular point of aim and an
uncommonl y good judgment of
distance for the astonishing ly
k swift protrusion of the adhesive
tongue. Lacerta viridis, by com-
I parison, has only a 40° range of
vision. The eyes of the sluggish
Gila Monster (Heloderma) (p. 497)
r are fixed.
- m A pecten (pp. 481 and 595) is
generally present. Internally
the eyes of reptiles, other than
g snakes, are essentially like those
of the lizard (p. 481). The eyes
of snakes have many peculiari-
ties, such as the absence of
the scleral skeleton. In the
chamreleons there is a single cir-
cular eyelid with a central aper-
ture. In the greater number of
FIG. 376.-Sphenod o11: Pineal apparatus. g,
blood-vessels; h, cavity of the eye filled with fluid; geckos, some skinks and other
k, capsule of connective- tissue; l. lens; m. molecular
layer of the retina; r. retina; st. stalk of the pineal lizards, and in all the snakes,
eye; x, cells in the stalk. (From Wiedersheim, after movable eyelids have been re-
Baldwin Spencer.)
placed by a spectacle, probably
developed from the eyelid rudiments. In such forms the nictitating mem-
brane appears to have been lost. On the basis of the structural peculiarities
of the ophidian eye, which differs in many ways from that of other reptiles,
Walls has argued that the snakes originated as burrowing creatures, and later
emerged above the surface of the ground (see Fig. 375).
Largely because of the profession of snake-charm ing there is perennial
controversy concerning hearing in serpents. The middle ear cavity is virtually
absent. A columella auris (p. 361) occurs embedded in muscular and fibrous
tissue and attached externally to the quadrate. Thus, despite the piping of
 PHYLUM CHORDATA 547

the charmer and the dancing of the cobra (or other species), it seems likely that
the snake is insensitive to air-borne sound. Very probably it is optically
influenced by the movements of the man and his instrument. There is sug-
gestive evidence that a rattlesnake, for example, cannot hear its own rattling.
On the other hand, the attachment of columella to quadrate undoubtedly
makes the snake highly sensitive to earth-borne vibrations such as approaching
feet (see also p. 448). Thus most snakes secrete themselves long before the
approach of Man (see, however, p. 537).
Developed in close relation to the epiphysis there is in many lizards (Lacerta,
Varanus, Anguis, Amphibolurus, and others), and particularly in Sphenodon,
a remarkable organ-the parietal organ or pineal eye (Fig. 376) . This is situated

F1c. 377.- Crotalinre: Special senses. Bushmaster (Lachesis muta) of tropical

America. Note the vertical pupil (which probably protects dusk.sensitive
retina from bright light) and the long forked tongue, which is functionally
associated with the organ of ]acob~o n . (From photograph by Ditmars.)

in the parietal foramen of the cranial roof just under the integument, and
covered in the young (e.g. Scleropus) by a transparent scale (Eakin, Quay and
Westfall). The pineal eye is developed from a hollow outgrowth of the
diencephalon in front of the epiphysis. The distal end of this becomes con-
stricted off as a hollow sphere, while the remainder is converted into a nerve.
The wall of the hollow sphere becomes divergently modified on opposite sides.
The distal side gives rise to a lens-like thickening (l.). The proximal side forms
a membrane several layers in thickness-the retina (r.). The whole is enclosed
in a capsule of connective-tissue (k.). The nerve usually degenerates before the
animal reaches maturity. It has been sometimes claimed, but not proved, that
this organ retains a photo-receptor function in some modern reptiles. The
paired facial pits that occur in pit-vipers (Crotalinre) (Fig. 377), but which are
absent in the Viperinre, function as thermoreceptors (Bullock and Cowles).
A pit occurs between eye and nostril and consists of two cavities separated by
 ZOOLOGY
a membrane. These organs (and the pits in the marginal scales along the
mouth of pythons and some boas) are supplied by branches of the trigeminal
nerve. The apparatus perhaps helps the animals to locate precisely and
attack warm-blooded prey.
Endocrine Glands.-These fall into the characteristic vertebrate pattern
(p. 147), possibly including the formation of a post-ovulatory corpus luteum in
viviparous and ovoviviparous snakes and lizards. A substance resembling
progesterone has been extracted from the ovaries of ovoviviparous snakes and
there is some evidence that, in at least one viviparous lizard (Hoplodactylus),
progesterone or its homologue may be involved in pregestational changes in the
female tract. The hypophysis is necessary for the maintenance of pregnancy in
snakes. There is evidence, too, that in some species ovariectomy is followed
by freta! resorption in the early stages of development, but, at the same time,
injection of progesterone does not prevent such effects as in mammals. It
has been suggested, but not proved, that something not unlike the chorionic
secretions of the Mammalia (p. 905) may take over in late pregnancy.
Excretion.-In common with the 'higher' amniotes, the metanephros takes
on the function of excretion. With the elimination of gills, and the cutaneous
permeability that accompanied the final conquest of land, the kidney in the
Reptilia assumed additional importance. Life depended partly upon con-
servation of body-fluids. Glomeruli were reduced in size and vascularity.
Excretion became uricotelic in at least some terrestrial stocks, including,
apparently, that which gave rise to the birds. Such animals abandoned urea
excretion, which involves considerable water loss. Uric acid, on the other
hand, is comparatively non-toxic and insoluble, and is precipitated in the uro-
da:um, the vital fluid being reabsorbed into the circulation and the now semi-
solid or solid nitrogen compounds discharged as urates with the fa:ces. Among
extant reptiles, the land snakes and lizards (to which water conservation is of
great importance) retain drastically reduced glomeruli. Chelonians which have
generally returned to water require no such protection and possess glomeruli
very like those of typical anurans.
The Crocodilia, snakes, and some lizards (e.g. Varanus) lack a urinary
bladder. Most lizards, Sphenodon, and the chelonians possess one. This
opens into the uroda:um. Aquatic reptiles, to whom water retention is less
important, appear generally to excrete relatively large proportions of com-
paratively soluble, and therefore toxic, urea and ammonia, and little uric acid.
Perhaps significantly, the land-living European tortoise Testudo grceca excretes
principally uric acid. In the North American Desert Gopher (Gopherus
agassizi) the accessory bladders developed in other tortoises are reduced to
tiny pouches which contain small amounts of urine. In this, the waste
nitrogen is held, not as soluble urea but as uric acid, which can be retained for
a considerable period without ill-effects. In the case of ovoviviparous lizards
 PHYLUM CHORDATA 549
and snakes, which retain a relatively impermeable egg-' shell', it is advantageous
to excrete embryonic nitrogenous waste principally as insoluble uric acid and
not as the more toxic urea.
There is evidence that females of some species use bladder-fluid to moisten
and firm the sand during nest-digging and possibly to moisten the eggs.
Reproduction.-The description already given of the reproductive organs
of the lizard (p. 483) applies, so far as all the principal features are concerned,
to all the Lacertilia and to the Ophidia.
In snakes and many lizards, however, a very distinct so-called sexual
segment has been demonstrated in the male kidney. In Vipera berus the organ
makes up almost half of the total kidney volume. The epithelium of the sub-
terminal portion of each individual renal tubule (nephron) is sharply marked
off into an area of columnar cells. No specific secretion has been demonstrated
but the cells themselves are said to exhibit a cyclical activity that runs hand
in hand with that of the secretory interstitial Leydig cells and the epididymis.
A feebly developed corresponding segment occurs also in the female.
There is no evidence that the sexual segment in reptiles is homologous with
the sexual component that has been described in the kidney of certain fishes
and urodeles. It has not been shown in Testudo.
Anal glands are a characteristic of snakes. These lie at the base of the tail
and discharge into the cloaca. There is some evidence that their odorous
secretion enables the sexes to locate each other.
In the Crocodilia and Chelonia there is, instead of paired penes (Fig. 378), a
median solid penis attached to the wall of the cloaca. A small process, the
clitoris, occurs in a corresponding position in the female. Though fertilisation
is always internal, most reptiles are oviparous, laying eggs enclosed in a tough,
parchment-like or calcified shell. These are usually deposited in holes, and
left to hatch by the heat of the sun. In the crocodiles they are laid in a rough
nest. Turtles of various species lay from one to 300 eggs which they usually
deposit in holes dug in the sand, often, in the case of marine species, on the
beaches of remote tropical atolls after dark (Fig. 327).
Some lizards and snakes are ovoviviparous. Their eggs are enclosed in a
thin membrane and retained in the oviduct during the developmental period.
Diverse modes of parturition may have no taxonomic significance, for, within
the genus Lacerta, one species, L. agilis, is oviparous and another, L. vivipara, is
ovoviviparous. A few northern ovoviviparous or viviparous lizards and snakes
are sometimes compelled to hibernate before the birth of their young, which are
then retained in the oviduct until the following spring. Some lizards and snakes
(as well as some fishes, p. 278) have developed yolk sac, and for chorio-allantoic
placentation. In such reptiles the eggs lie in expanded incubating chambers
in the oviducts. A single large artery and vein runs dorsally along each
oviducal wall, the artery giving off branches which pass around the incubating
 sso ZOOLOGY
chambers. Where the arterioles break up into capillaries the vein is likewise
joined by capillaries. Three types of primitive placentation have been
described.
The first is a simple arrangement whereby change in the maternal and
embryonic epithelium allows the
capillaries of the two blood-streams
to approach each other (the lizards
Lygosoma, Egernia, Tiliqua, M a-
buya, and probably Chalcides and
the snake Denisonia). A second
simple type involves the raising of
the maternal capillaries into small
ridges which are pressed against
the chorionic ectoderm. Although
some pseudopodia! processes from
the embryonic tissue penetrate the
maternal epithelium, the union is
superficial and without real inter-
penetration of parental and em-
bryonic material (Lygosoma). In
the third and most elaborate placen-
tation, the oviducal wall is raised
rt. pr. mus.
into a series of vascular folds in the
form of an ellipse which is applied
to a likewise elliptical area of
thickened chorionic ectoderm (Lygo-
soma, Chalcides).
A remarkable post-partum func-
tion of the yolk sac (see below) has
PIN been recently suggested in the
European Adder (Vipera berus)
lt. penis

FIG. 378.- 0phidia: Bilateral copulatory

organs. The penes (so-called 'hemipenes ')
of Vipera berus in ventral view with right
or gan artificially evaginated. On the left
side the ventra l caudal muscles have
been removed and the invaginat ed penis
exposed by retraction. ap. opening of left
penial sac; c. gr. copulatory grapples on right
penis; lt. penis, extending cauda lly under
pin holding dissection flaps apart; lt. r. mus.
large retractor muscle of left penis; med.
sept. m edia n septum; pr. mus. propulsor
muscle; x . end of left penis and place of
insertion of large retractor muscle. (Modi-
fied after Volsoe.)
 PHYLUM CHORDATA 55 I

(Bellairs, et al). The yolk-sac, which remains substantial at birth, is retracted

into the juvenile body cavity and may provide a source of energy for the young
snake for the first eight months of its life, including the period of winter
hibernation.
The embryos of lizards and snakes possess a small egg-tooth which,
when functional, is used to puncture and enlarge a hole in the egg-membrane.
Two types of egg-breaker are known in amniotes. In lizards and snakes the egg-
breaker is formed by a true tooth. This is composed of dentine and probably
has an enamel cap. It projects forwards in the midline from the premaxilla.
This structure-the egg-tooth-is present even in viviparous forms, though
in these it may be degenerate. It is reduced, too, in ovoviviparous species, the
young of which push through with the snout. The geckos are peculiar in
having paired egg-teeth projecting side by side. In Sphenodon, chelonians,
crocodiles, and birds there is no egg-tooth. In them its place is taken by
a horny carttncle on the tip of the snout. Monotremes (p. 688) have
both egg caruncle and egg-tooth though the latter may have little function.
Among the marsupials rudiments of both structures have been found in the
embryo.
Development.-In all Reptilia segmentation is meroblastic, being confined
to a germinal disc of protoplasm situated on one side of the yolk. This divides
to form a patch of cells which gradually extends as a two-layered sheet, the
blastoderm, over the surface of the ovum. The upper of the two layers is
the ectoderm, the lower the yolk-endoderm. The latter is comparable with the
yolk-cells of the frog, and the shallow space between it and the yolk represents
the segmentation-cavity. As the blastoderm extends (Fig. 379), it becomes
distinguishable into a central clearer area (area pellucida (a. pel.)) and a peri-
pheral whitish zone (area opaca (a. op.)). On the former now appears an
elliptical thickened patch, the embryonic shield (emb. s.), which is formed by
the ectoderm cells in this region assuming a cylindrical form while remaining
flat elsewhere. Behind the embryonic shield appears a thickening, the so-
called blastoporal plate or primitive knot. Cells invaginate through this region
to form a mesochordal canal which runs in the direction of the long axis of
the future embryo. The opening to the surface of the mesochordal canal is
known as the blastopore (bl. p.). This subsequently takes the form of a narrow
slit, the notochordal canal, running in the direction of the long axis. The
cavity of the invagination corresponds to the archenteron of the frog, and the
cells lining it are the notochord dorsally and the endoderm laterally and
ventrally. The latter subsequently (Fig. 380) coalesces with the yolk-endo-
derm below the floor of the archenteron, and in this position an aperture is
formed through which the archenteron opens freely into the shallow space that
lies between the yolk-endoderm and the yolk. It is from the common cavity
thus formed that the lumen of the enteric canal is derived. At a somewhat
552 ZOOLOGY
earlier stage a thickening (pr. pl.) has appeared in the yolk-endoderm in the
region whichwill give rise to the head of the embryo. This is the protochordal
plate. It enters into intimate relationship with the notochordal cells that
roof over the archenteron and, when the floor of the latter becomes opened

FIG. 3 79.-A IIi gator: Development. A. stage with embryonic shield, primitive knot and blas-
topore; B, considerably later stage in which the m edullary groove has become formed, together
with the head-fold of the embryo and the head-fold of the amnion; C, somewhat later stage with
well-developed medullary folds and medullary groove; D, later stage in which the medullary
groove has become partly closed in by the medullary folds and in which six pairs of protovertebroe
have become developed. amn. amnion; a. op. area opaca; a. pel. area pellucida; blp. blastopore;
emb. s. embryonic shield; f. br. fore-brain; h. br. hind-brain; ltd. f. h ead-fold; m. br. mid-brain;
med. f . medullary folds; prot. v. somites; pr. st. primitive knot. (After Clarke).

out, forms with them a continuous plate. In this, the notochord proper
originates along the middle line. The mesoderm of all the region in front of
the blastopore grows out from it at the sides. The aperture of invagination
becomes narrowed, and is eventually closed by the approximation and coal-
escence of its edges. In the region in which the coalescence of the edges
takes place there is for a time complete union of the layers, as in the region of
 PHYLUM CHORDATA 553
the primitive streak of birds and mammals. The anterior part of the aperture,
however, remains open for a time as the opening of the neurenteric canal.
In front of the blastopore a longitudinal depression, bounded by a pair of
longitudinal folds (Fig. 379, med. f.), is the beginning of the medullary groove.

FIG. 380.-Lacerta: Development. Longitudinal sections through the embryonic area of

blastoderms illustrating successive stages in the formation of the invagination-cavity (archenteron)
and its communication with the segmentation-cavity. blp. blastopore; ec . ectoderm of embryonic
shield; kn. primitive knot; pr. pl. protochordal plate; y. e. yolk-endoderm. In D the opening
below and in front of blp. points to aperture of communication established between the invagina-
tion-cavity and the underlying space. (:\1odified after Wenckebach.)

As this becomes closed it encloses, in its posterior portion, the blastopore or

dorsal opening of the neurenteric canal. At the sides of the medullary groove
appear the somites (prot. v.). The general history of these parts has already
been sketched in the section on the Craniata (p. 160), and further details will
be given in the account of the development of birds (p. 645), which agrees
with that of reptiles in most essential respects. In the section on avian
VOL. II. MM
554 ZOOLOGY
embryology will also be found an account of the formation of the character-
istic fretal membranes, the amnion and the allantois, which applies in all
essential respects to the Reptilia as well.
There are numerous accounts of the alleged great longevity of reptiles,
especially chelonians (p. 489). There are a'pparently valid records of
the survival in captivity of crocodiles and pythons for thirty years, of an
Anaconda (Eunectes) to the age of twenty-nine, and of a Sphenodon to twenty-
eight.
 CLASS AVES

INTRODUCTION
In many respects birds are the most highly specialised craniate class.
Almost every part of their organisation is modified in accordance with
aerial life. All birds possess feathers, and only birds possess them. The
fore-limbs are modified as wings. The sternum and shoulder-girdles are
strikingly altered and serve as origins for the great wing-muscles and the
mechanical framework to support their activity. The pelvic girdle and hind-
limbs have changed to support the entire weight of the body on the ground.
The perfection of the respiratory system, enabling a more complete absorption
of oxygen and the production of a higher constant temperature than in other
animals, has come hand in hand with the evolution of incomparable powers of
flight. Important negative characteristics of modem birds are an absence of
teeth and the left aortic arch. The right ovary and oviduct have generally
disappeared. The brain is of a highly specialised character. The elevation of
the group from near the ground has allowed the olfactory apparatus in most
species to degenerate, and this has been accompanied by an extraordinary
enlargement of the eye, and enhanced efficiency in vision. Despite the general
validity of the old assertion that birds are essentially glorified reptiles, the
above series of strongly-marked characteristics are hardly equalled in distinc-
tion in any other class. Moreover, the organisation of existing birds is, in its
essential features, singularly uniform. Even taking flightless types into con-
sideration, the entire class presents less diversity of structure than many
single orders of fishes, amphibians, and reptiles.
Although fossil forms have been found that possessed teeth, feathers, the
peculiarly avian synsacrum, and a long reptilian tail, the exact ancestry of
birds is not yet known. Avian fossils are comparatively rare for mainly three
reasons. In the first place, flight renders birds less likely to be bogged or
drowned and embedded in silt. Secondly, birds that die on dry land or sea
are usually eaten by predators, and have the hard parts scattered before
inundation. Thirdly, the generally pneumatic bones of birds, though possessed
of great tensile strength, are apparently less durable than those of other
groups.
Probably the best that can be at present said with certainty is that the
556 ZOOLOGY
class is derived from a stock of Mesozoic archreosaurian bipedal reptiles (p. 498).
The reptiles most closely allied to the birds appear to be the dinosaurs (p. 507) :
certainly both groups had a common ancestry. The ancestral birds developed
a feathery or feather-like covering probably first as a heat-conservation device
(like the fur of mammals), and later as an aid to flight while gliding from trees
or while volplaning off the ground at speed (p. 503).
The upper Jurassic genus Archceopteryx (? = Archceornis), by a lucky
accident of petrifaction in fine-grained lithographic limestone, is available in
fossil form, even including exquisitely preserved feather-structure. These small
(crow-sized) birds provide, as does Seymouria (p. 427), splendid examples of
mosaic evolution, i.e. the possession of well-developed characters thoroughly
typical of both groups between which the animal is transitional.
Thus, Archceopteryx had the following reptilian characters : a long tail of
many (zo) free vertebrre, simple articulation between vertebral centra, short
sacrum (of six vertebrre), separate metacarpals, clawed digits on the fore-
limbs, separate metatarsals, simple ribs and gastralia, and simple brain with
long, narrow cerebral hemispheres and small cerebellum behind the optic
lobes. At the same time it had the following avian characters : feathers
which were identical in structure with those of modern birds, arm-feathers
arranged in an identical way, a furcula formed from fused clavicles, back-
wardly projecting pubes, and an opposable hallux on each foot. There was
no obvious trace of a sternum in the fossil specimen of either Archceopteryx or
Archceornis. However, by means of X-ray examination, de Beer has now
shown that in the former it was flat and bony, and that it therefore accords
with other features revealing that in life Archceopteryx was incapable of flapping
flight and must have used its wings to glide from the branches of trees
(Fig. 414, P· 599).
The subsequent development of true flight, as distinct from the occasional
gliding that is found among fishes, amphibians, reptiles, and mammals (exclud-
ing the true flight of bats), enabled birds successfully to exploit a new environ-
ment and to radiate rapidly into one of the most successful groups of modern
animals.
Extant flightless birds are all descended from true fliers. However bene-
ficial flight is in escaping enemies and for the exploitation of both sea and
land, it has nevertheless been lost independently in a number of groups.
The 'older' palreognathous birds (cassowaries, Emu, Ostrich, rheas, kiwis,
moas, tinamous, and others) have all become flightless except the tinamous.
The penguins (apparently an early offshoot from flying birds like petrels and
albatrosses) have likewise become flightless. Among the more 'modern'
Neognathre many members of various groups have also, in special circum-
stances, lost the ability to fly. A classical case was the recently extinguished
Great Auk or Garefowl (Alca impennes), a Razorbill-like auk whose wings
 PHYLUM CHORDATA 557
became converted to paddles not unlike those of penguins: a striking example
of convergent evolution. Several species of rails have become earth-bound,
likewise gallinules (of which one flightless type, Notornis mantelli, still survives
in southern New Zealand). There became flightless, too, at least one species of
grebe (Centropelina of Lake Titicaca, in upland Peru) and a cormorant (Nan-
nopterum of the Galapagos). Several ground-dwelling pigeons have ceased to
fly, with a concomitant degeneration of wings and pectoral apparatus. Classi-
cal among these is a group of recently extinguished species which inhabited
islands of the Indian Ocean. These were the awkward, swan-sized Dodo
(Didus ineptus), which was common in Mauritius until the late 17th century,
the Solitaire (Pezophaps solitarius) of nearby Rodriguez, and the White Dodo
(D. bonbonicus) of Reunion. Representatives of other orders, including even
parrots (e.g. Strigops habroptilus or Kakapo of New Zealand), also became
flightless or almost so.
The reduction or complete loss of flight has often taken place in special
circumstances in which the absence of predators has made flight no longer
greatly beneficial. Oceanic islands, to which carnivorous mammals could not
penetrate, have often been a nursery of flightless birds. The abandonment of
flight allows a reduction in wing size, and often the assumption of a bulk
greater than the muscle mass of any flying bird could possibly lift. Likewise,
an increase in egg-size is possible. Once it becomes flightless, a bird must
stay that way and remain at the mercy of later, more ferocious arrivals, includ-
ing particularly predators introduced by Man. The flightless dodos, rails,
Great Auk, and many others almost all vanished at the onslaught of Man and
his domestic animals, although it is possible that some were already fading
before the Europeans began to circumnavigate the world. A remarkable
survival is the Tooth-billed Pigeon (Didunculus), which was in Samoa a ground-
nester rapidly approaching extinction. It, however, still could fly. It began
to nest in trees and has survived. Of the flightless 'ratite' (raft-breasted)
species such as moas, kiwis, ostriches, rheas, etc., only those that are rigidly
protected (e.g. kiwis) or large, powerful, and elusive (e.g. cassowaries, Emu,
and the Ostrich) survive in considerable numbers.
As long as a bird remains a good flier it cannot become very large : it is
probable that the size-limit for flying birds has been almost reached several
times (e.g. pigeons, turkeys, birds-of-prey (p. 6r5)). Efficient flight requires
a large surface-mass ratio. It is significant that the heaviest of the flying
birds that are almost constantly on the wing (albatrosses, vultures, eagles) are
soaring fliers which make very considerable use of long wings and rising air-
currents. The heavy Mute Swan (Cygnus olor) is unable to take off from the
surface of a frozen pond on a still day. A twelve-stone angel with pectoral
muscles only as efficient as the very efficient muscles of birds would be faced
with the gravest aerodynamical difficulties. Certainly there would be nowhere
558 ZOOLOGY
to house the relatively enormous muscle-mass required to get it airborne and
to keep it so (p. 582).
Bird-flight, like any other, depends on the interaction of forces between
the wings, the tail-rudder, and the atmosphere. Two principal forms of flight
-gliding and flapping-exist. The first is relatively simple, yet still incom-
pletely understood. All flapping birds (e.g. swallows) use gliding flight to some
degree. Except when taking off and landing, the gliders ride almost exclusively
on thermal air currents rising from the earth (e.g. birds-of-prey) or on perhaps
more or less horizontal air-currents above the ocean (petrels, albatrosses).
The pectoralis major muscle (p. 580) is the chief motive force in the powerful
down-beat of the flapping wing. This muscle is especially heavy, powerful,
and copiously supplied with blood in flapping fliers. In this sort of flight not
only streamlined wing-shape, and the principal up-and-down movements, but
many essential subsidiary actions, involving individual groups of feathers and
the tiny bastard-wing (p. 566), play a part. While a wing is actually beating,
its shape, due both to muscle movement and to feather arrangement, is con-
stantly changing. These influences, and postures, differ according to whether
the bird is taking off, gliding, flapping, or landing. To a great degree, too,
they differ according to the special modes of flight of individual genera. Thus,
the heavy, though swift, flapping flight of swans ; the delicate hovering of a
Kestrel; and the alternative backwards-and-forwards flight of a humming-
bird vibrating before a flower-all call into play different structural com-
ponents. In humming-birds which can fly backwards a little, as well as hover
for long periods, the up, as well as the down, strokes are powerful. (An analogy
is a sculled boat compared with one that is rowed.) The supracoracoideus
(so-called pectoralis minor) muscle of humming birds has almost as great a
mass as the pectoralis major. The wings beat at a rate of more than 50 per
second. The astonishing activity of these tiny creatures (some of which are
less than 3 inches long and weigh only 3 grammes) involves a respiration rate
of about 250 per minute (domestic fowl 30). The heart-beat of many small
birds under stress or excitement exceeds 8oo (domestic fowl3oo). The temper-
ature of passerines (p. 582) usually ranges from I02° to III° F. (see also below).
The enormous surface in relation to the tiny mass of some humming-birds
has made it necessary for them to evolve special behavioural and metabolic
mechanisms to enable them to survive at night, since they are strictly diurnal
feeders and therefore cannot replenish body-fuel after dark. By day, humming-
birds have by far the highest metabolic rates (as measured by oxygen consump-
tion in relation to mass) of all vertebrate animals. This metabolic rate, of
course, goes hand in hand with a relatively enormous ingestion and com-
bustion of fuel. One species, studied in a Californian laboratory, was observed
to increase its feeding ratio about an hour before night-fall. Then the animal
perched, and went into a state of torpidity in which its metabolic rate
 PHYLUM CHORDATA 559
dropped sharply, and its temperature equalled that of the environment. A
few humming-birds that inhabit cold places-e.g. the high Andes-are said to
retreat at night into caves. A North American night-jar, the Poor-will
(Phalamoptilus nuttalli), undergoes true hibernation involving a prolonged
winter torpidity at reduced body temperature.
Under experimental conditions the Poor-will, Anna Humming bird (Aero-
nautes saxatilis), and the White-throated Swift (M icropus apus) will reduce their
body temperature and become torpid (Bartholomew, et al.). A Poor-will
became torpid at 3"5° C. and its body temperature fell to 4·8° C. The bird was
aroused by gradually increasing the external temperature to 22° C. Several
hours passed before the bird became fully active, but in a well-camouflaged
species such slow recovery may not be disadvantageous. Humming-birds
recovered rapidly. Reduced body temperature and torpor reduce energy
consumption, and are associated with high metabolic rates during activity
(humming-birds) or survival during long intervals of fasting (swifts and Poor-
will).
The speed of birds, like their longevity (p. 562), has been greatly exag-
gerated, and few exact data are available on either. Most passerines (perching
birds) of the size of a sparrow fly relatively slowly (rS-35 miles per hour).
Even the Carrier Pigeon averages no more than so miles per hour, but swifts
(Apus) are believed to travel at between 6o and go miles per hour in short
bursts.
The remarkable respiratory rate, oxygen consumption, and high tempera-
tures of small flying birds result in a nervous and muscular efficiency that
makes possible the seemingly tireless activity that is one of their most obvious
characteristics. Heat generated by muscle activity is conserved principally
by the relatively non-conducting plumage, which, in times of reduced tempera-
ture, can be erected by a mechanism essentially similar to that which achieves
pilo-erection in mammals (p. 683). But even thickly plumaged birds must
(like all animals with a large surface and a small volume) eat enormously in
order to replace the body heat that is continuously flowing away. Thus, in
countries with severe winter, birds that do not migrate die in great numbers.
A few species such as swallows (Hirundo), bee-eaters (Merops), and Australian
wood-'swallows' (Artamus) have developed a curious secondary conservation
device-the 'clustering' habit of clinging perched together in dozens with only
wing- and tail-tips exposed, to conserve heat for a few days until the probable
advent of warmer weather. In recent severe winters swarming, sleeping
swallows have been sent south over the European Alps by aeroplane in artificial
migration to save their lives.
The colonisation by birds of areas too cold to support reptiles has been
made possible partly by the development of their habit of incubation by which
the embryo is kept at an appropriate temperature by heat from the brooding
s6o ZOOLOGY
bird's body. In neognathous species a special adaptation in the form of bare,
heavily vascular ventral brood-patches facilitates the transfer of parental
heat to the eggs. The seasonal development of brood-patches is under hor-
monal control, and apparently involves a synergism between prolactin from
the anterior pituitary gland (p. 150) and cestrogen from the ovary (p. 598).
Several days before incubation begins, specific ventral areas shed their feathers
and become richly vascularised. After the young fly the skin returns to the
pre-nuptial condition.
A few tropical and sub-tropical species have reverted, with specialisations,
to the reptilian habit of burying the eggs. Thus, the megapodes of Australia,
New Guinea, and adjacent Pacific islands scratch together piles of decaying
vegetation and therein lay their eggs. Some of these incubators, added to
over the years, ultimately reach a height of 20 feet. The heat generated by
the fermenting vegetation combined, in some species, with direct solar heat,
incubates the eggs. The Australian Mallee Fowl, Leipoa ocellata, controls the
temperature by altering the amount of covering material. In most species
there is no such regulation by the parents. Some species lay their eggs in warm
volcanic earth, while others depend on solar heat, burying their eggs in tropical
beaches. Young megapodes are precocial, i.e. they are able to leave the nest
and take care of themselves from the time they are hatched.
All small birds are perilously dependent on day-to-day food supplies, and
a few species, including particularly the European Nuthatch (Sitta), gather
stores for the winter as do certain mammals. Very young birds are particu-
larly susceptible to low temperatures, being almost poikilothermous (p. 386)
and in high, or even in temperate, latitudes are often in danger of freezing or
starvation. The breeding season in non-tropical species is 'timed' to coincide
with relatively high temperatures and abundant supplies of food for the young,
although a few birds (e.g. Emperor Penguin, Aptenodytes Jorsteri) have evolved
special anatomical and behavioural mechanisms that enable them to breed
during mid-winter. Very young birds of some species are compelled to eat
the equivalent of more than half their own weight in food per day in order to
satisfy their metabolic requirements and keep warm. Not only must the food
be sufficient in quantity, but it must also contain essential amino-acids. Thus,
most vegetarian species give their young at least a partially insectivorous diet
during the early stages of growth. Pigeons, which produce proteinous crop-
'milk' (p. 583), are among the few species that can often successfully reproduce
during autumn and winter.
Juvenile mortality in birds is extremely high, but once maturity is reached
some species live to a considerable age. An Eagle-owl (Bubo) lived in captivity
to the age of sixty-eight, and a Sulphur-crested Cockatoo (Kakatoe) to fifty-
six. Eagles, pelicans, and condors (V ultur) have lived for more than fifty
years and gulls, geese, and pigeons to between thirty and forty. A great
 PHYLUM CHORDATA

FIG. 381 .-Aves: Epigamic plumage. The basic structure of the feather lends itself to diversi-
fication both in gross form and in microscopic structure, the latter sometimes producing weirdly
beautiful 'interference' effects. Birds of widely different classes possess spectacular specialisa-
tions: e.g. the covert- (not tail-) feathers of the Indian peacock (Pave), and the lyre-shaped
feathers of Australian Menura; the racquet-tails of certain New Guinea Kingfishers (Tanysiptera) ;
the trailing wing-plumes of the African nightjars Macrodipteryx and Cosmetornis, and so on. It
is in the Paradiseid<l) (New Guinea and Australian rain-forests) that epigamic specialisation (in
males only) reaches its most astonishing extremes. Birds-of-paradise range in size between that
of a thrush and a small pigeon. The selection of males above (not to scale) is as follows:
r. Ribbon-tail (Tamiparadisea meyeri). The white, dark-tipped tail-feathers, relatively jhc
longest of any bird, were procured by Europeans from the head-dress of a stone-age Melanesian
and were sent to the U.K., where they were found to b e so characteristic that a new species could
be described while the rest of the anima l was still unknown. The bird is coloured green, purple,
black, copper, brown, and white. 2. Superb (Lophorina superba) . Principally metallic green
and velvety black. 3· Twelve-wired (Seleucidis m elanoleucus). Metallic green, dark glossed with
purple, yellow flank-plumes (wit h sh afts recurved). and brilliant red legs. (In the museum skin
the yellow commonly fades to white.) 4· Sunset (Paradisea apoda) . Chestnut, green and
yellow, with golden plumes. This was probably the first species to reach Europe. No legs
remained on the rough, native-preserved skins; hence Linn<l)us' specific name, and the sailors'
legend (originating in the sixteenth century) of heavenly birds that came to earth only upon their
death. s. Blue-bird (Paradisorni s rudolphi) . Vivid blue, velvet black, and maroon. This,
perhaps the loveliest of all, was a cquired (but not collected) by Finsch a nd Meyer, who quickly
named it after 'the high a nd mighty protector of ornithological resea rches over the whole world '
who was later to finish his career at Mayerling. 6. Enamelled (Pteridophora alberti). The
twin h ead-plumes have pale blue enamel-like 'flaps' projecting from the outer side of the shaft .
The rest is velvet black, yellows, and brown. 7· King (Cicinnurus regius) . During display the
emerald-tipped pectoral fans are erected, the snowy v entral feathers are fluffed, and the ' metal-
lic' green-tipped tail wires are swung forward over the crimson head. The apple-green gullet is
meanwhile exposed ; and the legs are blue. 8. Six-plumed (Parotia sefilata) . Principally velvet-
black, with a short glittering crest and a p eculiar bronze-and-white tuft above the nostrils. Large
erectile feathers form a breast-plate. The six head plumes are moved during display. g. Repub-
lican (Schlegelia wilsoni), described by Prince Bonaparte . The face is black and the head is bare
and blue; the nape is yellow, the back crimson, and there is a large emerald-coloured breast-plate.
(Redrawn after Iredale and other authnrs. )
s6z ZOOLOGY
variety of birds have lived for thirty years in captivity. The greatest recorded
age at liberty (determined by ringing) is that of a Black-headed gull (Larus)
at twenty-five. The most controversial, and doubtless fictitious, age of any
individual bird is the 120 years attributed to the celebrated sulphur-crested
'Cocky Bennett', for many years an institution at a public-house at Tom Ugly's

FIG. 382.-Aves: Sexual and aggression display. The scintillating blue-black male Satin
Bower-bird (Ptilonorhync!ms violaceus) of the forests of Eastern Australia builds an avenue-like
display ground or bower which faces north and south. At one end he accumulates with great
discrimination display objects broadly matching the colours of rival m a les. With these he
performs a noisy flashing display that attracts a female a nd repels riva ls. This is probably
partly a displaced combat-drive. The male paints the inner walls of his bower with a mixture of
charcoal ground up with saliva, or with pulped fruit, or other ma.terial. A special bark tool (see
also Fig. 426, p. 6zr) is made for use in the plastering operation, which may be of the nature of
displaced courtship feeding. After some weeks of m ale display, the watching female adopts a
specific squatting position in or n ear the bower: this constitutes a sign-stimulus whic h leads to
copulation. By now the forest has become seasonally full of the insect food on which the young
will be fed.

Point, Sydney. Year after year pictures in avicultural and other journals
showed this venerable bird with a grossiy overgrown beak and a body almost
completely devoid of feathers. He was extremely voluble, his favourite utter-
ance being: 'One more bloody feather and I 'll fly! ' He came to a much-
lamented end in 1916. Investigation showed his authentic history to cover a
mere twenty-six years, although he was unquestionably somewhat older.
 PHYLUM CHORDATA

The behavioural activities of birds, however intelligent some species seem to

be, are mostly reflex in character and essentially under the control of environ-
mental events (including activities of the opposite sex) and the internal hor-
monal condition of the individual. For example, a parent passerine bird will
work desperately to satisfy the squeaking, gaping maw of a young cuckoo and
pass unheeding her own offspring, which, thrown from the nest by the parasite,
lie dying in full sight a foot or so below. Her inherent urge is to feed a noisy
coloured gape in the nest. The urge to seek, obtain, and defend territory is
inherent and dependent upon the secretory condition of the testes. The same
is true of display, which in birds reaches unparalleled beauty and complexity,
and involves (e.g. in birds-of-paradise (Fig. 381)) the exhibition of epigamic
plumes of sometimes almost indescribable beauty, and sometimes (in bower-
birds) the building of special structures which are enhanced with coloured and
otherwise distinctive objects (Fig. 382). At the same time 'in some contexts
it is doubtful whether birds are exceeded in learning ability by any organisms
other than the highest mammals .... In their powers of coordinating their
sense impressions for purposes of orientation ... they appear supreme'
(Thorpe).

DISTINCTIVE CHARACTERS AND CLASSIFICATION

Aves are Craniata in which the epidermal exoskeleton takes the form of
feathers over the greater part of the body, of a rhamphotheca or horny sheath to
the beak, and of claws on the digits of the foot and sometimes of the hand.
In the standing position the body is entirely supported on the hind-limbs, the
articulations of which are thrown forward. The fore-limbs are modified to
form wings, usually provided with large feathers for the support of the body
during flight. The cervical and free thoracic vertebrre are usually hetero-
crelous, but may be procrelous or amphicrelous. The sacral vertebrre are fused
with the lumbar and with more or fewer of the posterior thoracic and anterior
caudal to form a synsacrum for the support of the ilia. The posterior caudal
vertebrre are usually fused to form a pygostyle around which the tail-quills are
arranged in a semicircle. The bones of the skull undergo early ankylosis.
There is a single, rounded, occipital condyle. The united premaxillre form
nearly the whole of the upper jaw. The lower jaw is composed originally of
five or six bones in each ramus, and is supported by a freely articulated
quadrate. The vertebral ribs are double-headed, provided with bony
uncini, and articulate with the bony sternal ribs by synovial joints. The
sternum is broad, and is typically produced into a longitudinal ventral keel,
having a separate centre of ossification. The keel may ~e partially or com-
pletely aborted. The coracoid is usually more or less pillar-like, the scapula
is sabre-shaped, and the clavicles and interclavicle unite to form a furcula.
 ZOOLOGY
Except in one extinct species the distal carpals and the metacarpals are united
to form a carpo-metacarpus. There are usually only three digits in the wing,
which probably represent the first, second, and third of the typical hand.
The ilium is of great size, having large pre- and post-acetabular portions.
The acetabulum is perforated in the dry bone. The pubis and ischium are
directed backwards and, except in one case of each, there is neither pubic
nor ischiatic symphysis. The head of the femur is at right angles to the shaft.
The proximal tarsals are fused with the tibia to form a tibia-tarsus. The fibula
is much reduced. The distal tarsals are fused with the second, third, and fourth
metatarsals to form a tarso-metatarsus ; the first metatarsal is free. The fifth
digit of the typical foot is absent.
In all Tertiary and Recent birds teeth are absent. The resophagus is
frequently dilated into a crop and the stomach is usually divided into pro-
ventriculus and gizzard. The junction between the large and small intestines
is marked by a pair of creca. The lungs are spongy and non-distensible. The
bronchi give off branches which open on the surface of the lung into thin-
walled air-sacs, and these in their turn usually communicate with pneumatic
cavities in more or fewer of the bones. The voice is produced in a syrinx
situated at or near the junction of the trachea with the bronchi. The heart is
four-chambered, the right auriculo-ventricular valve is muscular, and the right
aortic arch alone is present in the adult. The renal portal system is reduced.
The red blood-corpuscles are oval and nucleated. The temperature of the
blood is high (p. 558). The optic lobes are displaced laterally owing to the
meeting of the large cerebral hemispheres and cerebellum. The lumbar
region of the spinal cord has a sinus rhomboidalis. The olfactory organ is
usually poorly developed. The eye is usually large, and has sclerotic plates
and a relatively substantial pecten. The ear has a large curved cochlea, and
a columella. The kidney is three-lobed, and is developed from the meta-
nephros, the mesonephros undergoing atrophy. There is no urinary bladder.
The ovary and oviduct of the right side are more or less completely atrophied.
Birds are all oviparous, and the large ovum, containing much food-yolk,
becomes invested with albumen, a double shell-membrane, and a calcareous
shell, in its passage down the oviduct. The embryo has an amnion, an allan-
tois, and a large yolk-sac. The newly-hatched young may be able to run or
swim and to obtain their own food, in which case they are said to be precocial
or nidifugous ; or may be more or less naked and dependent for a time upon
the parents for their food supply, when they are altricial or nidicolous.
There is no general agreement with regard to the classification of birds.
Owing to the singular uniformity of the class in essential matters of structure,
and its vast diversity in detail, avian classification is a matter of great difficulty.
The following scheme is perhaps as good as any :
 PHYLUM CHORDATA

CLASS AVES
Sub-class Archreomithes
Order Archreopterygiformes (Jurassic)
Sub-class Neornithes
Super-order Odontognathre
Orders Hesperornithiformes (Cretaceous)
Ichthyornithiformes (Cretaceous)
Super-order Palreognathre
Orders Struthioniformes (Pliocene-Recent)
Rheiformes (Pliocene-Recent)
Casuariiformes (Pleistocene-Recent)
Apterygiformes (Pleistocene-Recent)
Dinornithiformes (Pleistocene-Recent)
JEpyornithiformes (Oligocene-Recent)
Tinamiformes (Pliocene-Recent)
Super-order Impennre
Order Sphenisciformes (Oligocene-Recent)
Super-order Neognathre (Cretaceous-Recent) 1
Orders Gaviiformes, Podicipitiformes, Procellariiformes, Pelecani-
formes, Ciconiiformes, Anseriformes, Falconiformes,
Galliformes, Gruiformes, Charadriiformes, Diatrymiformes,
Columbiformes, Psittaciformes, Cuculiformes, Strigiformes,
Caprimulgiformes, Micropodlformes, Coliiformes, Trogoni-
formes, Coraciiformes, Piciformes, Passeriformes.

EXAMPLE OF THE CLASS.- THE COMMON PIGEON (COLUMBA

LIVIA, var. DOMESTICA)
The Common, or Domestic Pigeon is known by many varieties, which
differ from one another in size, proportions, coloration, and details in feather
arrangement. The pouter, fantail, and tumbler illustrate extreme forms
which have arisen by artificial selection. These quaint monsters have been
produced by breeders who selected, generation after generation, individuals
which most nearly attained to some arbitrary standard of perfection, bred from
them alone, and killed off the 'inferior' strains. In the same way the Carrier
Pigeon was bred for speed and stamina. It is probable that the wild ancestral
species from which all these domestic breeds have arisen is the Rock Pigeon
(Columba livia), which is widely distributed in the Palrearctic and Oriental
regions. The following description refers especially to the common Dovecot
1 For reasons mentioned on p. 555, relatively little is known of the fossil history of modern
birds. Brief data are given under various orders, pp. 612-620.
s66 ZOOLOGY
Pigeon, which, although often feral, restricts itself to cities and so retains its
association with its creator, Man.
External Characters.-The head (Fig. 383) is small, with a rounded cranium
and prominent beak covered with a rhamphotheca or horny sheath. The head,
the long mobile neck, and trunk are invested in a close covering of feathers,
all directed backwards and overlapping one another.

F I G. 383.- Sub-class Neornithes, Super.

order Neognathre, Order Passeriformes.
Columba /i d a. Domestic P igeon. from t h,•
left s1de with most o f t he feathers removed.
ad. dg. rmx . ad-d1gita l rcmcx; al. sp. ala
sp uria ; an. a nus; au. ap. a ud itory aperture:
cb. rmf?. c u bital remiges ; cr. cere; dg. r, 2 , 3.
digits of manus; dg. r', 2', 3', 4', digits of pes;
lm. pt. humeral pteryla: /g. ligament of
remiges; m d. dg. rmg. mid-digital remiges:
na. nostril : net. m. nictitating membra ne ; o.
gl. oil-gland: pr. dg. rmg. pre-digital remiges:
pr. ptgm. prcpatagium; pt. ptgm. post -
patagium; ret. mesial rectrix of right side ; ret' .
sacs of left rectrices ; sp. pt. spinal ptcryla ;
ts. mtts. tarso-met atarsus; v. apt. ven tral
a pterium.

Upon removal of the feathers it can be seen that the true tail is a short,
conical projection of the trunk, known as the uropygium, which gives origin
to the series of large feathers-rectrices-(rct.) to which the word 'tail' is
usually applied. On the dorsal surface of the uropygium is a papilla bearing
on its summit the opening of a preen, coccygeal, or oil-gland (o. gl.). This is
the only integumentary gland found in birds. The skin is dry and no sweat-
glands are present. Thus, birds cannot cool themselves by the principal
means available to the Mammalia. In hot weather they do so by panting as
do dogs, which also lack sweat-glands, except in specialised regions, i.e. the
nose and the soles of the feet.
Connected with the lungs of birds are air-spaces which are widespread
throughout the body (pp. s86, s8g), and these are no doubt involved in the
loss of body heat upon the operation of the panting reflex. Probably, too, a
 PHYLUM CHORDATA

certain amount of heat is otherwise lost by radiation, convection, and conduc-

tion, by removal via the fceces, and by the evaporation of moisture from the
epithelial surfaces of the tongue, and the buccal cavity. A vasomotor nervous
system helps regulate heat loss. If the external temperature rises, the peri-
pheral capillaries dilate, allowing the loss of heat. If it is depressed they
constrict, with the result that heat is conserved. An apparatus (p. 559)
probably broadly similar to the pilomotor mechanism of mammals also exists.
The skin contains nerve-endings and the papilla of each feather has its own
nerve-processes.
The wings show the three typical divisions of the fore-limb: upper arm,
fore-arm, and hand (p. 383); but the parts of the hand are closely bound
together by skin, and only three imperfectly-marked digits, the second (dg. 2)
much larger than the first (dg. I) and third (dg. J), can be distinguished. In
the position of rest the three divisions of the wing are bent upon one another
in the form of a Z ; during flight they are straightened out and extended so
that the axis of the entire wing is at right angles to that of the trunk. On the
anterior or pre-axial border of the limb a fold of skin stretches between the
upper arm and the fore-arm; this is the alar membrane or pre-patagium (pr.
ptgm.). A similar but much smaller fold extends, postaxially, between the
proximal portion of the upper arm and the trunk; this is the post-patagium
(pt. ptgm.).
In the hind-limb the short thigh is closely bound to the trunk, not standing
well out as in a reptile, but directed downwards and forwards; the long shank
extends from the knee downwards and backwards. The foot is clearly divisible
into a proximal portion, the tarso-metatarsus (ts. mtts.), and four digits, of
which one, the hallux (dg. I'), is directed backwards, the others, the 2nd, 3rd,
and 4th of the typical foot, forwards. The entire hind-limb is in a plane
parallel with the sagittal plane of the trunk.
The mouth is terminal, and is guarded by the elongated upper and lower
beaks ; it has, therefore, a very wide gape. On each side of the base of the
upper beak is a swollen area of soft skin, the cere (cr.), surrounding the nostril
(na.), which has thus a remarkably backward position. It has been suggested
that the cere may be an exteroceptor organ, but there is no experimental
evidence that this is so. The eyes are very large, and each is guarded by an
upper and a lower eyelid and a transparent nictitating membrane (net. m.). A
short distance behind the eye is the auditory aperture (au. ap.), concealed by
auricular feathers and leading into a short external auditory meatus, closed below
by the tympanic membrane. The anus or cloacal aperture (an.) is a large,
transversely-elongated aperture placed on the ventral surface at the junction
of the uropygium with the trunk.
Exoskeleton.-The exoskeleton (including the feathers) is purely epidermal,
like that of lizards, which also resemble birds in possessing protective horny
 ZOOLOGY
scales. These cover the tarso-metatarsus and the four digits of the foot. Each
digit of the foot is terminated by a claw, which is also a horny product of the
epidermis. The rhamphotheca is of the same nature.
A feather (Fig. 384) con-
sists of a hollow stalk, the
calamus or quill, and an ex-
panded distal portion, the
vexillum or vane. At the
proximal end of the quill is a
small aperture, the inferior
umbilicus, into which fits a
small conical prolongation of
the skin, the feather papilla.
Here each feather has its own
basal musculature which en-
ables its erection in flight, in
cold weather (avoiding heat
loss) and at times of emotion-
al excitement. A second, ex-
tremely minute aperture, the
superior umbilicus, occurs at
the junction of the quill with
the vane on the inner or ven-
tral face of the feather, i.e.
the face adjacent to the body.
A ···. A small tuft of down in the
8 neighbourhood of the superior
umbilicus represents the after-
shaft of many birds-includ-
ing some pigeons (see pp. 6o6,
627).
The vane has a longitudi-
FrG. 384.--Aves: Contour feather. Callus domesticus. nal axis or rachis continuous
The circular inset magnifies the interlocking mechanism
between minute barbules and adjacent barbs (see text) .
proximally with the quill,
A. distal barbule, B. proximal barbule, C. downy barbule but differing from the latter
from barb of basal, fluffy part of feather, lacking inter-
locking apparatus. (H.edrawn after Rawles.) in being solid. To each side
of this shaft is attached a
kind of membrane forming the expanded part of the feather and composed of
barbs-delicate, thread-like structures which extend obliquely outwards from
the rachis. In an uninjured feather the barbs are closely connected so as to
form a continuous sheet. A moderate amount of force separates them, for they
are bound together only by extremely delicate oblique filaments, the barbules,
 PHYLUM CHORDATA s6g

which have the same general relation to the barbs as the barbs themselves to
the rachis.
The precise mode of interlocking of the barbs can be determined only by
microscopic examination. Each barb (Fig. 384) is a very thin and long plate
springing by a narrow base from the rachis, and pointed distally. From
its upper edge (that furthest from the body) spring two sets of barbules, a
proximal set directed towards the base of the feather, and a distal set
towards its tip. Owing to their oblique disposition the distal barbules of a
given barb cross the proximal barbules of the next, each distal barbule being
in contact with several proximal barbules of the barb immediately distal to
it. The lower edge of the distal barbule is produced into minute hooklets or
barbicels. In the entire feather the hooklets of each distal barbule hook
over prominent flanges of the proximal barbules with which it is in contact.
In this way the parts of the feather are so bound together that the entire
structure offers great resistance to the air.
Among the contour feathers which form the main covering of the bird and
have the structure just described are filoplumes. These are delicate hair-like
feathers with a long axis and a few barbs and devoid of locking apparatus at
the distal end. The feathers of different parts of the body vary extremely in
their individual structure in relation to function. Nestling pigeons are covered
with a temporary investment of down-feathers in which also there is no inter-
locking of the barbs (Fig. 385).
Feathers, like scales, arise in the embryo from papill<e of the skin formed
of derm with an epidermal covering (Fig. 385). The papilla becomes sunk in a
sac, the feather-follicle, from which it subsequently protrudes as an elongated
feather-germ. Its vascular dermal interior is the feather-pulp. The Malpighian
layer of the distal part of the feather-germ proliferates in such a way as to form
a number of vertical radiating ridges. Its proximal part becomes uniformly
thickened. In this way is produced the rudiment of a down-feather which has
a number of barbs springing, at the same level, from the distal end of the quill.
The horny layer of the epidermis forms the temporary sheath which is thrown
off as the feather grows and expands. The pulp of the permanent feather
is formed from the lower or deep end of that of the down-feather. Its
development is at first similar, but, instead of the ridges of the Malpighian
layer remaining all of one size, two adjacent ones outgrow the rest and become
the rachis. As the latter elongates it carries up with it the remaining ridges
which become the barbs.
The feathers do not spring uniformly from the whole surface of the body,
but from certain defined areas (Fig. 386), the feather tracts or pterylce (sp. pt.,
hu. pt., etc.). These are separated from one another by featherless spaces or
apteria (v. apt.), from which only a few filoplumes grow. The feathers are,
however, long enough to cover the apteria by their overlap, and the body is
VOL. II. NN
570 ZOOLOGY

FIG. 385.-Aves: Down.feather and its lormation. Gallus domesticus. A. Beginning of

development, showing aggregation of mesodermal cells (primordium of dermal papilla) beneath
ectoderm. B. Feather-germ rising above skin level due to increase in mesoderm and overlying
ectoderm (epidermis). C. Later stage showing elongation of feather-germ. A thick cylindrical
wall of epidermal cells encloses the central mesodermal pulp. At the apex of the feather (shown
slightly at a tangent) epidermal cells are beginning to undergo rearrangement into longitudinal
barb-ridges (see below). D. Feather near time of hatching, in tubular follicle lined with epidermis.
The dermal papilla is now permanently located at the base of the follicle. The epidermal walls of
the feather cylinder are divided into a series of longitudinal barb-ridges (primordia of barbs and
barbules) surrounding the central pulp and protected by an external sheath. D. Transverse
section, showing I I barb-ridges, central pulp (with adjacent melanocytes) and external sheath. At
bottom left is a mature down-feather of newly hatched chick. The sheath is split and the barbs
and barbules are free. Note the absence of a shaft (cf. Fig. 384) and the circular arrangement of
barbs around a short calamus. (Redrawn after Rawles.)
 PHYLUM CHORDATA 57I
thus completely covered with a thick, very light, and non-conducting invest-
ment.
In the wings and tail certain special arrangements of the feathers occur.
When the wing is stretched out at right angles to the trunk, twenty-three
large feathers (Fig. 383) are seen to spring from its hinder or postaxial border:
these are the remiges or wing-quills. Twelve of them are connected with the
ulna and are called cubitals or secondaries (cb. rmg.). The rest are known as
primaries. Seven of these are attached to the metacarpal region, and are hence
called metacarpals (mtcp. rmg.). The remaining four or digitals are attached
to the phalanges of the second and third digits. These are again distinguished
into a single ad-digital (ad. dg. rmx.), connected with the single phalanx of the
third digit (Fig. 395, ph. J, p. 577), two mid-digitals (md. dg. rmg.) with the

FIG. 386.-Colmnba: Pterylosis.

Left, ventral; Right, dorsal. al. pt. alar
pteryla or wing-tract; c. pt. cephalic
pteryla or· head-tract; cd. pt. caudal
pteryla or tail-tract; cr. pt. crural
pteryla; cv. apt. cervical apterium
or neck-space; fm. pt. femoral
pteryla; hu. pt. humeral pteryla; lat.
apt. lateral apterium; sp. pt. spinal
pteryla; v. apt. ventral apterium; v.
pt. ventral pteryla. (After Nitzsch.)

proximal phalanx of the second digit, and two pre-digitals (pr. dg. rmg.)
with its distal phalanx (Fig. 395, ph. z'). A special tuft of feathers on the
anterior border of the wing, arising from the pollex (Fig. 396, ph. I), forms the
ala spuria (al. sp.) or bastard wing. The spaces which would otherwise be
left between the bases of the remiges are filled in, both above and below, by
several rows of upper and lower wing-coverts. In the tail there are twelve long
rectrices (Fig. 383, ret.) or tail-quills, springing in a semicircle from the uropy-
gium. Their bases are covered, as in the wing, by upper and lower tail-coverts.
The whole feather-arrangement is known as the pterylosis.
Endoskeleton.-Every part of the bird's skeleton presents characteristic
and indeed unique features. The vertebral column, the skull, the sternum,
the ribs, the limb-girdles, and the limbs themselves are all so highly specialised
that there is hardly a bone, except the phalanges of the toes and the free caudal
vertebrre, which could possibly be assigned to any other vertebrate class.
572 ZOOLOGY
A further peculiarity is that many of the bones are light and contain no
marrow. They are filled with air, and are pneumatic (p. 502). Large pneu-
matic bones are internally strutted (Fig. 387) in a manner known to engineers

FIG. 387.-Neornithes: Internal strutting and pneumatism in bone. Lightness is achieved without
undue sacrifice of strength. The present example is from the metacarpal of a vulture. A com-
parable arrangement arose by convergence in the pterosaurs (p. 502). (Redrawn after Prochnow.)

as a Warren truss. This device is sometimes used in the wings of aeroplanes.

The cavities of pneumatic bones open externally in the dried skeleton by
apertures called pneumatic foramina (Fig. 395, pn. for.), by which, in life, they
communicate with the air-sacs and ultimately the lungs (see p. 586). In the
pigeon the bones of the fore-arm and hand, and of the leg, are non-pneumatic.

th..v.s

cd..v

__._,. . _,1 ~ t.u:r/Jwst

FIG. 388.- Columba: Endosketon. acr. cor. acrocoracoid; a. tr. anti-trochanter; acib. aceta-
bulum; car. carina sterni; crt. v. caudal vertebra'; cor. coracoid; cv. r. cervical ribs; f. irs. probe
passed into foramen triosseum; f ur. furcula; gl. cv. glenoid cavity; il. ilium; is. ischium; is. for.
ischiadic foramen; obi. n. obturator notch; pu. pubis; pyg. st. pygostyle ; scp. scapula; s. scr.
syn sacrum; st. sternum; st. r. sternal ribs; tit. v. I, first, and tit. v. 5. last thoracic vertebra;
unc. uncinus; r•r. r. vertebral ribs.

The vertebral column is distinguished from that of most other Craniata by

the great length and extreme mobility of the neck, the rigidity of the trunk-
region, and the shortness of the tail. As in Reptilia, the cervical passes
almost imperceptibly into the thoracic region. Tne first vertebra with its ribs
united with the sternum is conventionally considered as the first thoracic
 PHYLUM CHORD AT A 573
(Fig. 388, th. v. I) . There are fourteen cervical vertebrce. The last or last
two of these have double-headed ribs (cv. r.), each having its proximal end
divisible into the head proper articulating with the centrum of the vertebra,
and a tubercle articulating with the transverse process. Their distal ends
are free and do not unite with the sternum. In the third to the twelfth there
are vestigial ribs (Fig. 38gB, rb.), each having its head fused with the centrum,
and its tubercle with the transverse process. The whole rib thus has the
appearance of a short, backwardly-directed transverse process perforated at
its base. The perforation transmits the vertebral artery, and is called the
vertebrarterial foramen (vrb . f.).
The centra of the cervical vertebrce differ from those
of all other Craniata in having saddle-shaped surfaces.
A
The anterior face (Fig. 389, A en) is concave from side to.
urhf
side and convex from above downwards. The posterior
face (B) is convex from side to side and concave from
above downwards. Thus the centrum in sagittal section
appears opisthoccelous, in horizontal section proccelous.
8
This peculiar and complicated form of vertebra is dis-
tinguished as heteroca:lous. The centra articulate with
one another by synovial capsules. Each of these is en
traversed by a vertical plate of cartilage, the meniscus, FIG. 389.-Columba:

with a central perforation through which a suspensory Cervical vertebra. A, an-

terior, B, posterior face.
ligament passes from one centrum to the other. a. zyg. anterior zygapo-
physis; en. centrum ; n.
The first two vertebrce, the atlas and axis, resemble a. neural arch; p. zyg.
those of the lizard, but have their various elements com- rb. posterior zygapophysis;
rib; vrb . f. verte-
pletely fused. The ring-like atlas is small. brarterial foramen.
Between the last cervical vertebrce and the pelvic
region come either four or five thoracic vertebrce (Fig. 388) fused into a rigid
girder. When only four are present the first three are united. \Vhen there
are five, the second, third, and fourth are joined, the last remaining free. The
anterior thoracic as well as the posterior cervical vertebrce have the centrum
produced below into a compressed plate, the hypapophysis, for the origin of the
flexor muscles of the neck. They all bear ribs, each consisting of a vertebral
(vr. r.) and a sternal (st. r.) portion, and articulating with the vertebra by a
double head. The sternal, like the vertebral rib, is formed of true bone, not of
calcified cartilage as in reptiles. It articulates with the vertebral rib by a
synovial joint. Springing from the posterior edge of the vertebral rib is an
uncinus (unc.), resembling that of Sphenodon and the Crocodilia, but formed
of bone and ankylosed with the rib.
Following upon the fourth or fifth thoracic are about twelve vertebrce, all
fused into another single mass (Fig. 388, s. scr.). This second rigid girder
gives attachment laterally to the immense pelvic girdle. The whole of this
574 ZOOLOGY
group of vertebra! has, therefore, the function of a sacrum, differing from
that of a reptile in the large number of vertebrre composing it. The first
vertebra bears a pair of free ribs, and is, therefore, the fifth or sixth (last)
thoracic (th. v. 5). The next five or six have no free
ribs, and may be looked upon as lumbar vertebrre (Fig.
390, l1.-s3 .). Their transverse processes arise high up
on the neural arch. The ligament uniting them is
ossified, so that the lumbar region presents dorsally a
continuous plate of bone. Next come two sacral
vertebrre (c1 .) homologous with those of lizards. Be-
sides the transverse processes springing from the neural
arch, one or both of them bears a second or ventral
outgrowth (c. r.) which springs from each side of the
centrum and abuts against the ilium just internal to
the acetabulum. These distinctive processes are
FrG. 390.-Columba: ossified independently and represent sacral ribs. The
Synsacrum. Nestling
(fourteen days old) in remaining five vertebrre of the pelvic region are caudal.
ventral aspect. cl. cen-
trum of first sacral verte- Thus the mass of vertebrre supporting the pelvic girdle
bra; c'. centrum of fifth in the pigeon is a compound sacrum, or synsacrum,
caudal; c. r. first sacral
rib ; 11. centrum of f1rst formed by the fusion of the posterior thoracic, all the
lumbar; t•. third lumbar;
s . of fourth lumbar; s •
1 3
lumbar and sacral, and the anterior caudal vertebrre.
of sixth lumbar; tr. p. The synsacrum is followed by six free caudals.
transverse process of first
lumbar; lr. p'. of fifth The vertebral column ends posteriorly in an upturned,
lumbar; tr. p". of first compressed bone, the pygostyle or ploughshare-bone
sacral. (After T. J.
Parker.) (Fig. 388, pyg. st.). This is formed by the fusion of
four or more of the hindmost caudal vertebrre.
Thus the composition of the vertebral column of the pigeon may be expressed
in a vertebral formula as follows :
Synsacrum .
Cerv. 14. Thor. 4 or 5 + I. Lumb. 5 or 6. Sacr. 2. Caud. 5 + 6 + Pyg. 4 = 43·

The skull (Fig. 391) is distinguished at once by its rounded brain-case,

immense orbits, and long, pointed beak. The foramen magnum (f. m.) is
directed downwards as well as backwards, so as to be visible in a ventral
view. On its anterior margin is a single, small, rounded occipital condyle
(o. c.). Most of the bones, both of the cranial and facial regions, are firmly
ankylosed in the adult, and can be distinguished only in the young birds.
The occipitals, parietals, frontals, and alisphenoids have the usual relations
to the brain-case, the basi-occipital (b. o.), as in the lizard, bearing the occipital
condyle. The basisphenoid (Fig. 392, B. SPH.) is a large bone forming the
greater part of the basis cranii and continued forwards, as in the lizards, by a
slender rostrum (Fig. 391, pa.s., Fig. 392, RST.), which represents the anterior
 PHYLUM CHORDATA 575
portion of the parasphenoid. On the ventral aspect of the basisphenoid
paired membrane bones, the basi-temporals (Fig. 392, B. TMP), are developed.
These become firmly ankylosed to it in the adult and probably represent the
posterior portion of the parasphenoid. The tympanic cavity is bounded by
the squamosal (Fig. 391, sq.), which
is firmly united to the other cranial A
bones. The main part of the audi-
tory capsule is ossified by a large
pro-otic (Fig. 392, PR. OT.). The
small opisthotic of the embryo early
unites with the exoccipital, the
epiotic with the supra-occipital.
The parasphenoid and mesethmoid
together form the interorbital septum
(Fig. 391, i. o. s.). This is a vertical
partition, partly bony, partly carti-
laginous, which separates the orbits
from one another. The immense
size of the eyes has produced a com- c
pression of this region of the skull. p. m.r
The ecto-ethmoids or turbinals are ~~!!!@!!!~
comparatively poorly developed, in
correspondence with the small size
of the olfactory organs. There are
large lachrymals (Fig. 391, lc., Fig. FIG. 39I.- Col11mba: Skull. Young bird,
392 LCR.), and the nasals (na, na', from A , dorsal; B. ventral ; C, left view. a/. s.
alisphenoid; an. angular; ar. articular; b. o.
na", N A) are forked bones, each basi-occipital; d. dentary; e.o. exoccipital; eu.
of Eustachian tube; f. m. foramen
furnishing both an inner and an aperture magnum; fr. frontal; i. o. s. interorbital
outer boundary to the corresponding septum; ju. jugal; lc.lachrymal; lb. s. lambdoidal
suture; m .eth. mescthmoid; mx. maxilla; mx. p.
nostril. The premaxillce (p.mx., maxillo-palatine process; na. na'. na". nasal:
P11,1X .) are united into a large o. c. occipital condyle ; or. Jr. orbital plate of
frontal; pa. parietal; pa.s. para sphenoid (ros-
tri-radiate bone which forms practic- trum) ; pl. palatine; p.mx. premaxilla; pt.
pterygoid; qu. quadrate; s. an. supra-angular;
ally the whole of the upper beak. s. o. supraoccipital; sq. squamosal; ty . tym-
A cranio-facial hinge occurs (see panic cavity; II- XII, foramina for cranial
nerves. (After Parker.)
p. 632). The maxillce (mx., MX.),
on the other hand, are small, and have their anterior ends produced inwards
into spongy maxillo-palatine processes (Fig. 391, mx. p. , Fig. 392, mx. pal. pr).
The slender posterior end of the maxilla is continued backwards by an equally
slender jugal (ju., JU.) and quadratojugal (QU. ]U.) to the quadrate. The
latter (QU.) is a stout , three-rayed bone articulating by two facets on its otic
process with the roof of the tympanic cavity. It sends off an orbital
process from its anterior margin, and presents below a condyle for
 ZOOLOGY

FIG. 392.-Neornithes: Skull. Diagrammatic, in sagittal section. R eplacing Bones : AL.SPH

alisphenoid; ART. articular; B. OC. basi-occipital; B. SPH. basi-sphenoid; EP.OT. epiotic
EX.OC. exocciptal; M.ETH. mesethmoid; OP.OT. opisthotic; ORB. SPH. orbito-sphenoid
PR. OT. pro-otic; QU. quadrate; S. OC. supra-occipital. Investing bones: ANG. angular
B. TMP. basi-temporal; COR. coronary; DNT. dentary; FR. frontal; ]U. jugal; LCR.
lachrymal; MX. maxilla; NA. nasal; PA. parietal; PAL. palatine; PMX. premaxilla; PTG.
pterygoid; QU.]U. quadratojugal; RST. rostrum; 5 . ANG. supra-angular; SPL. splenial;
SQ. squamosal; VO. vomer; fie. fos. floccular fossa; mx. pal. pr. maxillo-palatine process,
opt. for. optic foramen; orb. pr. orbital process; at. pr. otic process; pty. fos. pituitary fossa.

articulation with the mandible. It is freely movable upon its tympanic

articulation, so that the lower jaw has a double joint as in lizards and snakes.
The palatines (pl., PAL.) have their slender anterior ends ankylosed with
the maxilla, their scroll-like posterior ends articu-
lating with the pterygoids and the rostrum. The
pterygoids (pt., PTG.) are rod-shaped and set
obliquely. Each articulates behind with the quad-
rate, and, at about the middle of its length, with
the basipterygoid process, a small faceted projection
of the base of the rostrum. There is no vomer
in the pigeon.
The mandible of the young bird consists of a
replacing bone, the articular (ar., ART.), and four
investing bones. These are the angular (an.,
ANG.), supra-angular (s.an., S. ANG.), dentary
(d., DNT.), and splenial (SPL.). All have the
same general relations as in lizards. The hyoid
apparatus (Fig. 393) is of characteristic form,
FIG. 393.- Columba: Hyoid
apparatus. The cartilaginous having an arrow-shaped body (b. hy.) with a short
parts are dotted. b. br. I, b.
br. 2 , basi-branchials; b. hy. pair of anterior cornua (c. hy.) derived from the
basi-hyal; c. br. cerato-bran- hyoid arch, and a long pair of posterior cornua
chial; c. hy. hyoid cornu; ep.
br. epi-branchial. (c. br., ep. br.) from the first branchial, The
 PHYLUM CHORDATA 577
columella (Fig. 394) is a rod-shaped bone attached to the stapes, and bearing
at its outer end a three-rayed cartilage, the extra-columella (e. st., i. st., s. st.),
flxed to the tympanic membrane (p. 144).
The sternum (Fig. 388, st.) is one of the
most characteristic parts of the skeleton.
#
~
r- L.-.,-e.st.
As in all modern flying birds, it is a broad .._,,.
plate of bone produced ventrally, in the FiG. 394--Co/umba: Columella.
sagittal plane, into a deep keel or carina sterni Much magnified, with the cartilaginous
parts dotted. e. st. extra-stapedial;
(car.), formed, in the young bird, from a i. st. infra-stapedial; s. st. supra-
separate centre of ossification. The posterior stapedial; st. stapes. (After T. J.
Parker.)
border of the sternum presents two pairs of
notches covered by membrane. Its anterior
edge bears a pair of deep grooves for the
articulation of the coracoids. The broad
keeled sternum gives insertion, both on the
keel and on each side of it, to the massive
pectoral muscles of flight. The keel serves
also to strengthen the structure as a whole.
On this broad ventral girder much of the
weight of the bird is carried in flight.
The shoulder or pectoral girdle (Fig. 388) is
unlike that of any other craniate. There is
a pair of stout pillar-like struts, the coracoids
(cor.), articulating with deep facets on the
anterior border of the sternum, and directed
upwards, forwards, and outwards. The dor-
sal end of each is produced into an aero-
coracoid process (acr. cor.), and below this, to
the posterior aspect of the bone, is attached
by ligament a relatively flimsy sabre-shaped
scapula (scp.). This extends backwards over
the ribs, and makes an acute angle with the
coracoid-the coraco-scapular angle. The
glenoid cavity (gl. cv.) is formed in equal FIG. 395--Columba: Fore-limb.
(see also Fig. 417, p. 6o1 ). cp.
proportion by the two bones. Internal to it Left mtcp. carpo-metacarpus; lm. humerus ;
the scapula is produced into an acromion ph. r, phalanx of first digit; ph. z', ph.
z", phalanges of second digit; ph. 3,
process. The scapula holds the wing up and phalanx of third digit; pn. for.
out from the ribs. The coracoids, in con- radiale; pneumatic foramen; ra. radius; 1'a'.
ul. ulna; ul'. ulnare.
junction with the anterior, slender V-shaped
furcula (fur.) or 'wish-bone', braces the mobile wing out from the sternum.
The apex of the furcula nearly reaches the sternum. Each of its extremities
is attached by ligament to the acromion and aero-coracoid processes of the
 ZOOLOGY
corresponding side in such a way that a large aperture, the foramen triosseum
(f. trs.), is left between the three bones of the shoulder-girdle. The furcula is
an investing bone and represents fused clavicles and
interclavicle.
ra
Equally characteristic is the skeleton of the fore-
limb. The humerus (Fig. 395, hu.) is a large, strong
bone, with a greatly expanded head and a prominent
ridge for the insertion of the pectoral muscle. In it,
as in all the other long bones, the extremities as well
as the shaft are formed of true bone. The radius
ph r (ra.) is slender and nearly straight, the telna stouter
and gently curved. There are two large free carpals,
a radiale (ra'.) and an ulnare (ul'.). Articulating with
these is the carpo-metacarpus (cp. mtcp.) consisting of
two rods. The rod on the pre-axial side is strong and
FIG. 396.- Columba: nearly straight. That on the postaxial side is slender
Manus. Left, in nestling, and curved. These elements are fused with one
with the cartilaginous
parts dotted. cp. I, radi· another at both their proximal and distal ends ; the
ale; cp. 2 , ulnare ; mcp.
I , 2, 3. metacarpals; ph .
proximal end is produced, pre-axially, into an out-
I, phalanx of first d igit; standing step-like process. The carpo-metacarpus is
ph. 2 , ph. 2 ', phalanges of
second digit; ph. 3. formed by the union of the distal carpals with three
phalanx of third digit ; metacarpals (Fig. 396), the second and third of which
ra. radius; ttl. ulna.
(AfterT. ]. Parker.) are the two rod-like portions of the bone, the first
the step-like projection. Articulating with the first
metacarpal is a single pointed phalanx (Fig. 395, ph. I) . The second metacarpal
bears two phalanges, the proximal one (ph. 2') produced postaxially into a
flange, the distal one (ph. 2") pointed.
The third metacarpal bears a single a il.

pointed phalanx (ph. J).

The pelvic girdle (Fig. 388) re-
sembles that of no other vertebrate
with the exception of certain dinosaurs
(p. 506). In the walking or perching
bird the centre of gravity is postero- olJf ts
ventral and the concentrated body FIG. 397.-Columl)a: Pelvic girdle. Left.
weight is transferred by ribs and ab- in a nestling, with cartilaginous parts dotted.
A c. acetabulum ; a. tr. anti -t rochanter; i l . pre-
dominal muscles to the massive ilium acetabular, and i/' . postacetabular portion of
(il.) which is attached by fibrous union ilium; is. ischium ; i . s. f. ischiatic foram en;
ob . f. obturator notch; pzl. pubis. (After T. J.
to the whole of the synsacrum and is P arker.)
ankylosed with it in the adult. It is
divisible into pre-acetabular and post-acetabular portions of approximately equal
size. As usual it furnishes the dorsal portion of the acetabulum, and on the
 PHYLUM CHORDATA 579
posterior edge of that cavity is produced into a process, the anti-trochanter
(a. tr.). This works against the trochanter, a process of the femur. The ven-
tral portion of the acetabulum is furnished in about
equal proportions by the pubis and ischium (Fig. 397). hd
It is not completely closed by bone, but is perforated
by an aperture covered by membrane. Both pubis
and ischium are directed sharply backwards from
fe
their dorsal or acetabular ends. The ischinm (is.) is
a broad bone, ankylosed posteriorly with the ilium,
and separated from it in front by an ischiatic foramen
(Fig. 388, is. for.; Fig. 397, i. s.f.). The pubis (pu.) jJal:

is a slender, curved rod, parallel with the ventral cn.pT"

edge of the ischium, and separated from it by an fi
obturator notch (Fig. 388, obt. n. ; Fig. 397, ob. f.).
Neither ischium nor pubis unites ventrally: a pubic ti.ls -
symphysis exists only in the Ostrich.
In the hind-limb the femur (Fig. 398, f e.) is a
comparatively short bone. Its proximal extremity
bears a prominent trochanter (tr.) and a rounded head
(hd.). The axis of this is at right angles to the shaft
of the bone. Thus the femur, and indeed the whole
limb, lies in a plane parallel with the sagittal plane
of the trunk. Movements of the legs are essentially
antero-posterior in direction. The distal end of the
femur is produced into pulley-like condyles. There
is a small sesamoid bone (i.e. a bone developed in a
tendon), the patella (pat.) , on the extensor side of the
knee-joint.
Articulating with the femur is a long bone,
the tibia-tarsus (ti. ts.), produced on the anterior
face of its proximal end into a large cnemial process
(en. pr.) Its proximal articular surface is slightly
hollowed for the condyles of the femur. Its distal Hind-Limb. FIG. 398. - Columba:
Left. en. pr.
end is pulley-like, not concave like the corresponding cnemial process; Jr. femur;
fi. fibula; hd. head of femur;
extremity of the tibia of other Amniota. The pulley- mtts. I , first metatarsal ; pat.
patella; ph. I, phalanges of
like distal end of the bone (Fig. 399, tl. I) consists of first digit; pll. 4. phalanges
the proximal tarsals (astragalus and calcaneum) which of fourth digit; ti. ts. tibia-
tarsus; ts. mtts. tarso-meta-
at an early period unite with the tibia and give rise tarsus; tr. trochanter.
to the compound shank-bone of the adult. The
fibula is very small, much shorter than the tibia, and tapers to a point at its
distal end.
Following the tibio-tarsus is an elongated bone, the tarso-metatarsus (Fig.
s8o ZOOLOGY
398, ts. mtts.), presenting at its proximal end a concave surface for the tibia-
tarsus, and at its distal end three distinct pulleys for the articulation of the
three forwardly-directed toes. In the young bird the proximal end of this
bone is a separate cartilage (Fig. 399, tl. 2), represent-

tl!~
1- .
. tl
ing the distal tarsals, and followed by three distinct
metatarsals which belong respectively to the second,
third, and fourth digits. Thus the ankle-joint of the
~ tl? bird is a mesotarsal joint, occurring, as in lizards,
between the proximal and distal tarsals, and not, as
mllf'
in mammals (p. 665), between the tibia and the proxi-
mtl~
mal tarsals. To the inner or pre-axial side of the
tarso-metatarsus, near its distal end, is attached by
fibrous tissue a small irregular bone, the first metatar-
sal (Fig. 398, mtts. I). The digits have the same
Pes. Part of left foot in number of phalanges as in the lizard, the backwardly-
FIG. 399.-Columba:

embryo (highly magnified) directed hallux two, the second or inner toe three, the
with cartilaginous parts
dotted. mil. 2, second, nztl. third or middle toe four, and the fourth or outer toe
3, third, and mtl. 4. fourth five. In all four digits the distal or ungual phalanx is
metatarsal; li. tibia; tl. I,
proximal tarsal cartilage; pointed and curved, and serves for the support of the
tl. 2, distal tarsal cartilage.
(After Parker.) horny claw.
Muscular System.-The toes are flexed by two sets
of tendons. The deep tendons of the three forwardly-directed digits are formed
by the trifurcation of the tendon of a single muscle, the peroneus medius. That
of the hallux is derived from a separate muscle, the flexor perforans, which is
joined by a slip from the peroneus medius. Thus a pull upon one tendon flexes
all the toes. When the leg is bent, as the bird settles to roost, the flexion of the
tarso-metatarsus on the shank puts the flexor tendons on the stretch as they pass
over the mesotarsal joint, and by the pull thus exerted the toes are automatic-
ally bent round the perch by the simple action of flexing the leg. They are
kept in this position while the bird is asleep by the mere weight of the body.
It has been thought that the action is assisted by a small but characteristic
muscle, the ambiens, but this is now in dispute. The ambiens muscle arises
from the pubis and passes along the inner surface of the thigh. It continues
into a long tendon which comes round to the outer side of the knee, enclosed
in a special sheath, and continuing down the leg, joins the superficial flexors of
the digits. This muscle, however, is absent in some species and its section in
others does not affect perching ability.
As might be expected, the tetrapod musculature of the fore-limb is greatly
modified. The powerful downstroke of the wing by which the bird rises and
propels itself through the air is performed by the pectoralis major (Fig. 400, pet.),
an immense muscle having about one-fifth the total weight of the body. This
arises from the whole of the keel of the sternum (car. st.) and from the clavicle
 PHYLUM CHORDATA ssr
(cl.), filling nearly the whole of the wedge-shaped space between the body and
the keel. This is the so-called 'breast' of the bird, the meat of which is highly
vascularised and 'red' in the strongly flying pigeon, and white in the feebly
flying Common fowl (Gallus domesticus). Its fibres converge to their insertion
(pet.") into the ventral aspect of the humerus (hu., hu'.), which it depresses.
The elevation of the wing is performed, not, as might be expected, by a dorsally-
placed muscle, but by the supracoracoideus (sb . clv.) ('pectoralis minor '). This
arises from the anterior part of the body of the sternum, dorsal to the pectoralis.

It sends its tendon through the foramen triosseum between furcula, scapula,
and coracoid, to be inserted into the dorsal aspect of the humerus. Because
the foramen acts like a pulley, the direction of action of the muscle is changed ;
the backward pull of the tendon raises the humerus. Muscles concerned with
rotational movements of the wing in the glenoid socket are t he relatively small
scapulo-humerals and coraco-humerals , which extend from girdle to humerus.
The deltoid muscles are complex, each with three tensores patagii (tns. lg.,
tns. br., ins. ace.), the action of which is to keep the prepatagium t ensely
stretched when the wing is extended. A similar muscle (tns. m. p.) acts upon
the postpatagium. The most important intrinsic muscles of the arm are the
biceps, and the larger triceps, which operate the elbow and perform adjustments
during flight. Fore-arm muscles such as the extensor carpi radialis and the
extensor carpi ulnaris operate in conjunction with the two former, and are
important in the stretching and folding of the wing. The radius is rotated
medially by two brachioradialis muscles. Although the muscles of the digits
are much reduced, they are nevertheless important in that they alter the
582 ZOOLOGY
disposition of individual parts of the wing-and even of individual feathers-
during flight. The bastard wing (first digit) is independently movable by
policis muscles; it may help to control the leading edge of the wing, and to
produce extra forward momentum during the take off. Much is still unknown
about flapping, and even the simpler gliding, flight.
Locomotion.-Among vertebrates birds are uniquely adapted in their
capacity for locomotion both in the air and on land. Many are expert swim-
mers as well. Upon the skeletal foundation briefly outlined are borne the
feathers of flight and the individual muscle elements that allow simultaneous
alterations in shape, size, angle, and, consequently, air permeability of the
wings in response to the ever-changing requirements of different phases of
flight. Although bird flight (including even the simpler soaring flight that
depends primarily on rising air currents, or variations in air currents over
waves) is still incompletely understood, certain of its features are relatively
straightforward and closely analogous to the simpler flight of aircraft. The
leading edge of each flight-feather is the narrower, and both fore and rear
edges of most are relatively constant in width along its length. Depending
on the characteristic mode of flight of the species, the terminal parts of some
pinions, both forward and rear, are narrowed towards the central vein. Thus
some feathers are terminally and subterminally emarginated and provide, as
a whole, an effect not unlike that given by the slotting apparatus on the wings
of an aeroplane.
Lift is achieved not only by the pressure of air-flow below the temporarily
impermeable wing, but also by the semi-vacuum, producing a 'suction zone',
caused by the swift rush of air cascading over the slightly convex dorsal
surface of the wing. (This phenomenon can be crudely demonstrated by
holding, and blowing hard over, a piece of curved cardboard which will then
be slightly elevated.) The creation of the suction zone is assisted by the thin
trailing edges which minimise the eddies behind the wings (stream-lining) and
also by the convex upper surfaces (the camber). This all-important dorsal
zone varies with the changing shape and movement of the wing in flight. If
the angle to the direction of air-flow is increased, eddies (trailing edge vortices)
spread from the downwardly directed trailing edge of the wing towards the
suction zone, eliminating vacuum 'lift'. the animal begins to stall. The
emargination of varying numbers of flight-feathers reduces the terminal eddies,
and changes in the disposition of the small elements of the bastard wing (p.
566) perhaps further tend to reduce air turbulence, and tend also to allow a
slower stalling speed while soaring. Habitually soaring birds are characterised
by the pronounced emargination of their flight-feathers. There is, too, a broad
relationship between wing-shape and slotting. Wings of high aspect ratio, i.e.
those that are very long in comparison with breadth, are not as a rule markedly
slotted. Wings of low aspect ratio, i.e. short and broad, are often notably so.
 PHYLUM CHORDATA

Most, if not all birds employ both flapping and soaring modes of flight.
Flapping flight is infinitely the more complicated, involving constant changes
in the relative positions of both distal and proximal parts of the wing. The
intrinsic musculature and intimate structure (and even individual movements)
of different feathers, together with movements of the pre- and postpatagia
(Fig. 383), allow the wing to alter shape, so that the down-beat resembles a
thrusting oar, impermeable to air, whilst in the up-beat it can be swiftly
readjusted with a minimum of air resistance for the next powerful stroke. In
addition, the rate of wing-beat varies widely from family to family. Types
with a small wing-area in relation to mass flap quickly and have evolved special
landing techniques. Those with a greater wing-area are able to progress
quickly at a more leisurely rate of wing-beat. Generally speaking, fast-flying
birds have, like fighter 'planes, relatively small wings. The bigger a flying
bird becomes, the larger its wing must develop. Weight increases with the
cube, while the expanse of the wing is related to the square of the linear di-
mensions. On the other hand, as the surface of the wing increases, so the
edge losses decrease and a higher weight per unit area can be, and is, carried
(wing-loading). At the same time, it has been shown experimentally that
pigeons can fly when deprived of up to 45 per cent. of their wing-surface.
Again, certain eagles (e.g. Uroiietus) are for short distances able to carry prey
much heavier than themselves. Many, but not all, species fly well :when the
wings are in moult. Some birds, however (e.g. migratory shearwaters,
Puffinus), delay the wing-moult, but not that of the body, until the completion
of their long post-nuptial transequatorial migrations.
Alimentary Canal and Associated Structures.-The mouth (Fig. 401) is
bounded above and below by the horny beak. The tongue (tng.) is large and
pointed at the tip. It possesses what appear to be taste-buds and, in addition,
mucous glands. Salivary glands-the median sub-lingual and paired angle-
glands at the gape-produce saliva which moistens the food.
In some graminivorous species the saliva contains a diastatic enzyme.
The pharynx leads into a wide and distensible resophagus (gul.), which dilates
into a large reservoir or crop (crp.) situated at the base of the neck, between
the skin and the muscles, and immediately in front of the sternum. The crop
enables the bird to ingest and store very quickly a large amount of food which
it can later digest at leisure. The epithelium of the crop in both sexes sheds
a white, slimy, proteinous, and fatty crop-' milk' during the breeding season
(seep. s6o). Crop-gland activity is controlled by prolactin, an anterior pituitary
hormone.
The 'pigeon's milk' is regurgitated to the young squabs which can double
their hatching weight in only two days. A neonatal rabbit doubles its birth
weight in six days and a cat in nine. 'Pigeon's milk', which is composed of
from 65-81 per cent. water, 13·3-18·8 per cent. protein, 6·9-12·7 per cent.
 ZOOLOGY
fat, and I·5 per cent. ash, is 'a very much more efficient nutriment than
mammalian milk' (Needham); see, however, p. 790.
In the crop, the ingested food, consisting principally of grain, is macerated.
From the crop the cesophagus is continued backwards into the stomach,
which consists of two parts, the glandular proventriculus (prvn.) and the

FrG. 40r.- Columba: Vis-

ceral relation ships. Nearly
the whole of t h e ileum is re-
moved. a nd the duodenum is
displaced out wards. a. ao.
a ortic a rch; bd. I, bd. z, b ile-
ducts; b. jabr. b ursa F a bricii;
cbl. cerebellum ; cre. right
cl!'cu m; cpdw. coprod::eum ; cr. cere ; C7b. h. left cerebral
hemi,;ph ere; crp. crop; cr. v. r, first cervical vertebra;
di.ca:. diacrele; d·ut. den tary ; duo. duodenum ; eus. ap.
apertu re o f Eust achian t ubes; giz. gizzard (dot ted behinu
the liver) ; gl. glot tis ; gul. gullet; ilm . ileu m ; i . orb. sp .
interorb ita l septum; kd. right kidney; lng. right lung; lr. liver (right lob e) ; n a. bristle passed
from nostr il into mou t h ; obi. sep. obliqu e septum ; o. gl. oil-gland ; pcd . pericardium ; pmx. pre -
m axilla; pn . pancreas ; p n. b. p ineal b ody ; p u . d . 1 -3, pancrea tic ducts ; pr. cv. right p recaval;
prdm . proctodreum ; prvn. provent ricu lus (dotted b ehind liv er); pt. cv. postcaval ; p ty . b. p it uit ary
body ; pyg .st. pygostyle; r. a u. right auricle ; r. br. right b ronchus ; ret. rectum; r. unt. righ t
ventricle; sp. cd. spinal cord ; spl. spleen (dotted b ehind liver); s. rhb. sinus rhomboidalis; s.
scr. synsacrum; st. carina st erni ; syr. syrinx ; th . v. I , fi~st, a nd th. v . 5, fift h thoracic vertebra;
tng. t ongue; tr. t rach ea; is. righ t t estis; ur. a p erture of left ureter ; urdm . urod;eum ; v. df.
a perture o f left vas rle ferens.

muscular gizzard (giz.). The proventriculus appears externally like a slight

dilat ation of the cesophagus but it is a gastric structure, the walls of which
contain numerous glandular pockets large enough t o be visible to the naked
eye. The gizzard has the shape of a biconvex lens. Its walls are very thick
and its lumen is small. The t hickening is due mainly t o the immense develop-
ment of the muscles which radiate from t wo t endons, one on each of the convex
surfaces. In the epithelial lining of the gizzard are numerous minute tubular
 PHYLUM CHORDATA

glands, which secrete a fluid which becomes thick, horny, and of a yellow or
green colour and lines the gizzard. The cavity always contains small stones,
which are swallowed by the bird and aid the gizzard in grinding up the food.
The duodenum (duo.) leaves the gizzard close to the entrance of the pro-
ventriculus and forms a distinct loop enclosing the pancreas. The duodenum
is lined with villi and single or branched crypts of Lieberkiihn. Goblet cells are
also present. The pancreas (pn.) is a large reddish gland which discharges
its exocrine products by three ducts (pn.d. I-J) into the duodenum. The
pancreas contains also endocrine islets of Langerhans which discharge into the
blood-stream insulin or a hormone of similar function (p. 153). The hormone
secretin, which in mammals influences the secretion of pancreatic juice, has
been obtained from the walls of the pigeon's duodenum. Also opening into
the duodenum are two bile ducts (b.d. I, b.d. 2). These come direct from the
right and left lobes of the large liver nearby. There is no gall-bladder in the
pigeon, but one occurs in Gallus and many other species. The ileum (ilm.)
presents first a single loop and then follows its greater part coiled into a sort
of spiral and lastly comes a single loop which passes without change of diameter
into the rectum (ret.). The junction between the two is marked only by a
pair of small blind pouches or cceca (ca'.). These are much more capacious in
many birds (e.g. Gallus) and are the site of a certain amount of digestion (of
vegetable fibres) by enzymatic and bacterial action. There is some evidence
too that they are concerned with the absorption of water. The cloaca is a large
chamber divided into three compartments, the coprodceum (cpdm.), which
receives the rectum, the urodceum (urdm.), into which the urinary and genital
ducts open, and the proctodceum (prdm.), which opens externally by the vent.
A thick-walled glandular pouch, the bursa Fabricii (b. Jabr.), lies against the
dorsal wall of the cloaca in young birds and opens into the proctodreum. It
atrophies in the adult (see Fig. 413, p. 597).
Respiratory and Vocal Organs.-The glottis (Fig. 401, gl.) is situated just
behind the root of the tongue, and leads into the larynx. This is supported by
cartilages-a cricoid divided into four pieces, and paired arytenoids-but it is
uncomplicated and does not, as in other vertebrates, function as a vocal organ.
The anterior part of the trachea (tr.) has the usual position, ventral to the gullet.
Further back, however, it is displaced to the left by the crop, becoming ventral
once more as it enters the body-cavity, where it divides into the right (r. br.)
and left bronchi. The rings supporting the trachea are not cartilaginous but
bony, as also is the first ring of each bronchus. Those of the trachea completely
surround the tube; those of the bronchi are incomplete mesially.
At the junction of the trachea with the paired bronchi occurs the character-
istic vocal organ, the syrinx (syr.), found in no other class. The last three or
four rings of the trachea (Fig. 402, tr.), and the first or bony half-ring of each
bronchus (br.), are modified to form a slightly dilated chamber, the tympanum,
VOL. II. 00
ss6 ZOOLOGY
the mucous membrane of which forms a cushion-like thickening on each side.
At the junction of the bronchi a bar of cartilage, the pessulus, extends dorso-
ventrally and supports an inconspicuous fold of mucous membrane, the
membrana semilunaris. The membranous inner walls of the bronchi form the
internal tympanzjorm membranes. A pair of intrinsic syringeal muscles arise
from the sides of the trachea and are inserted into the syrinx, and a pair of
sterno-tracheal muscles arise from the sternum and are inserted into the trachea.
The voice is produced by the vibration of the semilunar membrane. Its pitch
is altered by changes produced by the action of the muscles. The syringeal
musculature (Fig. 444, p. 639) varies
z.,. widely from group to group and is of
diagnostic significance.
The paired primary bronchi do
not immediately branch within the
lungs in the manner characteristic
of other tetrapods. Instead, each
bronchus is continued through to the
posterior end of the lung (Figs. 403,
404) and then divides into paired
secondary or lateral bronchi which
branch again and send tubes which
, give rise to a system of tertiary or
FrG. 402.-Columba: Trachea, bronchi, parabronchi. This breaks up into an
and lungs. Ventral aspect. a. in. aperture of
anterior thoracic air-sac; br. principal bron- extensive ramification of fine branch-
chus; br'. br", br'", secondary bronchi; p.
aperture of abdominal air-sac; p. a. pulmonary ing and anastomising tubules or air
artery entering lung; p. in. aperture of pos- capillaries which come into intimate
terior thoracic air-sac ; p. v. pulmonary vein
leaving lung ; sb . b. aperture of interclavicular contact with the red cells coursing
air-sac; sp. b. aperture of cervical air-sac; sy. through the blood capillaries of the
syrinx; tr. trachea. (After T . J. Parker.)
lungs.
At the same time, each secondary bronchus continues and enters a bladder-
like air-sac, formed as a dilation of the mucous membrane of the bronchus.
One of these, the abdominal air-sac, lies among the coils of the intestine. The
other, the posterior thoracic air-sac, is closely applied to the side-wall of the
body. The bronchus also gives off, near its entrance into the lung, three
short branches. One of these becomes connected with an anterior thoracic
air-sac, situated just in front of the posterior thoracic. Another is connected
with an interclavicular air-sac, which is median, unpaired, and connected
with both lungs. The third enters a cervical air-sac placed at the root of the
neck. Each side of the interclavicular gives off a diverticulum, or axillary
air-sac, lying in the axilla. All these sacs are paired except the interclavicular,
which is formed by the fusion of right and left moieties. The sacs communi-
cate with the pneumatic cavities of the bones. Thus, some birds are able,
 PHYLUM CHORDATA

are very small in comparison

with the size of the bird. They
are only slightly distensible,
being solid, spongy organs,
not bags with sacculated walls
as in amphibians and many
reptiles. Their dorsal surfaces
fit closely into the spaces
between the ribs and have no
peritoneal covering. Their
ventral faces are covered by
a strong sheet of fibrous tissue, the pulmonary aponeurosis or pleura (Fig. 403,
B, put. ap.), a special development of the peritoneum. Into this membrane
588 ZOOLOGY
are inserted small fan-like costopulmonary muscles, which anse from the
junction of the vertebral and sternal ribs.
The ventral or free walls of the thoracic air-sacs of each side are covered
by a sheet of fibrous tissue, the oblique septum (obl. sept.). This is continued
forwards to the pericardium, and is united with its fellow of the opposite side
in the middle dorsal line. It divides the ccelom into two compartments-one
containing the lungs with the interclavicular and thoracic air-sacs. The other
(abd. cav.) contains the heart, liver, stomach, intestine, etc., with the abdominal

FIG. 404.-Colu?nba: Relationship between lungs and air-sacs. From vent ral (left) and dorsal
(right) aspects. On the left side is shown only the ventral surface of the lungs, together with
expiratory bronchi and air-sacs (dotted). On the right side the inspiratory bronchi and air-sacs
are shown in black. B. main bronchus, C. cervical bronchus; M. mesobronchus ; s. abd. abdom-
inal air-sac; s. cerv. cervical a ir-sac; s. clav. inter-clavicular air-sac; with diverticulum (ax.) in
axilla; s. thor. ant. and post. thoracic air-sacs. (From Young, after Brandes and lhle.)

air-sacs. Unsuccessful attempts have been made to homologise the oblique

septum with the mammalian diaphragm (p. 676). It has been sometimes
thought to be an important part of the respiratory mechanism, but its experi-
mental destruction does not appear to interfere with either inspiration or
expiration.
Inspiration is brought about in the resting bird mainly by means of move-
ments of the intercostal muscles, and expiration by the abdominal muscles
which cause an alternate enlargement and diminution of the capacity of the
ccelom, and thus pumps air in and out of the lungs. During flight the same
effect seems to be produced by the movement of the pectoral muscles. In
either case the inspired air rushes through the lungs into the air-sacs and
 PHYLUM CHORDATA ssg
thence by diffusion into the pneumatic cavities of the bones. Thus, while in
other animals a certain amount of unchanged or residual ' dead space' air is
always left in the lungs after each expiration, in birds this is confined to the
air-sacs and to the smaller branches of the bronchi (Fig. 404). Respiratory move-
ment forces a current of fresh tidal air through the lungs. As a result of this
the aeration of the blood is very complete, and thus allows an unusually high
degree of muscular efficiency and temperature (p. 558). There is evidence that
birds resemble the Mammalia in having the respiratory centre in the medulla
oblongata. A specific panting centre may occur in the dorsal part of the
diencephalon. There is some evidence that one or both of these centres are
sensitive to the carbon dioxide content of the blood-stream. Bilateral vago-
tomy and vagal stimulation have not led to consistent results.

FIG. 405.-Columba: Heart and large

vessels. Dorsal aspect. a. ao. arch of aorta;
br. a. brachial a rtery; br. v. brachial vein; c. c.
common carotid : j u. jugular ; l. au. left
auricle; l . p. a. left p ulmona ry a rtery ; l. vn.
left vent ricle; p. c. v. left precaval ; pte.
post-caval; p. v. pulmonary veins; r. au, r.
au' , right a uricle ; r . p. a . righ t pulmonary
a rtery; r . pre. right p recaval; r. vn. righ t
ventricle. (After T. J. Parker.)

Thyroid, Parathyroid, and Thymus Glands.- Paired thyroids occur on each

side of the trachea near its division into bronchi. They produce thy roxin, an
iodine-containing hormone important in general metabolic function and said
also to be especially concerned with the periodic moult. Thyroid activity is
under the control of the anterior pituitary. Paired endocrine parathyroids,
situat ed nearby, control calcium and phosphat e met abolism. Paired elongated
thymus glands are lodged on each side of the throat in the young bird. They
involute at maturity, and in widely unrelat ed species show a marked enlarge-
ment at the end of the breeding season (Hohn). There is, nevertheless, no
evidence of endocrine activity.
Blood-vascular System.-The heart (Fig. 405) is of great proportional size
and, like that of crocodiles, consists of four chambers-right and left auricles
and powerful right and left ventricles. These are lined with endocardium.
There is no sinus venosus, that chamber being, as it were, absorbed into the
right auricle (Fig. 405, r. au.). The right ventricle (Fig. 406) partly en-
circles the left , the former having a crescentic, t he latter a circular cavity in
590 ZOOLOGY

transverse sections. The left auriculo-ventricular valve has the usual mem-
branous structure, consisting of two flaps connected with the wall of the
ventricle by tendons, but the corresponding valve of the right side is a
large muscular fold, very characteristic of the class. The pericardia! sac con-

i.e

ec ·
FIG. 406.-Columba: Blood vascular
system. Ventral a spect. a. ao. arch of
c.c ---~+----l\1 aorta; a. nz. a. anterior mesenteric artery;
a. 1'. 1'. afferent renal veins; a. r. v'. vein
bringing blood from pelvis into renal portal
system; br. a. brachial artery; b1·. v.
brachial vein ; c. caudal artery and vein;
c. c. common carotid artery; c. m. v.
coccygeo-mesenteric vein, displaced to t he
right; ca:. a. cceliac artery; d. ao. dorsal
aorta; e. c. external carotid artery; epg.
epigastric vein; e. r. v. efferent renal vein;
f. o. femoral artery; f. v. femoral vein; h. v.
hepatic vein; i. c. internal carotid artery;
i . if. internal iliac artery and vein; i. m .
internal mammary artery and vein; in. a.
innominate artery; i. v. iliac vein ; ju .
jugular vein; ju'. a nastomosis of jugular
,·eins; l. au. left auricle; /. p. a. left
pulmonary ;:.rtery; l. pre. left pre-caval
vein; l. vn. left ventricle; pc. left pectoral
arteries and veins; pc. a. right pectoral
artery; pc. v. right pectoral vein; p. nz. a.
posterior mesenteric artery; pte. post-caval
v ein ; ra.i , ra.•, ra.s, renal arteries; r. a.t£
right auricle; r. p. renal portal vein, on t h e
left side of the figure, supposed to be
dissected so as to show its passage through
the right kidney; r . p. a. right pulmonary
artery; 1'. pre. right precaval vein; r. v.
renal vein; r. t·n. right ventricle; sc. a.
sciatic artery; sc. v. sciatic vein; scl. a.
subclavian artery; vr. vertebral artery and
vein. (After T. J. Parker.)

tains serous fluid. The right auricle receives the right and left precavals
(r. pre., l. pre.) and the postcaval (pte) ; the left, four large pulmonary veins
(p. v.). There is an elaborate coronary circulation (p. ns), the number of
arteries varying in different species of birds. The intrinsic nervous system of
the heart includes a sinu-auriwlar node or pace-maker (in the wall of the right
auricle), an auriculo-ventricular node (in the atrial septum), and a right atrio-
 PHYLUM CHORDATA 59 I

ventricular ring of Purkinie fibres (p. 882). As in mammals, the contraction

wave begins at the SA node, spreads through the atria to the AV node and
thence by branches to the rest of the heart. Sympathetic stimulation acceler-
ates heart action and parasympathetic (vagal) impulses slow it down.
The left ventricle (Fig. 406, l. vn.), as in crocodiles, gives origin to the
right aortic arch (a. ao.), but the right ventricle (r. vn) gives off only one trunk,
the pulmonary artery, which soon divides into two (r. p. a., l. p. a.). The left
aortic arch is absent in the adult, and it is the right alone which is continued
into the dorsal aorta. The result of this is that the systemic arteries receive
oxygenated blood from the left side of the heart. The only mingling of
oxygenated and non-oxygenated blood is in the capillaries. This is a tre-
mendously important physiological advance made by birds over reptiles.
The aortic arch curves over the right bronchus to reach the dorsal body-
wall, and then passes directly backwards as the dorsal aorta (d. ao.). Owing
to the immense size of the pectoral muscles, the arteries supplying them are of
corresponding dimensions. The right and left innominate arteries (in. a.),
from which the carotids (c. c.), subclavians (br. a.), and pectorals (pc. a.) arise,
are actually larger than the aorta itself beyond their origin. In correspondence
with the position of the legs, the femoral (f. a.) and sciatic (sc. a.) arteries
arise far forward. The caudal artery (c.) is small as would be expected.
The most characteristic feature in the disposition of the circulatory organs
is the very considerable reduction of the renal portal system. There are two
renal portal veins (r.p.) formed by the bifurcation of the caudal. Each,
however, instead of breaking up into capillaries in the kidney, sends off only
a few small branches (a. r. v.) which carry blood to that organ. The main
vein passes forwards, through the substance of the kidney, and joins the
femoral vein (f. v.) from the leg to form the iliac vein (i. v.). There occurs at
the junction of the renal and iliac veins a valve which has been claimed to
regulate blood-flow into the former vessel. The iliac unites with its fellow
of the opposite side and forms the postcaval (pte.). Thus the main part of
the blood from the caudal and pelvic regions is taken directly to the heart,
and not through the renal capillaries as in most fishes and all amphibians and
reptiles.
At the point of bifurcation of the caudal veins a large coccygeo-mesenteric
vein (c. m. v.) comes off, and, running parallel with the rectum (from which it
receives tributaries) joins the portal vein. The abdominal vein of amphibians
and reptiles appears to be represented, in part at least, by the epigastric vein
(epg.), which returns the blood, not from the ventral body-wall, but from the
great omentum. This fold of peritoneum, loaded with fat, lies ventral to the
intestine and gizzard. The epigastric discharges into the hepatic vein.
The spleen (Fig. 401, spl.) is an ovoid red body of unusually small propor-
tional size attached by peritoneum to the right side of the proventriculus.
592 ZOOLOGY
The red blood cells are oval and nucleated. Leucocytes (lymphocytes,
heterophils, polymorpho-nuclear-pseudo-eosinophilic granulocytes, basophils,
eosinophils, and monocytes) occur. The blood of pigeons contains no platelets
but it nevertheless clots quickly. Thrombocytes are present and may be
involved in the general mechanism.
Lymphatic System.-This is extensive. It includes the lacteals coming away
from the small intestine. The lymphatics unite eventually to form paired
thoracic ducts which enter the precaval veins, thus achieving continuity with
the general circulation (pp. 78, ns).

FIG. 407.- Coluntba:

Brain and cranial nerves.
A, from above; B , from
below; C, from the left side.
cb. cerebellum; c. h. cerebral
hemispheres; f. flocculus; inf.
infundibulum; m. o. medulla
oblongata; o. l. optic lobes;
olj. olfactory bulbs; o. t. optic
tracts ; pn. pineal body;
II-XII, cranial nerves;
sp. I, first spinal nerve.
(After T . J. Parker.)

Nervous System and Hypophysis.-The brain is remarkable for its short,

broad, rounded form. Next to those of the Mammalia, the avian brain is
relatively the largest. It retains several reptilian characteristics-the
medulla oblongata (m.o.), for example, has a well-marked ventral flexure, as in
lizards (p. 480). The cerebellum (cb.) is of great· size, as might be expected in a
flying animal in which control of equilibrium is of such importance. There are
large spina-cerebella~ tracts. The cerebellum has a large median portion and
two small lateral lobes or flocculi (f.). The surface of the middle lobe is marked
by transverse grooves, so that the superficial grey matter is greatly increased
in extent. The fourth ventricle (Fig. 408, v4) is completely hidden by the
cerebellum, into which it docs not extend. The cerebral hemispheres (c. h.) are
 PHYLUM CHORDATA 593
large, but consist mainly of the massive and complex corpora striata : cortex
(resembling that of the reptilian brain) is present in the pallial region of the
hemisphere, but it is unconvoluted and not extensive. The backward growth
of the hemispheres to meet the cerebellum has pressed the optic lobes of the
mid-brain outwards so as to take up a lateral instead of the usual dorsal
position. These are of large and rounded form. Each contains an extension
from the aqueduct (Fig. 408, o.v.),
a narrow passage which represents c.h
the original cavity of the mid- A
brain. A further result of the
growth of the hemispheres and
cerebellum (respectively back-
wards and forwards) is that no
part of the diencephalon (the.)
appears externally except on the
ventral surface. Elsewhere it is o.v
seen only when the hemispheres
are pressed aside. It contains a
narrow vertical cavity, the third
B
ventricle (v3 .), bounded laterally eh
by large thalami, and communi-
cating on each side by the fora-
mina of Monro (f.m.) (interven-
tricular foramina) with the lateral
ventricles or cavities of the hemi-
spheres. The optic nerves and
tracts are extremely prominent. FIG. 408.- -Colutnba: Brain. A, with the
cavities opened from above; B, in sagittal section.
The tracts are crossed, and termi- a. c. anterior commissure ; cb. cerebellum; c. h.
nate in the mid-brain (as in cerebral h emispheres; c. s. corpus striatum; f. m .
foramen of Monro; inf. infundibulum; m. o. medulla
anurans) and in the thalami. The oblongata; o. c. optic commissure; o. ch . optic
chiasma; o. l. optic lobes; olf. olfactory bulbs; o. v.
thalami receive sensory fibres of optoccele; p. peduncles of cerebellum; p. c. p osterior
other kinds and are directly con- commissure; pn. pineal body; the. diencephalon;
v3 . , diaccele ; v•., metaccele. (After T. J. Parker.)
nected with the fore-brain (in-
cluding the corpora striata (c . s.)) and spinal cord. The corpora striata (c. s.)
are of relatively immense size and form the greater part of the hemispheres.
The remarkable development and differentiation of these organs is one of the
most characteristic features of the avian brain. There is evidence that the
relatively enormous corpora striata are concerned in the control of the reflex
behaviourisms which largely govern the lives of birds.
From the hypothalamus there arises a hypophysial portal system that joins
it with the well-developed pars distalis of the anterior pituitary gland. This
compound endocrine structure lies protected in a bony pocket of the sphenoid,
594 ZOOLOGY
the sella turcica. The neural lobe of the posterior pituitary is also well
developed (Fig. g6, p. rso) .
The anterior lobe is said to be bipartite. It produces hormones that
influence at least the primary sex organs (gonadotrophins), the adrenal cortex
(adrenocorticotrophic or ACTH), the thyroid (thyrotrophic hormone) and the
crop-gland (prolactin) (p. 583). The administration of prolactin also causes
broodiness, and the collapse and metamorphosis of the testis that normally fol-
lows the completion of spermatogenesis in seasonal birds. Although the avian
pituitary gland has no recognisable intermediate lobe, there is evidence that
the anterior lobe of the fowl produces a principle that exerts an intermedin-like
effect : it expands the melanophores of
hypophysectomised frogs and lizards.
The anterior pituitary, in its close
relationship with the central nervous
system and its exteroceptor organs, is
ideally situated as a vital organ in the
complex structural and functional
chain by which environmental events
(external stimuli of various kinds) partly
l control the reproductive and other pro-
cesses. The sight of another pigeon in
a mirror, for example, will sometimes
reflexly lead to enhanced follicle growth
and ovulation. Ventral stimulation by
the normal clutch size appears to de-
FrG. 409.- Columba: Eye. Sagittal sec. press ovulation in many species. Thus,
tion. on. cornea; ch. choroid; cl. pr. ciliary if the female is prevented from complet-
processes ; ir. iris; l. lens; opt. nv. optic
n erve; pet. pecten; rt. retina; scl. sclerotic . ing the full clutch by the daily removal
(After Vogt and Jung .) of an egg, she may continue laying,
inhibiting broodiness for many weeks.
The olfactory bulbs (olf.) are extremely small, in correspondence with the
poorly developed olfactory organ (see, however, p. 641).
The spinal cord (Fig. 401, sp. cd.) presents large brachial and lumbar
enlargements from which the nerves of the fore- and hind-limbs respectively
are given off. In the lumbar enlargement there is a divergence of the dorsal
columns of the cord converting the central canal into a wide, diamond-shaped
cavity, the sinus rhomboidalis (s. rhb.), bounded above only by the membranes
of the cord. The autonomic nervous system is of essentially the same plan as
in other tetrapods (pp. rrg, 412).
Organs of Special Sense.-The olfactory organs are paired chambers at the
base of the beak, separated from one another by the mesethmoid and bounded
externally by the ecto-ethmoid. The latter is produced inwards into three
 PHYLUM CHORDATA 595
scroll-like processes, the turbinals, which greatly increase the surface of mucous
membrane. The anterior portion of the cavity, including the anterior turbinal,
is covered by laminated epithelium and serves as a vestibule. The posterior
portion, including the middle and posterior turbinals, is invested by the one-
layered epithelium of the Schneiderian membrane from which the fibres of the
olfactory nerve arise. Most birds have a very poor sense of smell (d. however,
p. 64!).
The eye (Fig. 409) is extremely large and has the form of a biconvex lens.
Bony sclerotic plates help maintain its shape. A prominent nictitating
membrane slides across the eyeball, as in reptiles.
In many species of birds this is the only lid that blinks,
although the other lids close in sleep. It has been
suggested that the nictitating membrane closes and
protects the cornea during flight. Certainly the sliding
of the nictitating membrane helps keep the cornea
clean, helped by the fluid (tears) from the lachrymal
glands which supply each eyeball.
Within the eye, the iris is controlled by striated
fibres. Accommodation is achieved by the action of
striated ciliary muscles (Crampton's and Brlicke's),
which alter the shape of the relatively soft lens. At FrG. 410.- Columba:
Audio-equilibration. Left,
the same time a special set of muscles alters the shape outer aspect of labyrinth.
of the cornea. Cones, as well as rods, are present, and FA, ampulla of posterior
canal; F B, posterior
the retina bears a well-marked fovea (see also p. 639). canal ; H A, ampulla of
Projecting into the cavity of the eye from the entrance hh orizontal
orizontal canal; HB,
cana l; lag.
of the optic nerve is a large pecten (pet.) in the form of lagena, a t end of cochlea;
m r . membrane of Reiss-
a pleated and strongly pigmented membrane. The n er ; pb. basilar part of
pecten, about the function of which there has been SA cochlea; S , sacculus;
, ampulla of anterior
much controversy, first appears in the Reptilia (pp. canal. (From \Vieders-
heim, after Hasse.)
481 and 640).
The ear (Fig. 410) is divided into outer, middle, and inner sections, and is
chiefly distinguished internally from that of reptiles by the greater development
of the auditory apparatus. The cochlea is proportionately much shorter
than in mammals and is straight, or almost so. It encloses a basilar membrane
composed of numerous fibres bearing hair-cells \vhich are believed to be sensitive
to vibrations of high frequency. At the apical end of the cochlea there occurs
another aggregation of hair-cells embedded in a mucoid cupola which, in
addition, contains calcareous particles. A lagena occurs, which perhaps
appreciates sounds of low pitch (seep. 642).
The tympanic membrane, cavity and the columella have the same essential
arrangement as in lizards. The fenestra ovalis is almost occluded by the foot-
plate of the columella (p. 146). This, and its extension, the extra-columella,
596 ZOOLOGY
conveys tympanic oscillations to the perilymph of the inner ear. Eustachian
tubes, of pressure-equalising and drainage function, run from the middle ear
and open by a special aperture (Fig. 401, eus. ap.) in
the roof of the pharynx.
There are prominent connexions between the
cerebellum and the vestibular apparatus. The
anterior canal (SB) is of great size, and the whole
membranous labyrinth is closely invested by a layer
of dense, ivory-like bone, the bony labyrinth. This
can be isolated by cutting away the surrounding
spongy bone.
Excretory and Reproductive Organs, and Adrenal
Glands. The kidneys (Figs. 401, kd., Figs. 4rr and
412, k.) have a very characteristic form. Each is a
flattened organ divided into three main lobes and
· ~eli'
fitted closely into the hollows of the pelvis. It is
formed from the metanephros ; the large mesonephros
cLJ

FrG. 411.- Columlm:

Urinogenital organs in male. or Wolffian body,
During breeding season. which forms the
adr. adrenal; cl 2 . urodreum;
cl proctod<Pum; k. kidney;
3• embryonic kidney,
ts. t estis, that of the right
side displaced; ur. ureter ; a trophi es . The
11r'. aperture of ureter; vd. glomeruli are sup-
vas deferens ; vd'. its cloacal
aperture; v. s. vesicula plied by branches
seminalis. (After T. ].
Parker.) of the renal artery.
A high degree of
k
water-conservation is achieved by the ad-
dition of a long loop of H enle to each ex-
cretory tubule (Fig. roo, p. r55). Through
this water is reabsorbed from the glomerular
filtrate. There is left for excretion a con-
centration largely of uric acid precipitate,
and sufficient fluid to carry it down the ureter
(ur.), which is said in some species to be under FrG. 412 . -Colu1nba: Urinogenital
sympathetic control and peristaltic in action. organs in 2 female. During breeding
season. c/ • urodreum; cl•. procto-
The ureters (ur.) are narrow tubes passing dreum; k . kidney; I. ad. left oviduct;
I. od'. its cloacai aperture; I. od" . its
directly backwards to open into the urodceum, ccelomic funnel; I. od"'. its ccelomic
the middle compartment of the cloaca. aperture; ov. ovary; r. od. right ovi-
duct; r. od' . its cloacal aperture ; ur.
There is a difference of opinion as to whether ureter; ur'. its cloacal aperture.
(as in the lizard) more fluid is reabsorbed (After T . J. Parker .)
from the cloaca. Finally, urine is dis-
charged with the fceces. The fceces of seabirds, deposited in great
quantities on certain coastal islands of Peru, South Africa and elsewhere, is
 PHYLUM CHORDATA 597
collected as guano (from: Peruvian huano or dung), since it JS a valuable
agricultural fertiliser.
The adrenal or suprarenal glands (Fig. 4II, adr.) are prominent yellow
bodies of irregular shape situated anterior to the kidneys and near the gonads.
Chromaffin tissue, secreting adrenalin or its homologue, is inter-digitated (as in
reptiles) with the lipoidal tissue that is homologous with the inter-renal gland
of fishes (p. 252) and the adrenal cortex in the Mammalia (p. 152). Bilateral
adrenalectomy generally results in death within twenty-four hours unless
replacement therapy (cortical extracts) is provided. Adrenal tumours have
caused virilism in fowls as also in mammals.
The testes (Figs. 401 and 4rr, ts.) are ovoid bodies, varying enormously in
size according to the season. They are attached by peritoneum to the ventral

FIG. 4 r 3.-Columba: Cloacal re-

gion. The rectum discharges into
the coprod<Eum; the urinary and
genital ducts into the urod<Eum.
Mixed f<Ecal and excretory materials
are discharged from the proctod;cum,
but only after fluid has been con-
served by reabsorption into the
general circulation through the epi-
thelium of all three cloacal chambers.
The lymphoid bursa Fabricii (b. jab.)
opens into the proctod<Eum and may
possess a local protective function in
the young. ap. vent; ep. epidermis;
m. sph. cloacal sphincter; nwc.
mucous glands; r. rectum; ur. &: vd.
papill<E of ureter and vas deferens
(or oviduct). (From Young. after
Stresemann.)

ap

surfaces of the anterior ends of the kidneys. Internally each testis is composed
of convoluted seminiferous (spermatogenetic) tubules in which are elaborated
spermatozoa. Between the tubules are aggregated Leydig (interstitial) cells
which produce the sex hormone testosterone. Tubules are under the control of
the gonadotrophic tubule-stimulating hormone (=follicle-stimulating hormone
in female) and the interstitial cells are influenced by the interstitial cell stimulat-
ing hormone ( = luteinising hormone) of the anterior pituitary. The testes are
permanently retained within the body cavity-there is no temperature-regulat-
ing scrotum, such as occurs in most mammals (p. 889). It has been suggested
that the abdominal air-sacs (p. 587), which extend into the testicular region,
may reduce testis temperature and be concerned indirectly with spermato-
genesis, but this is not proved (see also p. 644).
From its inner border each testis gives off a convoluted vas deferens (v. d.)
which carries spermatozoa posteriorly, parallel with the ureter, to open into
598 ZOOLOGY
the urodc:eum on the extremity of a small papilla (Fig. 413). The posterior
end of the sperm-duct is enlarged to form a vesicula seminalis (v. s.). There is
no copulatory organ in pigeons.
The female organs (Fig. 412) are remarkable for the more or less complete
atrophy of the right ovary and oviduct. The left ovary (ov.) is a large organ in
the adult bird. Its surface is studded with follicles each containing a single
ovum. The ovary is governed in its seasonal activity by follicle-stimulating
hormone ( = tubule-stimulating in male) and the ltdeinising hormone ( =
I.C.S.H. in male) of the anterior pituitary. It produces the female sex hor-
mone, cestrogen, which modifies the accessory sexual organs and behaviour.
Progesterone (p. 152) or a progesterone-like substance has been detected in the
blood of both hens and cocks, but not from castrated birds.
The left oviduct (l. od.) is long and convoluted. Its anterior end is enlarged
to form a wide membranous oviducal funnel (l. od".) into which the ripe ova
pass after extrusion from the follicles. The rest of the tube has thick muscular
walls, lined with glandular epithelium, and opens into the urodc:eum. An
obvious vestige of the right oviduct (r. od.) is found in connexion with the
right side of the cloaca. A more or less extensive vestige of the right ovary is
frequently present.
Insemination occurs when the proctodc:ea of male and female are everted
and brought together. The sperms are shed into the female tract and travel
up the oviduct, where fertilisation occurs. As the ova or' yolks' pass down the
oviduct they are invested with the secretions of its various glands Successive
layers of albumen or' white' are deposited first, next a parchment-like double
shell membrane, and lastly a white calcareous shell. Two eggs are laid in a rough
nest, and are incubated by the parents for fourteen days, the temperature
being in this way kept at about 38° to 40° C. (I00° to 103° F.). At the end of
incubation the young bird is sufficiently developed to break the shell and
begin free life. It is at first covered with fine down, and fed by both parents
with the so-called 'pigeon's milk' (p. 583).

GENERAL ORGAN/SA TION OF BIRDS

The range of structural variation in the entire class of birds hardly equals
that of a single order of reptiles (e.g. pp. 496, 498). Among existing birds, the
Emu and ravens, which stand nearly at opposite ends of the series, present
nothing like the anatomical differences to be found (for example) between a
common lizard and a chameleon. Hence, in dividing the class into orders, we
find none of those strikingly distinctive characters which separate the orders of
fishes, amphibians, and reptiles, but must be content with characters which in
other groups would be considered insignificant. Such characters include
details in the structure of the skull and sternum, in the arrangement of the
muscles of the syrinx, wing and leg, in the form of the foot and in the peculiarities
 PHYLUM CHORDATA 599
of the newly-hatched young. It is for this reason that in the classification (p.
6o6) few diagnostic details are given. To define each order adequately would
involve a degree of taxonomic detail beyond the scope of the present work.
The differences between the two avian sub-classes, the Archc:eornithes and the
Neornithes are, however, of a fundamental nature.

SUB-CLASS ARCHJEORNITHES
ORDE R ARCH.MOPTE RYGIFORME S
These Mesozoic (p. 3) birds had no pygostyle (ploughshare bone). They
possessed a long tail of many vertebrc:e, with the rectrices arranged in two lateral

F IG. 4r4.- Sub-class Archreomithes, Order Archreopterygiformes, F amily Archreopterygidre,

ArcllreO]Jtel·yx. Almost the first birds, these (le ft) were Jurassic contemporaries o f the early
pterosaurs ('pteroda ctyls ' ) such as Rltall!p!torhyncltus (right) (p. 504). The flying reptiles did
not surv ive into Cenozoic times (p. 3) a nd left no d escendants. Although the Arch <eopterygi-
formes (above), and a lso the H esperornithiformes and Ichthy ornithiformes (p. 603), seem t o have
d ied out in the Upper Cretaceous, a t least one related line flourished and led t o the m odern b irds.
(Hedra wn after B . M. cat a logue, H eilmann and others.)

rows. The carpals and metacarpals were free, and the hand had three clawed
digits. Enamelled-crowned teeth were present in both jaws. So far two
genera have been described, each with a single species. These are Archceo-
pteryx lithographica (Figs. 414, 415) and Archceornis siemensi, known only
6oo ZOOLOGY
from two specimens 1 found in the Upper Jurassic limestone of Solenhofen in
Bavaria. Each was about the size of a crow. They probably belong to the same
genus or even species, in which case the name Archceopteryx has priority. In
the fossils the impressions of many of the feathers are preserved. Had this not
been so, it would not be easy to
separate the fossils from those of
certain small bipedal dinosaurs.
A most striking feature in
their organisation is the tail,
which was composed of about
twenty free caudal vertebrre
gradually tapering to the distal
end, as in most reptiles. The
rectrices were arranged one on
each side of the caudal vertebrre.
The sacrum was made up of only
about six vertebrre, as in the
ornithopod dinosaurs (p. so8).
These centra had amphiplatyan
faces with no indication of the
heteroccelous condition found in
extant birds. In addition to
cervical and thoracic ribs (which
were apparently devmd of un-
cinates), there were thin abdomi-
nal 'ribs', like those of Sphenodon
and crocodiles. These gastralia,
or so-called' ventral ribs', are ele-
ments of dermal bone which lie in
the ventral wall of the abdomen.
There is no evidence that any of
the true ribs were articulated
FIG . 415.-Arcltreoptery:r: Berlin specimen. The
with the small sternum.
feathered, yet tailed, toothed, and fingered (Fig. 416) The skull (Fig. 416) was
Archceopteryx is an example of mosaic evolution like
Seymouria (p. 427) and the pictidosaurs (p. 517). c. proportionately large, with a
carpal; cl. furcula; co. coracoid; h. humerus; r.
radius; sc. scapula; u . ulna; I-IV, digits. rounded, bird-like brain-case and
strong jaws, in each of which
was a series of conical teeth lodged in sockets. The jaw-bones, however, were
heavier than those of modern birds, and the teeth, though characteristically
reptilian, were relatively small. The eyes were large and apparently avian.
1 A third, very incomplete, fossil has now (1959) been found in the Langenaltheimer Haardt
quarry, 6 metres lower than, and 97 years after, the London specimen.
 PHYLUM CHORD AT A 6or
The sternum, hidden in the limestone matrix, but revealed by the fluorescent
quality of fossil bone under ultra-violet light, was flat without a keel (de Beer).
The coracoids (co.) were small. The scapulre (sc.) were slender, curved bones,
and there was a U-shaped furcula (cl.) formed by the fusion of the clavicles.
The bones were apparently without air-spaces (Fig. 387, p. 572). The vertebral
articulation was simple and non-avian. The fore-limb skeleton was about the

FIG. 416.-ATchreopteTy:r:
Skull. The toothed rostrum,
sclerotic plates, and the large
orbital and antorbital aper-
tures are clearly shown. En-
largement of cranial bones
reduced the two temporal
apertures characteristic of
archosaurian reptiles. (From
Headley, after Dames.)

same size as the hind-limb. The shape of the humerus reveals that the biceps
muscles were not powerfully developed. The radius and ulna were well
developed. The metacarpals were free, as in reptiles, except for the third
metacarpal, which was fixed with the carpal ossification-an indication of things
to come. The number of phalanges followed the usual reptilian rule : two in
the first digit, three in the second, and four in the third. The ungual phalanx of
all three digits was claw-shaped, and doubtless supported a horny claw. Despite
its feathers, the fore-limb was still essentially a foreleg (Figs. 415, 417).

'7'" FIG. 417.-ATcllreopt-

,.,.u:r: Manus.
(Right). c.
carpal; d I, first digit; 2,
C second digit; 3, third digit;
m, m. metacarpals; r.
radius; u. ulna. (From
Headley, after Dames.)

The remiges were divisible, as usual, into primaries (or metacarpo-digitals)

and secondaries (or cubitals). The primaries were probably attached to the
second, or to the second and third of the digits just described. The feathers
were of the delicate structure shown in modern birds.
The pelvis and the hind-limbs combined both avian and reptilian characters.
The elongated and backwardly directed pubis was bird-like. The separate tibia
and fibula were of about the same length : there was no suggestion of fibula
reduction, as in modern birds. The foot consisted of a slender tarsometatarsus
and four digits. The hallux was small and opposable, as in many modern birds.
VOL. II. PP
6oz ZOOLOGY

In addition to the wing and tail-quills there are remains of contour feathers
at the base of the neck and wing-coverts. Moreover, the rectrices were con-
tinued forwards by a series of large feathers which extended for some distance
along the sides of the body, and a row of similar but smaller feathers was
attached along both anterior and posterior faces of the tibio-tarsus.
These birds could not fly efficiently. The small, flat sternum, relatively
slight wing-spread and elongated, feathered, lizard-like tail make it clear that
they merely volplaned from branches. The free, probably clawed, digits of
the fore-limbs suggest a scrambling progression among branches. The evolu-
tion of true flight by arboreal Archawpteryx-like animals is more plausible
than Nopcsa's surmise that avian flight was first developed in feathered terres-
trial, bipedal animals that volplaned off the ground while running at speed.
Recent studies of an endocranial cast of Archceopteryx confirm Edinger's
suggestion that the cerebral hemispheres were smooth, long, and narrow, and
that the cerebellum was small. The latter lay behind the mid-brain instead of
overlapping it from behind and pressing it down to a more ventral level, as in
modem birds. For the purpose of its limited powers of flight, Archceopteryx did
not require a high degree of cerebellar co-ordination (de Beer). Thus the brain
was in certain important respects reptilian rather than avian. It seems almost
certain, on the other hand, that the Archreomithes were homoiothermous.
There is every reason to believe that Archceopteryx was a forest bird, yet
all three specimens (and an isolated feather) were embedded in marine beach
deposits. It would seem that they were blown seawards (as are even expert
fliers to-day), and so fortuitously came to rest in the fine-grained mud in which
they were so wonderfully preserved.
SUB-CLASS NEORNITHES
This includes all other known birds (both extinct and living), which,
according to some authorities, fall into four super-orders-r. Cretaceous Odon-
tognathre (e.g. Hesperornis, Ichthyornis); 2. Palreognathre (e.g. Ostrich, Emu,
etc.); 3· Impennre (penguins), and4. Neognathre of more than a score of relatively
modem orders. In all these groups the greatly shortened tail usually ends in
a pygostyle, around which the rectrices, when present, are arranged in a semi··
circle. Except in a few extinct forms there are no teeth. The metacarpals
are fused with the distal carpals to form a carpo-metacarpus. Except in one
instance, not more than two digits of the hand bear claws ; in nearly all cases
claws are absent in the manus. The sternum is well-developed and usually
keeled or carinate (p. 577).
The Neornithes, as far as is known, first appeared in the Cretaceous.
Cretaceous representatives were specialised water-birds that are preserved in
chalky marine deposits in North America. These forms (e.g. lchthyornis, Hes-
perornis) therefore probably give a no more characteristic impression of the
 PHYLUM CHORDATA 603

late-Mesozoic avifauna than do divers (Cavia) or pelicans (Pelecanus) of that of

to-day. Very little direct evidence is available concerning Cretaceous land-
birds, although fragmentary fossils seem to make it clear that large flightless
'ratite' (p. 6o6) and other toothless birds already existed. Flying forest-birds
in particular, are very rarely entombed in silt. Further, the relatively light
bones of birds do not seem to preserve well, except in especially fortuitous

FIG. 418.-Sub-class Nec-rnithes, Super-order Odontognathre, Orders Hesperornithiformes and

lchthyomithiformes, Hesperornis and Ichthuornis. Birds attained their modern appearance in
the Cretaceous but the flightless, superficially loon-like Hesperornis (below) nevertheless retained
true teeth (Fig. 419). Ichthyornis (above) was a strongly-flying, keel-breasted, tern-like bird
(Fig. 420) the head of which has not yet been found. (Redrawn from various sources.)

circumstances (e.g. p. 6oz). Fossils become more plentiful in Tertiary deposits.

but these, too, are often fragmentary, and many of their relationships are
highly speculative. By the Eocene and Oligocene, however, we find examples
of most of the flying, swimming, and nmning types familiar today.
The Neornithes have been arranged as follows:

SUPER-ORDER ODONTOGNATHJE
This Cretaceous (p. 3) group represents probably the first of the several
successful avian attempts to exploit the prolific harvest of the sea. From the
 ZOOLOGY
time of the initial emergence of amniotes (p. 457), various reptilian, avian, and
mammalian stocks successively became partly, and occasionally wholly,
marine. Although in regard to numbers birds may have become more success-
ful seafarers than any other amniote class, none has ever become viviparous

FIG. 4rg.- Hespef"ornis: Skeleton.

H. re{!alis was a marine Cretaceous
bird that had become highly special-
ised to an aauatic life. The teeth.
adapted for fishing, were in grooves
and not in sockets like those of
Archmopteryx (Fig. 4 14). Teeth were
absent from the premaxilla : a
horny beak was probably already
present. Hesperomis seemingly
possessed a strong convergent re-
semblance to modem divers (loons) :
the posture shown above is therefore
much too erect and the feet should be
more laterally placed. The occurrence
of such widely divergent marine
types as Hesperornis and Ichthyornis
(Fig. 420) suggests t hat land-birds,
too, must have radiated considerably
in the la te Cretaceous. Both the
above-mentioned birds were pre-
served in Kansas chalk-deposits. T he
figure (right) is about I/7 na tural
size. See also Fig. 418. (After
Marsh.)

and entirely aquatic. The Odontognathre are especially interesting in their

retention of teeth-'so advantageous for catching fish' (Storer)-probably
after teeth had been lost in the main stem of avian evolution. It was for
many years believed that the Odontognathre possessed a brain more reptilian
than avian, but by the use of cranial casts Edinger has shown that the opposite
is true.
ORDER HESPERORNITHIFORMES

Hesperornis (Fig. 419) was a powerful marine diving and swimming bird
from the Cretaceous of North America which stood perhaps 3 feet high. Sharply
 PHYLUM CHORDATA 6os

pointed teeth were carried in grooves, not in distinct jaw sockets. The pre-
maxillre were without teeth. The neck was long, the body elongated; and
the sternum lacked a keel. The shoulder-girdle was much reduced, and the
bird was almost certainly flightless (Fig. 419). The hind-limbs were powerful,
and the toes probably webbed.
The order contains three families: Hesperornithidre (e.g. Hesperornis,
Hargeria, Upper Cretaceous of North America); Enaliornithidce (e.g. Enaliornis

Frc. 42o.-I cllt/1yoTnis: Skelet-on.

/chlltyomis is often shown as a toothed
bird but there is no evidence that it
was so equipped. The toothed jaws
with which it was associated p robably
l>elonged to a mosasaur (Squamata).
The keeled sternum and the structun:
of the pectoral girdle showed that it
was a powerful flier (Fig. 418). but
the vertebr.e were still amphicrelus.
About eight inches high , / . victor
possibly resembled a modern tern in
general form and economy. This was
an Upper Cretaceous form. True gu ll s
date from at least the Eocene. (Mocli-
ficcl after Marsh.)

of the European Lower Cretaceous; N eog{{'ornis of the European Upper

Cretaceous); and Baptornithidre (e.g. Baptornis, North American Upper
Cretaceous).
ORDER IcHTHYORNITHIFORMES

This order contains several species (e.g. Ichthyornis (Fig. 418) and Apatornis)
found in the same deposits as Hesperornis. The brain of these somewhat tern-
like marine flying birds was of the typically avian pattern. It was formerly
believed that small, recurved and pointed teeth were set in sockets, but Gregory
has shown that the toothed jaws attributed to lchthyornis probably belonged to
reptilian mosasaurs. It is not yet known whether these birds had teeth or
not. The neck vertebrae were amphiccelous. The sternum had a well-developed
keel, and the wings were not unlike those of modern flying birds (Fig. 420).
The order contains two families: Ichthyornithidre (e.g. Ichythyornis) and
6o6 ZOOLOGY

Apatornithidce (e.g. Apatornis). All known specimens are from the Upper
Cretaceous of North America.

SUPER-ORDER PALJEOGNATHJE
These are almost always flightless and usually of large size. The sub-
stantial cerebellum is an indication that they are descended from volant
ancestors. There is some evidence that the stock appeared at the end of
the Mesozoic, and ·some (Emu, cassowaries, Ostrich, Rhea) are sufficiently
powerful and adaptable to remain relatively plentiful to-day in the face of
unprecedentedly savage competition by Man and his domesticated animals.
One, the Ostrich, is farmed for its specialised filamentous plumage. Palceo-
gnathous birds, with a generally raft-like sternum, have been considered as a
flightless 'Order Ratitce ', as distinct from an 'Order Carinatee' of keel-breasted
flying birds. Such an arrangement is untenable : several flying birds in special-
ised habitats have lost the carinate condition, and become raft-breasted in
varying degree along with the abandonment of flying.
De Beer has now assembled evidence that the 'ratite' palate (see below) is
not primitive, but neotenous. It probably represents an early stage 'through
which the palate of many carinates passes during development'. In short, the
so-called old palate (Fig. 434, p. 631) is probably an arrested development of the
neognathous condition (see McDowell). Whether the flightless birds constitute
a single natural group, or whether they are a heterogeneous assembly that have
pursued more or less parallel lines consequent upon the loss of flight is at
present unknown. The palceognathous palate varies greatly, as will be seen
if the skull of an Ostrich, kiwi, cassowary, and tinamu are compared. Except-
ing only the Ostrich (which still exists commonly north of the equator), these
birds live to-day only in southern continents and islands. It is important to
remember, however, that ostriches were spread over much of the Eurasian land-
mass in the Tertiary, along with even larger ostrich-like forms.
The feathers of such birds are 'primitive', in that they have no hooked
barbules, so that the barbs are free (Fig. 381, p. 561; Fig. 384, p. 568). Such
feathers are a further neotenous character: they are essentially similar to down.
Apteria are usually absent in the adult.
The rectrices are absent or irregularly arranged, and the pygostyle is small
or undeveloped. The sternal keel is vestigial or absent. The coracoid and
scapula are comparatively small and completely ankylosed. The acrocora-
coid process is vestigial, and the coraco-scapular angle approaches two right
angles. The wing is reduced in size and may be vestigial or absent. There
are large basipterygoid processes developed from the basisphenoid. The
vomer is large and broad, and separates the palatines, which do not come into
contact with the cranium (cf. neognathous or so-called 'new jawed' condition,
 PHYLUM CHORDATA

p. 613). This condition occurs also in the toothed odontognathous forms (p.
603), and recalls to some degree that of archosaurian reptiles (p. 498). The
quadrate articulates with the skull by a single or partially divided facet.
Permanent sutures between the skull bones are another result of neoteny,
i.e. 'the secondary retention of features which were juvenile in the ancestors
of the ratites' (de Beer).
The males have an erectile penis and the females a clitoris. The young are
precocious.
ORDER STRUTHIONIFORMES
The Ostrich (Struthio camelo) of Africa and Southern Asia (Arabia) is the
largest living bird, standing nearly 8 feet high. Ostriches retain only two toes
(the third and fourth), and the feathers lack an aftershaft. They extend their
rudimentary wings while running at speed. The story that the Ostrich buries
its head in the sand in order to conceal itself is untrue. A squatting bird,
however, will often lower its head to a position parallel with the ground, and
thus render itself less easy t<r see because of its obliterative coloration. Ost-
riches occurred in the Pliocene.

ORDER RHEIFORMES
These (genus Rhea) are two species of large running birds of the South
American pampas which seem to occupy an ecological niche approximately
similar to that of the surviving Australian Emu. Rheas retain three toes.
The genus occurred in the Pliocene.

ORDER CASUARIIFORMES
Emus (Dromceus) and cassowaries (Casuarius) of Australia, New Guinea,
and the East Indies. Of the vegetarian emus, which are known also from
Australian Pleistocene deposits, only one species survives. This is a plain-
and savannah-dwelling form, which is in no danger of extinction. It stands
nearly 6 feet high. It is a swift runner and, in addition, has each inner toe (of
the three) equipped with a powerful claw with which it can rip the belly of a
dog. Next to the Ostrich, the Emu is the largest living bird. Its feather
after-shafts are about the same length as the main quills.
The fruit-eating cassowaries are confined to the dense tropical rain-forests
of Australia (one species only), New Guinea, and the East Indies. The wattled
neck and brilliantly coloured head of each species are surmounted by a tall,
horny casque, which probably helps the bird push through the thick 'jungle'.
Cassowaries, too, can disembowel a dog with a single kick, and occasionally
humans have been killed by the powerful, sharp inner toe. In both Emu and
cassowaries the wings are reduced to vestiges supported by a single digit. The
wing rudiments of cassowaries are distinguished by the possession of several
spine-like, barbless feather-shafts some inches long. The dorsal feathers are
6o8 ZOOLOGY
long, and fall over the sides in a thick, almost hair-like protection against the
spiky rain-forest tangles. Pre-Pleistocene fossils have yet to be found.
ORDER APTERYGIFORMES
The kiwis (Apteryx, Figs. 421, 446) are confined to New Zealand and ad-
jacent islands. They are relatively small, almost wingless birds, about the
size of, or a little larger than, a domestic fowl. They run swiftly and defend

F1c . 421.- Sub-class Neornithes, Super-order Palreognathre, Orders Dinornithiformes and

Apterygiformes. DinoTnis and ApteTy:r. l\Ioas (left) became extinct in geologically recent
times. The.y showed a degree of convergence with giraffes. The earth -probing kiwis (right) are
protected by law, and some are not uncommon in many localities. Both groups originated in
and arc confined to the isla nds of New Zealand.

themselves with their claws. Kiwis are nocturnal, and live principally on
worms and insects caught by a long probing beak, the nostrils of which are at
the tip of the maxilla. The eyes are small (p. 640). The eggs are relatively
the largest of all living birds (see also p. 6og). The feathers have no after-
shaft. Pre-Pleistocene fossils have yet to be found.
ORDER DINORNITHIFORMES
The remarkable moas (families Dinornithidre and Anomalopterygidre) of
New Zealand were of perhaps twenty species, ranging from the size of a turkey
 PHYLUM CHORDATA 6og
to the giant Dinornis maximus about ro feet high (Fig. 421). In most species
the wings and pectoral girdles had disappeared completely, and in no case was
there any trace of a keel on the sternum. The beaks were short and the massive
legs had four toes. They were probably vegetarian, some perhaps feeding in
the manner of giraffes. Many Pleistocene fossils are available. Bones and
even the loosely constructed feathers (without barbicels) have been preserved
in middens. Moa bones were used by the Maoris as implements, and it has
been generally thought that the birds were exterminated when New Zealand
was colonised by them about the year A.D. 1350. This view is now subject to
considerable doubt (Archey). Modern ex-
cavations show that moas were indeed
killed in great numbers, but probably by a
pre-Maori people who also buried moa-egg
containers (holding about two quarts) with
their dead. There is some evidence that
moas disappeared less than a century
before the arrival of the Maoris who in
turn exterminated the earlier New Zea-
landers.

ORDER lEPYORNITHIFORMES
The giant flightless Madagascan !Epy-
ornis ('Elephant-birds') were extinguished
in recent times, perhaps only a very fevv FIG. 422 . -0rder Tinamiiormes, Fam-
centuries ago and therefore rather later ily Tinamidre. Eud1·omia. E . elegans
i,; , like many tinamus, extremely success-
than the moas. They possessed rela- ful and widely distributed. In its various
races it occurs from the valleys of the
tively tiny wings, but had stout, power- Andes to the Atlantic, and from Northern
ful legs and four toes. Some were larger search Argentine to Santa Cruz. Future re-
may shift tiH' tinarnus from the
than ostriches: A~. titan was about 10 feet Pal:rognatha- to near the neognathous
galliforrnes (p . 615). (Redrawn after
high. Eggs have been found measuring Grasse.)
13 X 9·5 inches, with a probable capacity
of about two gallons. It is not improbable that !Epyornis and its allies
inspired the exaggerated stories (Marco Polo, Arabian Nights, etc.) of the giant
Madagascan Roc, which was claimed to be big enough to carry off an elephant.
Although it is possible that the egg of !Epyornis is the largest animal cell that
has ever existed, it still falls rather below the size attributed to Roc's eggs-
(' as big as a butt'). North African Pliocene fossils have been found.
Madagascan fossils are Pleistocene.

ORDER TINAMIFORMES

The affinities of the superficially partridge-like, almost tail-less tinamus

(Fig. 422) are uncertain. Traditionally they have been placed in the position
6IO ZOOLOGY
given them here but there is much to be said for placing them near the Galli-
formes (p. 6r5) in another super-order (Neognathce) altogether. Tinamus are
essentially cursorial but they can fly clumsily over short distances: the sternum
is keeled. At the same time most authorities believe that their palatal struc-
ture generally agrees with that of the palceognathous forms dealt with above.
There is but one family-the Tinamidce-of more than fifty species which
are confined to the mainland of southern Mexico, and Central and South
America. The various species have spread widely throughout open pampas,
equatorial forests, and high into the barren uplands of the Andes.
When flushed, tinamus (which are cryptically coloured) usually run rapidly
and may conceal themselves by squatting. They do not perch. They are
essentially vegetarian. Their eggs are remarkable: the shells are highly
glossy and of a single, often intense, colour, according to the species. Fossils
are rare; some South American Pliocene and Pleistocene remains have been
identified.

SUPER-ORDER IMPENN.tE
ORDER SPHENISCIFORMES
Although penguins (Fig. 423) are here placed in a separate super-order, it
should be remembered that, despite their remarkably different appearance
from all other birds, they undoubtedly exhibit fundamental similarities with
the birds of the following super-order (Neognathce), in which they are often
included. Some have suggested that penguins evolved from flightless raft-
chested sea-birds but this view has little in its favour. The balance of evidence,
summarised by Simpson, suggests that the group evolved from a volant stock
in which the wings were used both for flying and swimming (as, for example,
by the extant auks). With the loss of flight and increase in weight, the wing
became increasingly efficient as a flipper which is nevertheless still used in
exactly the same motion as that of a bird in flight.
Penguins have become wonderfully adapted to swimming and diving. The
bones, excluding certain skull-bones, are solid and there are no air-sacs within
the body. The closely packed plumage holds very little air. The bones of
the featherless wings have become flattened and united to form a powerful,
resistant paddle or flipper which moves only at the shoulder-joint. It seems
probable that this flipper 'never ceased to be a functional wing, but only
changed the medium in which it functioned' (Simpson). The paddles are
used also for fighting, and, in some species, for terrestrial locomotion. The
hind-limbs are strikingly modified for swimming. The metatarsals, unlike
those of other birds, are only partly fused and the feet are strongly webbed.
The short, heavily packed plumage and subcutaneous fat deposits are insulat-
ing in function. The body is sleek and streamlined, and offers the least
possible resistance to diving and submarine activities.
 PHYLUM CHORDATA 6rr

Although one species has followed the cold Humboldt current as far north
as the Galapagos, penguins are typically Southern Hemisphere birds. They

J".l1

FIG. 423.-Sub-class Neornithes, Super-order lmpennre, Orde r Sphenisciformes, Family

Spheniscidre. Apterwdytes: Environment, development, and behaviour. In its successful colon-
isation of the harsh Antarctic mainland, the Emperor Penguin, A. forsteri, has developed a series of
r emarkable, even bizarre, forms of behaviour. During blizzards the birds huddle tightly together
in thousands, backs to the wind, with the chilled outer rank frequently changing places with
warmer birds inside. The species establishes no t erritory and builds n o n est. The female lays
in the tempestuous darkness of winter and the single egg, a nd later the chick, is carried on the
feet covered by a fold of tissue. The egg is early tran sferred to the male and the female goes off to
the sea (often as far as 50 miles away) for approximately the whole incubation period of some
63 days. She returns laden with subcutaneous and peritoneal fat, and brings crustacean krill
'past e ' to the male, whom she now relieves. Correlated with the mode of its tra nsportation,
and the time of its hatching, is the slow developmenta l rat e of the Emperor chicle By comparison,
the young Adelie Penguin (Pygoscelis adelie). which hatches in a nest in the same region in mid-
summer, grows rapidly and is relatively large at t he start of the n ext winter. Also correlated wit h
the stringent environmental conditions is the development in the Emperor of a most exaggerated
parental drive. Stray chicks are sometimes crushed to death- killed by kindness-by over-
solicitous potential foster-parents. Levick says that chicks som etimes crawl under icy ledges
t o escape being cared for, and that ev en dead c hicks may be carried about until t he down is rubbed
off their bodies. (From photograph s by courtesy of F .I.D.S.)

occur in greatest numbers between the southern extremes of South America,

South Africa, Australia, and the Antarctic continent (p. 612). They appear
in mid-Tertiary deposits in South America and on Antarctic islands. Certain
Miocene forms were about 5 feet high and weighed perhaps 200 lb.
6I2 ZOOLOGY

FIG. 424.-Neomithes: Adaptation and geographical distribution. The minute spot {arrowed)
in eastern Australia roughly indicates the total distribution of the passerine Sydney Rock Warbler
(Origma rubricaia). The vast cross-hatched ocean area approximately covers the distribution of the
penguins (Spheniscidm).
In a restricted region of exposed Triassic sandstone, a small modern warbler has become essen-
tially a rock-hopper. It wedges spider-web into a horizontal crevice in the roof of a dim cavern
and to this it anchors its bulky moss and root-fibre p endent nest; its eggs have become white:
it has become Origma rubricata . During the past 50 y ears odd deviant nests have been reported
hanging in a limestone cave, culverts, a tent, an engine-room , a sewer shaft, a hollow log, and an
artist's studio. The Rock \Varbler is adapted to reproduction in spacious hollows. These are not
found in quantity in surrounding geological formations; hence the restricted distribution.
P enguins no doubt evolved in the south, and became remarkably adapted (p. 6II) to life in
cold seas. They thrust out in all directions, colonising and speciating. One species-the Gala-
pagos-is enabled by the cold Humboldt current to live on the equator ; but none has been able
to cross the warm seas to colonise the Arctic-as has, for example, the strong-flying Great Skua
(Catharacta skua). Another species (the Humboldt) has spread along the colder western coast of
South America, and the Gentoo and Rockhopper (both circum-polar in distribution) reach
southern South America. Another (the African) lives in South Africa, and yet another (Fairy) in
southern Australia and northern New Zealand. Four species inhabit the southern islands of New
Zealand, where one breeds in woodland. Of the 17 species, only three (Chinstrap, Adelie, and
Emperor) have successfully invaded th e Antarctic continent.

SUPER-ORDER NEOGNATHLE
This great group is characterised by the 'new jaw' arrangement as opposed
to that of the palceognathous or 'old jawed' condition (p. 6o6). If the conclu-
sion is accepted that the palceognathous condition is an arrested stage in the
development of the neognathous palate, it follows that it is the so-called
 PHYLUM CHORDATA

'new palate' that is the primitive condition (unless one chooses to believe in
the Theory of Recapitulation). In the neognathous condition the palatines
extend posteriorly, make contact with the base of the cranium, and come
into mobile articulation with the short pterygoid bones. The vomers, too,
are brief. (In palceognathous birds this palata-cranial articulation is rigid.)
Neognathous types arose in the Cretaceous (p. 3), radiated in the Tertiary,
and became the dominant flying vertebrates. More than twenty orders (in
the ornithological sense, p. 555) are represented, a few of which include the
numerous species that occur on every city building and garden. They are as
follows:
ORDER GAVIIFORMES
The long-necked loons or divers (Gavia = Colymbus) fall into a single family
and genus of piscivorous se.a- and lake-birds from North America, Europe,
and the Arctic. In some characters (e.g. posterior position of legs) they have
converged with Hesperornis (p. 604) but, on the other hand, they are powerful
in flight. They lay their eggs in exposed places in nests made of piles of
vegetation. Early Tertiary fossils are known. Some authorities include the
divers with the next order.
ORDER PODICIPITIFORMES
The compact-bodied grebes (single family, Podicipitidce) are also often called
'divers' because of their habits. They are freshwater birds of almost cosmo-
politan distribution. They are almost completely aquatic and build floating
nests .. They are commonly said to be sufficiently agile to dive after a gun-flash,
yet escape the bullet. This story has gained currency solely due to poor
marksmanship. The toes are lobate and the legs are placed far back. Tertiary
fossils occur.

ORDER PROCELLARIIFORMES
These are tube-nosed, long-winged sea-birds, of which the following families
are recognised: Diomedeidce (albatrosses), Procellariidce (fulmars, shearwaters,
etc), Hydrobatidce (= Thalassidromidce) (storm-petrels, etc.), Pelecanoididce
(diving petrels). Members of this order have penetrated the furthest limits
of every ocean. The albatrosses, however, belong essentially to the Southern
Hemisphere, although many cross the equator. Tube-nosed birds range in
size from tiny storm-petrels (e.g. Oceanites) less than 7 inches long to the great
soaring albatrosses (e.g. Diomedea) with a wing-spread of about n! feet.
Petrels got their name from St. Peter because they seem to walk on the water.
They are called by sailors 'Mother Carey's Chickens'. ('Mother Carey' is a
corruption of Mater Cara, the Blessed Virgin.) Procellariiformes breed general-
ly on islands or cliffs, often in very high latitudes (e.g. Fulmarus). One shear-
water, Pujjinus tenuirostris (the fledgling of which is salted as 'mutton-bird'),
6!4 ZOOLOGY
breeds on islands off the Tasmanian coast and then makes an enormous circum-
oceanic migration via New Zealand, Japan, Alaska, and California, arriving
back in southern Australia about the same rz days every year. All tube-nosed
birds lay one large egg, sometimes in a burrow. Some eat plankton and fishes;
others are scavengers. They are among the few birds with a well-developed
olfactory apparatus (p. 64r). They appear in Oligocene deposits.

ORDER PELECANIFORMES

A polymorphic assembly of voracious fish-eaters which occupy the following

families: Phcethontidce (Tropic- or Bos'un-birds), Pelecanidce (pelicans),
Balcenicipitidce (Shoebill), Pelagornithidce (gannets and boobies), Phalacro-
coracidce (cormorants and shags), Anhingidce (darters or snake-birds), and
Fregatidce (frigate-birds). They are generally colonial nesters and have four-
toed webbed feet, bodies adapted for diving (some from heights of 6o feet),
and long beaks with wide gapes for catching and swallowing struggling prey.
Pelicans (Pelecanus) have buccal pouches for storage. Darters (Anhinga) first
spear the fish, then toss, catch, and swallow. Elopteryx, which probably comes
into this group, occurred in the Upper Cretaceous and may be one of the
earliest known neognathous birds. The Tertiary Odontopteryx had a saw-edged
beak.
ORDER CrcoNIIFORMEs

This long-legged and correspondingly long-billed group is composed of the

following: Ardeidce (herons, egrets, and bitterns), Cochleariidce (boat-billed
herons), Scopidce (hammer-heads), Ciconiidce (storks, openbills, Jabiru),
Threskiornithidce (ibises, spoonbills, etc.), Pha:nicopteridce (flamingoes).
The last-named alone have retained webbed feet, and have the middle toe
relatively short. The egrets seasonally develop filamentous nuptial plumes
which are marketed, generally illegally, as 'osprey plumes'. (Ospreys are
fish-hawks.) All the above birds feed principally in marshes and mudflats,
and this is probably responsible for the relative abundance of fossils from the
Eocene onwards.
ORDER ANSERIFORMES

The swans, geese, and ducks are strong fliers and almost cosmopolitan:
Anhimidce (screamers, etc.), Anatidce (all others). They may be vegetarian,
mollusc- or fish-eaters, or filter-feeders with, in addition, special tactile organs
on the beak. The feet are webbed, even in the tree-ducks. They are not
uncommon from the Eocene onwards.

ORDER F ALCONIFORMES
The diurnal birds-of-prey are grouped as follows: Cathartidce (American
vultures, condors), Sagittariidce (Secretary-bird), Accipitridce (harriers, kites,
eagles, Old World vultures, and ospreys), Falconidce (falcons and allies).
 PHYLUM CHORDATA

These powerful fliers have spread almost everywhere except Antarctica. They
are notable for the spread of their sharp claws for catching and holding prey,
and their hooked beaks for cutting and tearing. They generally eat living
vertebrates, including fish (ospreys and sea-eagles), but some eat carrion
and a few weak-billed forms devour insects. The Wedge-tailed Eagle (Uro-
mtus audax) of Australia, which has in Tasmania a wing-span of from 6 to ro
feet (about 7 to IO lb. weight), has been known to attack a seven-year-old girl
when hungry after escape from captivity. An eagle of even greater size is
Pithecophaga, the Monkey-eating Eagle of the Philippines. Two of the sea-
eagles (Halimtus pelagicus and H. albicilla) may be even bigger.
The order is represented from the Eocene. The Pleistocene condor T era-
tornis incredibilis was the biggest flying-bird yet known and may have had a
wing-span of 16 feet.
ORDER GALLIFORMES
This heterogeneous group of game-birds is almost cosmopolitan. It in-
cludes the Megapodidre (incubator-birds, brush-'turkeys', etc.), Cracidre
(curassows, etc.), Tetraonidre (grouse, ptarmigan), Phasianidre (partridges,
quail, true pheasants), Numididre (guinea-fowl), Meleagrididre (true turkeys),
and the extraordinary sub-order Opisthocomi containing the single genus
Opisthocomus (hoatzins) (Fig. 427). Notable for their palatability, massive
scratching feet and short, powerful flight, galliform birds are largely gramini-
vorous, but eat quantities of insects. They are essentially terrestrial. The
palate is different from that of raft-breasted types (p. 6o6), and most modem
groups as well, suggesting an ancient departure on an independent line. Pre-
Eocene fossils, however, remain unknown.
Opisthocomus of South America is especially notable in that the nestling
has, on the first two digits of the wing, claws used in clambering among branches.
This character, however, is almost certainly of independent development, and
not an inheritance from archreomithian (p. 599) ancestry.

ORDER GRUIFORMES
This is a polymorphic assembly including the Tumicidre (bustard-quails),
Pedionomidre (collared hemipodes), Gruidre (cranes, Brolga), Psophiidre
(trumpeters), Rallidre (rails, gallinules and coots), Heliomithes (sun-grebes),
Rhynochetidre (kagus), Eurypigidre (sun bitterns), Otidre (bustards), and the
Cariamidre (cariamas).
Excluding polar regions, the order is almost universal. Many (e.g. rails)
are laterally compressed reed-dwellers. The bustards have left the water and
colonised grass-land and savannah. The order dates from the Eocene, and
perhaps includes the South American Phororhacidre and allies which, probably
related to the cariamas, were common in the Miocene. These long-legged,
 ZOOLOGY
heavy-beaked, flightless birds stood nearly 6 feet high (e.g. Phororhacus) .
They were carnivorous ground-nesters, and their temporary success was possibly
related to the current absence of placental carnivores in South America.

ORDER DIATRYMIFORMES
These (Diatrymidre, Gastornithidre) were large, flightless European and
North American forms which were common in the early Eocene and probably

Frc. 4::!5.- Sub-class Neornithes, Super-order Neognathre, Order Diatrymiformes, Family

Diatrymidre. Diatryma. D. steini was aN orth American Eocene cursorial form about 7 feet high
which was neverth eless a 'modern' bird of an order perhaps allied to the Gruiformes (cranes,
bitterns, bustards, etc.) (Redrawn after Matthew and Grainger.)

related to the cariamas (see above). They had massive beaks, heads, and
necks and, among them, Diatryma reached a height of about 7 feet (Fig. 425).

ORDER CHARADRIIFORMES
This universally distributed (excepting the ice-caps) polymorphic group
contains the Jacanidre (lotus-birds, Fig. 429, p. 624), Rostratulidre (painted
snipe), Hrematopodidre (oyster-catchers), Charadriidre (lapwings, plovers), Scolo-
pacidre (woodcock, sandpipers, etc.), Recurvirostridre (stilts, avocets), Phalaro-
podidre (phalaropes), Dromadidre (crab-plovers), Burhinidre (thick-knees),
Glareolidre (coursers, pratincoles) , Thinocoridre (seed-snipe), Chionididre
 PHYLUM CHORDATA 6!7
(sheath-bills), Stercorariidre (skuas, jaegers), Laridre (gulls, terns), Rhyn-
chopidre (skimmers), and Alcidre (auks, puffins). The above shore-dwelling
groups, as well as their aquatic and sea-going derivatives, have become highly
successful. Among the most interesting are the jacanas, which have developed
slender, and enormously elongated, fore- and hind-toes adapted to progres-
sion over aquatic lily-leaves (p. 624). In Phalaropus the female is the
more brilliant of the pair and also assumes at least some of the typically
male pattern of behaviour, including territory selection. The male, on the
other hand, incubates. Waders (Charadriidce) occur in Tertiary deposits,
gulls and auks in the Eocene.
ORDER COLUMBIFORMES
The pigeons and doves are universal, excluding polar regions. The order
is often held to inc1ude the aberrant sand-grouse as follows: Pteroclididce
(sand-grouse), Raphidce (extinct dodos, etc., p. 577). Columbidce (all others).
The last-named family are mostly graminivorous or frugivorous and produce
'pigeon's milk' (p. 583). The crested Goztra cristata of New Guinea and adjac-
ent isles may stand nearly 2 feet high. Pre-Miocene fossils are rare. The sand-
grouse are ground-nesters which lack the characteristic raised hallux of the
galliformes (p. 615), which they greatly resemble in several ways.

ORDER PSITTACIFORMES
(Single Family Psittacidre)
The parrots and cockatoos are common only in the tropics and sub-tropics,
and reach their greatest number and diversification in the Australasian region.
They have powerful hooked beaks (Fig. 436, p. 632) adapted to husking food
and tearing dead timber in search of wood-boring insects, although many
forms eat nectar or are almost entirely frugivorous. They possess a character-
istic zygodactylous foot in which the rst and 4th digits are backwardly directed,
allowing for a pair of toes on each side of the branch. Their powers of mimicry,
their longevity (p. 560) and vivid coloration are well known. They do not
seem to mimic other species in the field, as do many passerines. The vivid
coloration is obliterative amongst foliage, making them surprisingly difficult to
see. At least one parrot (Geopsittacus) has become nocturnal. They occur
in Miocene deposits.
ORDER CucuLIFORMES
The cuckoos (Cuculidce), plantain-eaters, and turacos (Musophagidre),
possess a zygodactylous foot not unlike that of the parrots. Some species of
cuckoos (e.g. Centropus) build their own nests. The eggs of parasitic species
often match those of the host with remarkable fidelity. The Cuculidre
are widely distributed throughout tropical and sub-tropical regions and are
VOL. II. QQ
618 ZOOLOGY

common in temperate zones. Musophagidre occur in Africa and may not be

truly referable to this group. Cuckoos have been found in Oligocene deposits
but fossils are rare.
ORDER STRIGIFORMES
The almost universally distributed owls occupy the ecological niche vacated
at dusk by the diurnal birds-of-prey, with which they show notable convergence
in beak and claw. They are the Tytonidre (bam-owls) and Strigidre (all others).
In texture, as well as in its 'loose' arrangement, the plumage is adapted to a
relatively soundless approach, and the retina contains principally rods (p. 140).
The eyes of owls are directed forwards (as are those of the nocturnal hunter
Podargus, (p. 621)) and have become so large as to be almost immovable:
the animal must rotate its neck to look sideways. It is believed that some
owls hunt largely by sound. Prey (e.g. a rodent) is as a rule swallowed whole,
much of the food residue being regurgitated as casts or pellets with the fur
surrounding the sharp, broken bones. Owls occur in Oligocene and later
deposits.
ORDER CAPRIMULGIFORMES
The nightjars and allies are nocturnal insectivorous birds arranged as
follows: Steatornithes (oil-birds), Podargidre (frog-mouths), Nyctibiidre
(potoos), iEgothelidre (owlet nightjars), and Caprimulgidre ('night-hawks',
'goat-suckers'). These birds possess wide gapes, and 'loose' soft feathers
recalling the plumage of owls (p. 643). They have prominent, probably
sensory, bristles around the gape, and are protectively camouflaged and difficult
to see when at rest. They are best developed in the tropics and sub-tropics
but are not uncommon in temperate zones (e.g. Caprimulgus, Podargus).
Fossils have been rarely found, but occur in Pliocene deposits.

ORDER APODIFORMES (MICROPODIFORMES)

The swifts and humming-birds-Apodidre (swifts), Hemiprocnidre (crested
swifts), and Trochilidre (humming-birds)-are a homogeneous group. The
swifts, however, have diverged sharply into fast-flying, wide-mouthed, large-
eyed migratory insect-catchers, whilst the humming-birds have become gener-
ally tiny, protrusible-tongued, darting and hovering, relatively 'stationary',
nectar (and insect) feeders. Swifts are almost universal (excluding polar
regions), whereas humming-birds are confined to the central zone of
the Americas. Many swifts, and particularly the edible-nest swiftlets (Col-
localia), use a certain amount of saliva from sub-lingual glands (Fig. 442, p. 637)
as nest cement. Swifts are known from the Oligocene.

ORDER COLIIFORMES
The small distinct monogeneric group of mouse-birds or colies (Coliidre) is
confined to Africa. Insectivorous and frugivorous, they creep through the
 PHYLUM CHORDATA

branches of trees using their beaks, as well as peculiar feet on which the hallux
can be brought forward. Some species of Colius have bodies little bigger than
sparrows, but tails are as long as ro inches. Fossils are unknown.

ORDER TROGONIFORMES
These fall into the single widespread family Trogonidre of the tropical
Americas, Africa, Asia (including the East Indies), and the Philippines. They
range in size between that of thrush and crow, and have bristled gapes and
invariably loose, beautiful plumage. They are known from the Miocene.

ORDER CORACIIFORMES
This is a widespread, colourful, and perhaps polyphyletic group as follows:
Alcedinidre (kingfishers), Todidre (todies of the West Indies), Momotidre
(motmots of Central America), Meropidre (bee-eaters), Leptosomatidre (kirombos
or Madagascan rollers), Coraciidre (rollers), Upupidre (hoopoes), Ph~niculidre
(wood hoopoes), and Bucerotidre (hornbills). The order is known from the
Miocene.
ORDER PICIFORMES
The woodpeckers and their allies, absent only from Australasia and polar
regions, are as follows: Galbulidre (jacamars), Bucconidre (puff-birds), Capit-
onidre (barbets), Indicatoridre (honey-guides), Picidre (woodpeckers, wrynecks),
Ramphastidre (toucans). All have zygodactylous feet with the second and
third digits directed forwards, and the first and fourth backwards. The honey-
guides are especially noteworthy. The family is a predominantly African
(south of the Sahara) group, although two of the I I species are Asiatic. Certain
species exhibit noisy excitement and appear purposefully to lead honey-
badgers (Mellivora capensis) and Man to bees' nests. When the nest is broached
the bird eats fragments of discarded comb. Friedmann describes the peculiar
'guiding' behaviour of Indicator indicator as a' form of excitement reaction ...
when meeting with a potential foraging symbiont ... that the bird associates
with the act of opening bees' nests'. This meeting usually takes place near
a bees' nest but ordinarily the bird does not in fact 'lead' in a direct route but
noisily performs, often very circuitously, from tree to tree. It is not necessary
for the bird to know in advance where the bees have built; the excitement
fades, however, when a nest is reached. Then the bird sits quietly until it
receives a meal of wax. These birds are the only vertebrates known to be
capable of extracting nourishment from bees wax. Two micro-organisms of
high lipolytic activity-a Micrococcus and a strain of the yeast Candida
albicans-occur in the birds' freces.
Several species of honey-guides resemble cuckoos in laying in the nests of
other birds.
6zo ZOOLOGY

ORDER PASSERIFORMES
This-by far the largest avian order-is divided into four sub-orders:
Eurylremi (containing the single family Eurylremidre-broadbills of Indo-
Malaya and Africa); Tyranni (a predominantly American assemblage including
the tyrant-flycatchers (Tyrannidre) and their allies, as well as the New Zealand
'wrens' (Xenicidre)); Menurre (the Australian lyrebirds (Menuridre) and scrub-
birds (Atrichomithidre)); and Passeres or Oscines (all the remaining, and more
familiar, song-birds). The more primitive sub-orders Eurylremi and Tyranni
were formerly brigaded as the Mesomyodi, and the Menurre (also known as the
Sub-oscines, Pseudoscines, Subclamatores, and abnormal song-birds) were
grouped with the Passeres (the normal song-birds or Oscines) as the Acromyodi.
The sub-order Passeres (Oscines), numbering about 4,000 normal song-birds,
includes the more familiar passerines such as (for example) larks (Alaudidre),
swallows (Hirundinidre), bulbuls (Pycnonotidre), 'flycatchers' (Muscicapidre),
dippers (Cinclidre), shrikes (Laniidre), true tree-creepers (Certhiidre), tits (Pari-
dre), honey-eaters (Meliphagidre), silver-eyes (Zosteropidre), finches (Fringillidre),
weaver-finches, including sparrows (Ploceidre), starlings (Sturnidre), crows
(Corvidre), birds-of-paradise (Paradiseidre), and bower-birds (Ptilonorhynchidre).
There is some evidence that passerines occurred as long ago as the Eocene,
but few fossil types have been found in .deposits earlier than late Tertiary.
Although the Oscines are unquestionably a relatively modem, and still
expanding group, it is well to remember that their small, fragile bodies (and the
habits of many) render them less likely to be fossilised.
Information in great detail concerning many Oscines and their complicated
behaviour-patterns is available in numerous regional and other publications.

GENERAL ORGANIZATION OF THE NEORNITHES

External characters.-In the general build of the body the Neornithes differ
from the Archreornithes chiefly in the shorter and stouter trunk, and in the point
of articulation of the hind-limbs being thrown forward, so as to be almost directly
below the centre of gravity of the body. The animal is thus enabled without
effort to support itself on the legs alone. Birds are essentially bipedal. The
only exception appears to be the young of the Hoatzin (Opisthocomtts) (p. 615),
which uses its wings in climbing (Fig. 427).
The neck is always well developed, and is often, as in the swan and flam-
ingoes, of great proportional length. The cranial portion of the head is usually
not large. The beak (Fig. 426) may attain a remarkable size, and exhibits a
wide range of form. It may be extremely short and wide for catching moths
and other flying insects, as in swifts and nightjars; short and conical for eating
seed, as in finches; strongly hooked for tearing the bodies of animals, as in
 PHYLUM CHORDATA

FIG. 426.- Neognathre: Rostral and associated specialisations. Not to scale. r. Beak serra-
tions of the Australian Tooth-billed Catbird (Scenopa;tes dentirostris) used for sawing through the
petioles of leaves gathered for the daily d ecoration of its display-ground. 2 . Fish-holding
denticulations of the European Goosander (Mergus merganser) . 3· Termina l hook and beak-
pouch of the North America n White Pelican (Pelecanus erythrorhynclms) . The compressed horny
beak adornment is seasonally developed and falls off without trace after reproduction. 4· Tube-
nosed arrangement (in Wandering Albatross, Diomedea exulans) characteristic of ocean-going
Procellariiformes (petrels and albatrosses) . 5· Hooked beak of Egyptian vulture (Trigoniceps
occipitalis) used for tearing dead flesh. The head and neck, often buried in sup purating material,
are a lmost bare. 6. Marginal h ooks and leaflets which, with a spiny tongue, form a filter-feeding
mechanism in the old world Greater Flamingo (Pha;nicopterus antiquormn). 7· Obliquely
crossed mandibles of the passerine Pal<earctic Crossbill (Loxia curvirostra) . A special articulation
and musculature enables a swift powerful movement that rips open fir-cones, after which t he tongue
scoops out the exposed seed. 8. The Pacific passerine Neodrepanis coruscans, illustrating con-
vergence of beak form with that of a flower from which it brushes necta r a nd perhaps helps
fertilise. 9· European Green Woodpecker (Cecinus viridis) , which excavates with its beak, and
then probes for organisms with its protrusible tongue (see also Fig. 440) . 10. The Galapagos
Woodpecker-finch (Camarhynchus pallidus) trenches with its beak and probes with a cactus spine;
it is one of the few avian tool-makers (see also Fig. 382, p. 562). II. Broad gape in Australian
Tawny Frogmouth (Podargus strigoides), a nocturnal insectivorous species. The eyes are pro-
tected by tactile bristles and are directed forwards (seep. 643). A tuft of relatively filamentous,
and probably tactile, feathers surmounts the beak. (After various authors.)
6zz ZOOLOGY
birds of prey, or for rending fruits of various kinds, as in parrots; long, conical,
and of great strength, as in storks ; slender and elongated, as in ibises and cur-
lews ; broad and flattened for feeding in mud, as in ducks and geese; expanded
at the end, as in spoonbills; immensely enlarged, as in hornbills and toucans.
It is most commonly bent downwards at the tip, but may be straight or curved
upwards, as in the Avocet, or bent to one side, as in the New Zealand Wry-
billed Plover. Among ducks and geese the beak has often a lateral straining
device for the retention of food and the discharge of water. Mergansers have
sharp horny tooth-like lateral projections with which to hold struggling fish.
Tropical snake-birds (Anhinga) have stiletto-like beaks with which they spear
fish, toss them up, and then swallow them head-first. The Tooth-billed Cat-

FIG. 427.-Neornithes: Clawed

wings. A, Nestling Opisthocomtts
(with two clawed digits) ; B,
Adult Apteryx (kiwi). Both from
inner (ventral) aspect. cb. I.
first cuhital remex; dg. I , dg. 2 ,
dg. 3, digits ; pr. ptgm. prepatag.
8 ium; pt. ptgm. postpatagium.
(A, after Pycraft; B, after T. J.
Parker.)

bird (Scenopates) has a serrated beak with which it saws through petioles in
order to free green leaves for use on its display-ground.
The buccal arrangements of the flamingos (Phoenicopteri) deserve special
mention: there has been developed a complicated filter-feeding mechanism,
analogous to that of baleen whales, which enables them to subsist on organisms
living in under-water mud. This apparatus varies in complexity among the
six species (of three genera). The Lesser Flamingo (Phmnicopterus minor) has
a filter sufficiently fine to permit the dredging of blue-green algre and diatoms.
The Greater Flamingo (P. antiquorum) (Fig. 426), on the other hand, eats
chironomid larvre and seeds: the two species can co-exist without direct com-
petition. The filter-feeding apparatus involves both spiny tongue and lateral
lamellre that vary from species to species. Other vertebrate filter-feeders are
ammocoet e larvre (p. 196), certain fishes, and the larvre of the toad Xenopus.
The beak is sometimes, as in the toucans, brilliantly coloured, and there may
also be bright coloration of the cere, as in the macaws, and of naked spaces on
the head, as in the cassowaries. In the latter the head is produced into a great
 PHYLUM CHORDATA

horny prominence or casque, supported by an elevation of the roof of the skull.

The cere is frequently absent. The nostrils are placed at the base of the beak,
except in Apteryx, in which they are at the tip.

7 9

FIG. 428.-Neognathre: Pedal specialisations. Not to scale. r. Cormorant (Phalacrocorax),

adapted to swimming and defence. 2 . Grebe (Podica of African swamps), with laterally lobed
toes adapted to diving, swimming and defence. 3· Jungle fowl (Gallus of India), running, scratch-
ing and spurred for aggression and defence. 4· Megapode (Megapodius of New Guinea), heavy-
limbed for incubator-building, as well as food-gathering, running and defence. 5· Hawk (A ccipi-
ter) grasping prey (note rough raised pads) and holding it while being torn with beak; also perching
and defence. 6. Ptarmigan (Lagopus of Greenland tundras), running and defence, and feathered
in relation to thermo-regulation. 7· Weaver (Quelea of African savannah), with 'typical' passerine
foot adapted to perching, hopping and sometimes defence. In this and a few other genera the
feet are used also to weave nesting material. 8. a ,b. Sand-grouse (Syrrhaptes of Central Asian
steppe), running, defence a nd thermo-regulation. 9 · Swift (A pus). Pamprodactyle foot adapted
for clinging to perpendicular surfaces. (For the unwebbed 'wader' foot see Fig. 445, p. 641, and
for that of the lily-trotters, Fig. 429.) (Red rawn after Thompson, Grasse, Friedmann.)

The essential structure of the wing-apart from its feathers-is very uni-
form. As a rule all three digits are devoid of claws, as in pigeons, but the
ostriches have claws on all three digits. The rheas have claws on the first and
sometimes on the second and third ; and the cassowaries, Emu, and kiwis
624 ZOOLOGY
(Fig. 6o8) on the second digit. The Crested Screamer (Chauna) and two other
species (and, as a rare abnormality, the Common Fowl and Goose) have them
on the first digit. With these exceptions, and that of the young of the Hoatzin
(Fig. 427), the hand of the hatched bird has lost all the characters of the fore-

FIG. 429. -Jacanid~e: Adaptation to the lily surface. The jacanas (lotus-birds, lily-trotters, or
'Christ-birds ') are often imagined to 'walk on the water', but are in fact exquisitely adapted for
life on floating leaves. The negligible body-weight is distributed by elongated anterior digits and
grossly hypertrophied hind-claws. The eggs are laid in a shallow nest on leaves or floating debris
and arc stone-coloured a nd bear a disruptive pattern of 'hieroglyphic' markings. The casque of
Irediparra gallinacea (above) is yellow, like the centres of the surrounding lotus flowers. The
extreme specialisation of jacanas has resulted in a curiously discontinuous distribution a m ong
living forms (see also Fig. 424, p . 6r z). Restricted to lily lagoons and lakesides in warm climat es,
t h ree species inhabit Africa and :VIadagascar, one (of several races) the Americas, a nd three Asia- ·
Australasia, ·of \vhich only one (above) reaches the Australian continent. (From photographs,
and B. M. (N. H .) specimens.)

foot. Besides the true claws, horny spurs are sometimes present on the carpo-
metacarpus (plovers, etc.). A recently extinguished pigeon, the Solitaire
(p. 557), had a knob-like protuberance 'the size of a musket-ball' which is said
to have made a rattling noise during the wing-whirring display.
There is almost every gradation in the proportional length of the hind-
limb, from birds in which nothing but the foot projects beyond the contour
 PHYLUM CHORDATA 6zs
feathers (and even the toes may be feathered), to the long-legged storks and
cranes, in which the distal part of the tibio-tarsus is covered with scales as
well as the foot (Fig. 428). In aquatic forms a web is stretched between the
toes. This sometimes includes all four digits, as in cormorants. Sometimes
it leaves the hallux free. Again, it may form a separate
fringe or lobe to each digit, as in coots and grebes. As
to the toes themselves, the commonest arrangement is
for the hallux to be directed backwards, and Nos. 2, 3,
and 4 forwards. Perhaps the most remarkable feet with
this arrangement are those of the jacanas or parras
(J acanidre), a widespread tropical and sub-tropical family
of Charadriiformes (p. 6r6). In these all four unwebbed
toes are grotesquely elongated and clawed, resulting in
a distribution of weight that enables the' Lotus-bird' to
walk on the floating leaves of water-lilies (Fig. 429).
Various species are known as Lily-trotters, or Christ-
birds-because they apparently walk on the water. In
owls No. 4 is reversible, i.e. can be turned in either
direction. In the parrots, wood-peckers, etc. , it, as well
as the hallux, is permanently turned backwards in the
zygodactylous condition. In swifts, on the other hand,
all four toes turn forwards. The hallux is frequently
vestigial or absent, and in the Ostrich No. 4 has also
disappeared, producing the characteristic two-toed foot
of that bird.
In some groups (e.g. nightjars, owls, and various
aquatic or semi-aquatic birds such as bitterns, cormo-
rants, gannets, and various waders) one claw on each
43o.-Neogna-
foot of certain species may be pectinated, i.e. it has taken thre:FIG.Toilet adaptation.
on a more or Jess comb-like structure. This varies A comb-like toilet claw
has arisen by conver-
greatly in complexity, perhaps reaching its highest ex- gence in several unrelated
pression in bitterns (Fig. 430). Such feather-cleansing at erns,
vian groups (e.g . bit-
as above). See
structures present an interesting analogy with the digital also pp. 739 , 846 for
ana logous structures in
toilet implements of mammals. mammals.
Toilet-claws in birds are extremely erratic in their
distribution. Thus, the Barn Owl (Tyto alba) possesses them, whereas many
other owls do not. Among waders they are lacking in the Bar-tailed godwit
(Limosa lapponica), yet present in the congeneric Black-tailed species (L.
limosa).
It is probable that the possession of cleansing comb-claws is broadly related
to diet: they appear to have arisen most often in species that feed on mucus-
covered shore- or water-organisms. Certainly, in the Bittern (Botaurus
626 ZOOLOGY
stellaris) powder from the powder-down patches (see below) appears to reduce
the viscosity of adherent mucus and thus make it more easily disposable by the
toilet claws. It has been suggested, but not proved, that in moth-eating night-
jars such specialised claws are used to clear moth scales from the hair-like
tactile processes surrounding the gape.
pterylosis.-With the exception of the penguins, most birds (excluding
flightless' ratites' (p. 6o6)) have the feathers arranged in distinct feather-tracts
or pterylce, separated by apteria or featherless spaces. These are commonly
much more distinct than in the pigeon (Fig. 386), and their form and arrange-
ment are of importance in classification. In the flightless birds apteria are
usually found only in the young, the adult having a uniform covering of feathers.
Such birds also have nothing more than the merest trace of hooklets on the
barbules, so that the barbs do not interlock and the vanes of the feathers are
downy or hair-like. In penguins (p. 6w) the wing-feathers are degenerate. In
nearly all neognathine birds except passerines (p. 6zo) there is an absence of
the fifth secondary wing-quill: the condition of diastataxy or aquintocubitalism.
The distribution of this phenomenon is erratic.
Many birds are naked when hatched, but in most cases the body is more or
less completely covered by temporary feathers, the nestling-down. These are
of various forms, but always have a short axis, soft loose barbs devoid of inter-
locking apparatus, and, except in the Emu, no after-shaft (Fig. 431). They
are succeeded, as already descy;ibed, by the permanent feathers.
Many birds, such as the swans, possess down-feathers or plumulce throughout
life, interspersed among and hidden by the contour feathers or pennce. Semi-
plumes are down with a well-developed axis. Filoplumes have an elongated
axis and vestigial vexillum. A singularly specialised type of feather is the
powder-down, well-developed in tracts or powder-down patches in herons,
parrots and cockatoos, birds-of-prey, pigeons, and especially frog-mouths
(Podargus). This down gives off a powder not unlike talc in its imperme-
ability to water. The powder is not formed by the breakdown of a part of the
fully developed feather, as has been sometimes claimed, but is a derivative of
the innermost walls of the Malpighian layer of the feather papilla.
In many birds there springs from the under-side of the quill, near the
superior umbilicus, a second vane, the after-shaft (Fig. 431), usually smaller
than the main shaft, but sometimes of equal size. Both among flying and
flightless birds there are genera with double-shafted feathers as well as allied
forms in which the after-shaft is rudimentary or absent. Feathers are always
shed or moulted at regular intervals, as a rule annually after the breeding season,
though some species have a second lesser moult later on. During moult the
old feathers drop out and new ones are formed from the same pulps.
Feathers may be anointed in some species with greasy secretion from the
coccygeal, uropygial, oil- or preen-gland, a bilobular structure on the dorsal
 PHYLUM CHORDATA 627
surface of the body above the base of the tail (p. 566). The function of the
secretion is unknown. It has been variously suggested that it may waterproof
the feathers, produce a specific recognition scent, and that
it elaborates ergosterol, which, when exposed on the
feathers, is transformed by sunlight to Vitamin D. It is
assumed that the vitamin would be ingested during preen-
ing. Grasse suggests that birds which are addicted to ex-
cessive feather-eating may be suffering from Vitamin-D
deficiency.
The colours of feathers present great variety. Black,
brown, red, orange, yellow, and occasionally green are due
to the presence of definite pigments, i.e. are absorption
colours. The turacos possess the red copper-containing
pigment turacin which is soluble in dilute alkali but not, as
is often claimed, in ordinary rain-water! In this colourful
group the green pigment turacoverdin also occurs. In avian
plumage, white, and in some cases yellow, is produced by
the total reflection of light from the spongy air-containing
substance of the feather, there being, as in nearly all other
natural objects, no such thing as a white pigment. Blue,
violet, and in some cases green are produced by the light
from a brown pigment becoming broken up as it passes
through the superficial layer of the feathers in its passage
to the eye. No blue or violet pigments occur in feathers ,
and green pigments are rare. The beautiful metallic tints
of many birds are structural colours. In such feathers the
colour changes according to the relative position of the bird
and of the eye of the observer with regard to the source of
light. · The American flamingo (Phamicopteru s ruber) gets
its pink and vermilion integumentary (skin, beak and
feathers) colours from dietary carotenoids. The colours
gradually fade in captivity unless the diet is carefully
regulated. There is some evidence that crustacea are an FIG. 431.- Casu-
Feather struc-
important source of carotenoids in nature. Fox has evi- arius:ture. In cassowaries
dence that the metabolic fractionation of dietary carote- the aftershaft is al-
m ost as long as the
noids seems to involve oxidative processes but that some primary shaft. The
barbs
astaxanthin may pass unchanged from crustacean to the aunconnected
re probably a neo-
avian integument. tenous feature (see
text). (After Head-
There is also infinite variety in the colour pattern of ley.)
birds. In many the colouring is concealing, harmonising
with the environment. In the ptarmigan (Lagopus mutus) the colour changes
with the moult from greyish-brown in summer to white in winter, the former
628 ZOOLOGY
plumage helping to conceal the bird among herbage, the latter on snow. Fre-
quently, as in pheasants and birds-of-paradise, the female alone is protectively
coloured, while the male presents the most varied and brilliant tints, enhanced
by crests, plumes or tufts of feathers, lappets of skin, etc. (Fig. 38r). The
factors influencing sexual dimorphism differ from group to group and are some-
times complex. Comb-growth in the Domestic Fowl in both cock and hen is
under the control of androgens from testis and ovary. The comb is smaller in
ovariectomised females than in those left intact. Thyroxin (p. rsr), as well
as sex hormones, is influential in feather development and pattern. When the
hens of certain breeds of fowl are unsexed the next growth of plumage is male-
type, whereas that of the castrated cock is unchanged; the difference is con-
trolled by restrogen. There is evidence that in some species the female
plumage is neutral, whereas the male colour is controlled by pituitary, not sex,
hormones. In the Herring Gull (Larus argentatus) the cock-type plumage is
under the influence of androgens. The plumage condition of some species
(e.g. House Sparrow, Passer domesticus) appears to be under genetic control,
in that in neither sex does castration cause change, nor does the administration
of sex hormones. Parenthetically it may be mentioned that the colour of the
beak and various soft parts also can be under either genetic or endocrine con-
trol. Witschi says that the change of bill colour from black to yellow in
the Common Starling (Sturnus vulgaris) is a most delicate indicator for andro-
gens: it 'heralds the start of the breeding season at a time when testicular
changes are barely observable under the microscope and the deferent ducts
still remain in the eclipse condition'. The colour reaction occurs about two
weeks earlier in the males than the females.
Sexual dimorphism is common in monogamous as well as polygamous
species. In conjunction with specific postures and calls, colours and erectile
plumes become sign stimuli which evoke or release specific reactions, and indeed
whole patterns of behaviour in rivals and mates. Such ornaments as the bars
and ?pots on the wings and tail, fully exposed only during flight in many gre-
garious birds, and often widely different in closely allied species, are probably
recognition marks which enable stragglers to distinguish between a flock of
their own and other species.
Endoskeleton.-The vast majority of birds have saddle-shaped or hetero-
crelous cervical and thoracic vertebrre, but opisthocrelous thoracic vertebrre
occur in the Impennre (penguins), Laridre (gulls), some Charadriiformes (e.g.
stone-curlews and godwits), and some Pelecaniformes (e.g. cormorants and
darters). In the Ichthyornithes alone they were biconcave.
The spaces between adjacent centra are traversed by a meniscus with a sus-
pensory ligament, as in pigeons (p. 573). The number of vertebrre is very
variable, especially in the cervical region, where it rises to twenty-five in swans
and is reduced to nine in some song-birds. There is very commonly more or less
 PHYLUM CHORDATA 629

fusion of the thoracic vertebrre, and the formation of a rigid synsacrum by the
concrescence of the posterior thoracic, lumbar, sacral, and anterior caudal
vertebrre is universal. The posterior cervical and anterior thoracic vertebrre
commonly bear strong hypapophyses or inferior processes for the origin of the
great flexor muscles of the neck. The number of true sacral vertebrre varies
from one to five. A pygostyle, formed by the fusion of varying numbers of
caudal vertebrre, is of general occurrence, but is small and insignificant or
absent in certain 'ratites' (excluding, for example, the Ostrich).

(\
A scp
sp . IT
c

FIG. 432.-Neomithes: Sternum and pectoral articulation. K eel (cari na te) a nd raft (ratite)
types. A, Gallus (common Fowl, young); B, Turdus (Thrush); C, Vultur (Vulture); D , P ro-
cellaria (Pet rel); E, Casuarius (Cassowary). ant. lat. pr. anterior lateral process; car. carina;
cl. clavicle; cor. coracoid; jon . fontanelle ; fur. furcula ; obl. lar. pr. oblique latera l process; os.
p a ired ossification of sternum in E ; os. r , carina! ossification in A; os. 2, os. J, la t eral ossifications;
post. med. pr. posterior median process ; post. lat. pr. posterior lateral process ; pr. cor. pro-coracoid;
scp. scapula ; sp. spina sterni. (A and E , after W . K. Parker; B , C, and D, after Bronn.)

The ribs are always double-headed. The sternal ribs are ossified, not merely
calcified, and are united with the vertebral ribs by synovial joints. Ossified
uncini are nearly always present, and usually become ankylosed to the vertebral
ribs.
The sternum of most flying birds is a broad plate, concave dorsally from side
to side, and produced ventrally into an antero-posterior keel which is ossified
from a distinct centre (Figs. 432, A, os. r). The posterior edge of the bone is
either entire (D), or presents on each side of the keel one or two more or less
deep notches (A, B) or foramina (C). In the flightless genera (E) the keel is
 ZOOLOGY
either absent or reduced to the merest vestige, and there is no trace of the carina!
ossification in the young. External to the coracoid grooves the anterior edge
of the sternum is produced into larger or smaller antero-lateral processes (ant.
lat. pr.). In the Emu these are of great size and are closely applied to the
pericardium.
It was upon the characters of the raft-like sternum that the group' Ratitre'
was founded, but the difference between them and the keel-breasted 'Carinatre'
in this respect is not absolute, the ratite condition having been acquired by
many 'Carinatre' which have lost the power of flight. The keel is very small in
the flightless rails, Gallirallus, Notornis, and the extinct Aptornis from New
Zealand. It is practically absent in the Dodo (Didus) and Solitaire (Pezo-

Nv/T.JJT.ff Nv.Y'
,\:Orb.,<" Tr I A l.Spk
dpr l
Ec.Elh

p,"' Na
.E.:c.Oc

nLC op Ec.Eilt. Pm~

D~n.l

FIG. 433.-Apteryz: Slmll. Specimen from young kiwi in lateral view in which the cartilagin·
ous parts are dotted. Al. Spll. alisphenoid; Ang. angular; Cn. I, en. 2, condyle of quadrate;
Dent. dentary; d. pr., d. pr. descending processes of nasal and frontal; Ec. Eth. ecto-ethmoid; Ex.
col. extra-columella; Ex. Oc. exoccipital; Fr. frontal; ju. jugal; Lac. lachrymal; lac. for. lachry-
mal foramen; Na. nasal; na. ap. nasal aperture; Nv. II, III, IV, optic foramen, transmitting
also the 3rd and 4th nerves; Nv. V', foramen for orbito-nasal nerve ; N v. VII', for facial; Pa.
parietal; Pal. palatine; pa. oc. pr. par-occipital process; P . mx . premaxilla; Pr. ot. pro-otic;
Qu. ju. quadratojugal; Qu. (orb. pr.) orbital process of quadrate; S. Orb. F . supraorbital foramen;
Sq. squamosal. (After T. J. Parker.)

phaps), two gigantic extinct pigeons from Mauritius and Rodriguez. It is like-
wise nearly absent in the Kakapo or Ground-parrot (Strigops) of New Zealand;
in the extinct Giant Goose (Cnemiornis) from the same country; and in Hes-
perornis. The absence of the carina must be considered as an adaptive modi-
fication of no phylogenetic significance.
The entire super-order of penguins (Impennre) and the extinct Great Auk
are also flightless, but their wings, instead of being functionless, are modified
into powerful swimming paddles (Fig. 422). There has therefore, in these
cases, been no reduction either of the pectoral muscles or of the carina.
The skull of birds is generally remarkable for its huge orbits separated by a
thin interorbital septum, and for the comparatively small size of the ethmoid
bone and its turbinals. A striking exception is afforded by the kiwis (Apteryx),
in which the orbits (Fig. 433) are small, while the olfactory chambers (Ec. Eth.)
extend backwards between the eyes. The orbits are therefore separated from
 PHYLUM CHORDATA

t;Ju.to~pr)

•• A'vl.r.r
Oc.Por ~ /nl.Car
OcCn \ A' v Xlf
SOo
FIG. 434.-Aplet·yx: Skull. Speci- FIG. 435. -Anas: Skull. Duck,
men from juvenile kiwi, Apteryx mantelli skull from palatal aspect. a. p. f . an-
(pa latal aspect). The cartilaginous parts terior palatine foramen ; b. o. basi-
are dotted. B . Oc. basi-occipital; B. occipita l; b. pg. basi-pterygoid
ptg. pr. basi-pterygoid process; B. Tmp. process; b. s. basi-sphenoid; b. t.
basi-temporal; Ec. Et/1. ecto-ethmoid; basi-temporal; e. o. ex-occipital;
Eus. T. Eustachian tube; E x . Col. extra- eu. aperture of Eustachian tube;
columella ; Ex. Oc. ex-occipital; Int. Car. f. m. foramen magnum; i. c. in-
carotid foramen; Mx. maxilla; Nv. VII', ternal carotid foramen; j. jugal;
fora men for facial; Nv. I X. X, for glosso- mx. maxilla; mx. p. maxillo-
pharyngeal and vagus; Nv . XII, for palatine process ; oc. c. occipital
hypoglossal; Oc. Cn. occipital condyle; condyle; pl. palatine; p. n. post-
Oc. For. foramen magnum; Pal. pala- erior nares; px. premaxilla; q.
tine; pa. oc. pr. par-occipital process; P. quadrate; qj. quadratojugal; v.
mx. premaxilla; Pig. pterygoid; Qzt. vomer; IX, X, foramen for nir.th
(orb. pr.) orbital process of quadrate; Qu. and tenth nerves ; XII. for twelfth
(ot. pr.) otic process; Rost. rostrum; 5. nerve. (After Wiedersheim.)
Oc. supra-occipital; 5. Orb. F. supra-
orbital foramen; Sq. squamosal; Vo.
vomer. (After T. J. Parker.)

one another by the whole width of the olfactory organ (seep. 641). The same
thing occurs, to a less degree, in moas. In its essential features the skull is re-
markably uniform throughout the class. The rounded form of the brain-case,
more or less concealed externally by ridges for the attachment of muscles; the
upper beak, composed mainly of great tri-radiate premaxillre; the single, small
 ZOOLOGY

rounded occipital condyle; the slender maxilla-jugal arch; the large para-
sphenoidal rostrum; the freely articulated quadrate, with its otic, orbital, and
articular processes; the absence of the reptilian post-frontals; and the early
ankylosis of the bones-all these characters are universal among birds. There
are, however, numerous differences in detail, some of which, connected with the
bones of the palate, are of importance in classification.
In the large flightless forms and the tinamus there are large basipterygoid
processes (Fig. 434, B. ptg. pr.) springing, as in lizards, from the basisphenoid,
and articulating with the pterygoids near their posterior ends. The vomer
(Vo.) is large and broad, and is usually connected posteriorly with the palatines
(Pal.), which do not articulate with the rostrum. The maxillo-palatine pro-
cesses are comparatively small,
and do not unite with one another
or with the prevomer. This
arrangement of the bones of the
palate is sometimes called dromceo-
gnathous. In many birds, e.g. the
pigeon and the fowl, the basi-
pterygoid processes are either ab-
sent or spring from the base of
the rostrum. The vomer is small
and pointed, or may be absent,
and the palatines articulate pos-
FxG. 436.-Psittacide: Beak and skull
articulation. In parrots (e.g. Ara, Macaw, teriorly with the rostrum. The
above) the hinge between beak and skull enables
mastication movements of a unique character.
maxilla-palatines do not unite with
(From a photograph by A. Hamilton.) one another. These peculiarities
characterise the schizognathous
arrangement. In passerines a similar arrangement obtains, but the prevomer
is broad and truncated instead of pointed in front. This gives the cegitho-
gnathous arrangement. Lastly, in the storks, birds of prey, ducks, and geese,
etc., the maxilla-palatines (Fig. 435, mx. p.) fuse with one another in the middle
line, often giving rise to a fiat, spongy palate and producing the desmognatltous
arrangement.
The most remarkably specialised form of skull is found in the parrots (Fig.
436). In many birds the nasals and the ascending process of the premaxillre
are very thin and elastic where they join the skull, and there is an unossified
space in the mesethmoid, so that the upper beak is capable of a considerable
amount of movement in the vertical plane. Thus there occurs a true cranio-
facial joint or hinge between the upper beak and the skull. This gives the
bird a 'sliding palate'. This mechanism is best developed in the parrots,
allowing the curious upper beak movement that is so striking in the living bird.
When the mandible is depressed, the contraction of the digastric muscle causes
 PHYLUM CHORDATA

a forward movement of the lower end of the quadrate, which pushes forwards
the maxilla-jugal bar and the palatines and pterygoids, the latter sliding upon
the rostrum. Both the maxillre and the palatines are articulated in front with
the premaxilla, and together push it upwards. In this way depression of the
lower produces an automatic raising of the upper jaw. The great size and
strength of both premaxilla and mandible are remarkable, as also is the fact
that the orbit is completely surrounded by bone, a backward process of the
lachrymal being joined beneath it by a forward process of the frontal.
The mandible contains in the young bird the six bones on each side charac-
teristic of reptiles. The coronoid is, however, often absent. As a rule the
head of the quadrate articulates with the roof of
the tympanic cavity by a single facet in the large
A
flightless birds, by a double facet in the rest.
The hyoid always agrees in essential respects with
that of the pigeon. In woodpeckers the posterior
cornua are curved round the head and attached
to the skull in the neighbourhood of the right
nostril, allowing the extreme protrusion of a long
and flexible tongue (Fig. 441, p. 636).
The structure of the shoulder-girdle (Fig. 437)
furnishes one of the most distinctive characters
between ratite and non-ratite forms but, as with pr.cor.fe9
the sternum, the differences are adaptive and not
of phylogenetic significance. In most of the latter F1G.43 7 . -Aptery~: Shoulder
girdle. (Left). A, anterior; B ,
both coracoid and scapula are large and united lateral (out er) surface. acr.
acr. cor. acrocora.
with one another by ligament. The coracoid has acromion;
coid; cor. coracoid; gl. glenoid
an acrocoracoid and the scapula an acromion pro- cavity; pr. cor. lg. procoracoid,
reduced to a liga ment ; scp.
cess. The coraco-scapular angle is acute. A scapula. (After T. J. Parker.)
furcula occurs. In the large flightless forms the
coracoid (Fig. 432, cor.) and scapula (scp.) are much reduced in proportional
size and are ankylosed with one another. The acrocoracoid (acr. cor.) and
acromion (acr.) processes are reduced or absent. The coraco-scapular angle
approaches two right angles. There is no furcula, although separate vestiges
of clavicles are present in the Emu and cassowaries. In the almost flightless
song-bird Atrichornis, the clavicles are reduced to mere splints and the furcula
has been lost altogether. At the same time, the absence of clavicles is not
always associated with reduced powers of flight, as, for example, in some parrots.
In some of the moas (Pachyornis, etc.) the shoulder-girdle is wholly absent.
But, as in the case of the sternum, the distinction is not absolute. In
R esperornis, the Dodo, the Solitaire, Aptornis, Notornis, Gallirallus and
Cnemiornis the bones of the shoulder-girdle are proportionally small. The
coraco-scapular angle exceeds 90° and in some cases, such as certain
VOL. II. RR
 ZOOLOGY
parrakeets and owls, the furcula is slight, or represented by paired vestiges, or
absent.
In most adult birds the procoracoid is reduced to a process on the dorsal end
of the coracoid, but in the Ostrich and in the
embryo of Apteryx it is well developed and
separated by a fenestra from the coracoid. A
small bone, the accessory scapula, is sometimes
found on the outer side of the shoulder-joint.
The variations in the structure of the wing are
mostly matters of proportion, but a remarkable
flattening of all the bones is very characteristic of
penguins, which are further distinguished by the
- d~• presence of a sesamoid bone (p. 74), the patella
ulnaris, in place of the olecranon process. In the
Emu and kiwis the first and third digits of the
normal wing have disappeared during develop-
FIG. 438.-Neornithes: Em-
ment, the middle one alone remaining. In the
bryonic forelimb. Tern (Sterna) moas (Fig. 431) no trace of a wing has been found,
showing pentadactyle hand in
which the fifth digit does not and in one species only is there even a trace of
develop. dg. 1-4, digits ; lm . the glenoid cavity. In the embryos of several
humerus; ra. radius ; ul. ulna.
(After Leighton.) birds an additional digit has been found on the
ulnar or postaxial side (Fig. 438, dg. 4). This
brings the total number of digits up to four, the fifth of the pentadactyle
hand alone being unrepresented.
The simplest type of pelvic girdle is found in Apteryx (Fig. 439) and the

. FIG. 439-- A1>terf1:r: Pelvic

gudle. (Left.) a. acetabulum;
it. ilium; is. ischium; p.
pectineal process; pt . pubis.
(From \>Viedersheim, after
Marsh.)

tinamous, in which both pubis and ischium are free along their whole length,
as in dinosaurs. In the Emu and cassowaries the pubis and ischium unite by
cartilage or bone at their posterior end with the ilium. In most birds the
 PHYLUM CHORDATA

union between the two last is extensive. The deep ischiatic notch is replaced
by a small foramen. In the embryonic condition the ilium has a very
small pre-acetabular portion. The pubis and ischium are nearly vertical.
There is a distinct pectineal process (pp) (retained in Apteryx (Fig. 439, p))
and the whole pubis is singularly like that of a dinosaur. In the Ostrich alone
the pubes unite in the middle ventral line to form a symphysis. Rhea presents
the unique peculiarity of a dorsal symphysis of the ischia, just below the
vertebral column. In the Emu the posterior end of the pubis gives off a slender
process, which extends forwards close to the
ventral edge of that bone and probably re-
presents the epipubis of reptiles.
In the sacral region of the larger running
birds there occurs a spinal enlargement reminis-
cent, in very minor degree, of the remarkable
condition found in certain dinosaurs (Fig. 342,
p. 5II).
The bones of the hind-limb are very uniform
throughout the class, but the form of the tarso-
metatarsus of penguins is worthy of notice. It
is short and wide. Its three constituent meta- dial
tarsals, though fused, are clearly distinguishable .Nl.lal.r
throughout their whole length, and the resem-
.Mt.lsl.2
blance to the homologous part in Iguanodon is
very striking. In the embryo (Fig. 440) a vestige
of the fifth digit (mt. tsl. 5) has been found in the
form of a small rod of cartilage on the postaxial FIG. 44o.- Apter7J:ll: Hind-limb
or fibular side. One or two free centralia may and pes. Embryo, in dorsal a spect .
dist. distale; Fe. femur; Fib.
occur in the mesotarsal joint. fibula; fib. fibulare; Mt.tsl. 1-5,
metatarsals; Tib. tibia; tib. tibiale.
Except in certain diving birds (e.g. the puffins) (AfterT. J. Parker.)
the skeleton is always more or less pneumatic
(Fig. 387). Often all the bones contain air except those of the fore-arm, hand,
shank, and foot. In Apteryx, penguins, and some song-birds the skull alone is
pneumatic. In the hom bills every bone in the body appears to contain air. As
the passerine bird matures, an erosive process occurs, converting the cranium
into a two-layered vacuolated structure in which innumerable little columns
of osseous tissue separate a system of air-spaces. This pneumaticity of
the skull is probably an adaptation towards keeping the head light and the
centre of gravity well back during walking, swimming, and flying. The
fragility of their skulls occasions great mortality from cerebral hremorrhage
among cage birds. Although there is no direct relationship between pneu-
maticity and flying power in specific groups, the typically avian bone structure
undoubtedly greatly reduces weight without undue loss of strength. The
 ZOOLOGY
dried skeleton of a large frigate-bird (Fregata), for example, weighs only
about four ounces although the wing-span may be approximately seven feet.
The bones, in fact, weigh less than the plummage (Murphy).
Alimentary Canal and Associated Structures.-In all existing Neornithes the
jaws are covered by a horny beak and there are no teeth. That teeth were
present in the more primitive birds, and have gradually been lost seems certain
from the fact that the Cretaceous birds were toothed. In Hesperornis (Fig.
418) for example, there are long conical teeth in both jaws, set in continuous
grooves. In Gastornis (Order Diatrymiformes) and in Odontopteryx (an extinct

connect;ve
&issue

geniohyoid
muscle

geniohyoid
muJc/e

c/eidolhyroid
muscle

FIG. 441.-Cecinus: Feeding specialisations. The convulsive movement of the probing tongue
of the European Green Woodpecker (C. viridis) is controlled principally by the massive genio-
hyoid muscle which is closely bound to the hyoid cornua. These extend around and above the
skull into the right moiety of the upper element of the beak. In the North American Dryobates
the apparatus is not housed in the beak, but curves grotesquely around the right eye to the region
of the ear. For the more typical hyoid apparatus see Fig. 393, p . 576. (Modified after Leiber.)

carinate form allied to the Anseriformes), the margins of the bony jaws are
produced into strong, pointed, tooth-like prominences.
The young of many species possess bright yellow or otherwise coloured
gapes. A few exhibit small light-reflecting areas which, along with head-
movement and noisy squeaking, helps ensure that the parents will deposit the
food. The tongue shows a great deal of variation. It may be relatively simple
as in pigeons, horny as in parrots, brush-like as in honey-eaters, tube-like or
brush-like as in humming-birds, or sticky at the tip as in ant-eating ground-
woodpeckers. The tree-woodpeckers have an elongated barb-tipped tongue
variously from two to five times the length of the powerful beak with which they
enlarge the holes made by wood-boring organisms (Fig. 426, p. 621; Fig. 441).
 PHYLUM CHORDATA

Buccal glands, some at least of which produce saliva, occur. The sub-
lingual glands of members of the Oriental, Indonesian, and Pacific genus
Collocalia (Micropodidre) become seasonally hypertrophied (Fig. 442) and
produce a copious secretion of saliva that is used as nest-cement by the
swiftlets, and as the basis for bird's-nest soup by the Chinese. European
(Micropus), African (e.g. Colletoptera), and probably many other species of
swifts also use saliva as nest-cement. The Palm-swift (Cypsiurus) of Africa is
said also to glue its eggs to the nest with saliva. As a result, the palm leaves
on which the nests are built can blow into any position without loss of eggs
(Bannerman).
In the alimentary canal the chief variations concern the size of the crop,
creca, gizzard, and the extent of the intestine. In tubinarine birds (petrels,
etc.) the proventriculus is especially
large, and the surface of the sec-
retory epithelium is increased by
longitudinal ridges. The cells of
the proventricular glands contain a
concentration of lipids such as occur
only sparingly in corresponding
situations in other birds. It is
possible that this material is the
precursor of the oily substance that
frightened petrels ejaculate at in-
truders. In grain-eating birds the FIG. 442. -Collocalia: Nest-cement production.
The sublingual salivary glands of Oriental and
gizzard has thick muscular walls Pacific swiftlets (Collocalia) become seasonally
as shown by C. brevirostris (left)
and is lined by a thickened horny hypertrophied
which builds some weeks before C. francica (right).
epithelium, as in pigeons (p. 584). Both specimens were taken at the same cave on the
same date. Art. Articular bone ; Oes. resopha-
In flesh-eaters, such as hawks and gus; S. G. salivary gland; T r. trachea. (Modi-
owls, it is thin-walled and lined fied after Marshall and Folley.)
with epithelium of a more usual
character. Nectar-eating birds possess relatively simple stomachs with ex-
tremely thin walls. In some birds, the indigestible hard parts of the prey are
ejected from the throat. Owls swallow mice and small birds whole and eject
the bones enclosed in a covering of fur and feathers. Piscivorous birds eject
pellets of scales and bones and some insect-eaters likewise regurgitate indigest-
ible chitinous exoskeletal structures.
In the Common Fowl and many other birds the creca are of great length.
A gall-bladder is usually present. The speed of digestion of course. varies
according to the diet, but in general digestion seems to be more rapid than in
most animals. In some frugivorous species the 'fieshless' stone of a berry may
be voided less than fifteen minutes after it is swallowed. Small carnivorous
birds are said to digest their prey within a few hours. Speedy digestion and
 ZOOLOGY
absorption are essential in all animals of small volume, large surface, and high
metabolic activity, and birds particularly fall into this category. 1
Respiratory and Vocal Organs.-The rings of the trachea are always ossified.
The respiratory tube is frequently deflected to one side by the crop, as in pigeons,
and may undergo such an increase in length as to extend beneath the skin of
the abdomen, or even into the keel of the sternum (Fig. 443). In the Whooper
Swan (Cygnus) and certain cranes, the looped trachea probably functions as a
resonator. An aberrant resonating chamber is found in the North American
Heath Hen (Tympanunchus cupido). The anterior end of the resophagus is
enlarged into a vocal sac which can be inflated only by air that comes up from
the respiratory system through the pharynx.

FIG. 443·- Grus: Vocal

specialisations. The sternum
in certain cranes (e.g. G. anti-
gone) is excavated to accom-
modate the convoluted trachea.
A convergent resonating
arrangement has been evolved
in swans. (Redrawn after
Portmann.)

The syrinx is either tracheo-bronchial, as in the pigeon, i.e. formed by the

distal end of the trachea and the proximal ends of the bronchi, or is exclusively
tracheal (Fig. 444) or exclusively bronchial. In singing birds it is complex,
provided with numerous muscles, and often more powerful in the male. In
the mute Turkey VuJture (Cathartes aura) the syrinx is absent altogether.
The lungs are always firmly fixed to the dorsal body-wall by a pulmonary
aponeurosis, and are only slightly distensible. The general arrangement of the
air-sacs has been described in the pigeon (p. 586). In A pteryx the abdominal
air-sacs are small, and are completely enclosed by the oblique septum, and do
not extend into the abdominal cavity among the viscera. The bronchi send
off branches at right angles.
1 Several Australian and no doubt other species eat with impunity fruits that kill mammals.
It might be expected that they detoxicate the poisonous principle in the iiver, but apparently t his
is not so. If dogs or cats eat the bones of Bronze-wing Pigeons which have eaten the seeds of
the heart-leaved poison (Gastrolobium bilobum), one of the Western Australian Leguminos~. they
die after convulsions and paralysis. The flesh of the pigeons is harmless to Man and dog. Here
we may have a remarkable aspect of drought adaptation. It enables birds to take advantage of
all possible foods in relatively unsust enable areas.
 PHYLUM CHORDATA

The Circulatory Organs agree in all essential respects with those of the
pigeon. Their most characteristic features are the large size of the heart, the
muscular right auriculo-ventricular valve, the atrophy of the left aortic arch,
and the vestigial character of the renal portal system. The red blood-cells are
always oval and nucleated.
Nervous System and Organs of Special Sense.-The brain is also very uniform
in structure, being characterised by its short, rounded hemispheres, large folded
cerebellum produced forwards to meet the hemi-
spheres, and laterally placed optic lobes. In the
embryo the optic lobes have the normal dorsal
position, and the whole brain resembles that of a
reptile. In Apteryx (see below) in correlation with
the reduction of the eyes, the optic lobes are very
small, and are situated on the under side of the
brain. Above the anterior commissure is a small
bundle of fibres which is probably the homologue of
the hippocampal commissure of mammals.
A bird's eyes sometimes weigh more than its
brain. A few relatively small hunting birds, such
as certain hawks and owls, have larger eyes than
Man. The avian eye contains no important struc-
ture absent in the Reptilia (taken as a whole), but it
has reached a grade of efficiency higher than that of
any other group. This is, of course, related t o the
flying habit; and it is a commonplace that, alone of Fig. 444·- Cyanochen:
the major groups, the Aves contains no member that Vocal specialisation. Male
African Goose (C. cyanopterus
has lost the power of sight. Both the muscles of the with the additional syringeal
iris and the ciliary muscles (of Crampton and elaboration of an osseous bulla
(4) improving resonation. I.
Brticke) , which procure accommodation by changing ypsilotracheal, 2. tracheo-
lateral, and 3· sternotracheal
the shape of the unusually soft lens, are striated, muscles, 5· inter-bronchial
and therefore relatively fast in their operation. The aperture, 6. resophagus.
(Redrawn after Grasse.)
shape of the eyeball (comparatively flat to ' tubular'),
and the corresponding arrangement of the internal structures, are related to the
habits of the species. For example, in raptores the unusually long axial dis-
tance between lens and retina, and the broadening of the retinal area, probably
allow a great er accuracy of vision in these diurnal hunters. The pit-like fovea,
concerned probably with the detection of movement, is often exaggerated in
birds. The retina of diurnal birds contain relatively few rods and a com-
paratively vast number of cones with coloured oil-droplets, the differential pro-
portions of which help accentuate the appreciation of various colours in the
environment. There is no truth in the repeated statement that diurnal birds
are blue-blind, but it is probable that many of them are relatively insensitive
 ZOOLOGY

to blue light rays. Such rays probably make distant small prey comparatively
hazy and difficult to distinguish. A kestrel can see the movement of a field-
mouse from a height at which most animals could probably detect nothing.
In the eyes of most nocturnal birds cones are also present, but rods pre-
dominate. There is also great variation in the number of cones from species
to species. A sparrow possesses only about 40o,ooo per square millimetre of
fovea, as against the r,ooo,ooo found in certain hawks, which have been
claimed to possess a resolving power about eight times that of Man (Walls).
The most inefficient avian eye appears to be that of the nocturnal kiwis of
New Zealand. Most night-hunting birds have eyes exceedingly 'well-adapted
to take advantage of the small amount of light available to them, but the kiwis,
on the other hand, have a powerfully developed olfactory sense (see below). In
these, even though the orbit remains large, the eye has become reduced and
myopic. The eyes of kiwis are probably unique in lacking the pecten (Fig. 409,
p. 594), although a vestige of this enigmatic organ occurs in the embryo.
The pecten, the homologue of which is first obvious in fishes, occurs promi-
nently as a pleated projection of highly vascular pigmented tissue from the
optic nerve ('blind spot') into the vitreous humour. The blood vessels of
which the pecten is largely composed are appreciably bigger than capillaries.
The pecten is smallest in nocturnal birds and biggest in far-sighted diurnal
hunters, in which it may extend from the retina almost to the lens. Although
the pecten was observed in the r8th century, there is still no agreement con-
cerning its function. Originally, and possibly still, it may have provided
nourishment for the internal structures of the eye by the diffusion of blood
solutes into the vitreous. Of the score or more other theories concerning its
function, one of the most plausible is that it assists in movement-perception
because, in strong illumination, the shadows of its pleats create numerous tem-
porary blind-spots, and so increase the number of on and off photo-stimuli on
the retina. Such may aid the detection, and continued observation of small,
moving distant objects. It is a matter of common experience that the rapid
blinking of the human eye may aid the perception of dimly seen objects by the
production of discontinuous images.
The visual field of birds differs in relation to their habits and the position
and shape of their eyes. Pigeons, Gallus, and passerines with laterally placed
eyes (hunted birds) have a total visual field of sometimes more than 300 degrees,
compared with a field of about roo degrees in the predacious owls. In general
the avian eyeball has a relatively restricted movement. Owls, which possess
binocular vision, have increased the size of their eyes to an extent which makes
it impossible for them to move in their sockets. In compensation there has
been developed an extraordinary cervical mobility which allows the bird to
rotate its head to such a degree that it can look backwards. Most birds move
their heads when keeping a moving object under observation. Many shore-
 PHYLUM CHORDATA

birds (e.g. snipe), however, have eyes placed in a relatively posterior position.
This enables them to see behind with little adjustment. A most peculiar eye
arrangement adapted to habit and
habitat is found in the bitterns, which
'freeze' among rushes when they ob-
serve a predator. As the streaked
throat is inclined upwards in line with
the vertical rushes, the eyes focus
downwards, enabling the motionless
camouflaged bird to see anteriorly, and
focus binocularly, below the tilted beak
(Fig. 445).
All birds possess a functional
nictitatingmembrane. Inmanyspecies
it is transparent. It has been suggested
that this membrane is drawn pro-
tectively across the eye during long
flights, but there is as yet no evidence
that such is the case.
The olfactory sense in most birds is
notoriously poor, but Apteryx is dis-
tinguished by the high development of
the olfactory chamber, which extends
from the tip of the beak to the level of
the optic foramina. The turbinals are
large and complex, and there is a vestige
of the cartilage of Jacobson's organ.
Also, sea-going Procellariiformes (pet-
rels, albatrosses), which emit a charac-
teristically strong and peculiar odour,
possess remarkably developed olfactory
FlG. 445.- Botaut-us: Adaptation to the
organs. Nasal tubes open about mid- reed-beds. The European Bittern (B. stellaris)
is disruptively
way along the beak and lead into thus nest when approached. patterned and 'freezes ' on its
The eyes are so situated
protected capacious, paired, olfactory that the bird can see under its elevated beak.
This and the longitudinal ventral str;.ping helps
chambers, which are separated on the the sitting bittern to m erge harmoniously with
mid-line by an insubstantial septum. the perpendicular r eeds. Nocturnal frog-
mouths (Podargus) of the Indo-Austra lian
Extensive scroll-like turbinals invest region' freeze' when approached in daylight and
look like the end of a fractured branch. In such
the olfactory chambers and are sup- species the head is moved almost imperceptibly
plied by the olfactory nerves (p. 130). so that the passing enem y can be kept in view.
(From photographs.)
The olfactory bulbs are extremely large.
There is some evidence that these sea-scavengers locate their food partly by
scent. It is possible (but not proved) that vultures, also possessed of a
 ZOOLOGY

relatively complex olfactory apparatus, may do likewise. (For the function of

the bilateral nasal glands see pp. 643-644.)
Taste-buds occur in birds, but not to the extent shown by the Mammalia.
When present, they occur principally on the tongue or on the soft palate. It
has been claimed that certain parrots are best equipped amongst birds. There
is evidence that the contamination of food with relatively strong, and some-
times unpleasant, substances (e.g. anise, asafretida, aloes, and camphor) does not
deter turkeys, and that fowls and gulls, too, have an inefficient sense of taste.
The structure of the apparatus and audition and equilibration is very uni-
form throughout the class. Both elements are highly organised. The auditory
apparatus has outer, middle, and inner parts like those of mammals (p. 144).
A tympanic membrane separates the external meatus from the middle ear,
which in turn communicates with the pharynx through the Eustachian tube,
and with the inner ear through oval and circular apertures (fenestrce), which
are covered by membranes. The fenestra ovalis is almost wholly occluded by
the footplate of the columella, a cartilaginous extension (extra-columella) of
which carries tympanic vibrations to the perilymph of the inner ear.
A cochlea (Fig. 410, p. 595) occurs as a relatively short, straight organ;
although not 'snail-shaped', it is probably nonetheless homologous with the
spiral-shaped mammalian structure. Cartilaginous shelves support the basilar
membrane and divide the cochlea into two channels which communicate by
means of an aperture near its apex, the helicotrema. One such channel is in
communication with each fenestra. Agitation of the perilymph is reflected
in movements of the basilar membrane, and its hair-like processes, which may
be sound-receptors, the hair-cells being innervated by fibres from the cochlear
ganglion. Contained in the basilar membrane are fibres which alter in length,
and probably tension, and which as a whole may act as a frequency analyser.
The adjacent lagena contains additional hair-cells, the processes of which
are sunk into a mucoid cupola, which is equipped with calcareous particles.
(It is possible, but not unequivocally proved, that the 'lagenre' of fishes and
amphibians are homologous with each other and with that of reptiles, birds
and monotremes.) It is surmised that in birds the lagena may respond to low,
and the basilar membrane to high, frequency vibrations. Pumphrey has
made the interesting suggestion that, on published evidence of vocal mimicry,
Australian bower-birds (Fig. 382, p. 562) seem to have an auditory spectrum
'substantially wider than the pass-band of a broadcast receiver'. Birds vary
a good deal in auditory power. The range of sounds audible to sparrows
(Passer) is 675-II,500 frequencies per second, to the Canary (Serinus) 1,1oo-
1o,ooo, and to various parrots 40-14,000. These values are inferior by
comparison with those recorded in some mammals. Even Man has a range
of 20-2o,ooo, whilst in their peculiar specialisation certain bats (p. 756) are
claimed to have an upper range limit of g8,ooo cycles per second.
 PHYLUM CHORDATA

The owls are of especial interest in that although they see very well by
starlight (and by day), they appear to hunt largely by ear, and possess a skin-
fold, hidden by feathers, that helps to direct sound towards the meatus. Some
species have, in addition, a projecting feather formation, which probably helps
to canalise sound in a manner analogous to the pinnre of mammals (seep. 765).
The plumage of owls, incidentally, is soft and fluffy, and makes the least pos-
sible warning sound during the approach to the prey.
F1c. 446. - Neornithes: Tactile
bristles. Convergence in widely un-
related groups. .Not to scale. Above:
Nocturnal terrestrial forest-dwelling
kiwi (Apt~ryx, Apterygiformes, p. 6ol!).
Below (left): Cave-haunting Guarcharo
or Oil-bird (Stealomis caripensis, Cap-
rimulgiformes, p. 618). (night). Dis-
position of tactile bristles from ven-
tral aspect. Steatornis now known to
possesses an echo-location apparatus
analogous to that of insectivorous bats
(see p. 765). Flying in darkness the
birds emit a stream of the 'shar pest
imaginable clicks' (Griffin). Only when
their cars were p lugged did the clicking
birds collide with the walls of a dark.
cned room. The bristles shown above
are probably used while alighting in the
<lark cavern and in nocturnal fru it·
gath ering outside. (Redrawn after
Grasse. Ingram.)

Birds possess tactile organs. Light pressure on the plumage is appreciated,

and it is probable that the bristly feathers near the mouth and eyes of many
species are tactile in function. (Fig. 446.) 'Mud-feeders' (e.g. ducks and geese)
have a considerable concentration of tactile corpuscles in the bill-tips, which
apparently aid in the discrimination of food. There is evidence, too, that it is
by means of ventral tactile organs that the sitting female becomes aware that her
clutch is complete. Broodiness and inhibition of ovulation follow.
Excretion.- Birds retain a uricotelic excretory mechanism in the manner
of reptiles (p. 548). Glomeruli are reduced in size and vascularity.
Whether marine birds drink sea-water, and by what means they maintain
 ZOOLOGY
an appropriate salt balance, have been matters of discussion for many years.
Until recently it was thought that the function of the bilateral nasal glands
(which are relatively large in sea-birds) is to protect the nasal epithelium against
sea-water. Schmidt-Neilsen and associates, however, have information suggest-
ing that the glands are concerned with the extra-renal excretion of sodium
chloride. They showed that cormorants (Phalacrocorax) excrete a high con-
centration of sodium chloride from these glands and confirmatory evidence has
been obtained by work on penguins (Spheniscus) and other species. An adult
penguin was not fed for 2r hours and was then given 5 gm. sodium chloride in
56 gm. of fish, the normal daily feed being about 8oo gm. Within ten minutes,
drops of clear fluid appeared at the beak-tip. This liquid was of high salt con-
tent; and two-thirds of the experimental dose was eliminated through the
nostrils within four hours. Concurrent examination of cloacal matter sug-
gested that 'the quantitative role of the kidney in sodium chloride excretion
in this particular experiment was thus perhaps one-tenth of that of the salt-
glands.'
It is clear that, in some species at least, the nasal gland is a far more efficient
salt-excreting organ than the kidney. Even among plankton feeders it pro-
bably allows a net gain in drinking water. The liquid excreted in the beak is
flung off with a characteristic head-shake that is a common sight to many who
know sea-birds and which, in fact, has been described in ornithological literature
without realisation of its significance (Matthews).
Reproduction.-In general, the avian urino-genital system resembles that
described in the pigeon (p. 596), the most characteristic feature being the more
or less complete atrophy of the right ovary and oviduct. In hawks, however,
both ovaries are sometimes functional.
There is experimental evidence that in the Domestic Fowl spermatozoa must
undergo some extra-gonadal maturation or other process. Spermatozoa taken
experimentally from within the testes will not fertilise ova. Those taken from
the epididymides will sometimes do so, whereas those from seminal vesicles
are highly effective. In some species (e.g. many passerines) the sperm-filled
seminal vesicles descend seasonally into cloacal protuberances, the interior of
which are cooler than the body cavity. In birds (in which spermatogenesis
occurs in abdominal organs of high temperature) spermatozoa can remain
viable in the female tract for periods longer than that of most (but not all)
mammals. Thus, a whole clutch of 20 or 30 eggs of the Domestic Turkey (laid
at about daily intervals) can be fertilised as a result of a single copulation. In
the Domestic Fowl eggs can be fertilised up to 20 days after insemination, but
with less assurance after about the first week.
The 'ratites', Anseriformes, and some other large species possess a penis in
the form of a thickening of the ventral wall of the cloaca : it has a groove on
the dorsal surface serving as a sperm-channel, and its distal end is invaginated,
 PHYLUM CHORDATA

in the position of rest, by an elastic ligament. In the Ostrich there is a solid

penis, not unlike that of Chelonia and crocodiles : it can be retracted into a
pouch of the cloaca. In the above groups a clitoris occurs in the female.
The rudimentary state of the right ovary in almost all species allows sex
reversal from female to male to occur much more frequently than in most
vertebrate animals. The traditional English couplet: 'A whistling maid, and
a crowing hen, are fit for neither God norMen' has its counterpart in the French,
German, and other languages. When the single functional ovary is obliterated
by disease (or experimentally), the rudimentary tissue of the other often develops
as an ovotestis, or even as a testis, and under the action of gonadotrophins
(p. 150) pours its male secretions into the blood-stream, with characteristic
results. It is claimed that occasionally hens have changed sex completely and
fertilised other hens. From pre-Christian times, men have regarded a hen's
assumption of male plumage, 1 and of an ability to crow, tread, and fight as a
portent of disaster. The French said :

A hen that crows,

A priest that dances,
A woman who speaks Latin,
Never come to a beautiful end.

It would seem that after a hen assumes some male characters it may
occasionally still lay a malformed egg. Ovarian function, it is thought, is not
altogether inhibited, and on its way down the shrunken oviduct the so-called
'cock's egg' gets twisted and attenuated, so that the chalaza (Fig. 447) comes to
assume roughly the form of a serpent. Such an egg has in the past often caused
most awesome forebodings. An egg laid by a cock was alleged to hatch into a
basilisk, regulus, or cockatrice, a creature mentioned in the Bible. A cocka-
trice was alleged to kill at a glance ; its fiery breath scorched all vegetation.
It was part hen, part serpent, and the monarch of all the snakes and the dragons.
It inspired in all a terrible dread. The cock alone was unaffected, and indeed
in turn inspired the cockatrice to terror. Therefore, prudent medireval tra-
vellers often took a cock with them. In Basel, as late as the 15th century, a
cock was solemnly tried for witchcraft after laying an egg. The defence ad-
mitted the offence, but pointed out that the laying of the egg was unpremedi-
tated and involuntary and, in fact, violated no law. But the prosecution
asserted that the cock had been entered by the devil. They secured a con-
viction: and the cock and its egg were solemnly and legally burnt at the stake.
Post-mortem examination showed that it contained no fewer than three more
unlaid cock's eggs. The partial change from male to female probably occurs
no more frequently in birds than in other groups.
1 In Gallus the male plumage is the neutral (asexual) type and would appear at the next moult.
 ZOOLOGY
Development.-The process of development in birds has been most
thoroughly worked out in the Domestic Fowl, but enough is known of the
embryology of other birds to show that the differences are comparatively
unimportant.

ch

FIG. 447-- Gallus: Structure of egg. Semi-diagrammatic view of the egg of the Domestk
Fowl at the time of laying. a. air-space; alb. dense layer of albumen; alb'. more fluid albumen;
bl. blastoderm; ch. chalaza; sh. shell; sh. m . shell-membrane; sh. m . r, sh. m. z, its two layers
separated to enclose air-cavity; yk. yolk. (After A. M. Marshall, slightly altered.)

pt:sl
o.r.op
mee

FIG. 448.-Gallus: Development. Blastoderm. Diagrammatic. ar. op. area opaca; ar. pl.
area pellucida; hd. head; med. gr. medullary groove; mes. mesoderm, indicated by dotted outline
and deeper shade; pr. am. pro-amnion; pr. st. primitive streak; pr. v. somites. (From A.M.
Marshal, in part after Duval.)

Briefly, it can be said that the ovum is always large owing to the great
quantity of food-yolk. The protoplasm forms a small germinal disk at the upper
pole. Fertilisation is internal, and as the oosperm passes down the oviduct it
is coated by successive secretions from the oviducal glands. It first receives a
coat of thick, viscid albumen (Fig. 447, alb.), which, as the egg rotates during
 PHYLUM CHORDATA

its passage, becomes coiled at either end into a twisted cord, the chalaza (ch.).
Next, more fluid albumen (alb'.) is deposited layer by layer, then a tough,
parchment-like shell-membrane (sh. m.), and finally a calcareous shell (sh). The
shell-membrane is double, and at the broad end of the egg the two layers are
separate and enclose an air-cavity (a.). The shell may be white or variously
coloured by special pigments. It consists of three layers, and is traversed by
vertical pore-canals. The calcareous material from which the shell is formed in
at least some species (including pigeons), appears to be withdrawn from the
substance of the bones. Thus females, but not males, form (possibly under the

Jl '

FIG. 449. -Gallus: Development. Late Stages. all. allantois; am.cutcdgeofamnion; an .

anus; au. ap. auditory aperture'; au. s. auditory sac; f. br. fore-brain; f. I. fore-limb; h. br.
hind-brain; h.l. hind-limb; hi. heart; lty. hyoid arch; m. br. mid-brain; mn. mandibular arch ;
ua. nostril ; t. tail. (After Duval. )

seasonal influence of cestrogen) endosteal medullary bone which is later with-

drawn and placed at the disposal of the shell-forming region of the reproductive
tract.
Segmentation takes place during the passage of the egg down the oviduct,
and results, as in reptiles, in the formation of a blastoderm (Fig. 448) occupy-
ing a small area on the upper pole of the yolk. In the newly-laid egg the blasto-
derm is divisible, as in reptiles, into two parts, a central, clear area pellucida
(Fig. 448, ar. pl.) and a peripheral area opaca (ar. op.), and is usually two layers
thick. The upper layer gives rise to the ectoderm and mesoderm, the lower
layer is the developing endoderm. At first the endoderm is an irregular col-
lection of cells, which is probably formed by a delamination from the upper
layer; it soon becomes a continuous epithelium.
A grooved, longitudinal band, the primitive streak (pr. st.), now forms in
 ZOOLOGY
the posterior three-quarters of the area pellucida. Through the primitive
streak cells from the upper layer invaginate, and migrate laterally between
the upper and lower layers to form the mesoderm. The primitive streak
thus resembles the blastopore of amphibians, although no archenteron forms in
birds.
When invagination has ceased, the head process forms by a forward migration
of cells from the anterior tip of the primitive streak (primitive knot or Hensen's
node). At almost the same time, the primitive knot migrates along the primi-
tive streak toward the posterior end of the area pellucida, laying down a trail
of notochord as it goes.
Immediately in front of the primitive streak the medullary groove (med. gr.)
appears, and the medullary folds which bound it on the right and left diverge
posteriorly, so as to embrace the anterior end of the primitive streak, in just
the same way as they embrace the blastopore in Amphioxus.
The blastoderm gradually extends peripherally so as to cover the yolk.
Thereby it becomes divisible into an embryonic portion, from which the embryo
is formed, and an extra-embryonic portion, which invests the yolk-sac and takes
no direct share in the formation of the embryo. The extension of the ecto-
derm and endoderm takes place regularly and symmetrically. The extra-
embryonic mesoderm, while extending equally in the lateral and posterior
regions, grows forwards in the form of paired extensions. These afterwards
unite, but for a time there is an area of the blastoderm in front of the head
of the embryo, formed of ectoderm and endoderm only. This is called the
proamnion (pr. am.).
At an early period the mesoderm lying on either side of the medullary groove
becomes segmented into somites (Fig. 448, B, pr. v.). The lateral plate, the
extreme lateral part of the mesoderm, splits into somatic and splanchic layers
with the ccelom between (Fig.
am
451, B).
Gradually the embryo becomes
folded off from the yolk-sac, as in
other large-yolked eggs; but, ow-
a ing apparently to the confined
space in which it is enclosed, it
soon turns over so as to lie with its
left side against the yolk and its
right sidefacing the shell (Fig. 450).
aU The body (Fig. 449, A) becomes
strongly flexed so as to bring the
FrG. 450. -Gallus: Relationship between em-
bryonic and extra-embryonic structures. a. air -space ; head and tail almost into contact,
all. allantois; am. amnion; ar. vase. area vasculosa;
emb. embryo; yk. yolk-sac. (After Duval.) and the head soon acquires a pro-
portionally immense size, with
 PHYLUM CHORDATA

very large projecting eyes. At first the head is quite like that of one of
the lower vertebrate embryos, with protuberant brain-swellings (f. br., m.
br., h. br.), large square mouth, ventrally placed nostrils, connected by
grooves with the mouth, and three or four pairs of gill-slits. As in
reptiles, there is never any trace of gills. In the chick embryo as well
as in most other birds, an opercular fold grows backwards from the hyoid
arch, and covers the second and third branchial clefts. Soon the margins
of the mouth grow out into a beak (Fig. 449, B). The clefts close,
with the exception of the first, which gives rise to the tympana-eustachian pas-
sage. The head becomes characteristically avian. The limbs are at first
alike in form and size (A,J.l., h. l.), and the hands and feet have the character
of paws, the former with three, the latter with four digits. Gradually the
second digit of the hand outgrows the first and third, producing the char-
acteristic avian manus (B), while the metatarsal region elongates and gives rise
to the equally characteristic foot. At the same time feather-papillre make
their appearance, arranged in narrow and well-defined pterylre.
At an early period capillaries appear in the extra-embryonic blastoderm
between the opaque and pellucid areas, and give rise to a well-defint-d area
vasculosa (Fig. 450, ar. vase.). They are supplied by vitelline arteries from the
dorsal aorta, and their blood is returned by vitelline veins which join the portal
vein and take the blood, through the liver, to the heart. The vascular area
gradually extends, until it practically covers the whole of the yolk-sac: its
vessels take an important share in the absorption of the yolk by the embryo.
Before the embryo has begun to be folded off from the yolk, the rudiment
of one of the two characteristic embryonic membranes, the amnion, has appeared,
A crescentic amniotic fold arises (Fig. 45I, A, am. f.}, in front of the head-end
of the embryo, from the region of the pro-amnion. It consists at first of ecto-
derm only, the mesoderm not having yet spread into the pro-amnion. The
fold is soon continued backwards along the sides of the body (B) and round the
tail (A). In these regions (am. f'.), however, it consists from the first of ecto-
derm plus the somatic layer of mesoderm, i.e. it is a fold of what may be called
the extra-embryonic body-wall. The cavity is a prolongation of the space
between the somatic and splanchnic layer& of mesoderm, i.e. is an extension of
the extra-embryonic crelom.
The entire amniotic fold gradually closes dorsally (C), forming a double-
walled dome over the embryo. Its inner wall, formed of ectoderm internally
and mesoderm externally, is the amnion (am.), the cavity of which becomes
filled with a watery amniotic fluid serving as a protective water cushion to the
enclosed embryo and to prevent desiccation. The outer wall, formed of ecto-
derm externally and mesoderm internally, is the serous membrane (sr. m.) or
chorion. This comes to lie just beneath the vitelline membrane which sub-
sequently disappears.
VOL. II. SS
 ZOOLOGY

FIG. 451.-Gallus: Development of fretal membranes. A, early stage in the formation of

the amnion, sagittal section; B, slightly later stage, transverse section; C, sagittal section
showing stage with completed arr.nion and commencing allantois; D, stage in which the
allantois has begun to envelop the embryo and yolk-sac. The ectoderm is represented by a blue,
the endoderm by a red line; the mesoderm is grey. all. allantois; all'. the same growing round
the embryo and yolk-sac; am. amnion; am. f., am. f'. amniotic fold; an. anus; br. brain; cml.
crelome; cml'. extra-embryonic crelom; ht. heart; ms. ent. mesenteron; mth. mouth; nch.
notochord; sp. cd. spinal cord; sr. m. serous membrane; umb. d. umbilical duct; vt. m. vitelline
membrane; yk. yolk-sac.
 PHYLUM CHORDATA 6sr

The second of the embryonic membranes, the allantois, is developed as an

outpushing of the ventral wall of the mesenteron at its posterior end (C, all.),
and consists, therefore, of a layer of splanchnic mesoderm lined by endoderm.
It has at first the form of a small ovoid sac having the precise anatomical rela-
tions of the urinary bladder of Amphibia (Fig. 451, A, all.). It increases
rapidly in size (Fig. 451, all.), and makes its way backwards and to the
right, into the extra-embryonic crelom, between the amnion and the serous
membrane (Fig. 451, C, D). Arteries pass to it from the dorsal aorta. Its
veins, joining with those from the yolk-sac, take the blood through the liver to
the heart. Next, the distal end of the sac spreads itself out and extends all
round the embryo and yolk-sac (D, all'.), fusing, as it does so, with the serous
and vitelline membranes, and so coming to lie immediately beneath the shell-
membrane. It finally encloses the whole embryo and yolk-sac together with
the remains of the albumen, which has by this time been largely absorbed. The
allantois serves as the embryonic respiratory organ, gaseous exchange readily
taking place through the porous shell. Its cavity is an embryonic urinary
bladder, excretory products being discharged into it from the kidneys.
Respiratory movements begin when the embryo pushes its beak into the
air pocket that lies at one end of the egg. The shell is now punctured, usually
by means of a little horny elevation, or caruncle, at the end of the beak. By
this time the remainder of the yolk-sac has been drawn into the crelom and the
ventral body-walls have closed round it. The aperture in the shell is enlarged.
The young bird hatches and begins a free life.

VOL. II. ss::z

 CLASS MAMMALIA
INTRODUCTION
All mammals possess dermal milk-glands, and only mammals possess
them. All mammals are at least partially hairy. A single dentary,
articulating with the squamosal, occurs on each side in the lower jaw. The
skull has a double occipital condyle. The brain, and particularly the fore-
brain, is relatively large. The vertebrre are gastrocentrous (Fig. 452). Each
vertebra consists of a centrum and a neural arch and, in addition, thin plate-
like disks of bone-the epiphyses-at each end. On the cessation of growth
these fuse with the body of the vertebra. The tympanic membrane or ear-
drum is supported by a tympanic bone and there are three minute auditory
ossicles (malleus, incus, and stapes) in the middle ear. A muscular diaphragm
completely separates the thorax from the abdominal cavity. Excluding the
Monotremata (p. 688) all mammals are viviparous, i.e. they produce their
young alive. Most of them develop a filtering placenta between the maternal
and embryonic tissues in the uterus and employ special means, both anatomical
and behavioural, for the protection and nourishment of the newly born young.
Like the birds, they are homa:othermous with a four-chambered heart separating
completely the oxygenated and deoxygenated blood. Birds and mammals
are characterised also by the retention of a single aortic arch, in Mammalia
the left one, in Aves the right.
The manifold physiological and behavioural advantages conferred by thermo-
regulation and by the hyper-development of the cerebrum (as well as by the
female's capacity to produce and carry in her body perfectly suitable food for
the unborn young) have enabled eutherian mammals (p. 704) successfully to
colonise most of the earth including some of its least hospitable parts. More
than 8,500 extant mammalian species have been described.
Although prodigious gaps occur in our present understanding of mammalian
evolution it is at least clear that they are derived from the Synapsida (p. 513),
mammal-like reptiles of an ancient stem which appeared late in the Carboni-
ferous (p. 3). The Synapsida prospered exceedingly in the Pennian, yet
were, surprisingly, seemingly on the wane before the great reptilian radiations
came to fruition in the Mesozoic. The transition from primitive reptile to
early mammal occurred between the Upper Carboniferous and the end of the
Triassic. By the Lower Triassic there had appeared advanced theriodonts
 PHYLUM CHORDATA

(e.g. Cynognathus) that were probably distinctly mammalian in appearance

(p. 517). Cynognathus possessed a well-developed secondary palate, a feature
that is characteristic of (though not peculiar to) sucking mammals. (A false
palate occurs also in phytosaurs, crocodiles and one dinosaurian group.) In
some advanced therapsids large infra-orbital foramina (which in mammals
carry a rich nerve and blood supply to sensory whiskers and moist nasal mucosa)
also occur. It must be emphasised, however, that these structures are no
more than suggestive. There is, of course, no absolute evidence that any of
these animals were homceothermous, had a four-chambered heart, were hairy
or milk-producing. It seems possible, in fact, that many extinct animals not
possessing the conventional squamosal-dentary articulation, and at present re-
tained in the class Reptilia, might be called mammalian if their soft parts were
available for study. In almost every fossil uncovered the bones alone are pre-
served or petrified, and although evidence can be got from endocranial casts, and
further suggestive information obtained from other studies, it is inevitable that
conventional definitions should be based wholly on osteological considerations.
After its successful radiation in the Permian the proto-mammalian stock
suffered a set-back, and from the Triassic until the late Cretaceous (p. 3)
it survived only as relatively inconspicuous, small animals that are very rarely
found as fossils. Five groups-Multituberculata, Triconodonta, Symmetro-
donta, Dryolestoidea and Docodonta have been recognised. Pre-Pleistocene
monotreme fossils are unknown, but certain of the reptile-like skeletal com-
ponents, the egg-laying habits, the imperfect thermoregulation and other
primitive characters (p. 688) of the Monotremata suggest that they are survivors
of some such early group. Specific cranial features argue that they are
descended from triconodonts (p. 687).
No agreement has been reached as to the classification that most adequately
expresses the relationships of these early mammals. Those particularly
interested in mammalian palreontology should refer to works included in
References (p. 909). It must be pointed out again that any phylogenetic
arrangement adopted for animals of whose origins so little is known can only be
provisional. For example, it was believed for many years that the Multituber-
culata were marsupials, but work done during the first half of this century has
tended to disprove this and multituberculates are now placed in a sub-class of
their own (see below). This arrangement indicates that they are mammals, but
that they do not appear to be closely related to any other mammalian group.
The principal divisions are as follows : 1
1 Gregory (see p. 689) has proposed a new monotremefmarsupialian Sub-class Marsupionta
involving a general arrangement as follows :
Class Mammalia
Sub-class Marsupionta
Orders Marsupialia
Monotremata
Sub-class Monodelphia (Placentalia)
 ZOOLOGY
CLASS MAMMALIA
Sub-class Prototheria
Orders Triconodonta (Triassic-Cretaceous)
1 Symmetrodonta (Triassic-Cretaceous)

Monotremata (Pleistocene-Recent)
Sub-class Allotheria
Order Multltuberculata (Jurassic-Eocene)
Sub-class Theria
Infra-class Pantotheria (Trituberculata)
Orders Dryolestoidea (Jurassic-Cretaceous)
2 Docodonta (Jurassic)

Infra-class Metatheria
Order Marsupialia (Cretaceous-Recent)
Infra-class Eutheria (Placentalia)

EXAMPLE OF THE CLASS.-THE RABBIT (Oryctolagus cuniculus)

Oryctolagus cuniculus, along with other rabbits and the hares, belongs to
the family Leporidre of the Order Lagomorpha. They and the pikas of North
America and Central Asia (Family Ochotonidre) were originally placed with the
Rodentia but modern work has shown that their many resemblances thereto
are adaptive and superficial (p. 774).
External Characters.-The European Rabbit is a four-footed or quadrupedal
animal. The whole surface of its body is covered with soft fur. Below the
anterior extremity of the head the mouth is a transverse slit bounded by soft
lips. The upper lip is divided by a longitudinal cleft, running backwards to the
nostrils, and exposing the chisel-shaped incisor teeth. Behind the incisor teeth
the hairy integument projects on each side into the cavity of the mouth. At
the end of the snout, above the mouth, are two oblique slits, the nostrils. The
large eyes, situated at the sides of the head, have each three eyelids, an upper
and lower hairy lid, and an anterior hairless third eyelid or nictitating membrane,
supported by a plate of cartilage. Tactile vibrissce-very long, stiff hairs-are
scattered above and below the eyes and on the snout. Behind the eyes, and a
little nearer the summit of the head, are a pair of very long flexible and movable
external ears or pinnce. These are somewhat spout-shaped, expanding distally,
and are usually placed vertically with the concavity directed laterally and some-

1 I ncertae sedis.
2 Patterson removes the Docodonta from the Theria, and so limits the order Pantotheria to
include only the dryolestids. He considers Morgmiucodon to be a docodont and not a tricono-
dont, and places the symmetrodonts among the Theria, in the Infra-class Pantotheria. Kermack
and Mussett have erected a Sub-class Eotheria including only the docodonts (mammals with a
dual jaw articulation). They place the symmetrodonts in the Infra-class Pantotheria. The
suggestion is again made (see also pp. 518, 688) that monotremes may have arisen independently;
likewise the triconodonts, listed as an Order incerlae sedis.
 PHYLUM CHORDATA 6ss
what forwards, leading to the external auditory opening. The neck is a distinct
constriction, and the trunk is distinguishable into thorax in front and abdomen
behind. On the ventral surface of the abdomen in the female are four or five
pairs of teats. At its posterior end, below the root of the tail, is the anus. In
front of this in the male is the penis, with a small terminal urinogenital aperture.
The testes are enclosed in prominent scrotal sacs to right and left of the penis.
In the female the genital opening is the vulva. In the space (perinceum) be-
tween anus and penis or vulva are two bare, depressed areas of skin into which
open the ducts of the perinceal glands which secrete a product of strong and
characteristic odour (p. 847). The tail is short and covered with a tuft of fur.
The fore- and kind-limbs, both of which take part in locomotion and in
supporting the weight of the animal, differ considerably in size. The fore-
limbs are much the shorter. Both have the same general divisions as in lizards.
The upper arm is almost completely hidden by the skin, being applied closely
against the side of the body. The manus is provided with five digits, each
terminating in a horny claw. The thigh is also almost hidden by the skin.
The pes has four digits only, all provided with claws.
Skeleton.-The spinal column of the Rabbit is divisible, like that of birds
and lizards, into five regions-the cervical, thoracic, lumbar, sacral, and caudal.
In the cervical region there are seven vertebrre, in the thoracic twelve (sometimes
thirteen), in the lumbar seven (sometimes six) in the sacral four, and in the
caudal about fifteen.
The centra of the vertebrre in a young rabbit consist of three parts-a
middle part which is the thickest, and two thin disks of bone. The epiphyses,
anterior and posterior, are applied respectively to the anterior and posterior
faces of the middle part or centrum proper. Between successive centra in an
unmacerated skeleton are plates of fibro-cartilage, the intervertebral disks.
The transverse processes of all the cervical vertebn.e, except the seventh or
last, are perforated by a canal, the vertebrarterial canal, through which runs the
vertebral artery. The first vertebra or atlas (Fig. 452, A) resembles the corre-
sponding vertebra of birds (p. 573) in being of the shape of a ring without any
solid centrum like that of the rest. On the anterior face of its lateral portions
are two concave articular surfaces for the two condyles of the skull. The second
vertebra or axis (A and B) bears on the anterior face of its centrum a peg-like
process, the odontoid process (od.), which fits into the ventral part of the ring
of the atlas. It has a compressed spine (sp.), produced in the antero-posterior
direction, and its transverse processes are short and perforated by a canal which
carries the vertebral artery. All the cervical vertebrre except the last have
their transverse process bifurcated into dorsal and ventrallamellre. The seventh
differs from the others in having a more elongated neural spine, in having its
transverse processes simple and imperforate, and in the presence on the
posterior edge of the centrum of a little concave semi-lunar facet.
 ZOOLOGY
The thoracic vertebrre (C) have elongated spines which are mostly directed
backwards as well as upwards. The transverse processes are short and stout.
Each bears near its extremity a small smooth articular surface or tubercular facet
for the tubercle of a rib. On the anterior and posterior borders of each verte-
bra is a little semi-lunar facet, the capitular facet (fac.), situated at the junction
of the centrum and the neural arch. The two contiguous semi-lunar facets of
successive vertebrre form between them a little cup-like concavity into which
the head or capitulum of a rib is received. The semi-lunar facet on the last
cervical vertebra forms, with that on the anterior border of the first thoracic,
the concavity for the head of the first rib.
In the lumbar region the spines are comparatively short, and both trans-
verse processes and bodies are devoid of facets. From the centrum of each of
the first two (or three) projects downwards a short flattened process-the

Frc. 452.-Class Mammalia, Infra·class Eutheria, Cohort Glires, Order Lagomorpha, Family
Leporidm, Sub.family Leporinm, Oryctolagus: Vertebr<e. A, atlas and axis, ventral aspect.
od. odontoid process of axis. B , lateral view of axis. art. articular facet for atlas; od. odontoid
process; pt. zy. post-zygapophysis; sp. neural spine. C, thoracic vertebr<e, lateral view. cent.
centrum; fac. facet for rib; met. metapophysis; pr. zy . pre-zygapophysis; pt. zy. post-zygapo-
physis; rb. rib; sp. spinous process.

hypapophysis. Certain accessory processes, the metapophyses (met .) and

anapophyses, are well-developed, the former being extremely long in the
posterior lumbar region. The metapophyses are situated in front, projecting
forwards and outwards over the prezygapophyses. The anapophyses are
situated below the postzygapophyses and project backwards. The transverse
processes are long, and are directed forwards and outwards. That of the last
lumbar is bifurcated. All bear short lumbar ribs at their tips.
The sacral vertebrre are firmly ankylosed together to form a single com-
posite bone, the sacrum. The vertebrre bear a close resemblance to those of
the lumbar region, but hypophyses and anapophyses are absent, and the
metapophyses are comparatively small. The first and second bear great
expanded lateral processes, or sacral ribs, with roughened external surfaces for
articulation with the ilia. These are the only sacral vertebrre in the strict
sense of the term, the following two being in reality anterior caudal.
Of the caudal vertebrre the more anterior resemble those of the sacral
region, and have similar processes. Passing backwards in the caudal region
 PHYLUM CHORDATA

all the processes gradually diminish in size. The most posterior vertebrre are
represented merely by nearly cylindrical centra.
There are twelve pairs of ribs, of which the first seven are known as true
ribs, i.e. are connected by their cartilaginous sternal parts with the sternum.
The remaining five, the so-called false or floating ribs, are not directly
connected with the sternum. All, except the last four, bear two articular
facets. One of these is on the vertebral extremity or capitulum, and the other
is on the neck of the rib (near the tubercle). The former is for articulation
with the bodies, the latter the transverse processes of the vertebrre.
The sternum consists of six segments or sternebrce. The first, the manubrium
sterni or presternum, is the largest and has a ventral keel. With the last is
connected a rounded cartilaginous plate, the xiphisternum.
The skull (Figs. 453, 454), ignoring the jaws, is not very unlike that of birds
in general shape. The length is great as compared with either the breadth or
the depth. The maxillary region, or region of the snout (corresponding to the
beak of birds (p. 575)), is long in proportion to the rest. The orbits are closely
approximated, being separated only by a thin interorbital partition, and the
optic foramina are confluent. But certain important differences are recognis-
able. One is the mode of union of the constituent bones. In the Pigeon,
long before maturity is attained, the bony elements of the skull, originally
distinct, become completely fused so that their limits are no longer distinguish-
able. In the Rabbit, on the other hand, such fusion between elements takes
place in a few instances only, the majority of the bones remaining more or
less distinct throughout life. The lines along which the edges of contiguous
bones are united (the sutures) are sometimes straight, sometimes wavy and
sometimes zigzagged serrations of the edges of the two bones interlocking.
In some cases the edges of the bones are bevelled off and the bevelled edges
overlap, forming a squamous suture.
Another conspicuous difference between the skull of the rabbit and that of
the pigeon is in the mode of connexion of the lower jaw. In the rabbit it
articulates directly with the cranium: the quadrate, through which the union
is effected in the Pigeon, is apparently absent. Certain large apertures are
readily identified with the large openings in the skull of birds. In the
posterior wall of the skull is a large rounded opening, the foramen magnum,
flanked with a pair of smooth rounded elevations or condyles for articulation
with the first vertebra. These obviously correspond to the single condyle
situated ventro-medially in the foramen in the pigeon. A large opening,
situated at the end of the snout and looking forwards, obviously takes the place
of the external nares of the Aves. A large opening in the roof of the mouth,
leading forward to the external nasal opening, plainly represents, though much
wider and situated farther back, the internal or posterior nares of birds. The
rounded tubular opening (aud. me.) situated at the side of the posterior part
 ZOOLOGY

FrG. 453--0Tyctolagus: Skull. A, lateral

view; B, ventral view. ang. pro. angular pro-
cess of mandible; a. s. alisphenoid (external
pterygoid process); aud. me. external auditory
meatus; b. oc. basi-occipital; b. splz. basi-
sphenoid; cond. condyle. Jr. frontal; int. pa.
interparietal ; i . o. f. infra-orbital foramen ; ju.
jugal; lcr.lachrymal; m. molars; max. maxilla;
nas. nasal; opt. fo. optic foramen; o. sph.
orbito-sphenoid; pa. parietal ; pal. pa latine;
pal. max. palatine plate of maxilla; par. oc.
paroccipital process; pal. p. max. palatine
process of premaxilla; p.m. premolars; p.
max. pre-maxilla ; pr. sph. pre-sphenoid; pt.
pterygoid; p. t. sq. post-tympanic process of
squamosal; s. oc. supra-occipital; sph. points
to position of sphenoidal fissure, not clearly
v isible in a lateral view; sq. squamosal ; st. fo .
stylomastoid foramen; ty. bul. tympa nic bulla ;
vo. vomer; zyg. max. zygomatic process of
maxilla.

a .oc

of the skull, some distance behind the orbit, is much the same as the auditory
aperture of birds.
Surrounding the large opening of the foramen magnum are the bones of the
occipital region of the skull, the supra-, ex-, and basioccipitals. The first of
these (s. oc.) is a large plate of bone whose external surface is directed backwards
 PHYLUM CHORDATA

and upwards, and elevated in the middle into a shield-shaped prominence.

The exoccipitals lie at the sides of the opening, and each bears the greater part
of the somewhat oval prominence or condyle with which the corresponding
surface of the atlas or first vertebra articulates. Each is produced below into
a process called the paroccipital (par. oc.), closely applied to the tympanic bulla.
At the end of this, embedded in the tendon of a muscle (the styloglossus), is a
small bony rod, the stylohyal. A small aperture, the condylar foramen, situ-
ated below the condyle, gives passage to one of the cranial nerves, the hypo-
glossal. The basioccipital is a median plate of bone, almost horizontal in
position, which forms the floor of the most posterior part of the cranial cavity;
it bears the ventral third of the occipital condyles. All these four bones of

FIG. 454.-0ryctolagus: Skull. Longitudinal vertical section. The cartilaginous nasa

septum is removed. a. sph. ali-sphenoid; e.oc. exoccipital; e. tb. ethmo-turbinal; eth . ethmoid;
fl. fossa for flocculus of brain; i . incisors; mx. tb. maxillary turbinal ; n. tb. naso-turbinal; pal' .
palatine portion of the bony palate ; peri. periotic (petrous portion); p . sph. pre-sphenoid; sph. f.
sphenoidal fissure; s. t. sella turcica, or depression in which the pituitary b ody lies ; I. point at
which the olfactory nerves leave the skull; II. optic foramen; V. mn. foramen for mandibular
division of trigeminal; VII . for facial nerve; VIII . for auditory n erve; IX, X, XI, for glosso-
pharyngeal, vagus,· and spinal accessory; X II. for hypoglossal. Other letters as in Fig. 453·
(After T. J. Parker.)

the occipital region are in the adult rabbit united to form the single occipital
bone. Articulating in front with the basioccipital, but separated from it by
a plate of cartilage, is a plate of bone, also horizontal in position, which forms
the middle part of the floor of the cranial cavity. This is the basisphenoid.
It is perforated at about its middle by an oval foramen, the pituitary foramen,
and on its upper surface is a depression, the sella turcica, or pituitary fossa (Fig.
453, s. t.), in which the pituitary body (hypophysis cerebri) rests. In front of
it is another median bone of laterally compressed form, the presphenoid, with
which it is connected by cartilage. The removal of this leaves a gap in the
dried skull. The presphenoid forms the lower boundary of the single, large
optic foramen (Fig. 453, opt. fo.). Connected laterally with the basisphenoid
and presphenoid are two pairs of thin irregular plates, the alisphenoids (a. s.)
behind and the orbito-sphenoids (o. sph.) in front. The alisphenoids are broad
66o ZOOLOGY
wing-like bones, each produced below into a bilaminate process, the pterygoid
process. A large foramen, the sphenoidal fissure (sph.), situated between the
orbitosphenoid and the alisphenoid of each side, transmits from the interior of
the skull the third and fourth cranial nerves, the first and second divisions of
the fifth, and the sixth nerves.
The boundary of the anterior part of the brain-case is completed by a
narrow plate of bone, the cribriform plate of the ethmoid (Fig. 453, eth.), per-
forated by numerous small foramina for the passage of the olfactory nerves.
This cribriform plate forms a part of a median vertical bone, the mesethmoid,
the remainder of which (lamina perpendicularis) forms the bony part of the
partition (completed by cartilage in the unmacerated skull) between the nasal
cavities. Fused with the mesethmoid are two lateral, thin, twisted bones, the
ethmo-turbinals (e. tb.). Articulating with the inferior edge of the mesethmoid
is a long median bone, having a pair of delicate lateral wings, the vomer (vo.).
None of these, save the cribriform plate, takes any share in the bounding of the
cavity of the cranium. Roofing over that part of the cranial cavity, the walls
and floor of which are formed by the sphenoidal elements, is a pair of investing
bones, the parietals (Fig. 453, pa.). Farther forwards is another pair, thefrontals
(Jr.). The parietals are plate-like bones, convex externally, concave internally,
which articulate with the supra-occipital behind by a transverse serrated
lambdoidal suture. The right and left parietals articulate mutually by means
of a somewhat wavy sagittal suture. In front a transverse serrated suture,
the coronal, connects them with the frontals. Between the supra-occipital and
the parietals is a median ossification or interparietal (int. pa.). The frontals
are intimately united in the median plane by means of the frontal suture.
Laterally their orbital plates form an important part of the upper portion of the
inner wall of the orbit. Above this, over each orbit, is a curved, somewhat
crescentic process, the supraorbital process. Between the alisphenoid below,
the parietal and frontal above, the frontal and orbitosphenoid in front, and
the parietal behind, is the broad squamosal bone (sq.), whose superior margin is
bevelled off. It is produced in front into a strong zygomatic process, which
curves outwards, then downwards, and finally forwards, to unite, with the
jugal in the formation of the zygomatic arch. Below the root of the process is
a hollow, the glenoid fossa. Posteriorly the squamosal gives off a slender pro-
cess, the post-tympanic process (p. t. sq.), which becomes applied to the outer
surface of the periotic.
Between the occipital and parietal bones, below and behind the squamosal,
are the tympanic and periotic bones. The tympanic forms the bony part of the
wall of the external auditory meatus. Below it forms a dilated process, the
bulla tympani (ty. bul.) projecting from the under surface of the skull. The
periotic is a bone of irregular shape. Its internal (petrous) portion (Fig. 454,
peri.) enclosing the membranous labyrinth of the internal ear. Externally it
 PHYLUM CHORDATA 66I

presents two small openings, the fenestra ovalis and fenestra rotunda, visible
only when the tympanic is removed. Internally it bears a depression, the
floccular fossa, for the lodgment of the flocculus of the cerebellum. Part of
the periotic (mastoid portion) is seen on the exterior of the skull between the
tympanic and exoccipital. The periotic and tympanic are not ankylosed
together, but are loosely connected with the surrounding bones, and held in
position by the post-tympanic processes of the squamosal. Between the
tympanic and periotic are two foramina of importance--the stylomastoid, which
transmits the seventh cranial nerve, and the Eustachian aperture, at which the
Eustachian tube opens (Fig. 93, p. I44)·
Roofing over the olfactory cavities are two flat bones, the nasals (nas.).
Each has on its inner surface a very thin, hollow process, the nasoturbinal.
In front of the nasals are the premaxillre (p.max.) which are large bones form-
ing the anterior part of the snout. They bear the upper incisor teeth and give
off three processes-a nasal, a palatine (pal. p.max.), and a maxillary. The
maxilla (max.), which form the greater part of the upper jaw, and bear the
premolar and molar teeth, are large, irregularly-shaped bones, the outer sur-
faces of which are spongy. They give off internally horizontal processes, the
palatine processes (pal. max.), which unite to form the anterior part of the bony
palate. Between the premaxillre and maxillre and the palatines on the lower
surface of the skull is a large triangular opening divided into two anterior
palatine foramina by the palatine processes of the premaxillre. On the outer
surface of each maxilla, above the first premolar tooth is the infraorbital
foramen (i. o. f.), through which the second division of the fifth nerve emerges.
A strong process given off from the outer face of each maxilla, and turning
outwards, then backwards, to unite with the zygomatic process of the squamosal
and thus to complete the zygomatic arch, is a separate bone in the young, the
malar or jugal (ju.).
The maxillre help to bound the nasal cavities externally, and each bears on
its inner aspect a pair of thin scroll-like bones, the maxillo-turbinals (Fig. 454,
mx. tb.). The rest of the narrow bony palate, forming the roof of the mouth
and the floor of the nasal cavities, is formed by the palatine plates of the palatine
bones (pal.). The so-called pterygoids (pt.) are small irregular bones, each of
which articulates with the palatine in front and with the pterygoid process
of the alisphenoid behind. These are probably equivalent to the fused ptery-
goids and ectopterygoids of reptiles (Parrington and Westall). The lachrymals
(lcr.) are small bones, each situated in the anterior wall of the orbit, perforated
by a small aperture--the lachrymal foramen.
In the interior of the skull (Fig. 454) are three cavities, the right and left
olfactory or nasal cavities in front, and the cranial cavity behind. The
two first-named are separated from one another by a median partition or
septum, partly cartilaginous, partly bony, formed, as above described, by the
662 ZOOLOGY
mesethmoid. Each contains the turbinals or turbinate bones of its side. Each
opens to the exterior by the large external nasal aperture and communicates
behind with the cavity of the mouth by a posterior nasal aperture.
The cranial cavity has its walls moulded to a considerable extent on the
surface of the contained brain, and, in consequence, concavities in the former
correspond to the prominent portions of the latter. These concavities are
termed the fossce, and they consist of the cerebellar fossa behind and the cerebral
fossa in front, with the inconspicuous olfactory fossa in the frontal region.
The mandible, or lower jaw, consists of two lateral halves or rami, connected
with each other in front by a rough articular surface or symphysis. Behind
they diverge like the limbs of a letter V. In each ramus is an anterior horizontal

MLP
I

FIG. 455--0..,ctolagus:
law muscles. M LS. lateral
superficial, M LP. deep lat-
eral, nuna anterior medial,
M M P . posterior medial, of
masseter; PTE'. insertion
ofexternalpterygoid. PTI'.
insertion of internal ptery-
goid; t. temporal; t'. inser-
tion of temporal. (Redrawn
after Tullberg.)

portion, which bears the t eeth, and a posterior vertical or ascending portion,
which carries the articular surface or condy le (cond.) for articulation with the
glenoid cavity of the squamosal. In front of the condyle is the compressed
coronoid process. The angle where the horizontal and ascending processes meet
gives off an inward projection or angular process (ang. pro.) (Fig. 453).
The jaw-muscles in the Rabbit are specialised for the action of gnawing,
though to a less extent than in many rodents (see Fig. 455). The muscles
chiefly concerned are the temporal, masseter, pterygoids, and digastric. The
temporal has its origin from a long, rather irregular area on the reduced tem-
poral fossa, and is inserted on the medial side of the coronoid process of the
lower jaw. The masseter is a large mass, and is divided into three sections,
deep and superficial lateral, and medial. The origin is along the whole border
of the jugal arch (the medial and deep lateral) and on the lower border of the
 PHYLUM CHORDATA

maxilla (superficial lateral). The insertion of all three sections is on the outer
face and border of the angle of the lower jaw. The internal and external ptery-
goid muscles arising from the pterygoid region are inserted on the inner side of
the jaws at the angle and coronoid, respectively. The digastric muscles which
open the jaws arise from the paroccipital process, and are inserted on the medial
lower surface of the ramus. On the front part of the jaws are inserted other
smaller muscles, such as the buccinator, which moves the lips, the geniohyo-
glossus for the tongue and so on.
The hyoid consists, in addition to the separate vestigial stylohyals already
mentioned, of a stout thick body or basihyal, a pair of small anterior cornua
or ceratohyals, and a pair of long, backwardly-directed cornua or thyrohyals
(Fig. 6rz, p. 854).
The auditory ossicles, contained in the cavity of the middle ear and cut off
from the exterior in the unmacerated skull by the tympanic membrane, are
extremely small bones, which form a chain extending, like the columella auris
of the pigeon, from the tympanic membrane externally to the fenestra ovalis
internally. There are three of these auditory ossicles-the stapes (which corre-
sponds to the columella of birds (p. 596)), the incus, and the malleus. The
last-named has a slender process (processus gracilis). These are derived from
the quadrate and articular elements (q.v.) of lower vertebrates. In addition
there is a small disc-like orbicular bone which is attached to the incus.
The elements of the pectoral arch are fewer than in lizards (p. 486). There is
a broad, thin, triangular scapula, the base or vertebral edge of which has a thin
strip of cartilage (the suprascapular cartilage) continuous with it. Along the
outer (dorsal) surface runs a ridge, the spine of which ends below in a long acro-
mion process. From this a branch process or metacromion is given off posteri-
orly. The part of the outer surface of the scapula in front of the spine is the
prespinous or prescapular fossa. The part behind is the post-spinous or post-
scapular fossa. At the narrow lower end of the scapula is a concave surface-
the glenoid cavity-into which fits the head of the humerus. Immediately in
front of this is a small inwardly curved process, the coracoid process, which is
represented by two separate ossifications in the young Rabbit. A slender rod.
the clavicle, lies obliquely in the region between the presternum and the scapula,
but only extends a part of the distance between the two bones. In the adult
it is connected with them only through the medium of fibrous tissue.
The skeleton of the fore-limb is more readily comparable with that of the
lizard than that of the bird. There is, however, a difference in the position of
the parts owing to the rotation backwards of the distal end of the humerus, all
the segments being thus brought into a plane nearly parallel with the median
vertical plane of the body, with the pre-axial border directed outwards, and the
original dorsal surface backwards. The radius and ulna are fixed in the position
of pronation, i.e. the distal end of the radius is rotated inwards, so that, while
 ZOOLOGY
the proximal end is external to the ulna, the distal end becomes internal, and
the digits of the manus are directed forwards.
At the proximal end of the humerus, are to be recognised r. a rounded head
for articulation with the glenoid cavity of the scapula; 2. externally a greater
and 3. internally a lesser tuberosity for the insertion of muscles ; 4· a groove,
the bicipital groove, between the two tuberosities. On the anterior surface of
the proximal portion of the shaft is a slight ridge, the deltoid 1·idge. At the distal
end are two articular surfaces, the large and pulley-like trochlea for the ulna and
the smaller capitellum for the radius. Laterally there is an internal, and an
external, prominence or epicondyle.
The radius and ulna are firmly fixed together so as to be incapable of move-
ment, but are not actually ankylosed. The radius
- 1 " ~ articulates proximally with the humerus, distally
' , . uln with the scaphoid and lunar bones of the carpus .
. ·. t .• ; ~ .
' ' ·:· ~ .' The ulna presents on the anterior aspect of its
\' ; proximal end a deep fossa, the greater sigmoid
·· lure
lrrmz cavity, for the trochlea of the humerus. The
r 1Jij!q~o.4li!o.T' cun
lrp.J ·c e rtl prominent process on the proximal side of this is
u.nc the olecranon process. Distally it articulates with
the wneiform.
The carpal bones (Fig. 456), nine in number,
FIG. 456.- Orydolagus: are all small bones of irregular shape. Eight of
Fore-arm and carpus. Distal end,
dorsal view, the bones bent to-
these are arranged in two rows-a proximal and
wards the dorsal side so as to be a distal; the ninth, centrale (cent.), lies between
partly separated. cent. centrale;
cun. cuneiform; lun. lunar ; mag. the two rows. The bones of the proximal row,
magnum; rad. radius; sc. sca- taken in order from the inner to the outer side,
phoid; trpz. trapezoid; trpm.
trapezium; uln. ulna; unc. are scaphoid (sc.), lunar (or semi-lunar) (lun.),
unciform; I-V, bases of meta- cuneiform (cun.), and pisiform. Those of the
carpals. (After Krause.)
distal row are likewise the trapezium (trpm.),
trapezoid (trpz.). magnum (mag.). and unciform (unc.). 1
The five metacarpals are all small but relatively narrow and elongated bones,
the first being smaller than the rest. Each of the five digits has three phalanges,
except the first, which has two only. The distal (ungual) phalanges are grooved
dorsally for the attachment of the horny claw.
The pelvic arch (Fig. 457) contains the same elements as in the pigeon, but
the union of the ilium with the sacrum is less intimate. The acetabuhtm is not
perforated, and the pubes and ischia of opposite sides unite ventrally in a
symphysis (sy.). The three bones of the pelvis-ilium, pubis, and ischium-
are separate in the young Rabbit but are completely fused in the adult animal.
1 The homologies of these bones are not quite certain, but are very probably as follows :-
scaphoid = radiale; lunar = intermedium; cuneiform = ulnare; pisiform = a sesamoid bone;
centrale = 1st and 2nd centralia; trapezium = 1st distale; trapezoid = 2nd distale; magnum =
3rd distale ; unciform = 4th and sth distalia.
 PHYLUM CHORDATA 665

The ilium and ischium meet in the acetabulum (to which they contribute),
the articular cavity for the head of the femur. The remainder of the cavity is
bounded, not by the pubis, but by a small intercalated ossification-the
cotyloid bone. The ilium (il.) has a rough surface for articulation with the
sacrum. Between the pubis (pub.) in front, and the ischium (isch.) behind, is
a large aperture-the obturator foramen (obt.). The femur is .rotated forwards
when compared with that of the lizard, so that the limb is nearly in the same
plane as the fore-limb, and the pre-axial border is internal and the originally
dorsal surface is anterior. The femur has at its proximal end a prominent head
for articulation with the acetabulum. External to this is a prominent process,
the great trochanter. Internally there is a
much smaller lesser trochanter, while a small
process or third trochanter is situated on the
outer border a little below the great trochan-
ter. At the distal end of the femur are two
prominences or condyles, with a depression
between them. Two small sesamoids or jabellce
are situated opposite this distal end on its
posterior aspect. Opposite the knee-joint, or
articulation between the femur and the tibia, is
a larger bone of similar character-the knee-
cap or patella. The tibia has at its proximal
end two articular surfaces for the condyles of
the femur. Distally it has also two articular
surfaces, one internal, for the astragalus, the
other for the calcaneum. The fibula is a FIG. 457·- Oryctolagus: Sac-
slender bone which becomes completely fused rum and innominate bones. V en-
tral aspect. acet. acetabulum; il.
distally with the tibia. ilium; isch. ischium; obt. obturator
pub, pubis; sacr. sacrum;
The tarsus (Fig. 458) consists of six bones of foramen;
sy. symphysis.
irregular shape, arranged in two rows, one of
the bones, the navicular (nav.), being intercalated between the rows. In the
proximal row are two bones, the astragalus (ast.) and the calcaneum (cal.),
both articulating with the tibia. The calcaneum presents behind a long cal-
caneal process. The distal row contains three bones, the mesocuneiform,
ectocuneijorm, and cuboid (cub.). The entocuneiform, which commonly forms
the most internal member of this row in other mammals, is not present as a
separate bone. 1
There are four m etatarsals, the hallux or first digit being vestigial and fused
with the second metatarsal in the adult. The proximal end of the second is
1 In all probability the homologies of these bones are as follows :-astragalus = intermedium
+ tibiale and proximal centralia ; calcaneum = fibulare; navicular = 1st and 2nd centralia;
entocuneiform = rst distale; mesocuneiform = 2nd distale ; ectocuneiform = 3rd distale; cuboid
= 4th and sth distalia and probably 3rd centralia.
666 ZOOLOGY
produced into a process which articulates with the navicular. Each of the
digits has three phalanges, which are similar in character to those of the manus.
The ccelom of the rabbit differs from that of birds and reptiles in being
divided into two parts by a transverse muscular partition, the diaphragm.
The anterior part, or thorax, contains the heart and the roots of the great
vessels, the lungs and bronchi, and the posterior part of the <:esophagus. The
cesophagus perforates the diaphragm to enter the posterior part, or abdomen
which contains the stomach and intestine, the liver and pancreas, the spleen,
the kidneys, ureters and urinary bladder, and the
organs of reproduction.
Alimentary Canal and Associated Structures.-The
teeth (Fig. 453) are lodged in sockets or alveoli in the
cal premaxillre, maxill::e, and mandible. In the premaxillre
nav are situated four teeth-the four upper incisors. Of
these the two anterior are very long, curved, chisel-
cun
shaped teeth, devoid of roots and growing throughout
life from persistent pulps. Enamel is present, and
forms a thick layer on the anterior convex surface,
which accounts for the bevelled-off character of the
distal (occlusal) end. The layer of enamel is much
harder than the rest of the tooth, which therefore wears
away more quickly at its cutting extremity. Along
the anterior surface is a longitudinal groove. The
second pair of upper incisors are small teeth lodged just
behind the larger pair. In the lower jaw are two
y
incisors, which correspond in shape with the anterior
pair of the upper jaw, but lacking any longitudinal
groove. The remaining teeth of the upper jaw are
FIG. 458.-0ryctolagus: lodged in the maxillre. Canines, present in most mam-
Pes. ast. astragalus; cal.
calcaneum; cub. cuboid; mals as a single tooth on each side, above and below,
cun. cuneiforms ; nav. nav.
icular. are here entirely absent, and there is a considerable
space, or diastema between the incisors and the pre-
molars, the teeth next in order. Of these there are three in the upper jaw and
two in the lower. They are long and curved with persistent pulps like the
incisors. The first upper premolar is smaller than the others and of simple
shape. The rest have each a longitudinal groove on the outer side and a
transverse ridge on the crown. The lower premolar has two grooves ; the
second is similar to those of the upper jaw. Behind the premolars are the
m olars, of which three occur on each side both in the upper and lower jaws.
Opening into the cavity of the mouth, or buccal cavity, are the ducts of four
pairs of salivary glands. These are the parotids (situated below the pinnre),
the injraorbitals (below the eyes), the su bmaxillaries (Fig. 459, s. mx. gl.) (near
 PHYLUM CHORDATA 667

the angles of the jaw) and the sublinguals (beneath the tongue). The saliva
moistens and lubricates the food. On the floor of the mouth is the muscular
tongue, covered with a mucous membrane beset with many papillre, on certain
of which the taste-buds (p. 137) are situated. The roof of the mouth is formed
by the hard palate, which is crossed by a series of transverse ridges of its mucous
membrane. Posteriorly the hard palate passes into the soft palate, which ends
behind in a free pendulous flap, the uvula, in front of the opening of the posterior
nares. When the animal propels food from the buccal cavity towards the
resophagus, the soft palate and uvula rise and block the flow of air from the

FIG. 459--ot"flcrolagus: Bead, neck, and thoraz. Lateral view. The head and spinal column
are represented in mesial vertical section. The left lung is removed. The greater part of the
nasal septum is removed so as to show the right nasal cavity with its turbinals. aort. dorsal aorta;
p. hy. basi-hyal; cbl. cerebellum; cer. cerebral hemispheres; cor. v. coronary vein; dia, dia-
phragm; ep. epiglottis; eu. opening of Eustachian tube into pharynx ; 1ar. larynx; 1. j. v, left
jugular vein; 1. sb. a. left subclavian artery; l. sb. v. subclavian vein ; max. maxilla; med. m edulla
oblongata; mes.eth. mesethmoid; mx. trb. maxillo-turbinal; res. cesophagus; olf. olfactory bulb;
pl. art. pulmonary artery; p.max. premaxilla; pr. st. presternum; pt. c. post-caval vein; rt.l. lng.
root of left lung with bronchus and pulmonary veins and artery cut across ; s. gl. sub-lingual
salivary glands; s. mx. gld. sub-maxillary salivary gland; st. sternebrre; tng. tongue: tr. trachea;
trb. ethmo-turbinals; vel. pl. soft palate.

nostrils, and at the same time prevent food from entering the nares. At the
anterior end of the palate a pair of openings, the nasopalatine or anterior pala-
tine canals, lead into the nasal chambers, and into them open a pair of tubular
structures, the organs of jacobson (Fig. 460, jcb.). These are enclosed in carti-
lage and situated on the floor of the nasal cavities. Behind the mouth or
buccal cavity proper is the pharynx, which in the rabbit is not sharply marked
off from the buccal cavity, but begins where the hard palate ends. The
pharynx is divided into two parts, an upper or nasal division (nasopharynx)
and a lower or buccal division (oro-pharynx) by the soft palate. The aperture
of the posterior nares is continuous with the nasal division, at the sides of which
are the lower or pharyngeal openings of the Eustachian tubes. The naso-
pharynx is continuous with the oro-pharynx round the posterior free edge of
668 ZOOLOGY

the soft palate. From the oro-pharynx leads ventrally the slit-like opening of
the glottis 1 into the larynx and trachea. Attached externally to larynx and
trachea are the paired lobes of the endocrine thyroid gland (p. 151) and directly
adjacent are the minute parathyroid glands (p. 152). These structures are endo-
crine glands and not of course part of the digestive system ; nevertheless they
are conveniently noted here. The thyroids secrete an iodine-containing hor-
mone thyroxin which is important in general metabolism. If iodine is in-
sufficient in the diet (as is often the case in certain mountain regions) the thyroid
in turn receives insufficient quantities
from the blood-stream and the patho-
logical condition of goitre results in
animals, including Man. The para-
thyroids secrete parathormone which
controls aspects of calcium meta-
bolism. Removal causes death. Over-
hanging the glottis is a leaf-like mov-
able flap, the epiglottis (Fig. 459, ep.),
formed of a plate of elastic cartilage
covered with mucous membrane. Just
inc as the uvula closes off the anterior part
of the respiratory tract during de-
glutition or swallowing, the epiglottis
closes off the latter part, thus protect-
ing the bronchi and lungs from entrant
and injurious food-particles. Accident-
ally aspirated particles are repelled by
Fro. 46o.-o-ryctolagus: Nasal region. a defensive and explosive cough-reflex.
Vertical section through the anterior part of the
head. inc. section of larger incisor tooth; jcb. Behind the pharynx comes the ceso-
lumen of Jacobson's organ, surrounded by phagus or gullet (ces.) This is a narrow
cartilage; lcr. dct. lachrymal duct; max.
maxilla; max. trb. maxillary turbinals; nas. but dilatable muscular tube which runs
nasal bone; nas. pal. naso.palatine canal; sept.
cart. cartilaginous nasal septum. (After Krause.) backwards from the pharynx through
the neck and thorax, enters the cavity
of the abdomen through an aperture in the diaphragm, and opens into_ the
stomach. Food is propelled into the stomach by waves of peristaltic smooth
muscle contraction under involuntary control.
The stomach (Fig. 461) is a capacious sac, much wider at the cardiac end,
at which the <:esophagus enters, than at the opposite or pyloric end, from which
the comminuted food passes through a circular muscle, the pyloric sphincter, into
the duodenum (du. and du ') the first part of the small intestine. The duodenum

1 The term glottis is more strictly applied not to this slit, but to the slit-like aperture between
two folds of the mucous membrane within the larynx-the vocal cords-which constitute the chief
parts of the vocal apparatus.
 PHYLUM CHORDATA 66g

FIG. 461 .- Oryctolagus: Visceral relationships. The stomach, duodenum , posterior portion of
rectum a nd liver (in outline) with their arteries, veins, and ducts. A, the creliac artery of a nother
specimen (both X f). The gullet is cut through and the stomach somewhat displaced backwards
to show the ra mifications of the cre liac artery (ere. a.) ; the duodenum is spread out to the right of
the subject t o show the pancreas (pn.); the bra nches of the bile-duct (c. b. d.), portal vein (p. v .),
and h epatic artery (h. a .) are supposed t o be traced some distance into t h e various lobes of the
liver. a.m. a. anterior mesenteric artery; catt. caudate lobe of liver with its artery, vein, and bile-
duct ; c. b. d. common bile-duct; cd. st. cardiac portion of stomach ; c. il. a. common iliac artery;
cre. a. creliac artery; cy. a. cystic a rtery; cy. d. cystic duct; d. ao. dorsal aorta; dtt. proximal,
and du' . distal limbs of d uodenum; du . a. duodena l artery ; du. h. a . (in A ), duodena -hepatic
artery; g. a. gastic artery and vein; g. b. gall-bla dder; h. a. hepatic artery; h. d. left hepatic
duct; l . c. left central lobe of liver, with its artery, vein, and bile-d uct; l. g. v. lieno-gastic vein ;
l . l . lateral lob e of liver with its artery, vein, and bile-duct; ms. branch of mesenteric artery and
vein to duodenum ; ms. r. m esorectum ; m. v. chief mesenteric vein; res. resophagus; p. m . a.
post erior mesenteric artery; p. m. v. posterior mesenteric vein; pn. pancreas; pn. d. pancreatic
duct; p. v. porta l vein; py. st. pyloric portion of st omach; ret. rectum ; r. c. right central lobe
o f liver, with artery, vein, and b ile -duct ; spg. Spigelia n lobe of liver with its artery, vein, a nd bile
duct; spl. spleen; sp. a. splenic artery. (After T. J. Parker .)

VOL. II. TT
 ZOOLOGY
is an elongated, narrow, greatly coiled tube curved into a U-shaped loop. It
is succeeded by the coiled £leum which ends at an ileo-col£c sphhzcter and a
rounded expansion, the sacculus rotundus. At the junction of small and large
intestine is the ccecum, a long, wide, capacious, thin-walled pouch which ends in
a small blind vernviform appendix. The crecum exhibits a spiral constriction
which indicates the internal presence of a narrow spiral valve. In herbivorous
animals (such as rabbits) the crecum is an important organ of digestion and
absorption. Leaving the crecum close to the entrance of the ileum is the colon,
a wide sacculated tube which is continued into a narrow, smooth-walled, pos-
terior rectum (ret.), which opens to the exterior by an anal orifice. The intestine,
like that of the pigeon, is attached throughout its length to the dorsal wall of
the abdominal cavity by a mesentery, or double fold of peritoneum.
The liver, attached to the diaphragm by a fold of peritoneum, has its sub-
stance imperfectly divided by a series of fissures into five lobes. From each
lobe arises a tiny hepatic duct and these unite mutually and with the cystic duct
from the thin-walled gall-bladder, situate in a depression on the right posterior
surface of the liver. The common b£le-duct (c. b. d.), formed by the union of the
cystic and hepatic ducts, opens into the dorsal aspect of the duodenum near the
pylorus.
The pancreas (pn.) is a diffuse compound gland of combined exocrine and
endocrine function lying in the fold of mesentery passing across the loop of the
duodenum. Its single duct, the pancreatic duct (pn. d.), opens into the distal
and ascending limb of the loop. In addition to the enzymes of the pancreatic
fluid a further complement is secreted in the succus enteriws produced by the
glandular walls of the duodenum. From this region, too, arises the portion of
the hepatic portal system which carries absorbed foodstuffs up to the liver
(p. II2).
Rabbits share with the Common Hare, shrews, and probably other herbi-
vores the habit of refect£on : they consume their own freshly voided freces.
It is probable that they thus obtain quantities of B complex vitamins that have
been produced by bacterial action in the large intestine.
Blood Vascular System.-The heart (Fig. 462) is situated in the cavity of the
thorax, a little to the left of the middle line and lies in a space, the mediastinum,
between the two pleural sacs enclosing the lungs. The mediastinum is divisible
into anterior, dorsal, median, and ventral parts. In the anterior part lie
the posterior part of the trachea, the neighbouring parts of the resophagus and
of the thoracic duct of the lymphatic system, the roots of the great arteries and
the veins of the pre-caval system, and the phrenic, pneumogastric, and other
nerves. In the dorsal part are situated the posterior part of the resophagus,
the thoracic part of the dorsal aorta, the pneumogastric nerve, the azygos vein,
and the thoracic duct. The middle part is the widest, and lodges the heart and
roots of the aorta and pulmonary artery enclosed in the pericardium, the pos-
 PHYLUM CHORDATA

innominate
ascending artery left subclavian a.
aorta
rt. superior
vena cava

semi-lunar
valves

rt. pulmonary v. ft. pulmonary v.

aortic
valves
rt.
ft. a-v. (bicuspid)
valve

ventricular
septum
ventricle

inferior
vena cava

oesophagus

FrG. 462.-0ryctolagus: Heart and associated structures. Ventral aspect, The arrows
indicate the general direction of blood-flow. On the left side oxygenated blood (received from
both left and right pulmonary veins) empties into the left atrium, flows through the left atrio-
ventricular (bicuspid, mitral) valve into the ventricle, and is propelled into the general circulation
through the aortic valve. Into the right side comes deoxygenated blood (received from the
general circulation via the right superior vena cava (top arrow) and left superior vena cava (lowest
arrow). It flows into the ventricle through the right atrioventricular (tricuspid) valve (partly
dissected out) and is propelled towards the semilunar valves into the pulmonary arteries and the
lungs. The ligamentum arteriosum is a solid fibrous vestige of the ductus arteriosus that con-
nects the pulmonary and systemic arches. It takes origin from the distal part of th(! left sixth
arterial arch of the embryo. (:\1odified after Young.)

terior portion of the pre-caval veins, the phrenic nerves, the terminal part of
the azygos vein, and the roots of the lungs. The ventral part contains only
areolar tissue with the lobular lymphoid thymtts gland of undetermined function.
The pericardia! membrane enclosing the heart consists of two layers, a parietal,
forming the wall of the pericardia! cavity, and a visceral, immediately investing
the heart. Between the two is a narrow cavity containing a little lubricating
 ZOOLOGY
pericardia! fluid. In general shape the heart resembles that of birds-it has
the apex directed backwards and slightly to the left, and the base forwards.
Like that of the Pigeon, it comprises right and left auricles and right and left
ventricles. The right and left sides of the heart have their cavities completely
separated from one another by inter-auricular and inter-ventricular partitions.
Venous blood comes into the right auricle from the right and left pre-caval
veins and the single post.caval. The first opens into the anterior part, the
second into the left-hand side of the posterior portion, and the third into the
dorsal surface (Fig. 462). In the wall of the right auricle is a small node of
specialised tissue, the sinu-auricular node or pace-maker from which originates
the heart-beat. The conduction apparatus of the excitation wave, involving
the auriculo-ventric~tlar node, auriculo-ventricular bundle and the Purkinje net-
work of the Mammalian heart, is described on p. 882. From the inner surface
of the auricle arise numerous cords of muscle fibres, the musculi pectinati.
A membranous fold, the remnant of the fretal Eustachian val·De, extends from
the opening of the post-cava forwards towards the auricular septum. The
opening of the left pre-cava is bounded behind by a crescentic fold, the valve of
Thebesius. On the septum is an oval area, the fossa ovalis, where the partition
is much thinner than elsewhere. This marks the position of an aperture, the
foramen ovate, that occurs in the fretus. The crescentic anterior rim of the
aperture is known as the annulus ovalis. The cavity of the right auricle com-
municates with that of the right ventricle by the wide right auriculo-ventricular
opening. This is guarded by a large valve, the tricuspid, composed of three
membranous cusps. These are so arranged and attached that whilst they flap
back against the walls of the ventricle to allow the passage of blood from the
auricle to the ventricle, they meet together across the aperture so as to close the
passage when the ventricle contracts. The cusps of the valve are attached to
muscular processes of the wall of the ventricle, the musculi papillares, by
means of tendinous threads called the chordee tendinece. The right ventricle,
much thicker than the auricle, forms the right side of the conical apical portion,
but does not extend quite to the apex. Its walls are raised up into muscular
ridges called columnce carnece. It gives off in front, at its left anterior angle,
the pulmonary artery, the entrance to which is guarded by three pouch-like
semilunar valves (Fig. 462).
The left auricle receives oxygenated blood from the lungs. The right and
left pulmonary veins open together into the cavity of this auricle on its dorsal
aspect. A large left auriculo-ventricular opening leads from the cavity of the
left auricle into that of the left ventricle. This is guarded by a mitral valve
consisting of two membranous cusps with chordee tendinece and musculi
papillares. In the walls of the ventricle are columnce carnece, rather more
strongly developed than those in the right. At the basal (anterior) end of the
left ventricle is the opening of the aorta, guarded by three semilunar valves
 PHYLUM CHORDATA

similar to those at the entrance of the pulmonary artery. The coronary arteries,
which supply the cardiac muscle, are given off from the aorta just beyond the
semilunar valves. The corresponding vein opens into the terminal part of
the left pre-cava. The pulmonary artery divides into right and left branches
which carry venous blood to the corresponding lungs for oxygenation.

til

FrG. 463.-0ryctolagus: Blood vascular system.

The heart is somewhat displaced towards the left
of the subject; the arteries of the right and the
veins of the left side are in great measure removed.
a. epg. internal mammary or anterior epigastric
artery; a. f. anterior facial vein; a. m. anterior
mesenteric artery; a. ph. anterior phrenic vein;
az. v. azygos vein; br. brachial artery; c. il. a.
common iliac artery; c. il. v. hinder end of post- a .ph.
caval; em. cceliac artery; d. ao. dorsal aorta; e. c.
external carotid artery; e. il. a . external iliac
artery; e. il . v. external iliac vein; e. ju. external
jugular vein; fm . a. femoral artery; fm . v. femoral
vein; h. v. hepatic veins; i. c. internal carotid
artery; i. cs. intercostal vessels; i . ju . internal
jugular vein; i. l . ilio-lumbar artery and vein; in.
innominate artery; !. au. left auricle; l. c. c. left
common carotid artery; l. pr. c. left pre-caval
vein; l . v. left ventricle; m. sc. med ian sacral
artery ; p. a. pulmonary artery; p. epg. posterior
epigastric artery and vein; p. f. posterior facial
vein; p. m. posterior mesenteric artery; p. ph .
posterior phrenic v eins; pt. c. post-caval vein;
p. v. pulmonary vein; r. renal artery and vein;
r. au. right auricle; r. c. c. right common carotid
artery; r. pr. c. right pre-caval vein; r. v. right
ventricle; set. a. right subclavian artery; scl. v.
subclavian vein ; spm. spermatic artery and vein ;
s. vs. superior vesical artery and vein; ut. uterine
artery and vein; vr. vertebral artery. (After
T . J. Parker.)

From the base of the left ventricle the aorta takes oxygenated blood to a
system of arterial vessels by which it is conveyed throughout the body. The
aorta first runs forward, then bends round the left bronchus, forming the arch
of the aorta (Figs. 462, 463), to run through the thorax and abdomen, in
close contact with the spinal column, as the dcrsal aorta (d. ao.). From the
 ZOOLOGY
arch of the aorta are given off two large arteries, the innominate (in.) and the
left subclavian. The innominate divides to form the right subclavian (s.cl. a.)
and the right (r. c. c.) and left (l. c. c.) common carotid arteries. The subclavian
passes to the fore-limb as the brachial artery, giving origin to the vertebral
artery. After passing up through the vertebrarterial canal the vetebral artery
supplies branches to the spinal cord, finally entering the cranial cavity and hind-
brain. The subclavian also gives off the anterior epigastric or internal mam-
mary, which supplies the thoracic parieties. 1
The right carotid divides opposite the angle of the jaw into internal and
external carotids. The left carotid and left subclavian correspond to the right
carotid and right subclavian in their branches and distribution. If carefully
looked for, the slight dilation of the carotid simts can be seen at the origin of
the internal carotid, the common carotid having been pulled to one side (the
dissector going in through the neck). The carotid sinus contains receptors
(baroceptors) that are sensitive to changes in blood pressure. These originate
reflexes that tend to stabilise blood pressure. Thus, nerve impulses are sent to
cardiac and vasomotor centres in the medulla oblongata. A rise in arterial
pressure causes a reflex slowing of the heart and vasodilation, thus restoring
blood pressure to near its previous level. The reverse process also occurs.
Passing through the thorax the aorta gives off a series of small paired
intercostal arteries (i. cs.). In the abdomen its first large branch is the cceliac
artery (cce.), which supplies the liver, stomach, and spleen (p. 105). Behind
this it gives origin to the anterior mesenteric (a.m.), which supplies the intestine
and the pancreas. Opposite the kidneys it gives off the two renal arteries (r.),
and a good deal farther back the spermatic (spm.) or ovarian arteries as the case
may be. Just in front of the origin of the spermatic arteries is given off a
posterior mesenteric (p. m.), which supplies the hinder part of the rectum. A
series of small lumbar arteries supply the side-walls of the abdominal cavity.
Posteriorly the dorsal aorta divides to form the two common iliac arteries
(c. il. a.) which supply the pelvic cavity and hind-limbs. A small median sacral
(caudal) artery (m. sc.) passes backwards in the middle line to supply the caudal
region.
The system of caval veins which bring venous blood to the right auricle
consists of the right and left pre-cavce and of the single post-cava. The right pre-
cava is formed by the union of the right jugular (e.ju.) vein and right subclavian
(scl. v.). The azygos vein (az. v.), the right anterior intercostal (i.cs.), and the
right anterior epigastric or internal mammary also opens into it. The left pre-
cava receives a series of veins similar to those forming the right, except that
there is no azygos on the left side.
The post-caval vein (pt. c.) is formed in the hinder part of the abdominal
cavity by the union of the internal iliacs (i. il. v.) which bring blood from the
1 In any species there may be much variation in the mode of origin of the aortic arch branches.
 PHYLUM CHORDATA

back of the thighs. Shortly after its origin it receives the two external iliacs
(e. il. v.) also coming from the hind-limbs. In front of this a pair of ilio-lumbar
(i.l.) veins join it. A little farther forward the post-cava is joined by a pair of
spermatic (spm.) or ovarian veins and opposite the kidneys by a pair of renal
veins (r.). The post-cava ultimately also receives by means of the hepatic
veins blood originally carried to the liver by the hepatic portal system, after the
passage of such blood through the liver. The principal veins of the portal
system are the lieno-gastric, duodenal, anterior mesenteric, and posterior mesen-
teric which unite to form a single, large portal vein (Fig. 461, p. v.). There is no
trace of a renal portal system (pp. rr2, 158).
The red blood corpuscles are circular, bi-concave, non-nucleated discs. The
spleen is an elongated, compressed, dark-red body, situated in the abdominal
cavity in close contact with the stomach to which it is bound by a fold of
peritoneum, the gastro-splenic mesentery. The spleen is highly vascular and its
functions include blood storage and the phagocytosis, by means of its reticulo-
endothelial cells, of effete red cells. It has a fibrous capsule containing some
smooth muscle from which anastomising trabeculre extend inwards. In the
resultant interstices lies the splenic pulp into which arterioles discharge into
sinusoids and from which venules take up blood for venous return. The stored
blood can be rapidly driven into the general circulation in emergency by the
contraction of the capsule which is under sympathetic control (see p. rr9).
Lymphatic System.-Although relatively difficult to demonstrate, the lym-
phatics, associated with the capillaries, ramify through almost every part of
the body. Except in the spleen and parts of the liver, blood does not come in
actual contact with the tissues-tissue-spaces lie between.
The capillary walls are highly permeable to all but the largest molecules,
i.e. proteins. These are retained in the capillary, and exert an osmotic pressure
into the capillary of about 25 mm. Hg. The otttward movement is determined
by the excess hydrostatic pressure over this osmotic pressure at the arteriolar
end. At the venule end, the hydrostatic pressure is less than osmotic pressure
and so water and its solutes (e.g. metabolites) move back in again.
The proportion of the tissue-fluid that passes into the lymph capillaries
(lymphatics) as lymph is still almost identical with tissue fluid. Lymph contains
much the same inorganic components as blood as well as some protein. It
contains also lymphocytes, but lacks red cells and blood platelets which cannot
penetrate the blood capillary walls. The lymph vessels are interrupted at
various points by lymph nodes (containing reticular cells) which produce lym-
phocytes and macrophages which phagocytose bacteria and other invasive
particulate matter.
Emulsified and partly digested fats are carried as chyle from the lymphatics
of the small intestine. Lacking lymph-hearts (p. rr6), but well-equipped with
valves and so situated that muscle movement moves the fluid onwards, the
 ZOOLOGY
immense mammalian lymphatic network finally canalises its contents into the
blood-stream via the thoracic duct (Fig. 464). This passes anteriorly through
the thorax and opens into the left external jugular vein near its junction with
the subclavian. Lymphatics supplying the left side of the head, neck, and
arm drain into the thoracic duct near its outlet. In some animals about three-
quarters of each lung, and a large proportion of the peritoneum, drain into the
right lymph duct, which enters the right external jugular directly.

lt. and rt.

lt. internal common rt. internal
jugular v. carotids jugular v.

subclavian v.
lt. subclavian v.
FIG. 464.-Homo: Cardiac
region and thoracic duct. The
ft. subclavian a. pericardium has been dissected at
trachea ------,1----tt-t{ its line of reflection from the vessels
entering the heart. The descend-
oesophagus ---;::t=~m­ ing thoracic aorta has been re-
aorta - - - - - r moved to reveal the course of the
vagal nerves which form the
<esophageal plexus and sub-
sequently separate. The azygos
vein has been cut near its entrance
rt. pulmonary v. into the superior vena cava. The
left recurrent pharyngeal nerve as
shown (after leaving the vagus)
inferior hooking anteriorly (*) around the
top of the descending aorta to the
ligamentum arteriosus (ductus
arteriosum) which is not shown.
The receptaculum chyli is easily
seen in the rabbit. (Modified after
Boyd, et al.)

Respiratory System. The larynx (Fig. 465) is a chamber with walls supported
by cartilage, lying below and somewhat behind the pharynx, with which it
communicates through a slit-like aperture, the glottis. The cartilages of the
larynx are, in addition to the epiglottis (p. 465), the large thyroid (th.), which
forms the ventral and lateral walls, the ring-like cricoid (cr.), the two small
arytenoids (ary.), and a pair of small nodules, the cartilages of Santorini (sant.),
situated at the apices of the arytenoids. The vocal cords extend across the
c(lvity from the thyroid below to the arytenoids above. The larynx is essen-
tially a mechanical valve at the entrance to the air-way, capable of preventing
or of regulating both the entrance and the exit of air from the bronchial pas-
sages and the lungs, in response to changing physiological requirements. Its
 PHYLUM CHORDATA

permanent patency is ensured by the cricoid cartilage being completely ring-

shaped, unlike the remaining laryngeal cartilages and the incomplete rings of
trachea and bronchi.
Leading backwards from the larynx is the A
trachea or wind-pipe (Fig. 465, tr.), a long
tube the wall of which is supported by
cartilaginous rings which are incomplete ar,;y
dorsally. The trachea enters the cavity of
the thorax, and there divides into the two
bronchi, one passing to the root of each FIG. 465.-0ryct.olagus: Larynx.
lung. A, ventral view; B, dorsal view. ary.
arytenoid; cr. cricoid; ep. epiglottis;
The lttngs (Fig. 466) are enclosed in the sant. cartilage of Santorini; th. thyroid
cartilage; tr. trachea. (From Krause,
lateral parts of the cavity of the thorax. after Schneider.)
Each tri-lobed lung lies in a cavity, the
pleural sac, lined by a pleural membrane. The right and left pleural sacs are
separated by a considerable interval
owing to the development in the partition
between them (mediastinum) of the heart
and other organs. The lung is attached
to the mediastinum at its root only, and
there the pleural membrane is reflected
over it as a visceral layer (p. 107). In
this respect the Rabbit's lung differs
widely from that of birds. It differs also
in its minute structure. The bronchus,
entering at the root, divides and sub-
divides to form a ramifying system of
increasingly smaller tubes, each of the
ultimate branches of which, or terminal
FIG. 466.-0ryctolagus: Thoracic rela- bronchioles, opens into a minute chamber
tionships. Transverse section in the region or infundibulum, consisting of a central
of the ventricles to show the relations of
the pleur<e, mediastinum, etc. The lungs passage giving off a number of thin-
are contracted. aort. dorsal aorta; az. v.
azygos vein ; cent. centrum of thoracic walled alveoli. Each alveolus is en-
vertebra; l. lng. left lung; l. pl. left pleural meshed externally by a dense network of
sac; l . vent. left ventricle; my. spinal cord;
ws. resophagus; par. per. parietal layer of capillaries, which in the total lung com-
pericardium; pt. cav. post-caval, close to its prise a relatively immense
entrance into right auricle; r. lng. right
respiratory
lung ; r. pl. right pleural cavity; r. vent. surface. As the blood flows through
right ventricle; st. sternum; v. med. ventral
mediastinum; vis. per. visceral peritoneum. the capillaries, gaseous exchanges take
place between it and the air entering
and leaving the alveoli of the bronchial tract.
A group of infundibula (supplied by a single bronchiole, which divides therein
to form the terminal bronchioles) is termed a lobule of the lung. In gross shape
 ZOOLOGY
the lung is roughly conical, with the apex directed forwards. The base, which
is concave, lies, when the lung is distended, in contact with the convex anterior
surface of the diaphragm. The outer or costal surface is convex in adaptation
to the form of the side-wall of the thorax ; the internal or visceral surface is
concave.
Nervous System. The brain (Figs. 467, 468) of the Rabbit displays the
same principal parts as that of birds but with many differences in detail. The
A

m .b.

h.h

F IG. 467.-0ryctola.gus: Brain

and cranial nerves. A , d orsal, B ,
ventra l, C, lat eral view. b. o. ol-
factory bulb ; cb' . m edian lobe of
cerebellum (vermis); cb" .laterallobe
of cerebellum; cr. crura cere bri; ep .
epiphysis; f. b. parencephala; f. p.
lon gitudinal fissure; lt. b. cerebel-
lum; hp. hypophysis; m. b. mid-
b rain (corpora quadrigeminal ; md.
m edulla oblongat a ; pv. p ons Va-
rolii, the transverse fibres of which
are h ere not indicated; i- xii, cranial
nerves. (After Wiedersheim. )

surface of the two relatively long and narrow cerebral hemispheres of the
fore-brain (prosencephalon) present certain depressions or sulci , which, though
few and indistinct, indicate surface lobes and convolutions (gyri) not dis-
tinguishable in the case of pigeon or lizard. The cerebral cortex is arbitrarily
divided into paired frontal, parietal, temporal, and occipital lobes. Superficially
the hemispheres comprise a neopallium of grey cortical tissue, several cell-
layers thick, to which are carried impulses resulting from visual, auditory,
tactile, and other forms of peripheral stimulation. From circumscribed cortical
 PHYLUM CHORDATA 679

areas are initiated voluntary motor impulses which are carried via a bundle of
motor axones, the pyramidal tract. Association pathways of considerable
complexity link the various cortical areas, while the hemispheres are func-
tionally united via the commissural fibres of the corpus callosum (Figs. 468, cp.
cl.) and anterior commissure. Very large, club-shaped olfactory bulbs lie below
the anterior extremities of the cerebral hemispheres; behind each, on the ven-

FIG. 468.-oryctolagus: Brain. Two dissections from above (nat. size). In A the left
parencephalon is dissected down to the level of the corpus callosum : on the right the lateral
ventricle is exposed. In B the cerebral hemispheres are dissected to a little below the level of
the genu of the corpus callosum; only the frontal lobe of the left hemisphere is retained; of
the right a portion o f the temporal lobe also is left; the velum interpositum and pineal body are
removed, as well as the greater part of the body of the fornix, and the whole of the left posterior
pillar; the cerebellum is removed with the exception of a part of its right lateral lobe. a. co.
anterior commissure ; a. fo. anterior pillar of fornix ; a. pn. anterior peduncles of cerebellum;
b. fo. body of fornix; cbl. superior vermis of cerebellum; cb 2 • its lateral lobe; c. gn. corpus
geniculatum; c. h. cerebral hemisphere ; ch. pl. choroid ple xus; cp . cl. corpus callosum; cp. s.
corpus striatum; c. rs. corpus restiforme; d. p. dorsal pyramid; fl. flocculus; hp.m. hippo-
campus; m. co. middle commissure ; o. 11 • anterior, and o. 12 • posterior lobes of corpora quadri-
gemina; olf. olfactory bulb; o. th. optic thalamus ; o. tr. optic tract; p. co. posterior commissure;
p. fo. posterior pillar of fornix (tamia hippocampi); pn. pineal body; pd. pn. peduncle of pineal
body; p. pn. posterior peduncles of cerebellum; p. va. fibres of pons Varolii forming middle
peduncles of cerebellum; sp. lu. septum lucidum; st. I. stria longitudinalis; I. s. t renia semi-
circularis (narrow band of white ma tter between corpus striatum a nd optic t halamus) ; v. vn.
valve of Vieussens; v3 , third ventricle; v•, fourth ventricle. (After T . J. Parker.)

tral surface of the hemisphere, is the corresponding olfactory tract leading back
to a slight rounded elevation, the tuberculttm olfactorium. As mentioned above,
the two hemispheres are connected by a commissural structure which occurs
only in the Mammalia. This, the corpus callosum, runs transversely above the
level of the lateral ventricles. E'<amined in transverse section, i.e. in a longi-
tudinal section of the brain, the corpus callosum is seen to bend slightly down-
wards, forming the genu. Posteriorly it bends downwards and forwards,
forming the splenium, which passes forwards and is united with the subjacent
68o ZOOLOGY
fornix-a commissural narrow median strand of longitudinal fibres, which
bifurcates both anteriorly and posteriorly to form the so-called pillars of the
fornix-anterior and posterior (Fig. 468, p. fo.). Below the corpus callosum,
between it and the fornix, the thin inner walls of the hemispheres (septum
lucidum, sp. lu.) enclose a small, laterally compressed cavity, the so-called fifth
ventricle or pseudoccele. This is not a true brain-ventricle, but merely a space
between the closely-apposed hemispheres.
The lateral ventricles of the cerebral hemispheres are much more extensively
developed than in the brain of the pigeon. They are of somewhat complex
shape. Each consists of a middle portion, or body, roofed over by the corpus
callosum, a narrow anterior prolongation, or anterior cornu, a posterior cornu
(which runs backwards and inwards) and a descending cornu. This last passes
at first almost directly outwards, then downwards, and finally inwards and
forwards. On the floor of the body of the ventricle, and continued along the
whole extent of the descending cornu, is a prominent ridge of nearly semi-
circular transverse section-the hippocampus (hp. m.). This corresponds in
position with a groove, the hippocampal sulcus, on the inner surface of the tem-
poral lobe. Internally the two hippocampi merge in a median commissural
area-the psalterium or lyra.
Running along the anterior edge of the hippocampus is a ridge of fibres-
the tamia hippocampi or fimbria-which passes down into the descending cornu.
The union of the two trenire forms a median longitudinal strand, the body of the
fornix, which, as already explained, lies below the corpus callosum, continuous
with the splenium of the latter behind, but diverging from it anteriorly by
dipping down towards the base of the brain. In the angular space between
the corpus callosum and the fornix below is the septum lucidum. The trenire
hippocampi are the posterior pillars of the fornix (Fig. 468, p. fo.). The
anterior pillars (a. fo.) are a pair of vertical bands which pass from the anterior
end of the body downwards to the corpus mammillare (see below) at the base of
the diencephalon.
Lying immediately in front of the hippocampus is a vascular membrane, the
choroid plexus (ch. pl.) whose blood vessels secrete cerebrospinal fluid. This
passes inwards to join its fellow of the opposite side through a transverse
passage, the foramen of Monro, which opens behind into the diacrele. The
floor of the anterior cornu is formed of an eminence of grey matter-the
corpus striatum (cp. s.) which is, however, relatively restricted in mammals.
The right and left corpora striata are connected together by a narrow transverse
band of white fibres, the anterior commissure (a. co.), situated in front of the
anterior pillars of the fornix.
In the diencephalon the diac(J3le (v3), is a laterally compressed cavity, the
roof of which is formed by a delicate vascular membrane, the velum interpositum
(vl. ip.), in which there is a network of blood vessels (choroid plexus of the
 PHYLUM CHORDATA 681

diacrele) continuous with the choroid plexuses of the lateral ventricles. From
the posterior part of the roof of the diacrele arise the peduncles of the pineal body.
Forming the lateral portions of the diencephalon are large masses of mixed
grey and white matter, the thalami (o. th.). Various parts of these bodies are
in communication with the cortex and in particular, with the areas concerned
with olfactory, visual, auditory, visceral and other functions. The thalamus
is probably an important centre of integration and activity and has intimate
connexions with the underlying hypothalamus (see below) an ancient brain-floor
structure concerned with the regulation of fundamental visceral and somatic
activities. The thalami are connected by a mass of grey matter, the middle or
soft commissure (m. co.) (not represented in lower vertebrates), which passes across
the diacrele. A rounded elevation near the anterior end of the external surface
of each thalamus is the corpus geniculatum (c. gn.). The anterior boundary of
the diacrele is a thin vertical lamina, the lamina terminalis, of which the septum
lucidum is a mesial anterior prolongation. Below, the floor of the hypo-
thalamus is produced downward into a mesial process, the tuber cinereum or
infundibulum (inf.). Down this tract runs a hypophysial portal system of vessels
connecting the hypothalamus with the pars distalis or anterior 'lobe' of the
compound pituitary gland (see Fig. 96, p. 150). Anteriorly, on the ventral aspect
of the brain, is a thick transverse band of nerve-fibres, the united optic tracts,
from the anterior border of which the optic nerves are given off. Behind the
tuber cinereum, and formed as a thickening of its posterior wall, is a rounded
elevation, the corpus mammillare.
In the mid-brain (mesencephalon) the dorsal part is remarkable for the fact
that each optic lobe is divided into two by a transverse furrow, so that two
pairs of lobes (o.l 1., o.l2 .), the corpora quadrigemina, are produced. Between
the anterior lobes passes the delicate posterior commissure (p. co.). On the
ventral region of the mid-brain the crura cerebri are far more prominent than
in the lower vertebrate groups. In the hind-brain (rhombencephalon) the
cerebellum (Fig. 467, cb'., cb".) is large and complex. It is concerned with
neuromuscular co-ordination and equilibrium. It consists of a central lobe or
vermis and two lateral lobes, divided by very numerous fissures (or sulci) into a
large number of small convolutions (or gyri). Each lateral lobe bears an irre-
gularly-shaped prominence, the flocculus. On section (Fig. 468, cb.) the cere-
bellum exhibits a tree-like pattern (arbor vitce) from the arrangement of its
white and grey matter. On the ventral aspect of the hind-brain a flat band of
transverse fibres, the pons Varolii, connects together the lateral parts of the
cerebellum. The cerebellum is attached to the other parts of the brain by
three pairs of peduncles: the anterior, connecting it with the posterior optic
lobes, the middle, passing on each side into the pons Varolii, and the posterior,
connecting it with the dorsal portion of the medulla oblongata-a brain-
stem segment containing ancient centres concerned with fundamental metabolic
682 ZOOLOGY
activities such as respiration and heart-beat. Between the anterior peduncles
extends a transverse band 1 the valve of V ieussens connected by its anterior
edge with the corpora quadrigemina. Behind this is a short tract of transverse
fibres, the corpus trapezoideum, and behind this again is a slightly elevated area
marking the position of the olivary body. The floor of the fourth ventricle
presents a median groove which ends posteriorly in a pointed depression, the
calamus scriptorius, leading into the central canal of the spinal cord.
The Spinal Cord, Cranial, and Spinal Nerves.-As in reptiles and birds, there
are twelve pairs of cranial nerves (p. n6) associated with specific cerebral
regions, which, emerging through the several cranial foramina, convey afferent
and/or efferent fibres to various muscles and sensory organs mainly in the head
and neck although the Xth (vagus) and Xlth (accessorius) nerves range more
widely (p. 133).
The spinal nerves, too, are on much the same pattern as in reptiles and
birds. Each spinal nerve is attached to the cord by a sensory dorsal root and
a motor ventral root. From spinal nerves are given off dorsal and ventral
rami to various tissues and a ramus communicans ('white' or visceral ramus),
which is myelinated and in most cases communicates with a ganglion of the
sympathetic riervous system (see below).
The spinal cord in mammals is, like the brain, ensheathed in three meningeal
layers and consists of an outer white and an inner grey matter pierced by a central
canal which, with ciliated lining, continuous with the brain ventricles, contains
freely circulating cerebrospinal fluid. The grey matter is arranged into paired
dorsal and ventral columns which in cross-section appear as horns (p. 123).
Autonomic Nervous System.-This term covers two great functional com-
ponents of the visceral nervous system-the sympathetic and parasympathetic
-which involuntarily control many bodily activities and which are partly
independent_ of the central nervous system. When both sub-systems innervate
a particular organ they usually act antagonistically. Heart-beat, blood pressure
(to a considerable degree), the activity of smooth (unstriated) muscle of numer-
ous organs, secretion of certain endocrine and exocrine glands and many other
important activities are partly or wholly under autonomic control. The co-
ordination and control of the majority of such functions are located in special
centres in hypothalamus, medulla oblongata, and cord. The autonomic
system can be instantly stimulated to protective or stabilising reactions by
environmental stimuli acting through the exteroceptors and the central nervous
system, or by proprioceptive stimuli from within the organism.
The control of effector organs (glands, muscles, etc.) is not via single fibres
from the central areas of control. Instead, preganglionic fibres emerge from
the central nervous system to effect synapses with other distinct nerve cells
situated in sympathetic ganglia, the (postganglionic) fibres of which transmit
impulses to the target-organs (Fig. 82, p. ng).
 PHYLUM CHORDATA

Sympathetic System (Fig. 82, p. ng).-Prominent chains of sympathetic

ganglia extend from cervical to sacral regions on each side of the vertebral
column. In the thoracic and lumbar regions these receive usually white myelin-
ated preganglionic fibres from ventral nerve roots of the nearby cord (' thoraco-
lumbar outflow'). In these ganglia the preganglionic fibres may establish
synaptic relationship with cells giving rise to grey, unmyelinated post-ganglionic
fibres, or they may traverse the ganglia uninterruptedly to synapse in more
distant ganglia (e.g. cceliac, mesenteric ganglia). In either instance the efferent
impulses are effectively transmitted to the relevant organs.
The anterior and posterior ends of each sympathetic chain also present
ganglia, but these are not connected with the spinal cord. Like the other
ganglia they each give off a grey ramus composed of postganglionic fibres which
supply skin, the smooth muscle of blood vessels and other structures. From
the large anterior ganglia also arise afferent fibres supplying the eyes, tear-
glands, nasal mucosa, and salivary glands.
Sympathetic stimulation causes, among many other effects, increased heart-
beat, rise in blood pressure, bronchial dilation, peripheral vascular contraction,
sweat-gland activity, pupillary dilation, arrector pili contraction, salivary
inhibition, contraction of certain sphincters, splenic contraction (with resultant
increased volume of circulating blood), increased blood-sugar content-in short,
the animal exhibits symptoms of fear or anger and is made ready for fight or
flight. Increased output of adrenalin, the hormone from the medulla of the
adrenal gland (p. 152), leads to such effects. 'Cold sweats , pallor, 'goose
flesh' (and the related erection of hair in fear) are some equivalent phenomena
familiar to timid or suddenly startled persons. Sympathetic fibres are adren-
ergic or sometimes (as in sweat-glands) cholinergic (see below).
Para-sympathetic System (Fig. 82, p. ng).-Preganglionic fibres of the
broadly antagonistic parasympathetic system leave the central nervous system
by two disti.nct routes-(1) in the vagus (Xth) and other (IIIth, Vllth, and
IXth) cranial nerves and (2) through ventral spinal roots in the sacral region of
the spinal cord ('craniosacral outflow'). The ganglia which receive para-
sympathetic fibres are situated close to, or even within, the organs supplied,
hence their post-ganglionic fibres are short. This system is less extensive than
the sympathetic because it does not extend to the skin or the limbs.
Parasympathetic stimulation leads to dilation of blood vessels (except the
intrinsic coronary circulation of the heart), heightened salivary secretion,
constriction of bronchi, increased peristalsis, sphincter relaxation, and pupillary
constriction. In general the above states are relaxative and opposed to those
induced by sympathetic stimulation. Parasympathetic fib:.:es are cholinergic :
their axone terminals secrete acetylcholine upon the arrival thereat of neural
impulses. The acetylcholine is then quickly destroyed by an enzyme,
cholinesterase.
 ZOOLOGY
Organs of Special Sense.-The more obvious sense-organs, eyes (p. 139),
audio-equilibration (p. 143), olfaction (p. 138), taste (p. 137), as well as others
(pp. 135, 137), have been mentioned elsewhere.
Urinogenital Organs.-The kidneys are of somewhat compressed oval shape,
with a notch or hilum on the inner side. They are in close contact with the
dorsal wall of the abdominal cavity, the right being somewhat in advance of
the left. Towards the hilum, the tubules of the kidney converge to open into
a wide chamber, the pelvis, which forms the dilated commencement of the ureter.
When the kidney is cut across, its substance is seen to be divided into a central
mass or medulla and a peripheral portion or cortex (see p. 154). The ureter
(Fig. 469, ur.) runs backwards to open, not into a cloaca, but directly into the

FIG. 46g.-OTyctolagus:
Urinogenital system. Left, Male;
Right, Female; from the left side
(half nat. size). The kidneys and
proximal ends of the ureters, the
testes, ovaries, Fallopian tubes
and uteri are not shown. an.
anus; bl. urinary bladder; c. c.
corpus cavernosum; c. s. corpus
spongiosum; c. gl. Cowper's gland;
g. cl. apex of clitoris; g. p. apex of
penis ; p. gl. perineal gland; p. gl'.
aperture of its duct on the perineal
space; pr. anterior, pr'. posterior,
and pr". lateral lobes of prostate;
ret. rectum; r. gl. rectal gland;
u. g. a. urinogenital aperture ; u . m.
uterusmasculinus ; ur. ureter; va.
vagina ; vb . v estibule ; v . d. v a s
deferens. (After T. J . Parker.)

u.g.a. !l·cL rtg.a an

urinary bladder (bl.). The latter is a pyriform sac with muscular walls which
vary in thickness, according as the organ is dilated or contracted. In the male
the openings of the ureters are situated nearer the posterior narrower end or
neck than in the female. Adjacent to the anterior end of each kidney is a
compound adrenal (suprarenal) gland.
In the male rabbit the testes are oval bodies, which, though in the young
animal they occur in the body cavity, later pass backwards and downwards
and come to lie each in a scrotal sac situated at the side of the urinogenital
opening. The cavity of each scrotal sac is in free communication with the
cavity of the abdomen by an opening-the inguinal canal. The testes each
contain long, convoluted seminiferous tubules which elaborate spermatozoa and
between the tubules lies an interstitium formed of L eydig cells which produce
testosterone, the male sex hormone (see p. 152). The sperms are carried by a
 PHYLUM CHORDATA 6Bs
convoluted epididymis, closely adherent to the testis, which forms the proximal
part of the vas deferens. The vasa deferentia (v. d.) terminate by opening into a
urinogenital canal, or urethra, into which the neck of the urinary bladder is
continued. A prostate gland (pr.), which contributes to the bulk of the seminal
fluid, and which is influenced in its secretion by the male sex hormone, sur-
rounds the commencement of the urethra, the neck of the bladder, and the
terminal parts of the vasa deferentia. A dorsal diverticulum of the urethra,
the uterus masculinus (u . m .), lies embedded in the prostate gland close to the
neck of the bladder. A small pair of ovoid Cowper's glands (c. gl.), lie just
behind the prostate close to the side of the urethra. These, sometimes called
the bulbo-urethral glands, secrete
a clear alkaline fluid which may
neutralise urinary residues and, in
addition, vaginal acidity.
The terminal part of the
urethra traverses a cord of vas-
cular tissue, the corpus spongiosum
(c. s.), which forms the dorsal por-
tion of the penis. The greater part
of the penis is formed of two
closely approximated firm cords of
vascular tissue, the corpora caver-
nosa (c. c.). These are attached flt r.ut
proximally to the ischia, and FIG. 470.-0ryctolagus: Internal genitalia of
female. The a nterior end of the vagina, with the
terminate in a pointed apex (g. p.). right uterus, Fallopian tube and ovary (nat. size).
.
A Ioose f 0 ld 0 f Sk m, h Part of the ventral wall of the vagina is rem oved,
t e prepuce, and the proximal end of the left uterus is shown in
encloses the penis. A pair of longit ud ina l section. ft. t. Fallopian tube; ft. t'.
its peritoneal aperture; l. ut. left uterus; l. ut'.
glandS With an OdOfOUS Secretion, left OS uteri; OV. right ovary; r . ut. right uterus;
the perinceal glands (p. gl.), open r. ut'. right os uteri; s. vaginal septum; va. vagina.
(After T. J. Parker.)
on the perinceal space at the base
of the penis. Two other glands, the rectal glands (r. gl.), lie on either side of the
rectum.
In the female the ovaries (Fig. 470 ov.) are small ovoid bodies attached to
the dorsal wall of the abdomen behind the kidneys. These produce female sex
hormones (cestrogens) and ova. The Graafian follicles enclosing the ova form
extremely small rounded projections on their outer surface.
The eggs are shed into the abdominal cavity and enter the wide funnel-
shaped openings ((fl. t'.), with fimbriated or fringed margins) of the paired ovi-
ducts or Fallopian tubes (fl. t.). Posteriorly each tube passes into a thick-walled
uterus (r. ut.). The two uteri open separately into a median tube, the vagina
(va.). The vestibule (Fig. 469 B, vb.), or urinogenital canal, is a wide median
passage, into which the vagina and the bladder open. On its ventral wall is a
VOL. II. UU
686 ZOOLOGY

small, hard, rod-like body, the clitoris (c. c.), with a pointed apex (g. cl.), corre-
sponding to the penis of the male, and composed of two very short corpora
cavernosa attached anteriorly to the ischia, and invested internally by a soft,
grooved corpus spongiosum. The vulva, or external opening of the vestibule, is
bounded laterally by two prominent folds-the labia majora.
Development.-The Rabbit is viviparous. After the minute ovum has
escaped from its Graafian follicle it passes into the Fallopian tube, where it may
become fertilised by one of the approximately 70 million spermatozoa produced
at each ejaculation. (In the ram the
number varies from 300 to 450 million,
but there is great reduction after succes-
sive copulations.)
The fertilised egg undergoes cleavage
to become a morula and reaches the
uterus. In the endometrium nidation
takes place and, now a blastocyst, it
becomes a jcetus, or intra-uterine embryo.
The young animal escapes from the uterus
in a condition in which all the parts
have become fully formed, except that
the eyelids are closed and the hairy
covering is not yet completed. As many
FIG. 47I.-~-Oryctolauu.": Placentation.
Diagrammatic longitudinal section of em- as eight or ten young are produced at a
bryo at an advanced stage of pregnancy. a. birth, and the period of gestation, i.e. the
amnion; a. stalk of allantois; al. allantois
with blood vessels; c. embryo; ds. cavity time elapsing between the fertilisation of
of yolk-sac (umbilical vesicle); ed. endo-
the ovum and the birth of the young
dermallayer of yolk-sac; ed'. inner portion
animal, is thirty days. Fresh broods
of endoderm; ed". outer portion of endo·
derm lining the compressed cavity of the
may be born once a month throughout a
yolk-sac; fd. vascular layer of yolk-sac; pl.
considerable part of the year, and the
placental villi; 1'. space filled with fluid
between the amnion, the allantois, and the
volk-sac; sh. subzonal membrane; st. young rabbit may begin breeding at the
sinus terminalis. (From Foster and Bal-
four, after Bischoff.) age of three months.
Segmentation is of the holoblastic
type. An amnion and an allantois are developed much as in the case of birds
(p. 646). But the later history of these fcetal membranes is widely different in
the Rabbit owing to the modifications which they undergo in order to take
part in the formation of the placenta. Through the placenta (see p. 904) the
fretus receives its nourishment from the maternal circulation. The placenta
is formed from the serous membrane or chorion in a limited disc-shaped area,
in which the distal portion of the allantois coalesces with it (Fig. 471). The
membrane thus formed develops vascular processes, the chorionic villi, which
are received into depressions (the uterine crypts) in the mucous membrane
of the uterus. The completed placenta with its villi is supplied with blood by
 PHYLUM CHORDATA

the allantoic vessels. The placenta of the Rabbit is of the type termed
deciduate, the villi of the placenta being intimately united with the uterine
mucous membrane, and a part of the latter coming away with it at birth in
the form of a decidua, or after-birth.

SUB-CLASS PROTOTHERIA (MONOTREMATA)

INTRODUCTION
It now appears to be reasonably certain that the Jurassic Triconodonta are
ancestral to the extant prototherians or that the latter are derived from closely
related unknown forms. From the early Cretaceous (triconodonts) to the
Pleistocene (monotremes) nothing is known of the history of the group.
ORDER TRICONODONTA
Of the known groups of mammals, the Triconodonta are probably the most
primitive. In the past they have been variously claimed to be monotremes,
marsupials or to belong to an order with no close relationship to either. They
are almost certainly prototherians. They
(e.g. Triconodon, Priacodon (Fig. 472)
A mphilestes) are found in deposits ranging
from the Upper Trias to the Lower
Cretaceous. They were probably carni-
vorous and ra.:1g~rl in size from a small
shrew to a cat. The brain was small and
probably primitive. Like the monotremes,
but unlike all other mammals (and mam- FIG. 472-~Sub-class Prototheria,
Order Triconodonta, Family Tricono-
mal-like reptiles), the lateral wall of the dontidre. Priacodon. These small crea-
brain-case anterior to the ear is formed not tures lived in North America during the
Upper Jurassic and were perhaps the
by the alisphenoid but by the extension of most primitive mammals yet known.
The above restoration is about twice
the petrosal. There were three or four in- natural size. (Redrawn after Simpson.)
cisors in each jaw, well-developed canines,
and up to nine cheek-teeth which cannot be separated into premolars and
molars. The molar pattern was distinctive, with the three principal cusps
arranged in an antero-posterior line.
Recent discoveries in South Wales (Kermack et al) have led to the collection
of rich material of a triconodont related to Morganucodon. This makes avail-
able for the first time the almost complete osteological material of a Mesozoic
mammal.
ORDER SYMMETRODONTA
This group (e.g. Spalacotherium, Peralestes) ranged from the beginning of
the Jurassic to the Middle Cretaceous and is not unquestionably prototherian
(seep. 654). Only jaws and teeth have been discovered. The molar teeth were
688 ZOOLOGY
triangular, the base of the triangle being on the inner (lingual) side in the lower
jaw and on the outer (buccal) side in the upper jaw. The highest cusp was
placed at the apex of the triangle, buccally in the lower jaw and lingually in the
upper jaw. Cutting crests from the main cusp formed the anterior and
posterior edges of the triangle. As the jaws closed, the triangular teeth formed
an interlocking arrangement suited to the mastication of insects. The lower
molars may be compared with the trigonids of trituberculo-sectorial teeth (p.
865), but there is no trace of a talonid. The lower jaw was without an angular
process.
ORDER MONOTREMATA
The Platypus (Fig. 473) and the two echidnas (Fig. 474) are the only surviv-
ing prototherians. Their anatomy leaves no doubt of their great antiquity.
The two families, the Omithorhynchidre and Tachyglossidre, differ widely from
each other in external appearance, and in relatively minor anatomical characters
but they are Lound together as a single order by the common possession of
several deep-seated characters found in no other mammals.
Thus, while such essentially mammalian characters as milk-glands, hair,
complete diaphragm, four cerebral optic lobes, etc., are all present, certain of
their features indicate a wide cleft between them and all other mammals. Thus
the monotremes lay eggs: there is no uterine gestation. The urinogenital
organs are still not far from the reptilian condition (p. 482) : the erectile penis
is composed of a corpus spongiosum, a corpus fibrosum, and bears a groove
transmitting spermatozoa, but not urine. This last enters the cloaca through
a special urinary canal (Fig. 488, p. 701). In the skeleton the shoulder-girdle not
only retains well-developed coracoids (reduced to vestiges in other mammals)
but also an interclavicle (Fig. 484). Though milk is produced, the mammary
glands remain unspecialised, and devoid of nipples. The pouch which carries
the eggs in the Tachyglossidre is temporary.
The vertebrre are without epiphyses except in the tail region of Ornithorhyn-
chus. The mode of exit of the spinal nerves-in the middle of the vertebra,
instead of between adjacent vertebrre-is peculiar. The cervical vertebrre
bear free ribs. The limbs are specialised for rapid swimming or digging.
The skull, though very different in shape in the two families, is in each con-
stituted on the same general plan. The chondrocranium is typically mam-
malian, but differs in some respects from that of other mammals, e.g. in the
absence of an alisphenoid bone and in the presence of a bone additional to the
usual mammalian pterygoid (known as the 'Echidna pterygoid') which homo-
logises with the reptilian ectopterygoid.
The above brief osteological and physiological details make it seem that,
although pre-Pleistocene monotreme fossils are still unknown, these and the
triconodonts are extremely archaic mammals that have diverged from the
main stock (if they do not represent a separate stem altogether) perhaps even
 PHYLUM CHORDATA 68g

before any of the Upper Jurassic mammals next to be described. Granting that
the extant monotremes have become highly specialised in many of their features,
their possession of such t ypically mammalian characters as milk-glands (prob-
ably derived from sweat-glands as in other mammals), hair (but only imperfect
thermoregulation), and a large brain with a well-developed pallium (and
convolutions in echidnas) carries the suggestion that such characters are

F I G. 473----Sub -class Prototheria, Order Monotremat a, Fam ily Ornithorhynchidre. or.. itho-
rhy11chU!i . T h e Platy pus (0 . anatinus) is one o f t he three species of living monotrem es that a re
today confined t o Austra lia a nd New Guinea. The P lat ypus lives in secluded fresh-water streams of
East ern Australia and Tasmania. Each pair digs two t wisting burrows, each of whic h h as two
entra nces (above the water-line) and may be from 20 to 5 0 feet lo ng . W ithin one is built an
incubating nest made of grass and leaves. W h ile submerged t h e anim al swims blind (see text
a nd F ig. 475). (From photograph s.)

primitive features of early mammals (sec Figs. 643, 644, p. 886). Their presence
in the monotremes leads particularly to speculation concerning the possible
degree t o which synapsid reptiles may have progressed t owards lactation and
homceothermy, and to speculation, moreover, that the monotremes represent
an independent 'warm-blooded' line, derived from Triassic cynodonts (p. 517).
It has also been suggested by Gregory that the monotremes are derived from
archaic marsupial or pre-marsupial stock and that their peculiarities therefore,
are essentially due t o degeneration, neoteny, and specialisation, plus the
retention of certain marsupialian characters.
6go ZOOLOGY
The traditional, and probably correct view is that they derive from an
early stock on the principal mammalian line of descent.
Today the Monotremata are confined to Australia and directly adjacent
New Guinea. The Platypus 1 (Ornithorhynchzts anatinus) (Fig. 473), the sole
species of the genus and family, ranges widely from tropical Queensland along
the east Australian seaboard to Tasmania where it lives in the pools of secluded

FrG. 474 .- Sub-class Prototberia, Order Monotremata, Family Tachyg!ossidre. Tncii!JOlossus.

Spiny Anteater (T. aculeatus). When the echidna is threatened its spatulate claws (Fig. 483)
enable it to sink v ertically and rap idly out of sigh t with a minimum of disturbance to the surround-
ing soil. A defensive echidna presents a n unassailable bilateral barrier of spines that wholly
conceals rostrum (Fig. 482) and legs. The short (half an inch long) tail is protected by a termina l
tuft of spines. Even in rocky country, where burrowing is impeded, the claws and lateral spines
engage the earth and m ake dislodgement a lmost impossible. T. aculeatus has a wide continental
range : the illustration is from a photograph taken on a parched Queensland plain. Zaglossus
(= Proechidna) is confined t o New Guinea.

streams and rivers. Both it and Tachyglossus have been reported to undergo
a limited hibernation (p. 883). In monotremes heat regulation is imperfect
though of course far advanced over that of living reptiles. The temperature
of Tachyglossus falls to Z5° C. under an external temperature of 5° C., a condi-
1 Outside Austra lia the rgth-century terms 'Duck-billed Platyp us' or even ' Duckbill '
are often used . Such names are undesirable. The mu zzle of the Platypus bears only the most
superficial convergent resembla nce t o the bill of a duck. and the avian a ppella tion lends cu rrency
to the popular belief that the animal is a sort of non-missm g link between birds and mammals.
 PHYLUM CHORDATA

tion frequently experienced in some parts of its range. In the absence of sweat-
glands, panting mechanism (p. 708) or heat-reduction by vasodilation, the
temperature of T achyglossus rises with that of the environment above 30° C.:
the animal dies of heat-stroke at about 37° C. if unable to cool itself by burrow-
ing. The platypus has few sweat-glands but it lives mostly in the water or in
a cool burrow and does not encounter the heat-hazards faced by spiny anteaters.

cheek pouches
FIG. 475--0rnithorhyuchus:
Cephalic structures. Like Tachyglossus FIG. 476.-0rnitlwrhynchus: Buccal ap-
(Fig. 482) the Platypus lacks vibrissae paratus. The lower jaw has been exposed by
but its delicate flatt ened muzzle con- cutting through the cheeks. Three pairs of
tains minute sensory pores. The teeth are fused together into a calcified mass.
muzzle extends backwards protectively They have been la rgely replaced by horny
over the fac e . The deeply set eyes and material but in som e specimens calcification re-
ears open into a facial furrow that is mains evident. The cheek-pouches hold food
closed during submersion (Fig. 473): gathered under water. The whole beak is a
the animal hunts essentially by smell sense organ, for the animal swims with its eyes
and touch. (Redrawn after Wood and ears enclosed in pouches (Fig. 475). (Re-
Jones.) drawn after Wood Jones.)

The body of Ornithorhynchus is flattened and covered with a dense, soft

under-fur protected by harsher outer fur. The upper jaw is produced into a
depressed beak-like muzzle covered with soft, but tough skin (Fig. 475). In
life the muzzle is moist and flexible and quite unlike the dry horny bill of a
bird. The organ is pitted with tactile organs and bears also the nostrils.
Nostrils and eyes face upwards. While feeding below the surface, both the
small eyes and the ears (which lack pinnre) are enclosed within a curious back-
ward-directed facial-furrow. The inner ear remains primitive. The Platypus
appears to find its way, and its prey, chiefly by olfaction and touch. Animal
6g2 ZOOLOGY
food, along with mud and sand, is sieved into the buccal cavity between serra-
tions in the lower lips-an interesting convergence with the beaks of certain
ducks and other animals. The sand assists in mastication, and the animal
possesses storage cheek-pouches (Fig. 476).
The forelegs are short but powerful. The five digits end in strong claws
which are set in a web which joins them together and then projects below and
beyond the nails. This flap can be folded back so that the limb can be used
for walking, digging, or swimming. The hind limb is less specialised. The
proximal end of the fibula is prolonged in a manner analogous to the olecranon

I II III IV V V IV III II I

FrG. 477.- 0rnitlwrhync hus: Manus and pes: Left: Left hand, from dorsal surface . The
swim-webbing is carried beyond the extremities of the nails in a series of leathery extensions
prolonging the line of the digits. The whole can be folded back to expose the tips of the burrowing
(and defensive) claws. Right: R ight foot, from dorsal suriace. Beneath the first digit is a curious
prolongation of the web m embran e. This has nothing to do with the poison spur which is on the
ankle (Fig . 478). (Redrawn after Wood Jones.)

process of the ulna, and in the male a hollow tarsal spur is connected with a
small crural gland, the secretion of which is poisonous. In Ornithorhynchus
this gland is well-developed: although not dangerous to Man its protein,
including albuminous, secretion, can kill an experimental rabbit in go seconds.
In Tachyglossus the apparatus is less developed (Figs. 478, 483). The tail is
elongated, depressed, and is a powerful swimming-organ (Fig. 473).
Although the Platypus undergoes a courtship performance, and copulates
in the water, it is in fact much less aquatic than is generally supposed. It
spends most of its life in its burrow, and relatively few hours, generally in early
morning and near dusk, in search of aquatic food. It spends considerable time
performing its toilet beneath overhanging roots and rocks at the water's edge.
 PHYLUM CHORDATA

Two burrows are made and each has its concealed entrance above the water.
One is inhabited by the pair ; the other, between
IS and 6o feet long, is the female's nesting cham-
ber. This the brooding female blocks at intervals
with barriers of earth moulded by the flat tail.
The nest is made of grass, leaves or reeds and
during the approximately 14 days' incubation of
the two eggs, which tend to stick together (Fig.
479), the female does not leave the nesting
chamber. When the burrow is broached, she
makes no attempt to defend her young. The
female is devoid of nipples-the milk exudes
through relatively simple ducts and is licked up
by the young (Fig. 480). Unlike Tachyglossus,
the Platypus has a distinct voice-both growlings
and whistling notes have been described from
adults.
The teeth are remarkable (Figs. 476, 481).
In the embryo calcified teeth arise in each half of
the upper and lower jaws. They are very brittle,
clearly degenerate, and are soon shed and replaced
by underlying horny plates, in depressions of
which the teeth were originally set. The teeth
show some rather irregular tubercles arranged in a

OJ
pattern that gave rise to a view that the Proto- FIG . .f 78.- 0rnitlwrl1!1t1CI1Us:
Poison apparatus. The femoral or
theria are descended from, crural gland lies in deep fascia on
or distantly related to, the the dorsal aspect of the femur.
. Present only in the male, it dis-
Multituberculata. This charges through a duct which is
..
.
view cannot be upheld on of dilated into a reservoir at the base
the sharp, movable, fang-like
such evidence. spur (! inch long) on the inner side
of the ankle. A rudimentary spur
Frc. 479.-0rnitho-
rhynchus: Eggs. About
The dental formula is occurs in the young female but it
natural size. They are fre- peculiar (Fig. 481). There later disappears except for a mere
quently superficially con- depression. (Redrawn after \Vood
joined, and are of slightly appear to be no upper Jones.)
variable size. Two, some- incisors formed, but there
times three, are laid. They
are dirty white in colour are five in the lower jaw. A canine is present in both
and the largely keratinous
shell can be dented like jaws. The lower canine is followed by a problematic
those of many reptiles. calcification which it is suggested is a second canine.
The egg-cell is about 6 mm.
in diameter: some 30 times The rest of the series consists of two premolars, of which
its size in Man. (Redrawn the anterior
after Wood Jones.)
is replaced, and three molars in both jaws.
These teeth are not present together at any one time
and most of them are non-functional and abort as mere calcifications. At I I
weeks old there remain only two teeth in the upper, and three in the lower, jaw.
 ZOOLOGY
All known fossils are Pleistocene in age and have been found only in Australia.
The Platypus survived because of its highly secretive and specialised breeding-
and feeding-habits, and because it possesses few natural enemies apart from
pythons and goannas (Varanus). It was still plentiful when Europeans arrived
but rapidly became scarce because
of the warmth and beauty of its fur.
m. s.a. It is now rigidly protected and, al-
though rare, can be seen in secluded
streams a short distance from popu-
lous cities.
m.pyr. The spiny ant-eaters (Tachy-
glossidre = Echidnidre) (Figs. 474,
OS. m.
482) are represented by two closely
allied genera Zaglossus ( = Proechid-
na), of New Guinea, and Tachyglossus
( = Echidna), which ranges from
New Guinea to Tasmania. These
animals are land-living, insect-eat-
ing burrowers of astonishing abilit y
although they do not make tunnels.
The body is covered above with
strong pointed spines, between which
are coarse hairs. The lower surface
of the body has hair only. The weak
jaws are produced into a sensitive
pointed rostrum. The long, sticky
tongue is thrust through a buccal
aperture on the ventral surface, and
FIG. 48o.- OrnithorhytJchus: Mammary near the termination, of the muzzle
glands, cloaca, and tail. The mammary glands a re
extremely simple : the young lick the milk from and to this insects-mostly ants-
shallow longitudinal depressions. The cloaca, into adhere together with dirt which helps
which discharge the r ectum and the urinogenital
sinus (Fig. 487), recalls the reptilian condition in mastication. Teeth are absent
(Fig. 488A). The body wall is reflected t o show in all stages of development. The
the musculus subcutaneus abdominis, mammary
gland, musculus pyramidalis and os marsupium posterior aspect of the tongue is beset
(marsupial bone). (Redrawn after various
authors.) with horny serrations which grind
the food against corresponding ridges
on the palate. Large submaxillary, parotid and sublingual glands are present,
the secretions of which no doubt help neutralise formic acid discharged by
their prey.
The nasal passages open on the dorsal surface of the rostrum. It would
seem that along with its tactile muzzle, the animal is largely dependent on
olfaction; its repeated inspiratory 'sniffs' are a noticeable feature and variants
 PHYLUM CHORDATA 6gs

of these, in the apparent absence of a true voice, may be used in communication.

.,
Like Ornithorhynchus, the eyes are small and the ears without a pinna. The
tail is vestigial. The limbs are short and the feet provided with strong spade-
like claws (Fig. 483). With these the spiny ant-eaters are able to burrow rapidly
Canine Pre':'J>lars
.......
~

auu~r ~~w ~
o./Jcde f g lJ .X y z
du
Frc. 48r.-Oruitlwrhynchus: Dentition. The absence of upper incisors a nd the presence of
a n upper and lower replacing tooth should be noticed. a-e lower incisors; f. lower canine;
g.? second canine ; vw. p remolars; dv. replacing tooth; x . y. z. m olars. (After Green.)

in a vertical direction, presenting to the attacker only its formidable armoury

of stiff, sharp spines (Fig. 474). Once it begins to sink into the ea:::th it is almost
impossible to dislodge it alive. The second pedal digit is equipped with an
elongated toilet-claw-an adaptation to the cleansing of its body-hair and
armoury (Fig. 483).

Frc. 48z.-Tacl1yglossus:
Cephalic structures. The
small ventral buccal aperture
is just big enough for the
protrusion of the sticky
t ongue to which insects ad-
here. The face is covered
with bristle-like hair mixed
with finer structures. Some-
times there is developed the
suggestion of a pinna. The
nostrils are dorsaL Nearly l
natural size. (Redrawn after
Wood J ones.)

Oviposition in the spiny ant-eaters has not been observed. However, the
female is able to curve her body sufficiently to lay the single egg directly into a
ventral pouch (incubatorium) which develops during the sexual season. Here
the egg hatches. The baby is fed by milk (from ducts which open into the
pouch) until its spines become troublesome to the mother. It is then about
ro em. long. This habit of carrying the egg, and carrying and feeding the
young one in the pouch, together with the remarkable protectim afforded by
6g6 ZOOLOGY
spines and special burrowing habit, has been a principal factor in the animal's
survival. Today it is rigidly protected in Australia and is not rare in appro-
priate and very extensive habitats. As with the Platypus, fossils are known
only from the Australian Pleistocene.
Skeleton of Prototheria.-ln the Prototheria (Fig. 484) the epiphyses of
the vertebrre are not well developed in Ornithorhynchus, being represented
only in the caudal region, and they appear to be absent in Tachyglossus. In
both genera there is the normal number of vertebrre in the cervical region.
The odontoid process long remains separate from the centrum of the axis.
The ~cervical transverse processes are separately ossified, and only completely
unite with the vertebrre at a late period, sutures being traceable in all but very
old animals. Zygapophyses are absent in the cervical region. There are

Frc. 483.-Tachyglossus: Manus

and pes. L eft: Left hand (of female)
showing the spatulate claws adapted
for rapid digging (Fig. 474). Right:
Right foot (of male). The second
pedal digit in b oth sexes becomes a
curved elongated toilet-claw which
enables the animals to scratch deep
through hair and spines. The arrow
indicates the poison spur which is
lower down the leg than in the male
Platypus (Fig. 478). It is relatively
small and supplied by a pea-sized
gland. (Partly after Wood Jones.)

V IV III II I

nineteen thoraco-lumbar vertebrre in both genera. The transverse processes

are short, and the ribs do not articulate with them, but only with the sides of
the vertebral bodies. In the sacrum of Tachyglossus there are three or four,
in that of Ornithorhynchus two, united vertebrre. The caudal region differs
considerably in its development in the two genera. In Tachyglossus the tail
is very short, the vertebrre depressed, with no inferior spines, but with about
five subvertebral bones, which differ from ordinary chevron bones in being mere
flat nodules. In Ornithorhynchus the tail is long, and the number of caudal
vertebrre is twenty or twenty-one. Each has a distinct inferior spinous pro-
cess (infr. proc.). The sternum consists of a presternum and three keeled
sternebrre : in T achyglossus but not in Ornithorhynchus there is a xiphisternum.
The most remarkable feature of the sternal apparatus in the Prototheria is
the presence of a T-shaped interclavicle (epist.) corresponding to that of
reptiles. The sternal ribs are ossified, and are connected with the vertebral
ribs by imperfectly ossified intermediate ribs (int. rbs.).
 PHYLUM CHORDATA

FIG. 484.--Qf'nithot'hynchus:
Endoskeleton. Male in ventral
view. The right fore-limb has
.,._ maa::Jor
A
been separated and turned x--
' - '.

/t
round so as to bring into view
the dorsal surface of the manus ; IJ,-"'"lpa!Jor
the lower jaw is removed. ace. i 'tf.orbJor
tars. accessory tarsal bone sup- r ma;r
porting the spur; ant. pal. for. ..---.Po-•l.palfor
anterior palatine foramen ; atl. pter
atlas; ast. astragalus; ax. axis; 3///l
bs. oc. basi-occipital; bs. sph.
basisphenoid; calc. calcaneum;
cbd. cuboid; cerv. rb. cervical
rib; clav. clavicle; cond. for.
foramen above inner condyle
of humerus; cor. coracoid; cun.
cuneiform o f carpus; dent. posi-
tion of horny teeth; ect. cun.
ecto.cuneiform; ent. cun. ento-
cuneiform; ep. cor. epicora-
coid; epist. interclavicle; ep .
pb. epipubis; fb. fibula; fem.
femur; f or. mag. foramen mag- colldJor
num; glen. glenoid cavity of
shoulder-joint and glenoid
cavity for mandible; hum.
humerus; in. cond. inner con-
dyle of humerus; inf. orb. for. /I
points to position of infra-
orbital foramen; infr. proc. 1//
inferior processes of cauda l
vertebr<E; int. rbs. intermediate 17
ribs ; isch. ischium ; mag. mag-
num of carpus; max. maxilla;
max. for. maxillary foramen;
m etal. I, first metatarsal;
metal. V, fifth m etata rsal ; nas.
cart. n asal cartilage; obt. ob-
t urator foramen; ol. olecranon ;
out. cond. outer condyle of
humerus; pal. palatine; pat.
p atella ; post. pal. for. posterior
p alatine foramen; pr. max. pre-
maxilla ; pr. st. presternum;
pter. pterygoid; pub. pubis;
rad. radius; scap. scapula;
scaph. scaphoid o f tarsus;
scaph. lun. scaph a -lunar ; ses.
sesamoid bones of wrist and
a nkle; sp . tarsal h orny spur;
sq. squamosal; tib. tibia; trd.
trapezoid; trm. trapezium ;
tym. c. tymp a nic cavity; uln .
ulna ; unc. unciform; vom.
v omer ; x , dumb-b ell shaped
bone; zyg. zygomatic arch;
I-V, digits of manus.

The skull of the monotremes differs widely from that of other mammals.
The bones early become fused together, so t hat it is difficult to trace their exact
boundaries. The brain-case is larger and much more rounded in Tachyglossus
than in Ornithorhynchus, in accordance with the larger size of the brain in the
former genus. In both genera there is a pterygoid (investing) bone not separ-
 6g8 ZOOLOGY
ately represented in higher mammals, corresponding to the pterygoid of lower
vertebrates. The parasphenoid represents the lateral parts of the parasphenoid
of lower vertebrates and the inner lamella of the pterygoid process (usually
regarded as the pterygoid) of higher mammals. Perforating the posterior root
of the zygomatic arch is a temporal canal which is comparatively wide in the
full-grown Ornithorhynchus and
oc.cond narrow in Tachyglossus. This
structure is absent in higher mam-
mals, and apparently represents
cond
the post-temporal fossa of reptiles.
aud.oss In Tachyglossus (Fig. 485) the
squamosal extends farther for-
wards and the posterior root of
the zygomatic arch is more an-
terior than in mammals in general.
The zygoma is very narrow, and
there is no rudiment of postorbital
processes : the jugal is absent as a
separate ossification. The alve-
olar border of the maxilla (max.)
is narrow and devoid of teeth.
The nasal and premaxillary region
of the skull is drawn out into a
long, narrow rostrum. Near the
anterior end of this is a rounded
opening, the external nasal open-
ing, which is entirely bounded by
/
the premaxillre-the nasals not
extending so far forwards. An
aperture in the nasal septum corre-
sponds to an actual perforation by
l'rG. 485. -Tachyglossus: Skull. Ventral view,
wit\1 right ramus of mandible. ang. angle of mand- which the nasal cavities are in
ible; aud. oss. auditory ossicles; cond. condyle of
mandible; cor. coronoid process; max. maxilla; oc. direct communication in the living
cond. occipital condyle ; pal. palatine; p.max. pre- animal. The pterygoids (pt.) are
maxilla; pt. pterygoid; sq. squamosal; ty . tympanic
ring. in the form of flat plates con-
tinuous with the bony palate;
they extend back so as to form a part of the walls of the t ympanic cavities.
The tympanic (ty.) is an imperfect ring which does not become united with the
periotic. The mandible consists of very narrow styliform rami, loosely united
at the symphysis. The condyle (cond.) is narrow, rather more elongated
antero-posteriorly than transversely. There are very slight rudiments of the
angle and of the coronoid process (cor.).
 PHYLUM CHORDATA 6gg

In Ornithorhynchus (Fig. 484) the zygoma is stouter than in Tachyglosstts,

and there is a postorbital process, formed by the jugal. The maxillary
root of the zygoma develops a process which supports the horny tooth (dent.)
of the upper jaw. The nasal and premaxillary are expanded into a rostrum
which is much broader than in Tachyglosstts. The premaxillre (pr. max.)
diverge from one another anteriorly, and then curve inwards again, partly
enclosing a large space in which the nostrils are situated, and which is covered
over in the recent state by the tough but sensitive hairless integument investing
the cartilage of the rostrum, the latter being continuous with the nasal septum.
In this space between the premaxillre is situated a dumb-bell shaped bone (x.)
which arises from palatal processes of the premaxilla. The pterygoid (pter.)
is much smaller than in T achyglossus, and does not extend as far back as the
tympanic cavity. The mandible has its rami stouter than
in Tachyglossus; they meet for a short distance an-
teriorly, and then again diverge slightly. The condyle
is much larger than in Tachyglossus, and is transversely
elongate. In front of it is a broad process bearing the
horny tooth. acr
In the shoulder-girdle occur perhaps the most striking
peculiarities of the prototherian skeleton. There is a
FrG. 486.- 0,.nithor-
T-shaped interclavicle (epist.), as already stated, similar hynchus: Scapula.
to that of reptiles, the median limb articulating behind Outer surface of left
element. acr. process
with the presternum and the cross-piece closely applied corresponding to acro-
to the clavicles. There are two short and broad coracoids cularmion; glen. glenoid art i-
surface; sp. anter-
(cor.) (Fig. 484, p. 697). These articulate internally and ior border, correspond -
ing to the spine; x ,
behind with the presternum, and, externally, unite with slight ridge which
the scapula to form the glenoid cavity. In front of the bounds the surface of
origin of the sub-scapul-
coracoid is a flat plate, the epicoracoid (ep. cor.). The a ris muscle anteriorly.
scapula (Fig. 486) is unlike that of other mammals. It
has a well-developed acromion process (acr.) with which the clavicle articulates
and which t erminat es the anterior border, so that the latter would appear to
correspond to the scapular spine of other mammals, an interpretation con-
firmed by the arrangement of the scapular muscles. The anterior part of the
inner surface is in reality the pre-spinous fossa, the anterior portion of the
outer surface is the post-spinous fossa and the part behind this, separated from
it by a slight ridge, together with the posterior portion of the inner surface,
is the sub-scapular fossa.
The humerus is of remarkable shape, with greatly expanded extremities
(especially in Tachyglossus) and prominent tuberosities and condyles. In the
carpus the scaphoid and lunar are united ; there is no separate centrale. There
are a radial and two very large palmar sesamoids, which are sometimes united.
The pelvis presents a very long symphysis in which pubes and ischia take
700 ZOOLOGY

an almost equal share. The acetabulum is perforated in Tachyglossus. With

the anterior border of the pubes
are articulated a pair of large
epipubic or' marsupial' bones (Fig.
484, ep. pb.). The femur has ex-
panded extremities with prominent
external and internal trochanters.
There is a large ossified patella
(pat.). The fibula (/b.) has at its
proximal end a remarkable com-
pressed process which ossifies from
a separate centre, and resembles
the olecranon of the ulna. In the
tarsus there are the usual bones.
In Ornithorhynchtts the astragalus
and calcaneum are firmly united,
and an accessory ossification (ace.
tars.) on the inner side in the male
bears the tarsal spur. The meta-
o# tarsals are short and broad, as are
all the phalanges except the last.
Visceral Anatomy.-The stom-
ach of monotremes is almost
globular. Small and large intes-
tines are differentiated and there
occurs a crecum of modest size.
The large intestine enters the cloaca
some distance posterior to the
ureters and vasa deferentia (p. 488).
The liver is large and prominent
and there is a gall-bladder. The
bile-duct (in echidnas) traverses
the pancreas (lying in the loop of
the small intestine), where it is
FzG. 487.-Tachyglossus: Female urinogenital
system. The echidna has been opened from the joined by a short pancreatic duct.
ventral surface. The bladder has been sectioned
and the parts reflected to demonstrate its relation-
There is a capacious spleen. The
ships. A bristle (A) shows the connexion between lungs of the echidnas have been
the right oviduct and the m edian urinogenital sinus.
A rod (B) is passed through the intestine and its considered somewhat small in pro-
aperture into the cloaca. (Modified after Wood portion to total body size. The
Jones.)
heart of monotremes is peculiar in
that the tricuspid guards the left auriculo-ventricular aperture (cf. p. 671).
The kidneys are large, and the ureters do not empty directly into the bladder
 PHYLUM CHORDATA 701

but open into the urethra through a common aperture surmounted by a papilla
(Fig. 487). The urine flows through a short urinary canal into the large urino-
genital sinus: it does not, therefore, traverse the penis which carries only
spermatozoa (Fig. 488). The glans is divided into four lobes and lies on the
floor of the cloaca. The testes lie in depressions in the body wall. The urino-
genital arrangements in female monotremes are shown in Fig. 487. To some
extent the adrenal arrangement recalls that of the Reptilia: there is some

=======--./-
cl;~
FIG. 488.-Reptilian and mono-
treme affinities: Male urinogenital
system. The cloacal and p enial
arrangements of a chelonian (A ) and
A,_t;b,.~/
monotreme (B, C) compared. The c. spong.
basic plan is the same, but in the
monotremes spermatozoa from the
vas deferens (vas. d.) are discharged
through theurinogenitalcanal(u.g.c.)
(see Fig. 487, A') into a spernz.duct
tha t traverses the penis. The urine,
on the other hand, flows from the
ureter (ttr.) through the urinary B
canal (u. c. ) and is discharged from
the cloaca a s in reptiles and birds.
In B. the penis is erect; in C. with-
drawn. For remarks on the corpus
fibrosum (c. fibr.) and corpus spon-
giosum (c. spong.) see p . 890.
(Modified after Ihle.)
c
preputial
sac

bladder

intermingling of cortical and medullary tissues. There is however, an un-

mistakable aggregation of the two tissues towards the definite cortical and
medu1lary zones that have become stabilised in the Metatheria and Eutheria.

SUB-CLASS ALLOTHERIA
INTRODUCTION
Under this sub-class is provisionally included the order Multituberculata,
which ranged from the Jurassic until the Eocene (p. 3).

ORDER MULTITUBERCULATA

The affinities of the multituberculates (e.g. Ctenacodon, Plagiaulax) have

been debated almost from the time of their initial description. At different
VOL. II. XX
702 ZOOLOGY
times it has been held that they were ancestral to monotremes, or marsupials,
or eutherians. It has also been suggested that they constituted an archaic,
yet independent, branch of mam-
malian evolution; and this is the
generally accepted view to-day.
They were more highly specialised
-and successful-than the tricono-
donts and flourished over the long
period between Upper Jurassic and
the Eocene. They existed, in fact,
for as long a period as has been
proved for any other known mam-
malian group-for rather more than
70 million years. They seem to
have occupied an ecological position
not unlike that of some extant
rodents and to have been essentially
vegetarian. Of unknown ancestry,
they flourished in the late Creta-
ceous and were still common in the
Tertiary and Palreocene, when
occurred T ceniolabis (Fig. 489)
with a heavy skull some six inches
long.
The most noticeable character,
and the one from which the name of
the sub-class is taken, is the struc-
ture of the molar teeth. These have
from two to three longitudinal rows
of tubercles with from two to five
(or more) cusps in each row. There
were only one lower, and three
upper, incisors, the central one
being much enlarged. Canines
were absent. In many forms the
FIG. 489.- Sub.class Allotheria, Order Multi-
anterior lower cheek-teeth were
tuberculata, Family Taeniolabididre. Taeniolabis. much enlarged and specialised,
L atera l and palatal view o f skull. The lower a nd
upper (the lower of the two figures) m ola rs. in a manner analogous to that
Lower Palaeocene of N orth America. (After of some living marsupials, e.g.
Simpson.)
the rat-kangaroo, Bettongia. The
zygomatic arch arose far forward at the level of the anterior cheek-teeth,
and extended back almost to the occiput. The lower jaw was stout, but
 PHYLUM CHORDATA

without angular process, a character shared with the Triconodonta and

Symmetrodonta.
SUB-CLASS THERIA
INTRODUCTION
These constitute the 'modern' mammals. The group has been split into
three infra-classes, viz. r. the probably ancestral Jurassic Pantotheria ( = Tri-
tuberculata), including orders Dryolestoidea and Docodonta; 2. the mar-
supial Metatheria, and 3· the placental Eutheria, the most highly organised
and advanced mammals; see, however, footnote on p. 654.

INFRA-CLASS PANTOTHERIA (TRITUBERCULATA)

This is an artificial assemblage of Mesozoic forms which are very imper-
fectly known and of uncertain affinities. They are of considerable interest,
being possibly allied to the as yet unknown ancestors of the living non-mono-
treme mammals. All the Pantotheria were small; in the largest the jaw was
only 4 em. long. Their teeth are differentiated into incisors, canine, premolars
and molars, but the number of molars is often as many as eight. Two orders
may be distinguished.
ORDER DRYOLESTOIDEA
This group is known only from the Middle and Upper Jurassic. Amphi-
therium (Fig. 490) is the only known Middle Jurassic genus. Several genera
occurred in the Upper Jurassic, e.g.
Amblotherium, Melanodon, and
Paurodon, but apart from the jaws
and teeth hardly anything is known
of their structure. The lower molar
teeth consisted of an anterior tri-
angle of cusps, the trigonid, and a FIG. 490.-Sub-class Theria, Infra-class Panto-
posterior heel or talonid, which bit theria, Order Dryolestoidea, Family Amphith-
eriidre. Atnphithm·ium. Middle Jurassic of
against the inner apex of the tri- Europe. Inside lower jaw (over twice natural
angular upper molar. In the upper size). (Restoration after Simpson.)
molar the highest cusp was internal
(as in the symmetrodonts) but there was also a large cusp on the buccal side,
and some additional smaller cusps. The trigonids of the lower molars bit
between the triangular upper molars, and the diet was probably insectivorous.
The contour of the lower jaw was not unlike that of a primitive 'insectivores',
having a high coronoid and a well-marked angular process.

ORDER DocoDONTA
Usually united with the Dryolestoidea in an order Pantotheria, this small
group (e.g. Docodon) from the Upper Jurassic differs from them markedly in
 ZOOLOGY
the structure of the teeth and should probably be placed in a separate order.
The molars do not have the form of interlocking triangles, though in the upper
jaw they have three roots. The highest cusp of the upper molar is buccal in
position, not lingual as in the Dryolestoidea; there is an inner cusp which
however appears not to be homologous with that of Dryolestoidea, but shows
much resemblance to the protocone of tritubercular teeth.

INFRA-CLASS METATHERIA (= MARSUPIALIA, DIDELPHIA)

INTRODUCTION
Marsupials (of which some lack a pouch, and a few possess a placenta)
probably share a common Jurassic ancestry with the Eutheria or 'placentals'
(p. 723). Certain isolated molar teeth of a pre-tritubercular pattern from the
Middle Cretaceous of Texas could belong to marsupials or eutherians or to
ancestors of both groups. In the Upper Cretaceous definite representatives
of marsupials and eutherians are known. If one may judge from the small
number of specimens, nearly all from one limited area, the marsupials appear
to have been more numerous than the eutherians at that time, and it was not
until the Cenozoic that the Eutheria (p. 703) gained ascendancy. It was
possibly during the Cretaceous that marsupials entered New Guinea, Australia,
and adjacent islands, which were isolated from Asia probably in the late
Cretaceous (Fig. 492). Here they were able to survive free from competition
with eutherians (except for bats and later rodent invaders, p. 709) and to
radiate. It is probable that the basic stock was arboreal, though many have
become terrestrial. Tree-kangaroos (Dendrolagus (Fig. 493)) have become
secondarily arboreal from a fully terrestrial offshoot (see Macropodidre, p. 716).
Marsupials are mammals basically similar to the Eutheria, but whose young
are born in a rudimentary condition and are generally sheltered during their
later development within an integumentary pouch, the marsupium. A
pouch is lacking, however, in the Australian Numbat (marsupial ant-eater,
Myrmecobius fasciatus, p. 713) in which the young are dragged along, clinging
to the nipples. It is lacking also in the American Chironectes (Fig. 494),
Marmosa, and Monodelphis. A common sphincter-muscle surrounds the anal
and urinogenital apertures. The vaginre are distinct, and each has a separate
opening into the urinogenital canal (Fig. 648, p. 891). In both sexes the ureters
pass between the genital ducts: whereas in eutherians they pass outside the ducts.
The male has an erectile penis, which is sometimes bifid. In such species the
clitoris is likewise double. An allantoic placenta is rarely developed (p. 904).
When it occurs (e.g. in the Peramelidre) it is of relatively simple structure, and
functions for but a brief period. The brain lacks a corpus callosum and has
large olfactory bulbs and small hemispheres which do not extend posteriorly
over the cerebellum,
 Frc. 49r.- Animal distribution: Continental connections. The formation, destruction, and
som etimes secondary em ergence of inter-continental a nd other la nd -bridges (not necessarily
complet e at a n y one time ) h ave been p owerful m oulding influences on the composition of the
fauna of many areas. The disappearance of former land-bridges produced isolation which,
together with climatic and resultant environmental changes, had a spectacular influence on, for
example, the Marsupialia. Of northern origin, these radiated with enormous success in Australia
and, to a less degree, in South America . Isolation and changed external conditions explains the
present distribution of the relict Dip noi (F ig. 249, p . 361 ), L eiopelma (p. 429) Sphenodon (p. 495),
and many other groups. The dinosaurs (p . 504) were once a lmost universally distributed. Even
flying birds (e. g. the Australasian avifa una) and m a rine fishes (e.g . slightly differing populations
on each side of the Central Am erican isthmus) have b een affect ed. Ma ny p eculia r a spects of the
composit ion o f the fau na o f Australia, New Zeala nd, North a nd South America a nd perha ps
Madagascar can be b roadly understood in t erms o f former la nd-bridges (see Fig . 597, p. 827, in
relation to horses). At the same time it should be remembered that it is easy to p ostulate a land-
bridge t o a ccount for phenomena tha t m ay have t otally different, though less obvious, explana-
tions such as ch a nce dispersa l from island to island (see D a rlington). (The restricted striped areas
above indicat e extensive Jurassic lakes or inland seas.) (Modified after B . M. (N.H.) catalogue.)
706 ZOOLOGY
The tympanic chamber of the skull is often open, with the tympanic ring
exposed, a primitive feature shared with some eutherians: however, in a
number of forms, the tympanic chamber is covered and the ring is hidden or
'ensnared'. In such instances, the cover is always formed by the alisphenoid
bone, and not by an extension of the tympanic itself or by a combination of
bones, as in various eutherians. The optic foramina are confluent from side

FIG. 492. -Animal distribution: Faunal regions. The boundaries between the six regions (of
Sclater, and later \Vallace) are indefinite. Transitional areas are cross-h atched. The line north
of Australia was drawn by Wallace (and named a fter him by Huxley): it runs between Mindanao
and Talaud, Borneo and Celebes, Bali and Lombok. Weber's Line, that of ' faunal balance', is
somewhat to the east. The Sahara and Arabian deserts, the Hima layas a nd adjacent mountain -
ous or arid regions, the high and cool Mexican Plateau : all constitute barriers to certain groups.
New Zealand in particular (with its notable exclusions and its flightless birds, the frog L eiopelma,
and Sphenodon) as well as Madagascar (with tenrecs and lemurs and notable absentees) a lso
con stitute definite faunal regions.

to side, and the internal carotid artery enters the skull through a foramen in the
basisphenoid, and not through the foramen lacerum medium. The jugal bone
extends far back, so as to participate in the formation of the glenoid cavity
for articulation with the lower jaw, a feature universal in the Metatheria, but
one of sporadic occurrence only in the Eutheria (e.g. Hyrax, many rodents).
An inward inflection of the ventral posterior border of the lower jaw, the
'inflected angle', is to some degree a diagnostic feature, since it is not found
elsewhere save in a few rodents. It occurs in all marsupials with the sole
exception of the degenerate, thread-like jaw of the nectar-eating Tarsipes
 PHYLUM CHORDATA 707
and is greatly reduced in the phalangerid genera Dactylopsila and Dactylonax.
The seventh cervical vertebra in a large number of instances is pierced by a
foramen for the vertebral artery, but this feature is also found occasionally
in eutherians (some rodents, hyraxes, Hippopotamus, etc.). Well-developed
epipubic bones are commonly, but not invariably, present.

FrG. 493.-Infra -class Metatheria, Order Marsupialia, Super-family Phalangeroidea, Family

Macropodidre. Dendrolagus. Tree-kangaroos (e .g . the B lack-faced D. lumholtzi) are confined
t o the dense rain-forests of tropical Australasia. Secondarily arboreal, they have retained t he
remarkable leaping powers of the terrestrial kan garoos (which were themselves descended from a
primitive arboreal marsupial stock) . The tail of tree-kangaroos has become thinner, but has n ot
re-developed prehensility. The feet h ave become shorter and broader. Some nails have become
sharp a nd curved. The pads have become roughened and cushioned as a non-skid dev ice.
The hands have increased in size and power. The p osture is less erect than in terrestrial forms .
(From photographs.)

The dentition is curious in that there is only one set of teeth functional
throughout life, with the exception of one tooth, a milk-molar, which is replaced
by the last premolar. The pattern of the teeth varies according to function
in much the same general directions as in the Eutheria, carnivorous, insecti-
vorous, herbivorous and other adaptations being found.
In general it may be said that the marsupials represent a somewhat lower
grade of vertebrate organisation than the Eutheria. They retain a number of
primitive characters which they share with the more primitive eutherians (e.g.
708 ZOOLOGY
many Insectivora) such as a small brain, occupying but a comparatively small
portion of the skull and with its olfactory portion disproportionally large. The
palate is incompletely ossified ('fenestrated'). A humeral entepicondylar
foramen is very frequently present.
It has been pointed out that the monotremes are imperfectly homreotherm-
ous (p. 690). On the other hand, at least one marsupial, the Quokka Wallaby
(Setonix brachyurus), 'possesses a capacity to control its body temperature ...
as great as, or greater than, that shown by most placental mammals of similar
size' (Bartholomew). Further, marsupials employ the same general thermo-
regulatory mechanisms as eutherians-vasomotor changes, shivering, panting,
copious salivation, and, presumably, changes in metabolic rate. (Salivation and
licking is of particular importance in Setonix as well as other marsupials; it
has also been observed in rodents and the Domestic Cat in which it seems to be
essentially an emergency response.)
Although the terms 'placentals' and 'Placentalia' are frequently used to
distinguish the Eutheria from the Metatheria, these are undesirable. It should
be clearly understood that some marsupials have, seemingly independently,
developed an efficiently vascularised allantoic placenta. Perameles (p. 715},
in particular, has established a true chorio-allantoic placenta (p. 904), which is
not deciduate, but is absorbed after the birth of the young. In Phascolarctos
and Phascolomys (p. 716), too, there is allantoic vascularisation. All other
marsupials studied in this respect have a comparatively small allantois and
nourish the young through a yolk-sac placenta, which develops from the
universally large yolk-sac during the brief period of gestation.
Metatheria were widely distributed over many parts of the world as
recently as the Miocene period. They are now confined to the Australasian
region (with the exception of New Zealand), to South America (where during
the Tertiary period they had a wide expansion), and (a few species) to North
America. The earliest known forms are from the Cretaceous of Canada.
This assemblage of marsupials and other primitive animals (e.g. Dipnoi,
certain anurans, and flightless birds) in the southern hemisphere has
suggested, very plausibly, that they, and their specific parasites, have been
so distributed by means of long-vanished Antarctic land-bridges. On the
balance of evidence, however, it is more probable that these archaic, isolated,
southern survivors are remnants of formerly widespread stocks that have
lingered on in' species refuges' after their northern allies have been exterminated
by ecological competition with later-evolved, more progressive forms.
The Australasian zoogeographical region (Fig. 492), excluding New Zealand,
contains more than four times as many marsupials as occur in the Americas,
where their congregation is chiefly in the southern continent. But at the same
time it must be remembered that the Australasian region embraces also much
of Indonesia and that marsupials occur (south-east of Wallace's Line) in
 PHYLUM CHORDATA

the Celebes, Ceram, Amboyna, Timor, and New Guinea. In these countries,
and particularly· in the highly diversified Australian mainland environment,
they prospered and now exhibit many beautiful examples of convergence
with the more advanced, emergent, eutherians. Here marsupials developed
variously running, leaping, burrowing, climbing, gliding, forms (see pp. 707,
717), but, as far as is known, no habitual swimmers. Such natatorial forms
may have arisen and been later eliminated by rodent water-rats (see below).
It has been suggested that pouched mammals would be unlikely to become
successfully aquatic, but many marsupials became pouchless, including an
aquatic opossum (Chironectes, Fig. 494) that developed in South America.
No marsupial became truly a hooved animal, though Clueropus, the Pig-footed
Bandicoot, exhibits modifications in that direction; and the numerous
kangaroos, wallabies, wallaroos, rat-kangaroos, pademelons, and euros in
any case dominated the plains and hills. Some marsupials became mouse-
or rat-like, others somewhat sloth-like, others again cat- or wolf-like, while
the marsupial-moles (Notoryctes) (Fig. 497) became fossorial and remarkably
like the 'insectivoran' golden moles (Chrysochloridce) in appearance and habit.
The isolated Australian marsupials suffered competition from only four
types of eutherians. Into their habitat the bats flew, and the rodents probably
drifted on debris. These rodents are myomorphs (p. 777) and probably
invaded the region more than once. The invaders flourished and radiated,
giving rise to peculiar Australian murid genera (e.g. Pseudomys, Mastacomys,
Mesembriomys, Conilurtts, Leporilltts, Notomys, Ascopharynx, Melomys, Uromys)
including also a distinct sub-family of water-rats, the Hydromyince. Some
murids (Notomys and Ascopharynx) became jerboa-like. A later invasion by
the genus Rattus gave rise to several species of this genus that are found only
in Australia. These radiating rodents must have been severe competitors
with the smaller marsupials but the effect of this impact is still unknown.
About twenty genera of small terrestrial marsupials still exist to-day. Finally,
the third type of competitor, Man, brought with him the fourth, a dog (Canis
dingo) in his canoes. The Tasmanian Marsupial-wolf (Thylacimts) (Fig. 496),
once widespread in the mainland was, by the time Europeans arrived, limited to
Tasmania. The Tasmanian native (himself now extinct) had no dog. On this
general question Storr has argued that animals (whether marsupial or eutherian)
evolved in larger land-masses are able, when introduced into smaller countries,
successfully to compete with either marsupials or eutherians that are native there.
No wholly satisfactory means of marsupial classification has been devised.
Their taxonomy is usually based on the dentition, so that two groups are dis-
tinguishable : (r) the Polyprotodontia, with more than three incisors in each
half of the upper jaw, (2) the Diprotodontia, with never more than three
incisors in the upper, and one pair, in the lower jaw. Another method of
classification is based on certain peculiarities of the foot-structure. Some
 ZOOLOGY
marsupials have the second and third digits of the hind-foot united by a
common sheath, while others have the toes all separate. These conditions
are known respectively as syndactyly and didactyly. Most polyprotodonts are
didactylous and all diprotodonts are syndactylous, but one super-family, the
Perameloidea (bandicoots), are polyprotodont and syndactylous.
If the primitive condition be presumed to be polyprotodont and didactylous,
it would appear that this stock evolved along two lines, of which one, the
Didactyla, retained the simple foot-structure, and the other, the Syndactyla,
acquired the peculiar modification of the second and third toes. If this be so,
the second line may have undergone a further division in that some (the great
majority, indeed) reduced the number of incisors and became diprotodont,
while the Perameloidea, although becoming syndactylous, remained con-
servative as to their polyprotodont teeth.
With respect to dentition and foot-structure the marsupial groups arrange
themselves as follows :
INCISORS TOES
Didelphoidea Didelphoidea
. { Borhyrenoidea Borhyrenoidea
P oIypro t odont 1a Dasyur01"dea Didactyla {
Dasyuroidea
Perameloidea Crenolestoidea
Crenolestoidea Perameloidea
Diprotodontia { Syndactyla {
Phalangeroidea Phalangeroidea
There seems to be good evidence that the Didelphoidea (represented to-day
in the Americas only) may be the most primitive marsupial stock known and
that the remaining five groups (see below) arose from early representatives
of this group and have been phylogenetically separate ever since the late Cre-
taceous or very early Palceocene. Past attempts to relate the widely (geo-
graphically and phylogenetically) separated stems have proved unsuccessful
except in the case of borhycenoids and didelphoids (p. ]II) where a fossil
animal of intermediate structure (Eobrasilia) has been described from the
Brazilian Palceocene.
It has often been stressed that any classification based on an extremely
small number of characters in a large and diverse group is unlikely to be satis-
factory. Hence in the present state of palceontological ignorance of ancestral
Australian forms it is safest to adhere for the time being to a conservative
classification of six groups 'each of which is, beyond serious doubt, a natural
unit' (Simpson).
ORDER MARSUPIALIA
Super-families Didelphoidea (Upper Cretaceous-Recent)
Borhyrenoidea (Palceocene-Pliocene)
 PHYLUM CHORDATA JII
Dasyuroidea (Pleistocene-Recent)
Perameloidea (Pliocene-Recent)
Crenolestoidea (Paleocene-Recent)
Phalangeroidea (Pliocene-Recent)
SuPER-FAMILY DIDELPHOIDEA
These are the polyprotodont opossums 1 of North, Central, and South America
e.g. Didelphys, Marmosa, Chironectes) (Fig. 494). They are small, arboreal,

FIG. 494.-0rder Marsupialia, Super-family Didelphoidea, Family Didelphidre. Cllironectes.

The Water Opossum or Yapok (C. minimus) is a cat-sized South American forest -dweller that has
become web-footed, semi-aquatic, and pouchless. It lives in bank-burrows and feeds largely on
fish and crayfish. The single species o f Chironectes belongs t o the same sub-family (Didelphin;e)
as the originally-named opossums (Didelph_vs) of North a nd South Am erica. (From B.M. (N.H.)
specimens.)

generally insectivorous animals with an elongated, naked muzzle, a well-

developed (though nailless) opposable hallux and long prehensile tail. A
pouch is sometimes present, but it is incomplete or absent in most living
1 There is evidence that the North American opossum was really called by the 'Indians '
by the name possum, preceded by a grunt, hence 'o-possum' (Ha rtman) . The opossum-like
phalangers of Australia were called opossums as early as the r8th century by naturalists who
realised the similarity, and not by 19-century gold-diggers from Caliiornia as is often asserted.
The name phalanger was probably first applied (by Buffon) to the American and not the Australian
animals.
JI2 ZOOLOGY
forms. One species, the Water Opossum (Chironectes) of Central and South
America, has webbed toes. Some American opossums have the remarkable
habit, rare among mammals, of feigning death on capture-hence the expression
'playing possum'. Australian so-called opossums (phalangers, p. 716)
incidentally, do not 'play possum'. The Didelphoidea are early represented by
Eodelphis, Didelphodon, and several others in the Upper Cretaceous, Pera-
therium, found in both Europe and America from the Eocene to the Miocene,
hardly differ (as far as can be told from fragments) from small living didelphids.

SuPER-FAMILY BoRHYJENOIDEA
This super-family is represented by a South American group of polyprotodont
carnivorous animals of dubious affinity, which became extinct in the Pliocene.
They (e.g. the Oligocene-Miocene Borhyama, the Miocene Prothylacinus) were
large-headed, rather short-legged terrestrial animals, as big as bears, with
powerful claws and a small hallux. The Pliocene Thylacosmilus, a late survivor,
had long stabbing teeth not unlike those of the sabre-toothed 'tiger' (p. 796).
These animals have been claimed to show affinity with the Australian thylacines
(Fig. 496) and to constitute evidence for the existence of an Antarctic land-
bridge but any resemblance is probably due to convergent evolution.
The carnivorous Borhycenoidea probably radiated in seclusion after the
separation of South America early in the Eocene (Fig. 491). Although a few
eutherians (p. 723) had already reached that continent, these did not include
carnivores.
SuPER-FAMILY DASYUROIDEA
This Australasian group contains a large number of forms exhibiting an
astonishing range of adaptive radiation. The carnivorous, essentially terres-
trial Dasyuridce (native-' cats', Tasmanian Devil, Thylacine, etc.) generally
have the pollex rudimentary, the foot four-toed, the hallux, when present, small
and clawless, and the tail non-prehensile. The marsupium is well-developed
(but generally shallow) and the tail long. Body-length may be as great as two
feet, although many are smaller than a domestic cat. The early settlers called
the first-encountered species the 'spotted marten' (Fig. 495). The predaceous
Tasmanian Devil (Sarcophilus) is larger, with a more thick-set body. Its bones
have been found in aboriginal middens on the Australian mainland, although
good evidence is wanting of its occurrence there since European settlement.
The large carnivorous Tasmanian Marsupial-' wolf' (Thy/acinus) (Fig. 496)
is to-day extremely rare. It was formerly widespread on the Australian
continent, where it was exterminated before the advent of white settlement.
It bears a remarkc:..ble resemblance both in appearance and size to the true wolf.
Its skull too, is wolf-like in general outline, and its dentition is as dog-like as is
possible for a marsupial. Thylacintts is prominently barred across the dorsal
 PHYLUM CHORDATA 713

surface, making for concealment in the heavy timber where it lives. It is

inoffensive to Man, despite the vernacular, 'Tasmanian Tiger'.
Also included in the present super-family is the semi-arboreal genus
Phascogale, containing three slender, graceful, but ferocious brush-tailed
animals, each about the size of a small rat. Now rare, Phascogale raids poultry
farms, killing far in excess of its hunger. Allied to the above are several

FIG. 495.-0rder Marsupialia, Super-family Dasyuroidea, Family Dasyuridre, Sub-family

Dasyurinre. Da8yurops. The Tiger-cat or 'Spotted Marten' (D. maculatus) of the early British
colonists in Australia is a fearless hunter with which the Felidre, in some respects, converged.
The Tiger-cat sometimes kills wallabies, and has been known to keep a couple of Irish t erriers at
bay. The body is about two feet long. Dasyurops differs from Dasyurus in the possession of a
hallux, serrated foot-pads, and a spotted tail. The pouch is shallow and reversed- a crescentic
fold encloses only the front and sides of the mammary area which supports six nipples arranged in
two rows. 'Dasyures' easily hold their own against imported eutherians but have been decimated
in populous areas becau se of their blood-thirsty raids on domestic poultry. (From photographs.)

genera of pouched rats and mice (e.g. Dasycercus, Dasyuroides, Sminthopsis)

as well as long-tailed, hopping, pouchless (or almost pouchless) Jerboa-like
marsupials (Antechinomys). Some of the first-named accumulate fat stores
in their tails.
Two further genera of great interest are the curious numbats or banded
ant-eaters (Myrmecobius), and the Marsupial Mole (Notoryctes). Two species
of the former occur: these are rat-sized, and banded across the lumbar and
sacral regions. The female is pouchless, and so the small young are carried
JI4 ZOOLOGY

attached to the nipples. Numbats are very specialised gentle creatures, not
fast on the ground, not deep burrowers and not climbers. Probably they could
not have survived anywhere outside the isolation provided by Australia.
Although not persecuted by Man, they have become rare owing, possibly, to
the destruction of their native habitat by settlement and by feral domestic cats.

FrG. 496.-0rder Marsupialia, Super-families Dasyuroidea and Perameloidea. The banded

Tasmanian-wolf (Thy/acinus cynocephalus) (Family Dasyuridre, sub-family Thylacininre) inhabits
hilly heavily-timbered country. Although harmless t o Man it has b een known to tear open the
skull of a bull-terrier with a single bite. The four young are carried in the pouch for about three
months. The thylacine is shown holding a ba ndicoot (Thylacis) (Family Peramelid:e.) The
single species is extremely rare. (From photographs.)

Notoryctes, the Marsupial-mole (Fig. 497), is a small, burrowing, desert

animal which bears a superficial resemblance to the African golden moles
(Chrysochloridre, p. 729). It has short, powerful, furred limbs, each with five
toes. The third and fourth toes of the fore-foot are provided with remarkable,
large, flat, triangular claws. The rhinarium is covered with a horny shield
and the dorsal aspect of the head is also protected by a hard shield. The tail
is short and covered with bare, horny skin. An auditory pinna is absent, the
eyes are vestigial and the fur is silky and almost plush-like. A well-developed
pouch opens backwards. Pleistocene fossil examples have been found.
 PHYLUM CHORDATA 715

SUPER-FAMILy PERAMELOIDEA
The bandicoots (Fig. 496) are inoffensive Australasian terrestrial grass-
and forest dwellers (e.g. Perameles, Thylacis, Macrotis, Chceropus) usually of
burrowing habit. They range in size from that of a large rat to that of a rabbit
and subsist on a mixed diet. Although rarer than formerly, they are still
plentiful in the outer suburbs of Australian capital cities. They have an
elongated, pointed muzzle, and, in some cases, large auditory pinnce. The tail
is usually short but sometimes long. The first and fifth digits of the fore-feet
are vestigial or absent, the remaining three being nearly equally developed.
In the hind-foot the fourth toe is longer and stouter than the others, while the

Frc. 497. -0rder Marsupialia,

Super-family Dasyuroidea, Family
Notoryctidre. Notoryctes. Thexero-
philous Marsupial Mole (N. typltlops),
with which the African golden moles
(Fig. 510, p. 729) show extraordinary
convergence. There are two species.
See also Figs. 495, 496 for dog- and
cat-like forms within the same super-
family. (From Cambridge Natural
<:-.,~
f
History.)

second and third are small and slender, and united together by a web of skin:
the first is vestigial or absent. The marsupium opening is directed backwards.
Allied to the true bandicoots are the beautiful bilbies or rabbit-bandicoots
(Thylacomys). These are more carnivorous; and have long silky fur and long
rabbit-like ears. Fossil bandicoots have been found in Pliocene and Pleistocene
deposits.
SUPER-FAMILy C..£NOLESTOIDEA

These South American marsupials do not fall into either of the main
assemblages outlined above. They are represented to-day only by Cceno~
testes, Orolestes, and Rhyncholestes, small rodent-like forms which, with the
true opossums (p. 7II), represent the two marsupial types that survived the
late-Tertiary placental invasion from the north. The Ccenolestoidea is ob-
viously of considerable antiquity and Palceocene fossils are known. Ccenolestes
is a small, superficially very mouse-like animal with a polyprotodont upper
dentition and with the median pair of lower incisors enlarged as in the con-
ventional Australian diprotodont condition (p. 709). The toes have not
become syndactylous. The most likely explanation of their 'aberrant' form is
that they and the Australian diprotodonts were separated in the late Mesozoic
when still in the polyprotodont condition and that, in isolation, the Australian
forms became fully diprotodont, the Ccenolestoidea partly so. Polydolops
716 ZOOLOGY

(Paheocene-Eocene), Abderites (Oligocene-Miocene) and other forms pursued

this tendency much further than did Ca:nolestes.

SUPER-FAMILY PHALANGEROIDEA

Into this Australasian group fall the arboreal phalangers or possums (e.g.
Trichosurus, Pseudoche£rus) and flying-phalangers (marsupial gliders) (e.g.
Petaurus); the arboreal, clinging, superficially somewhat sloth-like Koala
(Phascolarctos) ; the aberrant, burrowing wombats (Phascolomys) ; and the
great assembly of kangaroos and their allies (Macropodidce) (Fig. 493, p. 707).
The phalangers have both fore- and hind-feet prehensile. The second and
third toes of the hind-foot are slender and united by a web, but the hallux,
which is nailless, is opposable to them. The fourth and fifth toes are nearly
equal. The tail is well developed and may be prehensile. The flying phal-
angers are provided with lateral folds of skin extending from the fore- to the
hind-limbs and, acting as a parachute, enable the animal (as in the true flying-
squirrels), to make leaping descending glides from tree to tree. Phalangers
are often of great beauty and were hunted extensively for their fur but they are
now generally protected and are not uncommon in the gardens of inner suburbs
of capital cities. The group contains also minute pigmy possums (CercaertHs),
pigmy 'feather-tailed' gliders (Acrobates) (Fig. 498) as well as the large slower-
moving cuscuses (Spilocuscus) of tropical Australasia. Remarkably specialised,
too, is the tiny, dorsal-striped, arboreal, monospecific, Honey-possum (Tarsipes)
which occupies a sub-family by itself. It has an extensile tongue, degenerate
cheek-teeth, and a long prehensile tail.
The wombats (Phascolomyidce) are large, heavy, thick-bodied vegetarian
animals that live in burrows, emerging at night. They have short, flattened
heads and short thick limbs, provided with strong claws on all the digits except
the hallux. The second and third toes of the hind-foot are partly skin-con-
nected. The tail is very short.
The rare arboreal Koala is the sole living representative of the family
Phascolarctidce. It is an inoffensive, sluggish, and extremely beautiful
marsupial. Once slaughtered for its pelt, it is now rigidly protected. It has
a thick body, vestigial tail, cheek-pouches, and an extremely capacious ccecum.
The Koala's feeding habits are inordinately specialised. It will eat the leaves
of about a dozen species only of Eucalyptus. It never drinks but licks up earth
containing, no doubt, essential mineral factors. The pouch of this animal is
peculiar in that its lateral recesses extend to the flanks. The gestation period
is said to be only 35 days. One young is produced every second year. Although
the adult is about 30 inches long the neonatus is only about ! inch in length and
weighs about s! grams. At birth the fore-limbs are relatively powerfully
developed. It is primarily by means of these that the blind, helpless young
one makes unaided a wriggling two-inch journey to the backwardly directed
 PHYLUM CHORDATA 717
pouch (see alsop. go8). Therein it fixes itself unaided to one of the two nipples.
At about six months the baby is some six inches long, but it does not outgrow
the pouch until two months later, after which it is carried about on the female's
back, or held in her arms as she sleeps in a tree-fork.
The kangaroos and their allies (Macropodidce), are remarkably adapted for

FIG. 498.-0rder Marsupialia, Super-family Phalangeroidea, Family Phalangeridre, Acrobates.

The Australian possums (phalangers) belong to the same super-family as the kangaroos, the Koala
and the burrowing wombats and are therefore only distantly related to the American opossums
(Fig. 494). The Feather-tailed, or Pygmy, Glider (A. pygmmus) inhabits the eucalyptus forests of
Eastern Australia and still survives in the suburbs of capital cities. About the size of a mouse, it
spends the daylight hours in hollow limbs, emerging at dusk to feed on insects and nectar. The
tail-structure has no interlocking device such as occurs in true feathers. Larger gliders have
fluffy tails : in the cat-sized Schoinobates volans the tail is about 20 inches long and steers the animal
from high branches to lower ones in glides of up to 120 yards. (Partly after Gould, and B.M.
(N.H.) specimens.)

swift terrestrial locomotion. They have a relatively small head and neck.
The forelimbs are small, and each is provided with five digits. The hind-legs
are long and powerful. Rapid progression is effected by great springing
leaps (as long as 26 feet) with the body inclined forwards and the fore-limbs
clear of the ground. The foot is narrow and provided with four toes, a hallux
being absent. The two inner (second and third) toes are small and united by
integument. The middle toe is very long and powerful, and is a formidable
VOL. II. yy
ZOOLOGY

FIG. 499.-0rder Marsupialia,

Super-family Phalangeroidea, Family
Diprotodoutidse. Diprotooon. These
animals, growing to the size o f a
rhinoceros (e.g. D. australis) were a
herbivorous t errestrial development
from arboreal phalanger s tock. Fossils
of diprotodons bogged in receding
Pleistocene lagoons have been found
in many parts of Australia. (From a
restoration by Stirling.)

, FIG. 500.-Diprodoutida!:
: Skull and dentition of Noto-
the rium. This P leistocene
a nimal was smaller than
Diprodotan (Fig. 499) but
shared its general resem-
blance to the ext a nt wom -
bats. (After Owen.)

FIG . soJ.-Supcr-family Phalaugeroidea,

Family Thylacoleonidse. Tl1ylacoleo. This
remarkable Pleistocene ma rsupial (T. camifex)
was a nother phalangeroid form . It was abou t
t h e size of a modern lion. The molars were
reduced and t he rear premolars were special-
ised into powerful shearing t eeth. (After
F lower.)
 PHYLUM CHORDATA

defensive weapon capable of disembowelling a dog. The tail is very long,

usually thick and helps support the body while at rest. There is a large
marsupium. The tree-kangaroos (Dendrolagtts) differ from ordinary kangaroos,
particularly in their shorter and thicker hind-limbs, in which the second and
third toes have become nearly as large as the fourth (Fig. 493, p. 707).
The numerous, highly varied and vegetarian Macropodidre range in size from
the rabbit-sized rat-kangaroos (Bettongia) to the magnificent plain-dwelling
Red Flier (M acropus rufus) (which has a blue-grey doe) and the Great Grey
(Forester or Boomer) (M. cangurtt) which stands about six feet high. The new-
born animal is little more than one inch long (seep. 908).
The aberrant and primitive Hypsiprymnodontidre contains only one species,
the Musk Rat-Kangaroo (Hypsiprymnodon moschatus), of dense North Queens-
land rain-forests. Alone among the kangaroos it possesses the first pedal digit,
and shows certain similarities with the phalangers.
Among the Phalangeroidea has been placed the largest extinct marsupial
yet discovered-Diprotodon (Fig. 499). This
cumbersome Pleistocene animal was about as
big as a modern rhinoceros, being perhaps six
feet high and 10 feet long. Diprotodon was
heavily quadrupedal \vith a massive head and
neck. Anteriorly the upper jaw was armed
with a pair of formidable chisel-shaped incisors Frc. 5oz.- Macropus: Atlas.
with a frontal enamel deposition. These teeth Note incomplete ossification of in·
ferior arch.
had persistent pulps (p. 864), growing con-
tinuously as they wore away. Behind them lay two pairs of lesser incisors.
The lower anterior pair of incisors was large and forwardly directed and
the lower cheek teeth were transversely ridged for grinding. Petrified
crushed vegetable material has been found with the bones. The animal was
plantigrade, with minute digits : its arboreal ancestry was indicated by the
structure of its hallux which appears to have been opposable. Hundreds of
fossils were recovered from a single lake site where the animals bogged in mud
(as do cattle to-day) in an attempt to reach water during a Pleistocene drought.
Another fossil marsupial of identical antiquity and site is Nototheri~tm (Fig.
soo), an ox-sized lumbering, broad-headed, herbivorous form, possibly armed
with a nasal horn. Some of its characters suggest a remote wombat affinity.
Thylacoleo (Fig. 501) is another Australian Pleistocene phalangeroid. It was
lion-sized, with peculiarly large posterior premolars, modified into shearing
teeth, suggestive of a carnivorous diet.
Endoskeleton of Metatheria.-In marsupials the inferior arch of the atlas
(Fig. 502) is often incompletely ossified, a gap being apparent in the macerated
bone. Sometimes this gap becomes closed by ingrowths from the lateral
parts of the arch, whilst at other times a small separate ossification is developed
 720 ZOOLOGY
therein, filling the opening. There are always nineteen trunk vertebrre. The
thoracic transverse processes are always well developed, and the ribs articulate

Ftc. soJ. -MaeroJ)us : Endo-

skeleton. Wallaby . The scapula
is represented as raised somewhat
higher t han it would be in the
na tural relations of t he parts. The
head of the femur has been sepa-
rated from the acetabulut'n. acet.
acetabulum; acr. acromion pro-
cess; ast. astragalus; calc. cal-
caneum ; cbd. cuboid; chev. chev-
ron bones; cl. clavicle; cun. cunei-
form of carpus ; epi. cpipubis ; fb .
fibula; f cm . femur; hd. head of
femur ; lm. humerus ; il. ilium;
-isch. ischium; obt. obturator fora-
men; orb. orbit ; pis. pisiform ; pub.
pubis ; rad. radius ; rb. 1 , first rib ;
rb. 13, last rib ; sc. scapula ; st.
sternum ; tb. tibia; troch. great
trochanter of femur; 11lt1. ulna ;
>m e. unciform; IV. fourth toe.

FIG. 504. - Phascolomy.'l :

Skull. Wombat, in lateral view.
~:xt.aud ang. angular; cond. cond yle of
'.Y mandible ; ext. aud. opening of
external auditory meatus; e.1:. oc.
exoccipital; ju. jugal;, max.
m axilla ; nas. nasal ; p. max.
premaxilla; sq. squamosal ; ty.
tympanic.

with them as well as wit h the vertebral bodies. Prominent metapophyses and
anapophyses are developed, largest in the lumbar region. In most marsupials
but one sacral vertebra is present though in some a second is ankylosed with it .
 PHYLUM CHORDATA 721

The caudal region varies greatly in length ; it is short in the Koala and the
wombats, long in the opossums, dasyures, phalangers, and kangaroos (Fig. 503).

oc.contl-

FIG. sos.-Dasyurus: Skull. Marsupial 'cat,' in lateral view. a/. sph. ali-sphenoid; ang.
angular process of mandible; Jr. frontal; ju . jugal; /cr. lachrymal; max. maxilla; nas. nasal; oc.
cond. occipital condyle; par. parietal; par. oc. par-occipital process; p. max. premaxilla; s. oc.
supra-occipital; sq. squamosal; sq'. zygomatic process of squamosal.

Chevron bones are generally present, except in the Koala and the wombats.
In the skull (Figs. 504, 505, 506) the brain-cavity is relatively small. The
pituitary fossa is not distinct, and there
are no clinoid processes. The zygoma is
complete, but the orbit is not completely
bounded by bone behind. The extinct
phalangeroid Thylacoleo, however, has a
complete post-orbital bar (Fig. 501). The
jugal extends beneath the squamosal root
of the zygoma to form part of the outer
wall of the glenoid fossa. The lachrymal
foramen is usually on the anterior margin
of the orbit, sometimes on the face. The
palate usually presents vacuities in its
posterior portion. The pterygoid is always
small. The alisphenoid is large, and
forms the anterior boundary of the tym-
panic cavity. In the kangaroos the ali-
sphenoid (Fig. 506, ali.) extends backwards
so as to join the great-ly elongated paroc-
cipital process. When an auditory bulla
is developed, it is formed by that bone, FIG. 506.-Petrogale: Skull. Rock
Wallaby, in ventral view. ali. alis.
the tympanic being always small, and phenoid; bas. oc. basioccipital; bas. sph.
never ankylosed to neighbouring bones. basisphenoid; ex. oc. exoccipital; ju.
jugal; max. maxilla; pal. palatine; par.
The internal carotid artery perforates the oc. paroccipital; p. max. premaxilla; pr.
sph. presphenoid; pt .. pterygoid; sq.
basisphenoid. The optic foramen is con- squamosal; ty. tympanic.
J22 ZOOLOGY
fluent with the sphenoidal fissure. In all except T arsipes the angle of the
mandible sends inwards a remarkable process (ang.), and is said to be inflected.
In the pectoral arch of the marsupials the coracoid process is, as usual,
developed from a special bony centre, and a distinct suture is often recognisable
between it and the scapula until a com-
paratively late stage. In the young
condition (when the fcetus is attached to
the teat) the coracoid is comparatively
extensive and reaches the presternum
ventrally. A clavicle is always present,
except in the bandicoots, but may be in-
lib
complete. There is never a distinct cen-
trale in the carpus. In the opossums
the ilium has the primitive form of a
straight, three-sided rod. In the kan-
garoos (Fig. 503, it.) it is still simple and
three-sided, but somewhat curved out-
ward. In the rest it is more or less com-
pressed. In nearly all the marsupials
cub there is a pair of elongated and com-
- ed.cu..n pressed epipubic or marsupial bones (Fig.
503, epi), which articulate posteriorly
with the anterior edge of the pubes. In
the Thylacine they are represented only
b y small unossified fibro-cartilages. The
fibula is always well developed, and in the
young condition of some marsupials there
is an accessory element situated outside
its proximal end. This apparently corre-
sponds to a bone known as the parafibula
FIG. 507.-Triclwsurus: Hind-leg and which occurs in some Lacertilia. In the
pes. Phalanger. ast. astragalus; calc. cal-
caneum; cub. cuboid; eel. ctm. ecto-cunei- phalangers (Fig. 498) and the Koala there
form ; ent. cun. ento-cuneiform; fb . fibula; is always a considerable range of move-
mes. cun. meso-cuneiform; nav. navicular;
tib. tibia; I-V, digits. (After Owen.) ment between the fibula and the tibia,
comparable in some degree to the move-
ments of pronation and supination of the radius and ulna. The foot (Figs.
507, 508), as already stated, presents a much greater range of modification
than the hand.
 PHYLUM CHORDATA

INFRA-CLASS EUTHERIA (PLACENTALIA, MONODELPHIA)

INTRODUCTION
The Eutheria and Metatheria are presumed to have arisen from some
as yet untraced branch of the Jurassic pantotheres (p. 703). Since undoubted
eutherians and metatherians are known from the Upper Cretaceous, they must
have diverged along their separate lines of evolution during the early part of
the Cretaceous period (p. 3). At the beginning of the Palreocene a great variety
of eutherians made their appearance, implying either a very rapid evolutionary
radiation, or perhaps more probably a rapid dispersal
from an area, at present undiscovered, where they had
been evolving during the Cretaceous. It must be re-
membered that only one Lower Palreocene mammalian
fauna, that of the Puerco Beds of New Mexico, is cl "
adequately known. At the dawn of the Eocene (some
50 million years ago) additional diversification took
place. Most of the orders of the infra-class Eutheria
were present in the Eocene.
In all, Simpson (whose classification is here generally
followed; see, however, p. 725) recognises twenty-six
eutherian orders, ten of which have become extinct.
The Eutheria includes all the more familiar mammals
of to-day, including Man and his domesticated beasts.
It embraces about 8,500 species.
The basic characters common to the group, which :rv
has in its fairly long history radiated into a considerable Fr G. soS.-·Mac-ropus:
number of lines, are naturally relatively few. Although Pes. Kangaroo, right. a.
astragalus; c. calcaneum;
an allantoic placenta occurs in certain marsupials cb. cuboid; e3 . ento-cunei-
form; n. navicular ; 11-
(p. 708), the chief eutherian character is nevertheless V, digits. (After Flower.)
the highly organised allantoic arrangement, whereby
the young can be nourished in the uterus for a comparatively long time
and be born in a far more advanced stage of growth than those of marsupials.
The uterus tends gradually to lose its original double formation (uterus
duplex) and becomes a single structure (uterus simplex). The urinary ducts
pass to the bladder outside the genital ducts (cf. pp. 704, 8g2). A common
sphincter is only rarely present and then only as a remnant, thus there is no
longer any cloaca. In the brain a new structure appears- the corpus callosum
(Fig. 486), which connects the large cerebral hemispheres. In the course
of its evolution the brain has become more and more highly organised,
especially by the remarkable development of the neopallia! region of the
hemispheres (pp. 679, 884).
The internal carotid enters the skull through the foramen lacerum medius
 ZOOLOGY
or through the bulla. (In marsupials it passes medially to the middle ear
to enter the skull through the basisphenoid.) The tympanic region in all but
the most primitive forms is protected by a bony covering, or bulla, an out-
growth of the petrous bone to which the tympanic bone becomes attached in
various ways to form a true tympanic bulla, as opposed to the alisphenoid bulla
of those marsupials with protected ear region. Some Eutheria resemble
marsupials in single features of the skull, but such resemblances have probably
been secondarily acquired. They include the perforation of the palate in
hedgehogs, the extension of the jugal back to the glenoid cavity in hyraxes, the
confluence of the optic foramen with the sphenoidal fissure in some Lipotyphla,
and the inflection of the angle of the lower jaw in some rodents. No eutherian
possesses epipubic bones or a marsupial pouch (cf., however, monotremes, p.
688). The Eutheria all have the dental formula of 3· 1·4·3, or some derivative
3"1"4"3
therefrom. (Some have lost all teeth, viz. Pholidota and some edentates.)
As mentioned above, most of the eutherian orders seem to have arisen
during the Palreocene. Some (e.g. Treniodonta, Tillodontia, Condylarthra,
Dinocerata) had a comparatively short career and probably did not survive
the Eocene. Others (Pyrotheria, Embrithopoda, Pantodonta) probably dis-
appeared towards the end of the Oligocene. The Notoungulata and Litop-
tema survived until a few million years ago. Some of the above groups are
still very imperfectly known, and the same can be said of the origins of many of
the extant orders: on the material available it is not always easy to determine
affinities.
Although the orders of Eutheria are so distinct to-day, their representatives
in the early Crenozoic were much more alike, and it is sometimes a matter of
dispute into which order some of the Palreocene forms should be placed. By
comparing the most primitive members of the various orders it is possible to
arrive at a conception of the characters of the hypothetical ancestral eutherian
stock. They must have been small mammals, probably not larger than a rat,
with a long trunk, arched back, rather short legs, and long, somewhat stout
tail. The manus and pes were plantigrade, with five digits provided with
claws. The clavicles were well developed, and the ulna and radius were free,
as were the tibia and fibula. In the skull the face was as long as the small
brain-case. The tympanic was ring-shaped and there was no bulla. The
snout possessed a well-developed rhinarium and vibrissre. The teeth were
adapted for an insectivorous diet, with sharp interlocking cusps and cutting
crests. There were three incisors, a moderately enlarged canine, four pre-
molars, and three molars, which were of a pretritubercular type (p. 868). As
regards soft paris, a good case can be made out that a crecum was present in
the gut. Both precaval veins may have been well-developed. The cerebral
hemispheres were small (not completely covering the corpora quadrigemina)
 PHYLUM CHORDATA

and unconvoluted. The olfactory lobes were large. The perineum was prob-
ably poorly developed, and traces of a cloaca may have been present. The
testes were probably abdominal and the uteri separate (duplex). The number
of young produced at a birth was probably large. Numerous mammary glands
formed a series along the body between axilla pit and groin. No living euth-
erian possesses this combination of characters, but many have retained some
of them. Primitive characters are most numerous in the orders Menotyphla
and Lipotyphla (generally united into a single order, Insectivora), and thus the
other orders are often regarded as having an insectivore ancestry. It must be
emphasised, however, that the living insectivores, though primitive in some
respects, are specialised in others. Further, some such primitive characters
have survived in many other orders.
We will follow Simpson's arrangement of living and extinct orders into four
cohorts. The Insectivora, however, are divided into Lipotyphla and Meno-
typhla.

INFRA-CLASS EUTHERIA

Cohort Unguiculata
Orders Lipotyphla (Upper Cretaceous-Recent)
Menotyphla (Upper Cretaceous-Recent)
Primates (Palreocene-Recent)
Dermoptera (Palreocene-Recent)
Chiroptera (Eocene-Recent)
Tamiodontia (Palreocene-Eocene)
Tillodontia (Palreocene-Eocene)
Edentata (Palreocene-Recent)
Pholidota (Oligocene-Recent)
Cohort Glires
Orders Rodentia (Palreocene-Recent)
Lagomorpha (Palreocene-Recent)
Cohort Mutica
Order Cetacea (Eocene-Recent)
Cohort Ferungulata
Super-order Ferre
Order Carnivora (Palreocene-Recent)
Super-order Protoungulata
Orders Condylarthra (Palreocene-Eocene)
Notoungulata (Palreocene-Pleistocene)
Litopterna (Palreocene-Pleistocene)
Astrapotheria (Eocene-Miocene)
Tubulidentata (Pliocene-Recent)
 ZOOLOGY
Super-order Prenungulata
Orders Hyracoidea 1 (Oligocene-Recent)
Proboscidea (Eocene-Recent)
Pantodonta (Pala:ocene-Eocene)
Dinocerata (Pala:ocene-Eocene)
Pyrotheria (Pala:ocene-Oligocene)
Embrithopoda (Oligocene)
Sirenia (Eocene-Recent)
Super-order Mesaxonia
Order Perissodactyla (Eocene-Recent)
Super-order Paraxonia
Order Artiodactyla (Eocene-Recent)

COHORT UNGUICULATA
This great assembly is composed of extinct and living orders in which
archaic mammalian characters are notably retained. Among living mammals
it includes the Lipotyphla (e.g. shrews, moles, and hedgehogs), Menotyphla
(tree-shrews and elephant shrews) Chiroptera (bats), Dermoptera (Colugo or
'Flying Lemur'), Edentata (restricted to armadillos, sloths, and true ant-
eaters among living beasts), Pholidota (pangolins) and the Primates (lemurs,
lorises, tarsiers, monkeys, apes, and Man).
The older order Rodentia (rodents and rabbits), properly divisible into
Rodentia, and Lagomorpha (p. 779), is often included among the above but
its two groups are here included in a separate cohort Glires (p. 774).

ORDER LIPOTYPHLA

This order, generally considered as a suborder of an order Insectivora, may

be divided as follows:
Sub-order Erinaceomorpha
Super-family Erinaceoidea (Upper Cretaceous-Recent)
Sub-order Soricomorpha
Super-families Tenrecoidea (Pala:ocene-Recent)
Soricoidea (Pala:ocene-Recent)
Chrysochloroidea (Miocene-Recent)
The Lipotyphla have retained a large number of the characters of the
Cretaceous stock from which all placental mammals have arisen. They are
small mammals, the largest, Solenodon (Fig. 509), being only about the size of
a cat. Some of the shrews, for example Sorex minutus and Suncus etruscus are
1 There is considerable controversy about the affinities of this group. Recently discovered
material has led Whitworth to place the order with the Perissodactyla in the Mesaxonia (p. 824).
 PHYLUM CHORDATA

the smallest living mammals, weighing only about three grammes (cf. p. 780).
The manus and pes are plantigrade, and with few exceptions all five digits are
retained. Clavicles have been lost only in Potamogale. In the vertebral
column there may occur small bony nodules between the vertebrre on the
ventral side which appear to be the remains of chevron bones (intercentra).
An entepicondylar foramen is usually present in the humerus. The tympanic
is usually ring-shaped. The cerebral hemispheres are small and smooth, and
the olfactory lobes are large. Remains of a cloaca are often present. The
testes are frequently abdominal in position, and there is never a fully developed
scrotum. The uterus is bicornuate. The number of young produced at a
birth may be very large: in Tenrec the average is 21. The mammary glands
are usually numerous: three to six pairs, except in the Chrysochloroidea,
where there are two pairs.
At the same time the Lipotyphla share a number of characteristic specialisa-
tions. The pelvic symphysis is reduced to a short contact between the ischia,
or may be completely absent (as in Soricoidea). There is never any crecum to
the gut. The snout is prolonged to form a mobile proboscis, moved by muscles
which are attached to the skull at the anterior edge of the orbit (in Erinaceo-
morpha) or pass back beneath the orbit as far as the squamosal (in Sorico-
morpha). The zygomatic arch is frequently incomplete: when it is present it
is usually slender, and the jugal is greatly reduced. The eyes tend to be reduced
in size, especially in the Soricomorpha, where the optic foramen is not separate
from the sphenoidal fissure.
The Lipotyphla are predominantly insectivorous, and have sharp molar
cusps and interlocking teeth. The incisors are frequently enlarged to function
as forceps, notably in the shrews (Soricidre), and the canines are then small or
absent. The molar patterns fall into two main types. In the dilambdodont
type the outer cusps (paracone and metacone) are both large, and usually form
a W of high cutting crests. In the zalambdodont type, which characterises
the Tenrecoidea (Fig. 509) and Chrysochloroidea (Fig. 510), the metacone is
undeveloped, and the paracone forms a V: the protocone is reduced, together
with the talonid of the lower molar, so that the zalambdodont molar teeth may
closely resemble those of the Dryolestoidea of the Jurassic.
The Erinaceoidea are to-day represented by a single family, the Erinaceidre,
with a wide distribution in Europe, Asia, and Africa, but extinct in North
America. The more primitive members of the group, found in South East
Asia, are hairy (e.g. Echinosorex = Gymnura), but in the hedgehogs (e.g.
Erinaceus) the hairs of the dorsal surface have become converted into spines
and, owing to a remarkable development of the dermal musculature, the
animal can roll itself into a spine-covered ball. A band of muscle passes down
the sides of the body and over the neck and the root of the tail to form a
sphincter round the edges of the spine-covered area. During contraction the
728 ZOOLOGY
spiny skin is pulled over the head and tail by other skin muscles. European
hedgehogs can kill vipers and are immune to their poison.
The Soricoidea include the families Soricidce (shrews) and Talpidce (moles
and desmans). Shrews are world-wide except in the Australian region and most
of South America; moles are Holarctic and Oriental in distribution. The
shrews have a higher metabolic rate than any other mammal, and are unable to

FIG. sog.-Infra-class Eutheria, Order Lipotyphla, Super-family Tenrecoidea, F a mily Soleno-

dontidre. Solenodon. The Lipotyphla (e.g. 5. paradoxus above) and t he Menotyphla (e.g . Fig.
514) are often united in a single polymorphic Order lnsectivora . Members of the genus Solenodon
are insectivorous, nocturnal, and gregarious, living by day in burrows and rock fissures. The
females m a ke n ests. They have well-developed inguinal and axillary glands and their saliva,
like that of some shrews, is said to be venomous. (From Bourliere, and B.M. (N.H .) specimens.)

survive more than ten hours without food. In some shrews (Blarina) the sub-
maxillary glands produce a poisonous saliva capable of killing a mouse. The
most primitive moles, e.g. Uropsiltts of China, are very much like shrews and
have no burrowing adaptations: their feet are unspecialised and pinnre are
present. The desman (Desmana = Mygale) is aquatic, possessing webbed feet
and a laterally compressed tail. The remaining Talpidce are specialised for
burrowing. Moles have been recorded to burrow one hundred yards in a
single night. The manus is very large and is held vertically at the side of the
 PHYLUM CHORDATA

head, with the palm outwards and the pollex ventrally. A falciform bone
develops from the carpus on the medial side of the pollex, giving greater breadth.
The humerus is very short and has its muscle crests powerfully developed.
The clavicle is very short and thick, and articulates with the humerus as well
as with the acromion process of the scapula. The humerus is entirely embedded
in the body, the shoulder girdle having been carried forward to the neck region
by elongation of the presternum. This aids in stream-lining. A prenasal bone
is developed in the snout. The second, third, and fourth cervical vertebrre are
fused. The hind limb pushes the body forward when burrowing, and the

FIG. 510. -0rdcr Lipotyphla, Sub-order Soricomorpha, Super -family Chrysochloroidea,

Family Chrysochloridre. Chry.<wcltlol·is. The fossorial golden m oles o f the African veldt (e.g.
C. lrevelyani, above) show in som e respects a remarkable convergence with t h e marsupial mole of
Australia (F ig. 497, p. 715). The eyes arc small, or covered by skin. The ears are externally
invisible a nd the legs arc short a nd, particularly the fore-paws (A). specialised for burrowing.
T he fur is silky and iridescent. True moles belong to a different super-family (Soricoidea). (From
Cambridge Nat ural H istory.)

sacrum is accordingly strongly constructed. The tail is short. The ~yes are
very small, and there is no ear-pinna.
The Tenrecoidea consist of two living families, the Tenrecidre ( = Centetidre),
found mainly in Madagascar, but with a few African representatives, and the
Solenodontidre, with a single genus, Solenodon (Fig. 509), in Cuba, Hayti and
other West Indian islands. Fossil Solenodontidre occurred in the Oligocene
and Miocene of North America. The Tenrecidre are varied in appearance.
Setifer is spiny like a hedgehog, but it cannot roll up. M icrogale is a small,
shrew-like animal. Hemicentetes, which has spines mixed with hair, has
reduced teeth and probably feeds on ants. Limnogale is aquatic, with webbed
feet. All these are found in Madagascar, where the family has undergone an
adaptive radiation. Potamogale, the otter-shrew of Africa, is also aquatic.
It swims by its long, flattened tail and webbed feet, and has lost its lachrymal
foramen and its clavicles. Solenodon (Fig. 509) is a relatively large, nocturnal
730 ZOOLOGY
animal with a long snout and a strongly developed tail. It uses its strong
claws to break open dead wood to obtain insect food.
The golden moles (Chrysochloridre) (Fig. 510) of East and South Africa,
the only family in the Chrysochloroidea, are so different from the other Lipoty-

F I G. jii. - Centetes: Skull.

Tenrec, lateral view. Jr. frontal;
max. maxilla ; pa. parietal; p. max.
premaxilla ; sq. squamosal. (After
Dobson.)

phla that some authorities have placed them in a separate order. In some of
their fossorial adaptations they resemble the Talpidre, which do not occur in
Africa. Thus, the eyes are greatly reduced, pinnre are absent, and the muscle
crests on the humerus are strongly developed.
However, the pectoral girdle is quite unlike
that of the Talpidre. The presternum is not
markedly elongated, but the anterior ribs are
concave to provide room for the humerus to
be embedded in the body. The clavicles are
long. The hand has only four digits, of which
the second and third are provided with large
claws. In these respects the golden moles re-
semble the marsupial Notoryctes (Fig. 497,
715). The skull is greatly modified. The
face is turned down on the globular brain-
case, and the whole skull is often very short
and broad. Its sutures unite early. In some
species the malleus is enormously enlarged,
but the significance of this is unknown. The
milk teeth are retained until the animal is
FrG. 51 2.-Talpa: Sternum and
part of shoulder girdle. Mole. cl. mature, and function with the permanent
clavicle ; H. humerus; M. manubrium. molars.
(From Cambridge Natural H istory.)
Skeleton of the Lipotyphla.- The neural
spine of the axis is usually well developed whilst those of the remaining cervical
vertebrre are small or obsolete. The number of trunk-vertebrre varies in the
different families from eighteen to twenty-four, and there is also great variation
in the development of the various processes.
 PHYLUM CHORDATA 73I
The number of sacral vertebrre ranges from two to seven. The caudal
region is variable in length and frequently has chevron bones. These sometimes
occur also in the lumbar region. The second, third, and fourth cervical verte-
brre are fused in the mole. The presternum is usually an expanded, T-shaped
bone, but in the moles it is greatly elongated.
The skull (Fig. 5II) varies greatly in the different families. The cranial
capacity is small, and the orbits and temporal fossre arc completely continuous.
A postorbital constriction occurs in the Erinaceidre, but
only rarely in the Soricomorpha, in which the anterior
part of the skull is tubular. The zygoma is often in-
complete (Soricidre, Tenrecoidea), and the jugal is small
or absent. With few exceptions, the tympanic is ring-
shaped and does not form a bulla. The palate is some-
times perforated.
In the pectoral girdle clavicles are always present
except in P otamogale. The pectoral girdle of the moles
has been described above. The humerus usually has
an entepicondylar foramen, and often also a supra-
condylar foramen. In the moles (Fig. srz) it is short,
greatly expanded at the extremities, and has a promi-
nent deltoid ridge, and two synovial articular surfaces
at the proximal end. One of these is for the glenoid
cavity of the scapula; the other is for the clavicle.
The radius and ulna are completely developed in all.
They are usually distinct, but are sometimes fused F1G .513.-- Talpa: Fore-
arm and manus. Mole. c.
distally. In the carpus the scaphoid and lunar some- cuneiform; ce. centrale; l .
nar; m . magnum; p. pisi-
times coalesce, sometimes remain distinct; an os cen- lu form; R . radius; rs. radial
trale is usually present. In the moles (Fig. 5I3) the sesamoid (falciform bone);
td. trapezoid; s. scaphoid;
manus is extremely broad, the breadth being increased tm. trapezium; U . ulna; u .
by the presence of a large, curved, radial sesamoid unciform; I - V. digits.
(falciform bone).
In the pelvis the symphysis pubis is short or sometimes absent, the pubes
remaining separated by a wide median ventral cleft. A third trochanter is
sometimes represented by a ridge. The fibula usually, though not always,
fuses distally with the tibia.

ORDER MENOTYPHLA
This order consists to-day of two very distinct families, the elephant-shrews
(Macroscelididce) of Africa and the tree-shrews (Tupaiidce) (Fig. 5!4) of the
Oriental region. The relationships of these animals are much disputed. The
Macroscelididce are often placed with the Lipotyphla as the ' Insectivora ', and
the Tupaiidce were included by Simpson in the Primates. It has also been
732 ZOOLOGY
suggested that the Macroscelididre should be raised to the rank of a distinct
order. (For a discussion of the problem see, for example, Evans, Butler.) In
spite of their differences, which are due largely to their adaptation to very
different modes of life, the two families show a number of resemblances which
are probably indicative of relationship. They may be regarded as survivors
from the stock of primitive mammals from which the Primates have evolved.

FIG. 514.- 0rder Menotyphla, Family Tupaiidre. 7'upaia. The tree-shrews (e.g. T. belangeri,
above) and the elephant shrews (Macroscelididae) are often lumped with the Lipotyphla (Fig. 509)
as an Order Insectivora. Including the pen-tailed species they are small, primitive, superficially
squirrel-like eutherians that show certain resemblances with the 'lower' primates (p. 734) with
which they are sometimes grouped. They are extremely active by day and some make relatively
elaborate a rboreal nests. They eat foliage, fruit, and insects, and sometimes small vertebrate~ .
(From Zoo!. Soc. London photographs and skins from B.l\1. (N.H .).)

The Menotyphla differ from the Lipotyphla in several respects. The pubic
symphysis is not reduced. A crecum is present. The eyes are relatively large
(Fig. 514). In the skull, the cranial cavity is larger, and extends forward
between the large orbits. The jugal is well developed. A vertical plate of the
palatine extends up the anterior medial wall of the orbit to meet the lachrymal,
whereas in the Lipotyphla this part of the orbital wall is formed by an extension
of the maxilla, and the palatine does not touch the lachrymal. The number of
young produced at a birth is never more than three, and there are one to three
pairs of mammary glands.
 PHYLUM CHORDATA 733
The Macroscelididre are ground-living animals with enlarged hind limbs,
resembling rabbits or jerboas in their manner of locomotion. The tibia is
elongated, and is fused with
the fibula at its lower end. The
metatarsals are also elongated,
and the pes is digitigrade.
There is a large bulla, formed
from several bones, including
the tympanic and the ali-
_•. -1/>PI
sphenoid. The last premolar
is molariform, and the last
molar is vestigial or absent. fos.ec
The testes are abdominal. Ex-
amples are Rhynchocyon, Mac-
roscelides, and Elephantulus.
feur···
The Tupaiidre are squirrel-
like and climb trees with the p.ty.S9.--

aid of claws (Fig. 514) . They

resemble the lemurs in many
features of the skull, but their
feet are not prehensile. The
tibia and fibula are free. The
olfactory organ is smaller than
in the Macroscelididre, with
fewer turbinal bones, and the
olfactory lobes of the brain are
reduced. The facial part of
the skull is relatively shorter,
and the eyes look somewhat
forwards. As in lemurs, there
is a postorbital bar, formed
from the frontal and jugal
bones. The ring-shaped tym-
panic is situated inside a bulla FrG. 5I5.-Ptilocercus: Skull. Tree-shrew, palatal
and lateral views. i 1 i.2 incisors ; c. canine; p.2, s, •,
formed from the entotympanic, pre-molars; M.'-3 molars. As. alisphenoid; can. as. ali-
as in the Madagascan lemurs. sphenoid canal; con. condyle; cr. tmp . temporal crest; en.
entotympanic; f. c. carotid foramen ; f. eus. eustachian
The molar teeth are dilamb- ty. opening; Fr. frontal; f. l. a. foramen lacerum anterius;
dodont resembling those of f. mal. malar foramen ; f. op. optic foramen; f. or. fora-
, m en ovale ; fos. ect. pt. ectopterygoid fossa; fos. ty. hy.
shrews. The testes descend t ympano-hyoid fossa; f. p. gl. post-glenoid foramen;
Permanently as in Primates · fo. pl. pata7ine foramen; f. st. m. styl?mastoid foramen;
Ms. mastmd ; p. ty . Sq. post-tympamc process of squa-
Examples are Tupaia and mosal; Pt. pterygoid ; Ty. tympanic. (After Gregory.)
Ptilocercus (Fig. 5I5)-
voL. u. zz
734 ZOOLOGY
Anagale, from the Oligocene of Mongolia, is in some respects intermediate
between the two living families, but it approaches the Primates in the flatten-
ing of its terminal phalanges, indicating the possession of nails. Some
'insectivores' from the Palreocene and Eocene may prove to belong to the
Menotyphla, but their structure has not been sufficiently elucidated. The
Leptictidre, which range from the Upper Cretaceous to the Oligocene (e.g.
Ictops), have hitherto been classed with the hedgehogs in the Lipotyphla, but
recent studies of their skulls indicate that they probably stand near the base
of the Menotyphla.
ORDER PRIMATES
There is still a considerable lack of knowledge as to many details of the
structure of the earliest Primates, due to the rarity of their fossils, a conse-
quence, probably, of their forest-living habits. 'Every scrap of fossilized
material that has become available has been studied intensively because of
Man's position in the order.' Yet a great deal of unnecessary confusion exists
in the Primate literature. A major reason for this is that' much of the work on
primates has been done by students who had no experience in taxonomy and
who were completely incompetent to enter this field, however competent they
may have been in other respects . . . Most primates have alternative names
. . . hardly two students use the same nomenclature for them. The im-
portance of distinctions within the group has also been so exaggerated that
almost every colour-phase, aberrant individual or scrap of bone or tooth has been
given a separate name, and almost every really distinct species has been called
a genus and a large proportion of genera have been called families. [Further]
many studies of this order are covertly or overtly emotional' (Simpson). 1
Linnreus included Man as a third genus in the Primates but many later
authorities, including Cuvier, either declined to consider Man as an animal or
regarded him as the unique occupant of a separate order. To-day no educated
person has any doubt that Homo sapiens Linn. is a primate; but there is still
considerable argument as to his exact taxonomical position and relationships,
as, indeed, there is concerning the origin of the Primates as a whole. There
is, however, enough evidence to show the great antiquity of the group. Lemuri-
forms, for example, occur first in the Palreocene. Anthropoids, on the other
hand, do not appear until the late Oligocene.
Absolutely diagnostic characters of a primate are not easy to find. The
most primitive species (p. 736) still bear traces of their origin from the cen-
tralised Cretaceous 'insectivore' stock. There are, however, definite trends of
evolution within the group, of which the beginning can be seen even in the
1 Upon a single tooth later proved to be that of Prosthenops, an extinct peccary (p. 837),
there was erected a 'new' genus and species with the resounding name Hesperopithecus harold-
cooki, which was claimed to represent a North American Pliocene ape that resembled' the human
type more closely than it does any known anthropoid ape' (see Wood-Jones).
 PHYLUM CHORDATA 735
earliest forms. The dentition, for instance, never has the full eutherian formula
of three incisors, a canine, four premolars, and three molars in each half of the
upper and lower jaws. From the first it is reduced to two incisors, and
(except in some early forms with four) to three premolars. Reduction may
eventually proceed further by the loss of another premolar and a molar. The
pattern of the molar teeth is at first trituberculo-sectorial, and in many later
forms becomes quadri- or quinque-tubercular. The generalised nature of
the primate diet has not resulted in the evolution of any great specialisation of
tooth-pattern.
Primates were originally, and for the most part still are, arboreal. A
return to a terrestrial habitat is a secondary and rare feature. This fundamental
arboreality of the group has been held to have been of primary importance in
the moulding of many of the most notable primate characteristics. Life away
from the ground has allowed the drastic reduction of the olfactory receptor
apparatus, olfactory bulbs, and rhinopallium. The consequent reduction of
the snout allowed both eyes to look forward (in the manner of broad-faced,
short-beaked nocturnal hunting birds) with the concomitant development
of stereoscopic vision. Arboreal life also made possible the evolution of the
grasping hand and opposable thumb. With all of these, and many other,
novelties there came the great increase in size and complexity of the brain-
especially in the neopallium of the cerebral hemispheres which grew so as
ultimately to cover a large area of the cerebellum. The primate face has
become more and more subcerebral as it became bent down on the basicranial
axis, which became more inclined to the vertebral axis. Concurrently with
the evolution of a grasping hand, the rounded articulation of the head of the
radius with the humerus and of the distal extremity with the ulna allowed the
hand to have a very free movement of supination, a characteristic primate
feature, the primitive position of the mammalian hand being, of course, one of
pronation.
Whilst the brain has become highly organised, with resulting modification of
the skull, the rest of the body has retained a larger number of unspecialised
features than is shown by many of the other orders of mammals. Progression is
still plantigrade, and even in Man the number of digits remains the primitive
five. Flat nails are characteristic of most primates, although claws occur on
at least some of the fingers and toes of the lemuroids. The skull always has a
complete orbital ring, and the eye-socket may either be confluent with the
temporal fossa or separated by a bony partition. There is always a tympanic
bulla formed by a flange from the petrosal with which the tympanic ring takes
part in various ways. These have some value in classification.
The order is commonly divided into three sub-orders : the Lemuroidea,
Tarsioidea, and Anthropoidea, but in this account the following condensed
arrangement is preferred :
736 ZOOLOGY
ORDER PRIMATES
Sub-order Prosimil
Infra-order Lemurlformes
Families Plesiadapidre (Palreocene-Eocene)
Adapidre (Eocene)
Lemuridre (Pleistocene-Recent)
Indridre (Pleistocene-Recent)
Daubentoniidre (?-Recent)
Infra-order Lorisiformes
Family Lorisidre (Miocene-Recent)
Infra-order Tarsiiformes
Families Anaptomorphidre (Palreocene-Oligocene)
Tarsiidre (?-Recent)
Sub-order Anthropoidea
Super-family Ceboidea (Platyrrhina)
Families Cebidre (Miocene-Recent)
Callithricidre (?-Recent)
Super-family Cercopithecoidea
Family Cercopithecidre (Oligocene-Recent)
Super-family Hominoidea
Families Parapithecidre (Oligocene)
Pongidre (Oligocene-Recent)
Hominidre (Pleistocene-Recent)

It need hardly be said that the above classification, like all others, will
most likely be modified as new data emerge.

SuB-ORDER PROSIMII
The Prosimii are the more primitive Primates. In the Palreocene and
Eocene they formed practically the entire order. They had a wide distribu-
tion, many forms being known from Europe and North America. With the
rise of the Anthropoidea in the Oligocene, the Prosimii were reduced in numbers,
and are at present confined to Africa, Madagascar, and the eastern part of the
Oriental Region (from Ceylon to Borneo). Three infra-orders may be distin-
guished: the Lemuriformes (Fig. 516) now confined to Madagascar; the
Lorisiformes (Fig. 518), from the continent of Africa and the Oriental Region;
and the Tarsiiformes (Fig. 519), represented to-day by a single genus from the
East Indies. Fossils show that the Lemuriformes and Tarsiiformes were
distinct groups even in the Palreocene.
 PHYLUM CHORDATA 737

INFRA-ORDER LEMURIFORMES
The lemurs share a number of generalised characters with the Tupaiidce
(p. 731), insectivores with whom they seem to have a common origin and which,
in fact, may be more appropriately placed in the Primates.
To-day true lemurs (e.g. Lemur (Fig. 516)) exist in Madagascar only. Here

Fro. 5 r6.-0rder Primates, Sub-order Prosimii, Infra-order Lemuriformes, Family Lemuridre.

Lemur. The Ring-tailed Lemur (L. calla) is unusual in living among rocks. It inhabits southern
and south-western Madagascar where trees are stunted and few. The animal has adapted itself
to a mixed diet including especially prickly pears and often birds' eggs. It is about the size of a
cat. The naked black area of the fore-arm covers the ante branchial ('carpal') gland (see also p.
84 7); nearby, on one or both limbs is a solid horny spur (not shown) . Another gland, the brachial
(not shown) occurs near the proximal end of the biceps muscle. (From photographs, and speci-
mens from the B.M. (N.H.))

Pleistocene fossils (e.g. Megaladapis) also occur. In Madagascar too (and in

Pleistocene deposits) is found the allied Indri. Elsewhere, other allies occur in
the European and North American Palceocene (e.g. Plesiadapis) and Eocene
(Notharctus, family Adapidce). In Madagascar also still survives the Aye-aye
(Chiromys = Daubentonia), yet another lemuroid of annectant family.
Compared with Anthropoidea, Lemuriformes show a number of distin-
guishing features, some explicable as primitive persistent characters, others
as arboreal adaptations, and others as specialisations peculiar to the lemuroid
 ZOOLOGY

line of evolution. Primitive characters include the low brain-case (with its
macrosmatic brain and uncovered cerebellum), the confluence under the
postorbital rim of the orbital and temporal fossa:, the lateral instead of forward
direction of the orbits (Fig. 517), the presence of four or even five ethmoturbinal
bones (instead of the usual three), the backward extension of the jugal, the
thickened hind border of the palate, the large nasal bones (wide at the back,
instead of narrow and pointed), the uterus duplex, non-deciduous placenta, an
entepicondylar foramen in the humerus and a third trochanter in the femur.
The dentition is on the primitive trituberculosectorial plan, but subject to

FIG . 517.-Lemur: Skull and dentition.

considerable specialisation in various forms. The procumbent lower incisors

and incisiform lower canines are specialisations that have arisen within the
group. The hands and feet are also specialised, in that the second pedal digit
at least is always furnished with a sharp claw. The second finger is likewise
always clawed. At least some of the digits have flat nails. The tympanic
ring is enclosed inside the bulla, where it lies free, as in the Tupaiida: (Fig. 6rr).
The aye-ayes (Daubentoniida:) are notable for their short skull, single pair
of enlarged incisors and reduced pre-molars.

INFRA-ORDER LORISIFORMES
The lorises are absent from Madagascar. They occur in Africa (e.g.
Galago, the bush-babies, and Perodicticus, the pottos, Fig. 5r8) and in Asia
(e.g. Loris, the Slender Loris of India), including the East Indies. They are
poorly represented in the fossil record. The oldest known form is Progalago,
from the Miocene of Africa.
Though sharing many characters with the Lemuriformes, and often grouped
with them in a sub-order Lemuroidea, the Lorisiformes are distinguished in
several ways. Thus the ring-shaped tympanic is not situated inside the bulla,
 PHYLUM CHORDATA 739
but is attached to its edge to form a short bony auditory meatus. The ethmoid
(os planum) appears in the medial wall of the orbit. Vibrissce are reduced.
In addition to the pair of mammary glands on the breast a second pair is present
in the inguinal region. The eyes are enlarged and the face shortened, as in
some Lemuriformes e.g. Indri. Also, as in Lemuriformes, the lower canine is
incisiform.

FrG. srS.-Order Primates, Sub-order Prosimii, Infra-order Lorisiformes, Family Lorisidre.

Perodicticu.~. This family, including the pottos (e.g. P. potto above), bush-babies, and lorises is
Asiatic and African in distribution, the present genus being confined to tropical Africa. Pottos
are nocturnaL The present spec ies possesses a peculiar defensive armoury of sharp vertebral
spines that project through the skin. (Note toilet claw (cf. Fig. 483, p. 6g6). (From a photograph
by Suschitzky.)

The galagos are leaping forms in which the calcaneum and navicular have
become greatly lengthened. This has also happened, to a lesser degree, in some
Lemuriformes, e.g. Microcebus.

T ARSIIFORMES
INFRA-ORDER
The tarsiers, to-day represented by only one genus, Tarsius (Fig. 519) of the
East Indies, appear to have had a separate lineage dating from the Palceocene
(Anaptomorphidce, e.g. Tetonius, Anaptomorphus) and were formerly common
in both the Old and New Worlds.
740 ZOOLOGY
Tarsiers are of considerable in-
terest in that they have been
claimed anatomically to occupy an
intermediate position between the
lemurs and the anthropoids, and
opinions have differed as to whether
they are merely moderately aber-
rant lemurs or representatives of an
ancient stock of remote affinity
thereto. T arsius differs from lemurs
and lorises in having the orbits
(Fig. 520) directed forwards and
almost completely separated from
the temporal fossa, by the micros-
matic brain, and by single olfactory
foramina. They resemble Lorisi-
formes in the arrangement of the
tympanic ring, which remains out-
side the bulla and forms the tubular
external auditory meatus (Fig. 6n).
The structure of the nostrils and
upper lip is nearer that of the
anthropoids than of the lemurs, the
jugal does not extend so far back,
and the carotid artery enters the
bulla wall.
There are specialised features
which are sufficient to separate the
tarsioids from the anthropoids. In
the first place, the hind foot re-
sembles that of Galago in having a
Frc. 5 r9.- 0rder Primates, Sub-order Prosimii, greatly elongated navicular and
Infra -Order Tarsiiformes, Family Tarsiidre. Tar- calcaneum, a feature which occurs
s ius. Celebesian Tarsier ( T . spectrum). Tarsiers
are rat-sized a nimals restricted t o the Indo - in all the early forms in which this
Malayian region. They a re essentia lly but not
exclusively a rboreal a nd inhabit rain-forests. structure is known, and one which
Nocturnal and crepuscular (note orbits, Fig. 520) alone is sufficient t o prevent the line
they cannot see well by day and generally sleep
clinging to an a lmost vertical branch. They pro- from being considered as directly
gress by leaps, adhesion on impact being aided by
pec uliar digital pads (Fig. 6o7); the tail is not ancestral. The digits of the hand
p rehensile. They eat insects and small reptiles have nails, as do all the toes except
(tree geckos) and crustaceans on which they spring
and capture by hand. Pre-r8th century natura- the second and third, which are
lists sometimes placed these bizarre primates clawed. The tibia and fibula are
variously with opossums (marsupials) or jerboas
(rodents). (From photographs.) fused distally-a specialisation to-
 PHYLUM CHORDATA 741
wards its peculiar saltatory progression. The great enlargement of the eyes
(a specialisation for nocturnal habits) has caused considerable modification in
the shape of the skull and, though the retinre contain only rods, there are
perhaps indications of a macula in the area where, in anthropoids, cones occur.
The head in general is more monkey-like than
that of the lemurs. The enormous eyes have
little mobility and in compensation the tarsier
can rotate its head to a degree that enables the
eyes to face backwards. The lemurine snout
is replaced by a short face, with a relatively
minute nose and large pinnre. The olfactory
areas of the brain are reduced, as might be
anticipated, but the brain as a whole is large :
the hemispheres, in accordance with the great
occipital expansiOn· corre1ated w1t · h unusua11y dentition.
FIG. szo.-Tarsius: Skull and
T. spectmm (see Fig. 519).
good vision, spread over part of the cerebellum:
this last is relatively small and the mid-brain large. The placenta is deciduous
and has similarities with that of higher forms. The external genitalia of both
sexes are of primitive unspecialised pattern.

SuB-ORDER ANTHROPOIDEA
The rest of the Primates-the monkeys, apes, and Man-are included in the
present group. Its precise origin remains in doubt, but fossils increasingly
appear from the Oligocene onwards. Eocene remains are still unknown.
In the anthropoids the cranium is expanded as the cerebral hemispheres
reach their greatest development. These overhang the cerebellum and the
medulla as well and the neopallium is generally markedly convoluted with the
development of the Sylvian fissure, and the central and other sulci. The
occipital visual areas are notably well-developed. The optic tract exhibits a
partial fibre-decussation ; the eyes are forwardly directed and vision is bino-
cular: the retina is variably, but often richly, supplied with cones and a macula
lutea is generally present. The enhancement of the visual sense is correlated
with a corresponding reduction in olfactory reception. The nose differs from
that of the lemurs. In lemurs the muzzle or rhinarium has the primitive naked
form, and the narial orifice is continued laterally as in most mammals. The
rhinarium itself is hairless, and extends to the non-protrusible upper lip, which
is restrained by a mucosal ligament. In the anthropoids the nostrils are com-
pletely ringed round by naked skin: there is no moist rhinarium. The upper
lip lacks a ligament, and so is freely protrusible. The pinnre are reduced and
generally are pressed to the head. The tympanic bone is united with the
squamosal. The sphincter colli is differentiated into muscles that enable a
new facial mobility and permit the expression of crude emotion (Fig. 521).
742 ZOOLOGY
The upper molars consist of the trigon of three cusps, to which a fourth (hypo-
cone) has been added. The cusps become joined by crests in various ways.
In the lower molars the talonid and trigonid become equal in height and the
paraconid is lost. There are nearly always three molars, but in the Calli-
thricid<e the last one is lost. The bicuspid premolars are limited to three and
later to two on each side. The incisors lose their spike-like form and become
broad cutting teeth. With the exception of the primitive marmosets (Calli-
thricid<e) the fingers and toes are all provided with flat nails and the pollex
and hallux are opposable. The foramen magnum tends to face downwards,

FIG. 521.- 0rder Primates, Sub -order Anthropoidea: New World Super-family Ceboidea ( =
Platyrrhina) and Old World Super-family Cercopithecoidea ( = Catarrhina, minus gibbons, apes,
and Man). L eft: P latyrrhine Spider Monkey Ateles geoffroyi (Family Cebidre). Right: Narrow-
n osed Rhesus Monkey, Macaca mulatta (Family Cercopithecidre). (From photographs by
Suschitzky.)

instead of backwards, in accordance with a semi-upright or upright gait.

Other important characteristics are the highly efficient h<emochorial placenta-
tion, a uterus simplex and the occurrence of menstruation. The last-named is
not a widespread phenomenon. Although common or perhaps universal in
the Cercopithecoidea and Hominoidea, it is not ubiquitous in the Ceboidea. It
occurs in Tarsius, but not in the lemurs (nor in the tree-shrew Tupaia, some-
times included in the Prosimii).
Menstruation has been suggested in the dermopteran Colugo (Cynocephalus),
as well as in the menotyphlan Elephantulus. It has been reported in bats.
Phenomena claimed as menstrual have sometimes proved to be merely cyclical
(or even non-cyclical) uterine h<emorrhages, rather than the sudden and total
 PHYLUM CHORDATA 743
destruction of the uterine mucosa specifically at the end of the luteal phase of
the restrous cycle (p. 8g6).
The Anthropoidea are generally subdivided into two groups-the Platyr-
rhina, or New World Monkeys, and the Catarrhina from the Old World (Fig.
521). There is, however, good reason for the creation of three divisions and
so we must abandon the classical dichotomy.

SUPER-FAMILY CEBOIDEA (PLATYRRHINA)

This includes two exclusively New World (Central and South American)
families, (1) Callithricidre (marmosets) and (2) Cebidre, composed ofthecapuchins
(Cebus), spider monkeys (Ateles) (Fig. 521), howlers (Alouatta), woolly monkeys
(Lagothrix) and others. Of these 'fiat-nosed monkeys', with widely separated
nostrils and a broad septum, some species are deceptively narrow-nosed (catar-
rhine) in appearance. The Platyrrhina have probably been isolated since the
Eocene, and although they are more primitive than the Old World monkeys,
the two stocks are obviously very similar in essentials. However, the tail in
platyrrhines is almost always primitively long and is sometimes prehensile,
the thumb is relatively unopposable and three premolars are retained, whereas
the second premolar had already been lost in the earliest known (Oligocene)
catarrhines. The tympanic ring is united with the petrosal. It resembles that
of the lorises, whereas the tympanic bone of catarrhines is tube-like and re-
sembles that of the tarsiers. A large bulla occurs; this is absent in catarrhines.
Within the group there is only the faintest suggestion of menstruation. The
howlers have developed a large ossified laryngeal chamber that acts as a
resonator.
Ceboidea are represented in the South American Miocene, but possibly
very significantly, they remain undiscovered in North America, a fact that has
led to the conjecture that they may have arisen independently from a once
widespread tarsioid stock.

SUPER-FAMILY CERCOPITHECOIDEA
Early last century, Hemprich grouped the Old World monkeys, apes, and
Man as the Catarrhina or narrow-nosed Primates, as opposed to the Platyrrhina
of the New World (see above). However, the Old World monkeys, which form
the family Cercopithecidre, stand apart from the other catarrhine forms, and
have been placed by Simpson in a separate super-family.
The Cercopithecoidea resemble the Hominoidea (see below) and differ from
the Ceboidea ( = Platyrrhina) in the reduction of the premolars to two, in the
presence of a long tubular bony auditory meatus (Fig. 6II, p. 853), and in the
reduction of the turbinals and nasal fossre (allowing of close-set nostrils). The
thumb is opposable, except in the guerezas (Colobus) where it has atrophied.
They are more primitive than the Hominoidea in the possession of a tail, which is
744 ZOOLOGY

often long, but never prehensile. In some forms, such as some of the macaques
or rhesus monkeys (Macaca) and the mandrill (Mandril/us), the tail is short.
Ischial callosities (areas of bare skin on the buttocks, often brightly coloured)
are well-developed. Cheek-pouches are present, except in the guerezas and
the langurs (Semnopithecus), which have complex lobulated stomachs. The
molar teeth have four cusps, which are arranged in two pairs, united by cross-
crests (bilophodont). Most cercopithecids are arboreal, but many are habitu-
ally terrestrial, e.g. baboons (Papio). On the ground they walk on all fours in
a plantigrade manner.
Cercopithecida: were present in the African Miocene and perhaps earlier.
By the Pliocene several living genera can be recognised.

SUPER-FAMILY HOMINOIDEA
The extinct Egyptian Oligocene Parapithecida: and the extant apes
(Pongida:) and Man (Hominida:) are now separated from the other narrow-
nosed anthropoids as a super-family Hominoidea. They differ from the
Cercopithecoidea in several respects. The tail has disappeared. The thorax
is broad, with a flattened ventral surface, the sternum having become short and
wide, whereas in cercopithecoids the thorax is narrow from side to side as in
most mammals. The lumbar region of hominoids is shorter than in cercopithe-
coids, and there are only 4 or 5 lumbar vertebra:, instead of 6 or 7· The brain
is proportionately larger than in cercopithecoids. Ischial callosities have
disappeared except in the gibbons, and cheek-pouches are never present. The
hair on the forearm points towards the elbow instead of towards the wrist.
The molar teeth are characterised by the retention of an oblique crest from the
protocone to the metacone, a survival from the tritubercular stage (p. 865).

FAMILy PONG IDlE

The Pongida: are to-day confined to the widely separated forests of Africa
(Gorilla (Fig. szz); and the chimpanzee, Pan), Southern China, Indo-Malaya
(Hylobates, gibbons), and the East Indies west of Wallace's Line (Pongo, the
Orang-utan). They are specialised in the elongation of the fore-limbs, which,
especially in the gibbons, are used in swinging from branch to branch (brachia-
tion). Gibbons can walk in an upright position using the fore-limbs as balanc-
ing rods, but all other apes use their long forearms as crutches to help swing the
trunk over the ground.
The Pongida: are represented from the Lower Oligocene by the Egyptian
PropliopithectJ,S. Several apes occurred in the Lower Miocene of East Africa,
of which Limnopithecus appears to be related to the gibbons, while Proconsul
and Sivapithecus represent the great apes. Limb-bones of Limnopithecus and
Proconsul show, however, that the brachiating habit had not been developed at
that time, and the brain of Proconsul was probably more primitive than in
 PHYLUM CHORDATA 745
existing apes. Several forms of ape have been found in the Miocene and
Pliocene of Europe and India (Pliopithecus, Dryopithews, Sivapithecus etc.).
Oreopithecus, from the Lower Pliocene of Italy, resembled Man in the small size
of its canine teeth. A much greater resemblance to Man is shown by the
South African, possibly cave-dwelling, Australopithecinre. These were almost

FIG. 522. - 0rder Primates, Sub-order Anthropoidea, Super-family Hominoidea, Family

Pongidre. Gorilla. The largest and h eaviest of the primates, the Gorilla (G. gorilla) sometimes
n~aches a height of about six feet and a weight of more than 400 lbs. It usually lives in
t erritorial family groups ( 10 to 15 animals) dominated by an old male that is probably polygamous.
A mixed diet includes fruit, foliage, and seeds. Contrary to popular belief, gorillas generally prefer
to retreat if the human intruder stands his ground. (From photographs.)

certainly bipedal, and approached man in the pelvis and the form of the teeth.
They are regarded by many as primitive members of the family Hominidre.

FAMILy HOMINID£

The third family, the Hominidre (if the Australopithecinre are excluded)
consists of only two genera-the extinct Pithecanthropus ( = Sinanthropus),
including the Java and Pekin 'men', and the genus Homo, first identifiable in
the Pleistocene, containing two species, H. neanderthalensis and H. sapiens.
The only surviving species is H. sapiens which embraces all the known 'races'
 ZOOLOGY

of men-negroid, mongoloid, australoid, and caucasian, irrespective of colour

and other superficialities.
Until recently, a third genus, Eoanthropus was recognised by most (but not
all) authorities on the evidence presented by fragments of a skull and part of a
lower jaw discovered in Pleistocene gravel at Piltdown in England. A few
workers (e.g. Waterston and Miller) claimed that the jaw bore too great a
resemblance to that of an ape to belong to a skull of such considerable cranial
capacity as the one with which it was associated and, in fact, asserted that
although the skull was human, the mandible was simian.

FIG. 523.-Hominoidea: Jaw Structure and dentition in Gorilla and Homo. Left, Gorilla
(Pongidre): The most anterior part of the face is formed by the premaxilla. The two elongated
dental arcades are almost parallel and the incisors project markedly. A diastema is present.
Right, Homo (Hominidre) : In Man alone among the Mammalia the premaxilla, although distin-
guishable on the palate by a premaxillo-maxillary suture, does not form an independent part of the
face (as pointed out by Vesalius). Opinion is divided as to whether the premaxilla is overgrown b y
the embryonic maxilla (Wood Jones) or whether it is lost as a distinct facial element because of
the fusion of both bones during early development. The general shortening of the dental arch
in Man is, despite a certain broadening, associated with reduced t ooth-size in a continuous series.
Other dental characteristics in Man are the small size and frequent absence of the maxillary
lateral incisors, the small, sexually undifferentiated canines, the reduction and arrangement of
premolar roots, and the often crowded, relatively small molars. (Redrawn after Wood Jones.)

In 1953 exhaustive inquiries by means of modern fluorine-determination

and other tests showed that the cranial fragments are probably of Upper
Pleistocene age, but that the mandible is unquestionably that of a modern
ape. The teeth of a young animal had been artificially abraded to simulate
those of an adult man, and the jaw-bone had been stained by iron salts and
potassium dichromate. Fragments from a second reputed individual were
proved also to be fraudulent, as also were alleged implements found nearby.
Inevitably the unmasking of this now-famous fraud was seized upon by funda-
mentalists to attempt to discredit all researches revealing the evolution of Man.
 PHYLUM CHORDATA 747
Actually, the erasure of 'Eoanthropus' clarifies the problem of human
ancestry. The work that led to the unmasking of the Piltdown fraud (see
Weiner) was undertaken largely because the mandible and teeth of Eoan-
thropus could not be reconciled with those of indubitably genuine modem
'finds'.
Man, the sole living representative of the Hominidre, differs anatomically
from all other primate mammals in the following (among other) particulars:
(r) in his habitual bipedal orthograde posture and its associated morpho-
logical and physiological specialisations ;
(2) in his distinctive external body-form, determined partly by the
locomotor system, partly by subcutaneous fat ;
(3) in the peculiarities of hair distribution and hair tracts.
(4) in the primitive morphological simplicity of the external genitalia
in both sexes and the absence of ' sexual' skin, baculum, and
ischial callosities;
(S) in the small canines which show very little sexual differentiation and
do not project beyond the other teeth, but bite edge to edge like the
mctsors.
(6) in the lack of the premaxilla (os incisivum) which does not persist as a
separate post-natal skeletal element.
(7) in the absolute size and structural complexity of the cerebrum.
(8) in the uniquely prolonged period of infancy and childhood and the
slowness of skeletal maturation.
Man alone performs true ('heel and toe') walking : the arched foot was
evolved for support, not prehension; the hallux cannot be abducted and is
never a potential 'thumb'. The skull is balanced upon, not slung from, the
spinal column, and the foramen magnum looks downwards and forwards :
the balancing muscles attach to relatively little of the skull base, hence the
occipital region protrudes. The large nasals remain permanently discrete:
early in ontogeny the premaxilla becomes incorporated with the descriptive
'maxilla', so that a premaxillary-maxillary suture is lacking. The facial
skeleton does not protrude muzzle-wise : the external nose and chin are
prominent. The teeth have the maxillary premolars single- or double-rooted
(triple-rooted in cercopithecids and anthropoid apes) and the mandibular pre-
molars generally single-rooted (double-rooted in cercopithecids). A philtrum,
superior labial tubercle and red everted lip-margins characterise the face.
Coarse body-hair is confined to the scalp and beard areas, the eyebrows and
eyelashes, the axillre and the pubic region; and the disposition of hair tracts is
much unlike that of anthropoid apes. Subcutaneous fat helps to mould the
body contour, especially in the female, wherein the mammre, even when non-
lactating, are prominent surface features.
 ZOOLOGY
Postnatal development is extremely protracted, the successive periods
of infancy, childhood and adolescence being unparalleled among mammals,
Skeletal maturity is not attained until about the twenty-fifth year. This
protracted period of dependence and immaturity is pre-eminently one of
learning, permitting the acquisition of physical skills and the development of
the specifically human intellect.
For Man is, uniquely, the rational animal, capable of forming abstract
concepts and of enunciating his thought in articulate speech. Correlated
with these higher faculties, and constituting the machinery of their expression,
is the absolutely large and structurally complex cerebrum, with its character-
istically extensive association areas and its intricacy of fibre-connexion.
Skeleton of the Primates.-The atlas is ring-like, the odontoid sub-
conical. The spines of the cervical vertebrce are usually well-developed and
simple. In Man they are short (with the exception of the seventh) and
commonly bifid. In some forms they are trifid. The number of thoraco-
lumbar vertebrce is usually nineteen, but there are only seventeen in Man,
the Gorilla, and Chimpanzee, and sixteen in the Orang. In some lemurs there
may be twenty-three or twenty-four. The number of sacral vertebrce varies
from two to five. The sacrum of Man, which comprises five ankylosed verte-
brce, differs from that of other Primates in its greater relative breadth and in
its backward curvature : it forms a well-marked angle (sacro-vertebral angle)
with the lumbar region, an angle much less pronounced in other primates.
The number of caudal vertebrce varies with the length of the tail-from four to
about thirty-three. In Man there are but four vestigial caudal vertebrce,
ankylosed to form the coccyx. In all forms wherein the tail is well developed
chevron bones are present.
The human skull (Fig. 524) presents a marked contrast in certain respects
to that of other mammals, but in some points is approached by that of the
Simiidce. An obtrusive characteristic of the human skull is the large size of the
brain-case, the cubic content of the Clanial cavity averaging 1500 cubic centi-
metres in the European male. This great development is most marked in that
part of the cavity which lodges the cerebral hemispheres, in adaptation to the
large dimensions of which the cranium bulges out both anteriorly and posteriorly
to such an extent that the entire length of the cavity greatly exceeds that of the
basicranial axis. A result of the posterior bulging of the brain-case is that the
foramen magnum (f. m.) is no longer situated at the posterior extremity of the
skull as in other mammals, but assumes a position farther forwards towards the
middle of the base. The anterior expansion, causing a strong arching forwards
of the frontal region, brings about an alteration in the position of the ethmoidal
cribriform plane. Instead of being perpendicular or inclined to the basi-
cranial axis this becomes horizontal, and forms the middle part of the floor
of the anterior cranial fossa. The fossa for lodgment of the cerebellum lies
 PHYLUM CHORDATA 749
entirely beneath the posterior portion of the cerebral fossa; the olfactory fossa
is comparatively small.
The outer cranial surface is smooth and rounded, devoid of any prominent
ridges or crests. The occipital crest of lower mammals is represented merely by

FIG. 524.- Homo: Skull and hyoid apparatus. J uvenile (a t fi rst dentition) with m andible
displaced downwards to show its en t ire form . The lettering reads clockwise from the upper jaw:
P. mz. anterior portion of maxilla; MT. maxillo-turbinal; ET. ethmo-turbinal; j'vf£. ossified
portion of the mesethmoid; Na. nasal ; cg. crista ga lli of t h e mcsethmoid; OS. orbitosphenoid, or
lesser wing of the sphenoid; AS. a lisphenoid, or greater wing of the sphenoid. Fr. frontal;
Pa. parietal; SO. supraoccipital ; M. mastoid portion of the periotic ; Sq. squa mosal ; P er.
petrous portion of the periotic; the large foramen b elow the end of t he line is the internal auditory
m eatus, the small d epression above it is the n early-obliterated floccular fossa. E1:0. exoccipital,
the line points t o the condylar foramen; fm. foramen magnum; BO. basioccipita l; BS. basi-
sphenoid; st. sella turcica; P S . presphenoid, ankylosed with the basisphenoid , forming the 'body
of the sphenoid'; Pt. pterygoid; Pl. palatine ; Vo . vomer; IV!z. maxilla. Lower .Taw: s. sym-
p h ysis of mandible; cp. coronoid process; cd. articular condyle ; a. angle . Hyoid apparatus:
sh. stylohyal, or 'sty loid process of temporal'; ch. ceratohyal, or lesser cornu of h yoid; bh.
basihyal, or body of hyoid; th. thyrohyal, or greater cornu of hyoid. (After Flower.)

a rough raised line- the superior curved line of the occiput. The paroccipital
processes are represented by slight eminences only-the j ugular eminences.
There is no auditory bulla; the mastoid portion of the periotic projects down-
wards as a prominent mastoid process. The periotic, tympanic, and squamosal
early fuse into one bone-the temporal bone. The post-glenoid process is very
slightly developed. The whole facial region is relatively small. The orbits,
VOL. II. A 3
750 ZOOLOGY
which are of moderate size, are directed forwards ; their bony margins are com-
plete, and a plate of bone, developed partly from the jugal, partly from the ali-
sphenoid, almost completely cuts them off from the temporal fossre, leaving
only a small, but characteristically wide, aperture of communication-the
spheno-maxillary fissure. The interfrontal suture usually disappears early.
The nasals rarely fuse in post-natal life. The suture between the premaxilla
and the maxilla becomes obliterated early in fcetallife, so that the entire upper
jaw appears to consist of a single bone. A peculiar spine, the nasal spine, is
developed in the middle line below the nasal opening. The most marked feature
of the mandible is the presence of a prominence, the mental prominence, in the
lower part of the symphysial region (5.). The stylohyal nearly always be-
comes fused together with the tympanohyal to the periotic and tympanic,
giving rise to a slender process, the styloid process (sh.) which projects down-
wards from the base of the skull.
None of the other primates has a cranial capacity approaching that of
Man. Therefore, in them those modifications in the shape of the skull which are
the concomitants of the great development of the human brain are accordingly
not recognisable, or are much less strongly marked. The various fossre of the
cranium, however, as a rule occupy the same relative positions as in Man.
The cerebellar fossa is entirely beneath the cerebral. The ethmoidal plane,
and that of the foramen magnum (occipital plane), are usually both horizontal
or nearly so. In all the Pongidre, with the exception of the orangs, the frontals
meet in the middle line below, over the presphenoid. In many monkeys the
outer surface of the cranium is smooth and free from prominent ridges. In
others, as in the baboons, the Orang, the Gorilla, and the Chimpanzee
there are strongly developed occipital, sagittal, and supraorbital ridges.
These are usually much more prominent in the male than in the female and
increase in size with age. The paroccipital processes are always rudimentary,
but there are well-marked post-glenoid processes. The mastoid does not form
a distinct mastoid process in many forms. In the Cebidre and marmosets
alone is there a tympanic bulla. The entire facial region is relatively larger
than in Man ; the premaxillo-maxillary region is always more prominent,
and in the baboons projects forwards as a distinct muzzle. The orbit is
separated from the temporal fossa as in Man. The nasals are usually fused in
the adult. The nasal spine is never developed. The suture between the pre-
maxilla and the maxilla becomes obliterated, if at all, only in old individuals.
The mental prominence of the mandible is never developed, the anterior
surface of the symphysial region sloping backwards and downwards from
the alveolar margin. The stylohyal never gives rise to an ossified styloid
process.
In the skull, as in many other respects, the lemurs occupy an intermediate
position between the higher Primates and the lower orders of mammals. The
 PHYLUM CHORDATA 751

FrG. 525.-Primates: Endoskeleton of Hylobates and Homo. a. astragalus (talus); ca. cal-
caneum; car. carpus; cl. clavicle; co. coracoid; ft. fibula; h. femur; hu. humerus; il. ilium;
is. ischium; me. metacarpals; pa. patella; pu. pubis; ra. radius; sc. scapula; st. sternum; t.
tibia; u . ulna. (After Young.)
752 ZOOLOGY
occipital and ethmoidal planes are usually vertical. The tympanic forms a
large bulla. The orbits, which are large, are usually separated from the tem-
poral fossc.e by a narrow rim of bone only. The lachrymal foramen is situated
on the face outside the margin of the orbit. The facial region is usually
elongated, and may form a prominent muzzle.
In all the Primates the clavicle is complete (Fig. 525), and in the scapula the
spine, acromion, and coracoid process all are well developed. In Man and
the higher apes the glenoid border of the scapula is much longer than the cora-
coid border. In the lower monkeys, on the other hand, these borders are nearly
equal. The humerus is comparatively long and slender; the tuberosities and
ridges are not, as a rule, very strongly developed. In Man and the apes the
bone is twisted around its long axis; in the lower forms this torsion is absent.
In Man and the higher apes the foramen above the inner condyle is absent but
it is present in many of the American monkeys and in
most lemurs. Characteristic of the ulna of Man and
the higher apes is the small upward extension of the
" olecranon process. The radius and ulna are discrete in
all. In the higher forms the shafts of the two bones
are bent apart, a wide interosseous space intervening
and facilitating considerable freedom of pronation and
supination. In the carpus (Fig. 526) the scaphoid and
FIG. 526 .-Papio:
lunar are always distinct, and a centrale is present in
Carpus. Baboo n. ce. all except some of the lemurs, the Gorilla, Chimpan-
centrale; c. cuneiform; l.
lunare; m . magnum; p. zee, and Man. A pisiform is present, and in most a
pisiform; r. s . radial sesa- radial sesamoid. As compared with that of the other
moid ; s . scaphoid; td.
trapezoid; tm. trapezium; Primates, the carpus of Man is short and broad ;
u . un c iform. (Aft e r
F lower.) the trapezium has a saddle-shaped articular surface
turned somewhat inwards. In Man, the Chimpanzee,
Gorilla, and Orang the carpus articulates exclusively with the radius. In all
the others it articulates also with the ulna. In Man the pollex has a re-
markable and characteristic freedom of movement in opposition to the other
digits.
The pelvis of Man is remarkable for its relative breadth, for the expanded
form of the ilia and the deep concavity of their inner surfaces, and for the short-
ness of the pubic symphysis. In the higher apes some of these features are
recognisable, though less pronounced ; but in the lower apes the ilia are long and
narrow, and usually curved outwards. In the Old World monkeys the tubero-
sities of the ischia are strongly everted and roughened for the attachment of the
ischial callosities.
The tibia and fibula are well developed and discrete in all. In nearly all the
hallux (owing to the form and direction of the articulation between it and the
internal cuneiform) is opposable to the other digits. This converts the foot into
 PHYLUM CHORDATA 753
a grasping organ. The human foot (Fig. 527) is distinguishable from that of the
other Primates by the absence of this power of opposition and by the relative
length of the tarsus, which exceeds that of the metatarsus.

FIG. 527.-l'rimates: Comparative anatomy of pes. Foot of Homo, Gorilla, and Pongo at the
same absolute length to show the difference in proportions. The line a' a' indicates the boundary
between tarsus and metatarsus; b'b', that between the latter and the proximal phalanges; and
c'c' bounds the ends of the distal phalanges; as. astragalus; ca. calcaneum; sc . scaphoid. (After
Huxley.)

ORDER DERMOPTERA
A single Oriental genus, Cynocephalus ( = Galeopithecus) (Fig. 528), with-
only two species, forms the family Cynocephalidce ( = Galceopithecidce) or
'flying lemurs'.
The 'flying lemurs' do not fly and are not lemurs : 'Colugo' is therefore
a better group name. The colugos are quite distinct and are aberrant 'insec-
tivoran' derivatives. They have close affinity with the Chiroptera within
which group they have sometimes been included.
The most noticeable character of Cynocephalus is the hairy, muscular
parachuting membrane or patagium which extends from the neck to the manus
(embracing all the digits) and down the sides of the body to the ankle, and
thence to the tip of the moderately long tail. It may be compared to the
similar, but not identical, structures in the gliding marsupials and rodents.
The brain, like that of the 'insectivores,' is primitive in being macrosmatic, and
in having the corpora quadrigemina uncovered by the cerebrum. The denti-
tion is peculiar. The lower incisors are procumbent and comb-like, the molars
multicuspidate. The bulla is formed by the tympanic ring, which also forms
754 ZOOLOGY
the lower border of the spout-like external auditory meatus. In several
characters the Dermoptera agree with the fruit-bats, the Tupaiidre, and the
lemurs, but how far these characters are merely convergent is indeterminate in
the present state of knowledge. The order appears to have had a separate
line of evolution from the Lower Eocene (cf. Plagiomene and its allies).

FIG. 528.-0rder Dermoptera, Family Cynocephalidre. Cynocepllalu.~. The Colugo or so.

called Fly ing L emur (C. variegatus) d oes not fly a nd is u nrelated to the lemurs. This East Indies
form, which shows certain' insectivoran' (p . 725) affinities, possesses gliding membranes that enclose
n eck, limbs, and tail. The feet, too, are webbed. J ourneys of up to 70 ya rds, sometimes wit h the
baby clinging to the mother 's ventral su rface, are made. Like other mammalian gliders (e .g.
Fig. 498, p . 717) the Colugo t urns sharply upwards as it reaches its t arget, thus presenting a fiat
surface a t the point of impact . Strictly n octurna l, the Colugo lives a lmost entirely on fruit and
leaves. The second surviving species (C. volans) occurs in the P hillipines (From a photograph
by Walker, a nd skins from B.M. (N.H.).)

ORDER CHIROPTERA
The Chiroptera or bats (Fig. 529) are the only mammals capable of true
flight. The fore-limbs have the segments greatly elongated (especially the
fore-arm and the four ulnar digits) , and these support a thin membranous fold
of the integument which stretches to the hind-limbs and constitutes the wing.
Curious arterio-venous connexions in the wings allow a speedy through-flow
and a considerable short-circuiting of the capillary bed. The sternum is keeled
 PHYLUM CHORDATA 755
for the attachment of the large pectoral flight muscles. The great development
of these muscles is sometimes associated with an axillary or even a dorsal
disposition of the mammary tissue.
There is often a cartilaginous rod (calcar) attached to the inner side of the
ankle which helps to support a second membranous fold (inter-femoral mem-
brane) extending between the hind-limbs, and sometimes involving the tail.

FIG. 529.-0rder Chiroptera, Sub-order Megachiroptera, Family Pteropidre. Pteropus. The

nocturnal, la rge-eyed fruit-bats or flying-foxes of lndo-Malaya and Australasia (excluding New
Zealand) congregate noisily by day in ' camps' tha t sometimes contain hundreds of thousands of
bats. They use both thumb and pedal claws while feeding and changing position in camp.
The biggest species (e.g. P. giganteus) are as large as a small cat and have a wing-spread of about
five feet. They are nomadic pests, following the fruit-crops, and eating flowers as well. (Partly
after a photograph by Suschitzky, and B.M. (N.H.) skins.)

The ulna is vestigial. The pollex, much shorter than the other digits, is
directed forwards, and terminates in a well-developed curved claw. In the
Megachiroptera, but not in the Microchiroptera, the second digit is also usually
clawed, but the other digits are always clawless. The position of the hind-
limbs is peculiar. Each is rotated outwards so that the knee is directed back-
wards instead of forwards as in other mammals. The five pedal digits are all
provided with claws. So complete is the adaptation of the limbs for flight and
suspension (in caves and trees) that bats are only able to shuffle along with
756 ZOOLOGY
great difficulty on the ground. In the Megachiroptera the muzzle is nearly
always elongated, and the pinna of the ear simple. In the Microchiroptera
the muzzle is generally short, the pinna usually complicated by the presence of
an inner lobe or tragus, and often produced into remarkable arborescent ap-
pendages. The nose is also often provided with elaborate leaf-like or arbores-
cent lobes (Fig. 530). The body-is usually covered with soft fur, except in one
group of Microchiroptera in which the integument is practically naked. The
tail is sometimes short, sometimes well developed. In the Chiroptera the
cerebral hemispheres are smooth and do not overlap the cerebellum. The
olfactory centres are reduced as in the generality of flying animals. The
dentition is complete, heterodont and diphyodont. The milk teeth are small
hooked structures whi.ch help attach the young to the mother. The penis is
large and pendent. The testes are abdominal but may descend seasonally, and
often unilaterally, to a prominent situation in the groin. The uterus is simple
or bicornuate. The placenta is deciduous and discoidal in the examples studied.

SuB-ORDER MEGACHIROPTERA
These are generally big frugivorous Chiroptera with large eyes, long snout,
without foliaceous nasal or auricular appendages and with the second digit of
the manus usually ending in a claw. The tail, when present, is not enclosed in
the inter-femoral membrane, but lies below it. The crowns of the molar teeth are
devoid of sharp cusps. This sub-order comprises the fruit-bats or so-called
flying-foxe.., (Pteropus) and other genera from the tropical and sub-tropical
parts of Asia, Australia, and Africa. Indian and Australian fruit-bats
(Pteropus) may have a wing-span of some five feet (Fig. 529).

SuB-ORDER MICROCHIROPTERA
These are widely distributed, small, mostly insectivorous, Chiroptera
usually with a short snout and, frequently, foliaceous nasal and auricular
appendages. The second digit of the manus is always clawless. The tail when
present is at least partially enclosed in the inter-femoral membrane. The
crowns of the molar teeth have typically sharp cusps.
The sub-order includes all other bats (e.g. Vespertilio, Minioptems, Rhino-
tophus, Desmodtts (Fig. 530) and Diphylla (true vampires) and dozens of other
genera). The little insectivorous forms have generally relatively small eyes,
but they are never blind. They fly with assurance in apparently totally dark
caves and in experimentally blackened rooms by means of an echo-location
apparatus analogous to the sonic depth-sounding devices of Man (cf. the radar-
like direction-finding apparatus of fishes, p. 335). Blindfolded bats do not
collide with objects placed in their line of flight. As early as the late 18th
century Spallanzani showed that only when the ears are obstructed do bats
strike such obstacles, but it was not until 1920 that Hartridge suggested that
 PHYLUM CHORDATA 757
they might find their way by means of echos from ultrasonic squeaking.
Recent work with Myotis has revealed that bats can indeed appreciate fre-
quencies as high as Ioo,ooo cycles (d. Man, with an upper range of about zo,ooo
cycles) and that only if both ears are left unobstructed do they successfully
avoid wires stretched across their path. Efficiency varies in relation to the

FIG. 530.-0rder Chiroptera, Sub-order Microchiroptera: Feeding adaptations. r. Myotis

lucijugus (Family Vespertilionid<e) of North America, a generalised insect-eater. z. Clueronycteris
mexicana (Family Phyllostomatid<E), a tropical American form that lives mostly on nectar and
pollen. It has a long snout, tactile vibrissa>, and brush tongue (cf. Fig. 426, p. 621). 3· Desmodus
rotund us (Desmodontid.e), a tropical American true vampire which feeds on vertebrate blood. The
flattened muzzle enables the highly specialised blade-like teeth deftly and almost painlessly to
operate on the sleeping host. Below are shown a sharp, narrow upper incisor (i 2 ) and canine (c).
A small depression is scooped in the skin from which blood seeps; this is transferred by the vampire's
tongue. The stomach is reduced . Little heavier than house mice, D . rotundus and its allies are
unique among bats in their ability to move with gentle agility on a horizontal surface. (Redrawn
after Wimsatt, Gregory.)

thickness of obstacles. Further, the more numerous the wires, the higher
becomes the rate of vocalisation. The chiropteran apparatus of vocal emis-
sion, auditory reception and muscular reaction has evolved to a refinement
that enables some species to turn sharply aside from obstacles less than
ten inches distant. Rottsettus, one of the large-eyed fruit-bats also possesses an
echo location device. There is, incidentally, evidence that the nocturnal, cave-
758 ZOOLOGY
nesting South American nightjar Steatomis caripensis (Fig. 446, p. 643) guides
itself by means of echo-location in a manner broadly comparable with that of
bats (Griffin).
Among the Microchiroptera is the family Desmodontidre which are true
vampire bats from tropical America and the West Indies. Of these, Desmodus
(Fig. 530) and Diphylla live solely on vertebrate blood. Desmodus painlessly
scoops out a fragment of skin, exposing the capillaries with its obliquely
flattened, razor-edged upper incisors, thereafter licking up the oozing blood.
The resophagus is narrow : the cardiac part of the stomach is expanded as a
diverticulum, but the pyloric portion is small. In legend-and often in modern
travellers' stories-the vampire is generally claimed to be a large animal, but
D. rotundus, the commonest species, is a mere three inches long. Dcsmodus has
in the past sometimes been sufficiently troublesome to livestock to cause the
abandonment of settlements, and is the proved carrier of a form of rabies which
may prove fatal to Man and domestic animals.
Often confused with the true vampires are the spear-nosed frugivorous, yet
unfortunately named Vampyrum ( = Vampyrus), and the long-tongued
Glossophaga, both of which are harmless to Man. Vampyrum was so named in
1815 before the identity of the small, less conspicuous, blood-suckers was re-
vealed. The archaic expression vampir originally referred to the hypothetical
'soul' which allegedly left a dead human individual to feast at night on the blood
of living men. These belong to the Phyllostomatidre.
Although the Microchiroptera are, excluding rodents, among the commonest
mammals, their small fragile bones are rarely preserved as fossils. It is of
great interest therefore that Pala:ochiropleryx, one of several genera from
the Eocene of Europe, seems already as fully developed as any of the
Microchiroptera. Zanycteris (known only from its teeth) may carry the
sub-order as far back as the Palreocene, but it is probable that it and its
congeners are really insectivores of uncertain affinity. In all, 17 families of bats
are recognised.
Skeleton of the Chiroptera (Fig. 531).-The cervical region of the vertebral
column is characterised by the absence of distinct neural spines, and the same
holds good to a less extent of the trunk-vertebrre. The transverse processes of
the lumbar region are also rudimentary. The tail varies in development.
When it is elongated the component vcrtebrre have long, cylindrical bodies
without processes. Sagittal and occipital crests are developed in the skull of
some species. The facial region is rather elongated, especially in the Mega-
chiroptera (Fig. 529). Postorbital processes of the frontal may be present or
absent. The zygoma is long and slender and the malar small and applied to
its outer surface. The long and narrow nasals are in some cases united; the
premaxillre are small and absent in one of the false vampires (Megadermatidre).
The mandible has an angular process in the Microchiroptera, but not in the
 PHYLUM CHORDATA 759
Megachiroptera. The segments of the sternum are sometimes distinct, some-
times united. The presternum has
a mesial keel developed in co-
ordination with the great size of the
pectoral muscles. The sternal ribs
are ossified.
The scapula is large and oval in
shape. The spine is near the an-
terior margin. The postscapular
fossa is ridged for the origin of
muscular fibres. The spine has
a well-developed acromion. The
coracoid is elongated and in some
cases bifurcated. The clavicle is
long. The procoracoid is repre-
sented by a separate ossification ;
there are rudiments of the sternal
end of the coracoid between the
clavicle and the first rib. The
humerus and radius are both elon-
gated. The ulna is reduced, and is
sometimes represented by its proxi-
mal end only, ankylosed with the
radius. A large sesamoid is de-
veloped in the tendon of the triceps
muscle near the ulnar olecranon pro-
cess. The carpal scaphoid and
..Ol
lunar are united, and sometimes
=
§-
the cuneiform, too, is united with t
them. The pisiform is small.
There is no centrale. The ungual
t
phalanges are absent in the nailless
digits. The pelvis is small, and the
symphysis pubis often deficient.
The fibula is sometimes well-
developed, sometimes rudimentary.
The tuber calcanei is an inwardly
curved process of the calcaneum,
attached to which (by means of
ligamentous fibres) is a slender
rod of bone or cartilage, the calcar which supports the inter-femoral
membrane.
760 ZOOLOGY

ORDER T JENIODONTIA
This order, of which representatives are known from the Palreocene
(Conoryctes, Ectogamts, Onychodectes, Psittacotherittm, etc.) to the Middle Eocene
(Stylinodon), is, like the tillodonts, a very obscure one. It has no representa~
tion after the Eocene.
Earlier forms were primitive and somewhat insectivore-like, with a complete
dentition and enamel-covered teeth. Stylinodon, the last mid-Eocene survivor,
showed more distinctive characters in its very hypsodont, rootless teeth, whose
enamel was confined to bands at the insides. These teeth were peg-like and
without molar pattern. Later treniodonts were probably herbivorous, but
unable to compete with the radiating Ungulata.
Both the Tillodontia and Treniodontia appear to have become specialised
early along side lines which were, comparatively speaking, of short duration.
Opinions differ as to whether they manifest any close mutual affinity.

ORDER TILLODONTIA
This is an archaic order of undetermined ancestry, which left no descendants
and of which specimens occur only in the Lower and Middle Eocene of North
America (e.g. Esthonyx, Anchippodtts, Tillotherium). Some have grouped the
Tillodontia with the rodents, though with little justification. The most pro~
gressive form known, the mid-Eocene Tillotherium, was the size of a bear.
There were two pairs of incisors, of which the first was small, the second much
enlarged, rootless, and rodent-like. The mandibular condyles, however, were
transverse, and entirely unlike the condition found in rodents. The canines
were small, the premolars and molars brachydont and bunodont. The brain-
case was small, the skull low, and the snout sharp. Tillotherium was planti-
grade and pentadactyle : it was probably herbivorous, but perhaps omnivorous.

ORDER EDENTATA
This polymorphic yet geographically restricted group has undergone very
considerable taxonomic rearrangement but the true affinities of some of its
sub-divisions remain still in doubt. The group as a whole has sometimes been
completely removed from any other and its independent reptilian origin has
been suggested more than once. Such views cannot be sustained. There
seems now to be adequate evidence that the edentates arose from proto-
insectivoran ancestry about the beginning of the Cenozoic. As at present
constituted the order contains the fossil North American Palreanodonta, and
the Central and South American armadilloes, true ant-eaters, sloths and various
extinct forms of close affinity. The arrangement followed below reduces the
Xenarthra to sub-ordinal rank as follows :
 PHYLUM CHORDATA

Sub-order Palreanodonta (Palreocene-Oligocene)

Sub-order Xenarthra
Infra-order Cingulata (Lorieata)
Super-families Dasypodoidea (Palreocene-Recent)
Glyptodontoidea (Upper Eocene-Pleistocene)
Infra-order Pilosa
Super-families Megalonyehoidea (Upper Eocene-Pleistocene)
Myrmeeophagoidea (Pliocene-Recent)
Bradypodoidea (Recent)

SuB-ORDER PAL.:EANODONTA

This North American group (e.g. Palaanodon, M etacheiromys, Epoicotherium)

appears to represent a primitive Palreocene offshoot from the ancestry that also
gave rise to the Xenarthra, which found a southern refuge before the partition
of the Americas. Four rare genera have been described.
Metacheiromys, an Eocene edentate about r8 inches long, probably lacked
dermal armour but resembled to-day's armadilloes in many details. Interesting
specialisations occurred in the enamel-covered canines (a unique feature in
edentates) and in reduction and degeneration of teeth. The vertebrre had not
acquired the xenarthran peculiarity of extra zygapophyses and there was no
ischio-caudal symphysis.

SuB-ORDER XENARTHRA
This sub-order exhibits a great diversity of appearance and behaviour, but
constitutes a natural and exclusive group, composed of essentially the Cin-
gulata ( = Loricata) containing armoured armadillos (Figs. 532, 533) and
extinct glyptodons (Fig. 535), and the Pilosa, embracing ant-eaters (Fig. 537),
sloths (Fig. 536), and extinct ground-sloths (Fig. 539).
The group consists almost entirely of South American forms. A few late-
Tertiary animals penetrated northwards after the subsequent re-establishment
of an inter-continental land-bridge (Figs. 491, 492, p. 705). However diverse,
the Xenarthra are still unmistakably united basically by certain specialised
characters which prove them a single natural group. Enamel is entirely absent
from the teeth, except in an Eocene armadillo, Utaetus, in which small enamel
caps were just persisting. In the remainder, the teeth (see p. 871) are trans-
formed to pillar-like persistently growing stumps (the armadillos), or are reduced
in number (the sloths) or may be altogether absent (ant-eaters (Fig. 546)). A
most noticeable character is the presence of an extra pair of zygopophyses
upon the posterior dorsal and lumbar vertebrre (Fig. 540) a condition found
elsewhere only in snakes and some lizards (e.g. Iguanidre). Among other
specialised characters are the frequent reduction (even absence) of the zygo-
matic arch, the frequent fusion of the coracoid process to the front border of the
 ZOOLOGY
scapula so as to enc1ose a large foramen (Fig. 548) and articulation of several
of the anterior caudal vertebrre to the ischium, producing an unusually long
ischio-sacral region. The simple type of brain, on the other hand, and the
persistently abdominal testes are primitive. All Edentata have heavily
clawed feet.
The Xenarthra diverged widely. The Infra-order Cingulata are covered
with bony plates bearing horny scutes, an unusual feature in mammals. In

FIG. 532.- 0rder Edentata, Sub-order Xenarthra, Infra-order Cingglata, Super-family Dasy-
podoidea, Family Dasypodidse. Chknnyphorus. The fairy armadillos (e.g. C. truncatus) possess
the armour characteristic of the group and in addition have a posterior shield that enables them
to close the burrow. The protruding tail is armoured. (From B.M. (N.H.) skins.)

this group the teeth are more numerous than in the hairy Pilosa and the jugal
arch is more complete. There is a considerable fusion of the vertebral column
elements subserving the same mechanical junctions seen in chelonians. The
super-families are the Dasypodoidea (Dasypodidre, the armadillos, Fig. 532, and
the Peltephilidre); the extinct Glyptodontoidea (Glyptodontidre, e.g. Glyptodon).
The glyptodons (Fig. 535) had a peculiar protection involving the fusion of a
mosaic of numerous small polygonal plates into a rigid hemispherical armour
supported by blocks of fused vertebrre-i.e. cervical (excluding the first), fused
dorsals, and finally a compacted group of posterior dorsals, lumbars, and
 PHYLUM CHORDATA

sacrals connected posteriorly with the carapace. The tail, too, was armoured,
sometimes with spikes. The head was protected by a bony casque and the
zygomatic arch had a peculiarly elongated ventral down-growth. The
clavicle was absent. These curious creatures survived from the Eocene until
the Pleistocene and some penetrated northwards from South America as far
as the southern United States. No doubt they represent a primitive offshoot
from early armadillo stock.

FrG. 533. -Family Dasypodidre, Tolypeutes. The Three-banded Armadillo (T . conurus) (and
a few other species) possesses a hinged dorsal armour that allows it to curl protectively, the
armoured head and tail fitting into place to form an impregnable sphere (Fig. 534). Most
armadillos live in savannah or pampas. The fore-feet are peculiarly specialised for rapid digging.
(From B.M. (N.H.) skins.)

In the armadillos (Dasypodidre) the head (Fig. 533) is comparatively short,

broad, and depressed. The number of complete digits of the fore-foot varies
from three to five. These are provided with powerful claws, so as to form an
efficient digging organ. The hind-foot always has five digits with smaller
claws. The tail is usually well developed. The most striking external feature
of the armadillos is their armour of bony dermal plates. This armour usually
consists of a scapular shield of closely-united plates covering the anterior part
of the body, followed by a series of transverse bands separated from one
another by hairy skin, and a posterior pelvic shield. In the genus Tolypeutes
 ZOOLOGY
these bands are movable, so that the animal can roll itself into a ball. The
tail is also usually enclosed in rings of bony plates, and a number protect the
dorsal surface of the head (Fig. 534).
The infra-order Pilosa contains the
following hairy groups: The super-family
Megalonychoidea, an extinct line of
ground sloths consisting of the Mega-
theriid::e (e.g. Megatherittm) the Mega-
lonychid::e (e.g. Megalonyx) and Mylo-
dontid::e (Mylodon , Fig. 536) : the
super-family Myrmecophagoidea com-
pnsmg the Myrmecophagid::e (e.g.
Myrmecophaga, the Great Ant-eater (Fig.
537), Tamandua, the Lesser Ant-eater
FIG. 534.- Tolypeutes: Defensive mec- (Fig. 538)): the super-family Brady-
hanism. (See also Fig. 533.) (From photo-
graph by Eisentraut.) podoidea comprising the Bradypodid::e
(e.g. Bradyptts, the Three-toed Sloth,
with nine cervical vertebr::e (Fig. 539), and Cholcepus, the Two-toed Sloth, with
six cervicals). The two last-named species are, with the exception of the dug-
ongs among the Sirenia, the only mammals without the typical number of seven
cervical vertebr::e (p. 850).

FIG. 535.- Infra.order Cingulata, Super. family Glyptodontoidea, Family Glyptodontidre.

GI1Jpfodon. This South American Pleistocene armadillo possessed a fused armour and was about
nine feet long. The tail ended in a defensive ' mace'. (After Owen.)

In their limb structure the sloths (Bradypodid::e, Fig. 540) are perhaps more
completely adapted to an arboreal life than any other mammalian group.
They have a short, rounded head, ears with small pinn::e, and long, slender
limbs, the anterior being much the longer. The digits, never more than three
in number, are long, curved, and hook-like, and thus enable the animal to hand-
climb, body downwards, among the branches. The three-toed sloth has
 PHYLUM CHORDATA

three toes in both manus and pes : the two-toed sloth has two in the manus,
three in the pes. The tail is rudimentary. The body is covered with long,
coarse hairs, which differ from those of other mammals in being longitudinally
fluted. On these hairs algre grow abundantly, conferring obliterative greenish
and bluish hues on the slow moving animals in the forest. Modem sloths live
on leaves, but a former insectivorous life is evidenced by the reduction of the
teeth and their enamel loss. This irreversible process has been compensated
by the provision of a grinding surface by cement and by persistent pulps.
The ground sloths (e.g. Megatherium, Nothrotherium, Mylodon (Fig. 536)),
which disappeared so recently as the Pleistocene, were once very common and
latterly as big as oxen. They
occurred in both American con-
tinents, and their fossils have been
found in the West Indies.
The ant-eaters (Myrmecoph-
agidre) have a greatly elongated
snout (Fig. 546), with the mouth
as a small aperture at its extremity,
small eyes, and auditory pinnre
which are sometimes small, some-
times well-developed. There are
five digits in the fore-foot, of which
the third has always a very large
curved and pointed claw, rendering
the manus an efficient digging
FIG. 536. - Infra-order Pilosa, Super-family
organ. The toes of the hindfoot, Megalonychoidea, Family Mylodontidse. Mylodon.
four or five in number, are sub- This group flourished in both the Americas during
the Pleistocene. M. robustus was as big as a
equal, and provided with mo- small rhinoceros. (After Owen.)
derate-sized claws. In walking,
the weight of the body rests on the dorsal surfaces of the second, third, and
fourth digits of the manus and on a thick callous pad on the extremity of the
fifth, and in the pes, on the entire plantar surface. The tail is always very
long, and is sometimes prehensile. The body is covered with long hair. In
the Two-toed Ant-eater (Cyclopes) the muzzle is short. There are four digits
in the manus, of which the second and third only have claws, that of the
third being the longer. The pes has four sub-equal clawed toes, forming a
hook not unlike that of the sloths. The tail is prehensile.
Skeleton of Xenarthra.-In armadillos more or fewer of the cervical verte-
brre are ankylosed t ogether both by their bodies and neural arches. In the
lumbar region the metapophyses are greatly prolonged. They are longer than
the transverse processes and support the bony carapace. A remarkable
peculiarity of the spinal column in armadillos is the fusion of a number of the
VOL. II. B 3
766 ZOOLOGY

anterior caudal vertebrre with the true sacrals to form the long sacrum which
thus contains as many as ten vertebrre. The caudal region is of moderate
length, with numerous chevron bones. In Manis and Myrmecophaga the
neck-vertebrre are not united. In the posterior thoracic and lumbar regions of
Myrmecophaga are developed complex accessory articulations between the
vertebrre; the sacrum contains, in addition to the true sacral vertebrre, a num-
ber derived from the caudal region.

F1G. 537.- 0rder Edentata, Sub-order Xenarthra, Infra-order Pilosa, Super-family Myrmeco-
phagoidea, Family Myrmecophagidre. Myrmecophaga. The South American Giant Ant-eater
or 'Ant-bear' seems to be rigidly insectivorous, feeding principally on ants which it gathers by
means of its sticky protrusible tongue. Anterior pedal specialisations allow the animal to walk
unimpeded by its powerful digging claws. (From B.M. (N.H.) skins.)

In the sloths none of the cervical vertebrre are ankylosed. In the three-
toed sloths (Fig. 540) there is an important divergence from ordinary mammals
in the number of vertebrre in the cervical region for there are nine or ten
instead of seven. In the Two-toed Sloth (Cholceptts hoffmanni), there are six
only. The neural spines of all the vertebrre are very short. A number of
the anterior caudal vertebrre are united firmly, though not quite fused , with
one another and with the true sacrals.
In the armadillos the sternal ribs, which are sub-bifid at their sternal ends,
are ossified, and articulate with the sternum by means of well-developed
synovial articulations. In the American ant-eaters there are similar synovial
 PHYLUM CHORDATA

joints, and the sternal ends of the sternal ribs are completely bifid. In the
sloths the sternum is long and narrow, and there are no costo-sternal synovial
joints. The anterior sternal ribs are ossified and completely united with the
vertebral ribs, but their posterior fellows are separated from the latter by inter-
mediate ribs which are less perfectly ossified.
In the armadillos the skull (Fig. 545) is broad and flat , the facial region
triangular. The tympanic (ty.) is in some developed into a bulla. The bony

F rc . 538.- Family Myrmecophagidre. TamaJidua. T he South a nd Cent ral American Collared

Ant-eat er ( T . tetradacty la) is a forest -dwelling arboreal off-shoot from t errestrial st ock (cf. F ig. 537),
tho ugh its diet remains t he sam e. The tail has becom e prehensile. (From photographs.)

auditory meatus is in some cases elongated. The zygoma is complete. The

pterygoids are small, and do not develop palatine plates. The mandible has a
well-developed ramus with a prominent coronoid process and a well-marked
angular process.
In the ant-eat ers (Figs. 546, 547) the skull is extremely long and narrow.
The facial region is drawn out into a long, narrow rostrum, with the external
nares at its extremity. The olfactory fossre are correspondingly greatly
developed. The rostrum is composed mainly of mesethmoid, vomer, maxillre,
and nasals; the premaxillre are small. The zygoma is incomplete, and the
 ZOOLOGY
orbit is not closed behind by bone, a postorbital process of the frontal being
entirely absent. The pterygoids (pter.), in all but Cyclopes, develop palatine
plates. There is no bony auditory meatus. The mandible is entirely devoid
of ascending ramus and consists of two long and slender horizontal rami, with
a very short symphysis.
In the sloths (Fig. 544) the cerebral component of the cranium is elevated

Fr c . 539.-0rder Edentata, Sub-order Xenarthra, Infra-order Pilosa, Super-family Bradypo-

doidea, Family Bradypodidre. Bradypus. The three-toed Sloth (B . tridactylus) is a nother South
American edentate that has becom e arboreal. It inhabits tropical forests and possesses very
inefficient thermo-regulation: exposure t o sun temperature of 35°- 40° C. causes death from heat-
stroke. The algce which give the fur a green tinge (p. 844) are reported partly to support a fur-
living population of mites, minute beetles, a nd moths. (From a photograph by Gross.)

and rounded, the facial short. The frontal region is elevated, owing to the
development of extensive frontal air-sinuses. The premaxillce are small, and
not firmly connected with the maxillce, so that they are commonly lost from the
macerated skull. The jugal (J'u.) develops a strong zygomatic process which
bifurcates behind into two branches. Neither of these is connected with the
vestigial zygomatic process of the squamosal, so that the zygomatic arch
remains incomplete. There are, at most, the rudiments of postorbital pro-
cesses of the frontals. The pterygoids develop vertical lamince and form no
 PHYLUM CHORDATA

F rG. 540.- BTadypus:

Skeleton. T hree-toed
Sloth (B. tridacty lus) .
(After Blainville.)

c I
uln rad
o::t::

' so
trd.m
metals

FrG . 542 . - BTadypus: F r G. 543. -BTadypus: Pes. ast.

Manus. Right. wn. cuneiform; astragalus ; calc. calcaneum; cbd.
FrG. 54 I .-Bmdypw~: Should-
lttn.lunar ; m . c. I , first met acar- cuboid; fb . fibula; meso.c. meso-
er girdle. L ateral view in three- pal ; m. c. s. rudiment of fifth cuneiform; metal. r, vestige o f first
t oed Sloth (B . tridactylus). acr. metacarpal; pis. pisiform ; rad. metatarsal; metal. 5, vestige of fifth
a cromion; cl. clavicle; col'. radius; sc. scaphoid ; trd. m . m etatarsal; nav. navicular; lib.
coracoid. tibia; x , peg.like process at distal
trapezoid a nd magnum united;
uln. ulna ; unc. unciform. end of fibula.
770 ZOOLOGY

FIG. 544.-Bradypus:
Skull. Lateral view in
Three-toed Sloth (B. tri-
dactylus).

sr- par

FIG. 545.-Dasypus: Skull.

Six-banded Armadillo (D. sex-
cine/us) .

m.a..x

I cond' t.r
al.spk Jy e.:;d.aud
cor

FIG. 546 .-Myrmecophaga: Skull. Lateral view in Great Anteater. al. sph . ali-sphenoid;
cond. condyle of mandible; cor. coronoid process of mandible; ex. oc. ex-occipital; ext. aud.
external auditory meatus; Jr. frontal; ju. jugal; lcr. lachrymal; max. maxilla; nas. nasal; occ.
cond. occipital condyle; pal. palatine; par. parietal; p. max. premaxilla; s. oc. supra-occipital;
sq. squamosal; ty. tympanic.

palatine plates. The ascending ramus and coronoid process of the mandible
are both well developed.
In the ant-eaters and armadillos the bones of the fore-limb are short and
powerfuL The scapula in the ant-eaters is broad and rounded. Its anterior
border unites with the coracoid process so as to convert the coraco-scapular
 PHYLUM CHORDATA 771
notch into a foramen. In the middle of the spine there is a triangular process :
a ridge on the postspinous fossa presents the appearance of a second spine.
The fibres of origin of the subscapularis muscle
extend on to the outer surface as far forward as s . oc

this ridge, so that the part of the outer surface

behind the ridge corresponds to a part of the sub-
scapular fossa, which in other Theria is co-exten- j.J l:A.! r
g Le n
sive with the inner surface. Except in Cyclopes sq
the clavicles are vestigial. All the carpal bones al .s,JJit. -
are distinct.
pat
In the armadillos the scapula (Fig. 548) has an
extremely prolonged acromion (acr.), sometimes
articulating with the humerus. A ridge (sp'.)
representing a second spine is present. The
clavicle is well developed. The humerus is short
and powerful, with well-developed processes and
ridges, and with a foramen (entepicondylar foramen)
above the inner condyle. The carpus consists of
the ordinary eight bones.
In the sloths (fig. 540) the arm bones are
comparatively long and p.ma.r -
slender. A coraco-scap- FrG. 547 --JlfyrmecopiiOga:
ular foramen is formed as Skull. Ventral view in Great
Ant-eater. Letters as in Fig. 546.
in the ant-eaters. In the In addition, b. oc. basioccipital;
glen. glenoid surface
Three-toed sloths (Fig. dible; pter. pterygoid. for man-
541) the acromion (acr.)
is at first connected with the coracoid process, but
becomes reduced and loses this connexion. In the
two-toed sloth the connexion persists. The clavicle
(cl.) is not directly connected medially with the
sternum. Laterally it is directly connected with the
coracoid process-a condition unique among the mam-
malia. The humerus is very long and slender, as are
FrG. 548.- Dasypus:
Shoulder girdle. Six-banded the radius and ulna, which are capable of a certain
Armadillo (D. sexcinctus). amount of pronation and supination. In the carpus
acr. acromion; cor. coracoid
process; pr. sc. prespinous (Fig. 542) the trapezoid and magnum are united in
fossa; pt. sc. post-spinous
fossa; sp. spine; sp'. ridge Bradypus, distinct in Cholcepus : in the former the
probably marking the ante- trapezium is usually fused with the rudimentary first
rior limit of origin of the
subscapularis muscle. metacarpal. The first and fifth metacarpals are repre-
sented by rudiments only. The proximal phalanges of
the three digits are early ankylosed with the corresponding metacarpals, so that
it might readily be supposed that a normal component of each digit was absent.
772 ZOOLOGY

The pelvis of the American ant-eaters is elongated, with a short symphysis

pubis. The ischia unite with the spinal column. There is no third trochanter.
The tibia and fibula are nearly
straight, and parallel. In Cyclopes
the pes is modified to form a climbing
organ.
In the sloths the pelvis is short
and wide. The spines of the ischia
unite with the anterior caudal verte-
bne so that a sacro-sciatic foramen is
ac formed as in ant-eaters. The femur
is long and slender; it has no third
trochanter. The tibia and fibula are
also long and slender. At its distal
end (Fig. 543) the fibula develops a
peg-like process (x) which fits into a
depression in the outer face of the
astragalus. The calcaneal process is
extremely prolonged in Bradypus, in
FIG . 549- -Dasyplt."': Pelvis and sacrum. which there is a t endency to ankylosis
Six-banded Arma dillo (D . sexcinctus) . ac. aceta-
bulum; il. ilium; isch. ischium; obi. for. obtur- between the tarsal bones, and the
ator foram en; p eel. tub. pectineal tubercle; pub.
pubis. proximal phalanges ankylose with the
metatarsals.
In the armadillos the pelvis (Fig. 549) is extremely long, and both ilia and
ischia are firmly fused with the spinal column. The femur has a prominent
third trochanter. The bones of the pes are not specially remarkable.

ORDER PHOLIDOTA

The peculiar pangolins or scaly ant-eaters (Figs. 550, 551) of tropical Africa
and Asia are often placed among the Xenarthra but ever since Huxley's day
opinion has tended increasingly to accord them separate ordinal rank, at the
same time retaining their undeniable connexions with the Edentata (in its
original sense). Little is known of their phylogeny. Pleistocene remains are
essentially similar to those of extant forms and the mid-Tertiary (European)
species are known from fragments only.
The most noticeable character of living forms is their covering of imbricating
horny scales over the head, body, and tail, among which scales grow a few
scattered hairs. The lower body-surface is hair covered. The head is pro-
duced into a short, pointed muzzle. The tongue is long and teeth are entirely
absent, as are also the jugal arch and clavicle. The skull is long and cylin-
drical and there is no bony separation of orbit and temporal fossa. The body
and tail may be five feet long.
 PHYLUM CHORDATA 773
There is but one genus, Manis, which contains some seven species. The
limbs are short and strong, and pentadactyle. The hind-feet are plantigrade,
the fore-feet provided with strong curved claws. In walking, the weight rests

FIG. 550.- 0rder Pholidota, Family Manidre. Manis. The pangolins (scaly ant-eaters) and the
Aardvark (Order Tubulidentata) (Fig. 576) are often called the 'Old vVorld edentates' but are
probably better separated from armadillos, ant-
eaters and sloths and, of course, from each other.
The tropical pangolins (e.g. M. javanica above)
are remarkably protected by an over-lapping
armour (Fig. 551) and have developed a remark-
able pedal specialisation that allows the possession
of powerful digging claws without undue inter-
ference with walking (see convergence illustrated in
Fig. 537). The tail may be prehensile (e.g. in
M. longicaudata) and contain as many as 50
vertebr;e, the largest number of any mammal
(cf. four retained by Man). Some pangolins are
arboreal. (From photographs and B.M. (N.H .)
skins.)

FrG. 551. - Manis: Defensive mechanism.

(From photograp h by Lang. )

on the upper and outer side of the fourth and fifth digits. Some forms are
expert climbers.
What resemblance there is to the edentate Myrmecophaga is probably due
774 ZOOLOGY
to convergent adaptation. Whilst a remote connexion with the American
Edentata is not absolutely precluded, it is more convenient in the state of
present knowledge to regard the Pholidota as a separate order.

COHORT GLIRES
In this great division are placed the orders Rodentia and Lagomorpha which
were previously included in a single order Rodentia. Despite their familiarity
to-day, and an abundance of Eocene fossils, there is still no certainty concerning
the origin of either. It is probable that they represent early departures from
the primitive eutherian insectivore stem. They show many adaptive parallels
in morphology, physiology, and behaviour.

ORDER RoDENTIA

This large and important order was formerly divided into two very unequal
sub-orders, the numerous Simplicidentata and the few Duplicidentata (Lago-
morpha) in which were placed only certain extinct Palceocene rabbit-like
animals, as well as the pikas, hares, and rabbits, including the American cotton-
tails. The Duplicidentata all possess a second pair of incisors, hence the name.
It is now known, however, that many of the numerous similarities between the
two groups are largely adaptive and that there are adequate grounds for the
erection of a separate order Lagomorpha. Some authorities, indeed, believe
that the two groups are related only to the extent that both are offshoots of
early Eutherian stock and it has even been suggested (but not accepted) that
the two orders arose independently from metatherians. Certainly both groups
were clearly distinct in the Eocene period some 50 million years ago. The fact
that these two widely differing, but apparently very similar, orders were so long
combined in classification is a cogent warning to students against the un-
questioning acceptance of systematic arrangements which are, in the state of
current knowledge, inevitably no more than provisional.
The Rodentia, comprising almost three thousand living species, is numer-
ically the largest of all the mammalian groups. Rodents are of enormous
economic and medical importance in that the rats carry plague-fleas and do
incalculable damage to stored foodstuffs and other goods, while plagues of mice
periodically inflict incomputable injury upon crops in various countries.
No rodent has ever attained any great size. A capybara (Hydrocha:rtts),
about the size of a small pig, is the largest living form, but its bulk may have
been exceeded by two extinct beavers (Trogontherium and Castoroides). Most
rodents are small and some, such as the Harvest-mouse (Micromys minutus), are
among the smallest known mammals and weigh a mere 7 gms. (see alsop. 727).
The distribution of the group is almost global. Rodents are among the few
eutherian mammals to occur in Australia, where they radiated successfully pre-
 PHYLUM CHORDATA 775
sumably after voyages thither on floating driftwood. Many rodents (such as
the Lemming (Lemmus lemmus) and various species of rats and mice) can
multiply rapidly to almost incredible numbers.
The characters of the order are as follows : Never more than a single pair of
upper incisors is present. The incisors are chisel-shaped in wear owing to the
absence of enamel on their posterior surfaces and, having persistently open roots,
they grow throughout life and are always of great size. There is no anterior
symphysis between the mandibles and special modifications of the mylohyoids
and masseters permit the separation and return of the prominent lower incisors
in a curious, almost scissor-action in feeding. Canines are never present, and
there is a wide diastema between the incisors and the cheek-teeth into which
the cheeks can be tucked so as to separate the front part of the mouth from the
hind during the process of gnawing. This habit of gnawing, universal in
the group, has induced noticeable modifications not only of the teeth, but of the
whole skull and jaws. The glenoid cavities for the mandibular condyles are
elongated in a fore-and-aft direction, an adaptation which enables the jaws to be
moved forwards, and thereby the upper and lower incisors to be brought into
contact for the purpose of gnawing. In this position, owing to the size of the
incisors, the cheek-teeth are thrown out of action and cannot operate. For
chewing, therefore, the jaws can be moved backwards when the lower incisor
fits in behind the upper, and the upper and lower cheek-teeth can be brought
into contact. The grinding-teeth vary in number, and frequently have per-
sistently open roots. In some cases the milk dentition is suppressed.
To provide the necessary power for gnawing, the jaw-muscles are very
greatly enlarged (Fig. 552). The increase in area for their attachments has
produced considerable modifications in the skull and jaws which differ in
different groups, and afford a basis for classification. The premaxillre and
maxillre are enlarged, not only to house the great curved incisors, but also for
the attachment of the anterior mass of the masseter muscle. This in some
forms passes through the enormously enlarged antorbital vacuity, which may
be as big as the orbit (in Ccelogenys, the pacas), or it may pass over the edge of
the jugal arch in front of the orbit (as in the enigmatic Pedetes). The fossre for
the pterygoid muscles may also be very large. The lower jaw may have the
external border expanded into a flange for the insertion of the masseter. It
is interesting to note a convergence in shape between rodents and Phascolomys
(marsupial wombats). These have a single pair of persistently growing incisors
and an enlargement of the masseter muscle inserted on to an everted mandi-
bular flange. In many rodents the jugal extends back to the glenoid cavity,
as in marsupials.
Additionally to its characteristic incisors and absent canines, the rodent
dentition is peculiar as regards the cheek-teeth. Two premolars occur above
and one below, though even fewer may be present. As few as one molar only
 ZOOLOGY
may occur in each maxilla and in each moiety of the mandible. (The recently
described water-rat Mayermys from New Guinea has the unique dentition of
one cheek tooth only in each jaw, thus taking a further step beyond its allies
in the sub-family Hydromyinre which may have as few as two.) An average
number for many rodents is four molar teeth. There is also a wide range of

: :
i lanfe~ior su~erf~c!a'}·····m r ... ..fposlerior deep~ i
•posTenor superftctill asse er \anterior deep--~

:.
i linemal pler~goid
~ex lerndl p er~goid

:.z~gomalicus :
lev<:~tor lab11 superiori5·:
FIG . 552.-RaUu.<J: Jaw musculature. (After E . Chase Green.)

difference in cheek-tooth crown-pattern as well as in size. Thus, the squirrels

(Fig. 553B) possess teeth of the short-crowned brachydont and rooted t ype,
which, especially in the earliest species, can be referred to as the tritubercular
type. The dentition exhibits gradations through various bilophodont patterns
(Fig. 553C) to the highly complicated multilophodont and hypsodont varieties
with permanently open roots, as, for example, in the capybaras (Fig. 553A).
In the rest of their anatomy the rodents are not highly specialised. They
are, for the most part, unguiculate, pentadactyle, and plantigrade. A clavicle
 PHYLUM CHORDATA 777
is usually present and, as in all herbivorous mammals, the crecum tends to be
large. The brain is little, if at all, convoluted, and the cerebellum is not
covered by the cerebral lobes.
The order is divided into three ill-defined sub-orders : r. Sciuromorpha
(squirrels, beavers, North American gophers, and allies); 2. Myomorpha
(jerboas, voles and lemmings, rats and mice, hamsters and jumping-mice), and
3· Hystricomorpha (porcupines, guinea-pigs, chinchillas, viscachas, agoutis,
capybaras, mole-rats, and allies).
A strong case, however, can be made that the above groups are essentially
morphological grades and not phylogenetic divisions indicating close relation-
ship of their contained forms. For example, Wood has shown that the New
and Old World hystricomorphs (see below) are each derived independently from
a world-wide sciuromorph stock. Rodents exist in great number and exhibit

FIG. 553 .-Rodentia: Molar dentition. A, Hydrochamts (Capybara) ; B , Sciurus (squirrel);

C, Ctenodactylus (of uncertain affinity, possibly a hystricornorph .) (From Cambridge Natural History,
after Tullberg.)

an extraordinarily wide range of adaptive radiation. Some such as the

beavers, are amphibious, with flattened swimming-tails. Some are burrowers
to such an extent as to have acquired a mole-like appearance (e.g. Spalax,
Georychus). The squirrels are tree-living. The Anomaluridre ('flying'-
squirrels) have proceeded a step further by the development of a parachuting
membrane closely resembling that of the gliding marsupials. The Dipodidre
are saltatorial (hopping) forms, somewhat resembling kangaroos in their mode
of progression.
The Sciuromorpha (of some 13 extinct and living families) are primitive
in their retention of two upper and one lower premolars, and in the antorbital
foramen not being enlarged. It does not transmit any part of the masseter
muscle. In some classifications five super-families are recognised as follows :
the Sciuroidea (squirrels, gophers, chipmunks, etc.), the Aplodontoidea
(Aplodontia, the Sewellel), the Ischyromyidre (Eocene and Oligocene rodents),
the Castoroidea (beavers), the Geomyoidea (a purely North American group
including the pocket-gophers, etc.), and the Anomaluroidea. The last-named
 ZOOLOGY
includes Anomalurus, a West African 'flying-squirrel', and possibly Pedetes,
the Cape Jumping-hare. The last-named possesses a large infra-orbital
opening, which, however, does not transmit any part of the masseter muscle,
so that the skull in this respect resembles structurally, but not functionally
that of the Myomorpha.
The Myomorpha (rats, mice, jerboas, hamsters, etc.) are differentiated into
nine families and are characterised by the superficial portion of the masseter
muscle passing forwards in front of the orbit, while its median portion runs,
via the orbit, through the enlarged infraorbital opening. The sub-order
Hystricomorpha (true porcupines, the guinea-pigs (Cavia), the Viscacha
(Lagostomus) etc.) comprise some 19 families. They have the infraorbital
canal enlarged to the greatest extent of all, and the median portion of the
masseter is enlarged at the expense of the anterior superficial portion, which
does not extend on to the face, but retains its more primitive attachment to
the jugal arch. The tibia and fibula remain separate.
Skeleton of the Rodentia.-Among the rodents the jerboas are exceptional
in having the cervical vertebrre ankylosed. Generally the transverse processes
of the lumbar vertebrre are elongated. As in the Ungulata, the sacrum usually
consists of one broad anterior vertebra followed by several narrower ones.
The caudal region varies in length in the different families. In some it is very
short, but in many (porcupines, squirrels, and beavers) it is elongated. The
sternum of the rodents has a long and narrow body. Sometimes there is a
broad pre-sternum. The posterior end is always expanded into a cartilaginous
xiphisternum.
The skull is elongated, narrow in front, broader and depressed behind.
The nasal cavities are very large, especially in the porcupines, with complex air
sinuses in their upper part. In some the optic foramina fuse into one. An
interparietal is often present; Paroccipital processes are developed. The
orbit and the temporal fossa are always continuous. The nasal bones
are large, and the nasal apertures are terminal or nearly so. The premaxillre
are always very large. A remarkable feature of the skull is the presence in
many of a large opening corresponding to the infraorbital foramen. The
middle part of the zygoma is formed by the jugal. The latter often helps to
bound the glenoid cavity, as in the marsupials. The anterior palatine foramina
are long. The periotic and tympanic may become ankylosed together, but not
to the neighbouring bones. The coronoid process of the mandible is sometimes
rudimentary or absent; the. angle is often produced into a process.
The scapula of the rodentia is generally long and narrow. The spine
sometimes has a metacromion process and a long acromion. The coracoid
process is small. The clavicle varies as regards its development. Vestiges
of the sternal end of the coracoid are sometimes distinguishable. The bones
of the arm and forearm show considerable variation. The radius and ulna
 PHYLUM CHORDATA 779
are in most instances distinct, though in close and firm apposition. The
scaphoid and lunare are usually united; a centrale may be present or absent.
The pelvis and femur vary greatly: sometimes there is a third trochanter.
The fibula may be distinct or fused with the tibia. In the jerboas the meta-
tarsals of the three digits are fused (see Fig. 6r9).

ORDER LAGOMORPHA
As mentioned (p. 774) these forms (the hares, rabbits, and pikas or conies)
were once included within the Rodentia (as the smaller sub-order Duplici-
dentata). They occur throughout Eurasia (including high arctic islands in
the case of the hare Lepus arcticus) and North America. Hares have penetrated
Africa as far as the Cape, and cotton-tails (Sylvilagtts) occupy all tropical South
America. The lagomorphs agree with the rodents in possessing a diastema,
incisors with persistent pulps, cheek-pouches, enormous masseter muscles and
relatively weak temporal muscles. They differ, however, in lacking the curious
mandibular specialisation (p. 775), in their simpler, more restricted masseter
muscle and in their additional small pair of upper incisors. There are always
three upper and two lower premolars and three molars. The orbital foramina
are small. Muscles never invade the infraorbital canal. The incisive fora-
mina are large; and the maxillc:e are curiously laterally fenestrated. The tail
is invariably reduced and the hind-limbs are highly specialised for a saltatorial
mode of progression. Some true rodents (agoutis, porcupines) also have re-
duced tails, and hind-limbs modified for jumping (jerboas). The apparently
close similarity of rodent and lagomorph cheek-teeth is taxonomically mislead-
ing, being merely the result of convergent evolution. In the rodents the rows of
lower cheek-teeth are more widely separated from each other than are their
upper counterparts. In the lagomorphs the opposite is true.
The lagomorphs fall into two families: the Leporidc:e (hares and rabbits)
and the Ochotonidc:e (pikas, Ochotona). Leporids are known from the Eocene
and the second group from the Oligocene.

COHORT MUTICA
This division contains only those widely divergent and highly specialised
mammals, the whales, whose phylogenetic history remains obscure. Their
perfection of adaptation to a marine existence has, to a remarkable degree,
obliterated evidence of their affinities to the other extant mammalian orders.
Although an abundance of cetacean fossils is available from the Miocene, and
archaic whales (Archc:eoceti) have been found in Eocene deposits, the early
stages of cetacean evolution remain unknown,
780 ZOOLOGY

ORDER CETACEA

Because their bodies are supported by water, and because of the manifold
physiological advantages of homreiothermy, whales have been enabled to attain

F rc. 554.- 0rder Cetacea, Sub-order Mysticeti, Family Balrenopteridre, Balamopte-ra, a nd

Sub-order Odontoceti, Family Delphinidre. 01•cinus. Among t he micro-feeding Mysticeti
or 'whale-bone' (baleen) whales the Blue, Sulphur-bottom, or Sibbald's Whale (B. musculus)
grows to a length of more than r oo feet and may weigh 120 t ons (cf. bull African elephant, 6 tons) .
It is p erhaps the most m assive animal that ever lived. A single Blue Whale (a rorqual) may
yield a bout 135 barrels (22~ tons) of oil, 3! tons of guano (from ground-up flesh) a nd about 336 lbs.
of baleen.
The second extant sub-order is the Odontoceti (toothed whales). The Killer Whale (0. orca)
eats fishes, birds, and marine mammals, attacking and eating the tongues of whales much bigger
tha n itself (Andrews). In the stomach of a single Killer was found the remains of 13 porpoises and
14 seals. T h e species is harmless to Man but has a ttacked boats. 0 . orca is sexually dimorphic:
the male (with a 'sharp ' triangular dorsal fin reaching 5k feet high) grows to a length of about
JI feet, and the female (with a slightly recurved dorsal fin) to about r8 feet. Although toothed
whales of course produce oil (6 barrels from Orca) they are of relatively less commercial value.
Some however (e.g. dolphins) have been hunted for food from earliest t imes, especially b ecau se
they a re considered as fish in cert a in countries a nd can t herefore be eaten on fa st days. Though
ra re in equatorial waters, both t h e above species are or were a lmost cosmopolitan in distribution.
(From photographs a nd B.M. (N.H.) specimens.)

the greatest bulk of known animals, living or extinct. The Blue Whale
(Balcenoptera musculus), for example, grows to more than roo feet long and may
weigh about 120 tons (Fig. 554). This animal is capable of a speed of up to
18 knots. The great torpedo-shaped body is naked, the skin is very smooth,
 PHYLUM CHORDATA

and structures likely to impede the even flow of water over the body surface,
such as ear pinnce, scrotum, and hind-limbs, have disappeared. The thrust
propelling the whale's enormous bulk through the water is produced mainly
by the up-and-down motion of the laterally expanded tail-flukes. The fore-
limbs, or flippers, and the dorsal fin (when present) act as rudders and stabilisers.
Dorsal fin and tail flukes are specialised skin folds supported by underlying
fibrous tissue and are devoid of any bony skeleton. The Blue Whale, incident-
ally, is sometimes called the 'Sulphur-bottom' because of a yellowish ventral
colouration imparted by an adherent diatom population.
Modem Cetacea, which occur with an apparent suddenness in the early part
of the Miocene, are characterised by the adaptation of many of their morpho-
logical characters to their special mode of life. Some of these structures are
new, some are normal mammalian structures much modified, and others
indicate a complete loss of ancestral equipment. Examples of the first cate-
gory include the formation of the horizontally expanded tail-flukes (Fig. 554),
the single dorsal fin, and baleen or whalebone (Figs. 558, 559). Further
examples are to be found in extra digits (hyperdactyly) and extra phalanges
(hyperphalangy) (Fig. 564) which both serve to enlarge the surface of the
flipper. Characters which have been modified from the normal include the
development in the hypodermis of a thick insulating layer of dermal fat
(the blubber)-which may constitute as much as 27 per cent of total weight-
the very extensive breaking up of certain blood vessels into fine networks (retia
mirabilia), the great obliquity of the diaphragm and its powerful musculature;
the enclosure of the proximal elements of the fore-limb inside the body and the
alteration of the hand to a skin-covered paddle; the peculiar shape of the
scapula with extremely reduced prescapular fossa, forwardly directed acromion
process and absence of spine; the relatively simple, bilobed liver and non-
lobulated lungs; and the subdivisioning of the stomach into several digestive
regions. The whole skeleton is naturally much modified. The skull has
greatly elongated facial, and dorsoventrally compressed ('telescoped') cranial,
portions. The nasal openings, often asymmetrical, are placed far back on the
upper surface of the head. The tympanic bulla is loosely attached, and the
cranial bones in general are much modified in shape. In the vertebral column
the cervical vertebrce are shortened, compressed, and fused together to a greater
or less extent (Fig. 56r). (In Balama, for example, there is complete fusion;
in Balamoptera, complete separation.) The sternum is reduced. The features
that have been lost are: hair, except for a very few facial bristles, finger-nails,
except for traces in the £cetus; external ears; all skin-glands; the lachrymal
duct and nicitating membrane in the eye; the heads of the ribs in some; the
odontoid process in most, and, except for traces, the hind-limbs. Of all
mammals the Cetacea show the greatest deviation from the ordinary.
The order falls into three sub-orders: r. the extinct Archceoceti (e.g.
VOL. II. C3
 ZOOLOGY
Protocetus, Basilosaurus = Zeuglodon); 2. the toothed Odontoceti, and 3-
the baleen-bearing Mysticeti.
SuB-ORDER ARCH.iEOCETI

The Archreoceti are first known from the Eocene and are of uncertain
relationship. They give little indication of their original land-living ancestors

FIG. 555.-Protocetus : Skull.

Palatal aspect in Middle Eocene
Whale.

FrG. 556.-0rder Cetacea, Sub-order Archreoceti, Family Basilosauridre. Basilosaurus ( =

Zeuglodon). Basilosaurus was common in Eocene seas. It was serpentine in shape, and reached
a length of about 55 feet. (From Romer, after Gidley.)

FrG. 557.-Celacea: Evolution of

blow-hole. Left: Dorsal view of
the Eocene Basilosaurus ( =Zeuglo-
don) showing primitive anterior po-
sition (arrowed). Right: Modern
animal, in which the nostrils and
surrounding bones have migrated
posteriorly to the top of the skull.
(Redrawn after Romer.)

beyond the slight suggestion of creodont-insectivore affinities. Archreocetes

grew to a length of 70 feet and had already lost their hind-limbs. They com-
prise three families: the Protocetidre, Basilosauridre ( = Zeuglodontidre), and
Dorudontidre. The Protocetidre (e.g. Protocetus (Fig. 555), Eocetus, Pappo-
cetus) are known from the Middle Eocene and are of great interest because the
dentition of Prolocetus still showed a division into three incisors: a canine,
three premolars, and three molars. The cheek-teeth had become elongated
 PHYLUM CHORDATA

and sectorial, but one at least (the third premolar) retained an inner tubercle
and is somewhat reminiscent of a creodont tooth.
In the Basilosauridce Prozeuglodon had evolved still further. Its anterior
teeth were simple cones. The posterior teeth (which, however, were still two-
rooted) were elongated, with secondary serrations. The skull and skeleton
as far as known, had advanced a long way towards the cetacean type. Members
of this family (e.g. Basilosaurus, Figs. 556, 557) became unusually elongated,
and were clearly a side-line. The Dorudontidce (e.g. Dorudon) were shorter-
bodied and persisted till the end of the Oligocene. While the Archceoceti were
definitely whales, they were more primitive than modern whales in some
respects. The head, for instance, had the front part elongated, as in other
whales, but the cranial part was not compressed and the skull-bones bore a
more usual relationship to one another than is the case in later forms.

SuB-ORDER 0DONTOCETI

The Odontoceti or toothed whales are represented by several extant

families, of which may be mentioned the Iniidce and Platanistidce (the blind
freshwater dolphins of South America and the Orient); the Physeteridce (e.g.
Physeter, the sperm whales); the Ziphiidce (bottle-nosed and beaked whales);
the Delphinidce (a large heterogeneous assemblage of porpoises and dolphin,
including the white Whale, Narwhal, Pilot Whale, and killer whales (Fig. 554)).
The Agorophiidce of the Upper Eocene, and Squalodontidae of the Miocene
are two extinct families.
The freshwater dolphins retain a larger number of primitive characters than
do other whales. Thus the cervical vertebrce are not fused, the interlocking
processes of the thoracic vertebrce are more prominent, the number of double-
headed ribs is large, and the hand is relatively simple and without increase in
the number of phalanges. In the Ziphiidce the snout is long and narrow but
functional teeth are restricted to the lower jaw in which one pair (at most two
pairs) persist and may in male animals be relatively massive. Two divergent
throat grooves, and tail-flukes without a well-defined median notch in the
hinder margin, are distinctive of the family. The Physeteridce, represented by
the Cachalot (growing to about 6o feet, and the biggest of all odontocetes) and
the Pigmy Sperm (only about 12 feet long when adult), are characterised by
the restriction of functional teeth to the lower jaw, by the long symphysial
contact of the two rami of the latter and by the possession of a spermaceti
organ. The last-named is a large receptacle situated in the snout to the right
of the single nostril. It contains a large quantity of clear, colourless oil which
in the air solidifies into a white, soft wax. Its function is uncertain.
A peculiar, and commercially valuable, intestinal product of the Physeteridce
is ambergris, a substance formerly used in Europe in pharmaceutical prepara-
tions, and still so used in the Orient as also in cooking. It has been employed
 ZOOLOGY
both in the East and West for centuries in the manufacture of perfumes.
When used in minute quantities it adds to the pervasive and absorptive qualities
of toilet 'washes' and perfumes. Ambergris is often said to be formed as a
'biliary concretion' in sick whales, but there is no evidence that such is the
case. Its exact formation is still in doubt, but would appear to be pathological.
Certainly, it is found in relatively few individuals. It has been variously
considered to arise from intestinal irritation by the horny beaks of squid (the
principal food of the sperm whales), or to be impacted, and otherwise altered,
unvoided, f:l!cal material. In addition to occurring in the intestine of
apparently healthy harpooned animals, or in carcases cast ashore, ambergris
may be netted as flotsam, or picked up on beaches. When freshly voided it
possesses a highly disagreeable odour, but after being well washed by the sea
this may be lost and there may remain a not unpleasant, somewhat sweet,
musty, earthy odour. As much as gz6 lb. (65 X 30 inches) has been found in
the intestine of a bull sperm whale only 49 feet long. The market price, which
has been fairly stable for many years, varies from zs. 6d. to 65s. per ounce
according to quality.
In appearance, after floating and hardening, ambergris resembles a lump
of dark, dull, often 'marbled', cobbler's wax. It is often impregnated with the
undigested remnants of squid and cuttlefish. It has a specific gravity ranging
from 0·780 to o·gz6, melts at about 62° C. to a resinous dark yellow liquid, and
is volatilised at 100° C. Ambergris dissolves in alcohol and ether (exhibiting
iridescence) and liberates the ambrein which gives it its special quality.
The Delphinida! display most pronouncedly the homodonty which charac-
terises the group generally. The single, enormously elongated, spirally scored
'horn' of the Narwhal is at the one extreme of number of teeth, the Common
Dolphin at the other with as many as fifty teeth in each row.
In the Odontoceti the skull shows a varying degree of asymmetry. The
external nostril or blowhole is single and usually crescentic in outline. The
upper jaw bones widen posteriorly to their snout portions and are squamous
above the frontal bones, so that the latter are almost completely obliterated
from dorsal view. The anterior ribs usually have capitulum and tuberculum.
The sternum is formed of two or more sternebra!. The mandibular rami are
wide at their articular end, gradually narrowing to the symphysis. The latter
may be of considerable length (as in the river dolphins and sperm whales) or
greatly shortened (as in the Common Porpoise and Pilot Whale).

SuB-ORDER MYSTICETI
The Mysticeti or baleen whales are rare in horizons earlier than the Lower
Miocene. During this period a now-extinct family, the Cetotheriida!,
flourished. In these whales the compression of the skull had not proceeded to
the extent found in later forms. The remaining extant families are the
 PHYLUM CHORDATA

Balrenidre (right whales, Balcena; and the Pigmy Right Whale, Caperea); the
Eschrichtidre (the Californian Grey Whale, Eschrichtius); and the Balrenopteridre
(rorquals, Balcenoptera (Fig. 554); and the Humpback Afegaptera). These are
represented by a few recent genera and many others now extinct.
The baleen whales have the very numerous, horny baleen (or whalebone)
plates arranged in two series, one on each side of th'e upper jaw (Figs. 558, 559).
The plates, roughly triangular in outline, have their shortest margin inserted
transversely in the roof of the mouth and are spaced about a quarter of an inch
apart. Of the two remaining, subequal margins the labial is smooth, while the

FIG. ss8.-Neobalrena: Adaptation to microphagy. The genera Baltena and Neobalama are
composed of the Hight Whales, including N. marginata (above), the Greenland and Southern
'whalebone' vVhales, and others. The bigger species were the 'right' ones to kill for 'whalebone',
since the baleen blades of the Greenland \Vhale (B. mysticetus) were as long as 15 feet (see also Fig.
559). During the late rgth-century heyday of voluminous corsetry, baleen fetched as much as
[2 ,ooo p er ton . Baleen had many other common uses-e. g . as umbrella ribs-and it was woven
into expensive fabrics that were guaranteed to 'stand of themselves' . Oil was relatively less
prized during this period. C. coron oid process of mandible ; Fr. frontal bone; L. lachrymal; IYix.
maxilla; 0. occipital; Sq. squamosal. (After Beddard.)

lingual is frayed out into a fringe of bristles. After water is taken into the
buccal cavity the combination of these bristles forms an effective sieve for
straining the plankton (usually crustacean krill) from the water that it is
now forced from the mouth either by the action of a very muscular tongue
(Balrenidre) or by the contraction of sub-cutaneous gular muscles (Balreno-
pteridre). As much as two tons of plankton have been found in the stomach
of one Blue Whale.
Skeleton of Cetacea.-The cervical region (Fig. 561) (cerv.) is always very
short, and the constituent vertebrre are often completely fused into a con-
tinuous bony mass (Fig. 56o), or the atlas alone may be separate from the rest.
Sometimes, however, all the vertebra: are complete and separate. In the latter
786 ZOOLOGY
case they have small arches and long transverse processes consisting of two
narrow bars with a wide space between. The epiphyses are very distinct discs
which often remain separate from the bodies up to a late period. The neural

FIG. ssg.-Baleen plates: Relation-

ship with tongue. The twin sieves each
consist of as many as 300 to 400 flexible
horny blades embedded in the ·gum'.
The baleen elements are longest and widest
in the middle of each series, and become
progressively shorter at each end. Each
blade is in turn produced laterally into
hair-like sieve processes. The water
taken in through the open mouth is dis-
charged through the sieve and leaves a vast
trawl of krill behind. The tongue in baleen
whales is immense compared with that of
odontocetes. For fcetal teeth in Baloeno-
ptera, see Fig. 637, p. 874.

spines are well developed. The zygapophyses are poorly developed, and are
absent in the posterior portion of the trunk. In the absence of hind-limbs there
is no sacral region. The caudal region consists of numerous vertebrce beneath
which, opposite the intervertebral spaces, are a series of chevron bones (chev.).
In the baleen whales only one pair of ribs
articulates with the sternum, and none articulates
with the body of the vertebra, but only with the
transverse processes. In the toothed whales the
anterior ribs (Fig. 561) have distinct tubercles and
heads articulating respectively with transverse
processes and vertebral bodies. The hinder ribs
lack heads, and articulation is simply between
tubercle and transverse process. The anterior
ribs are linked with the sternum by ossified sternal
ribs, while the posterior ones are progressively
slender and imperfectly ossified. The sternum
varies in shape. Sometimes it consists of a
presternum and a series of several sternebrce with- FIG. s6o.-BaltFna: Verte-
brre. Section through middle
out xiphisternum. Sometimes (Fig. 562) it is a line of united cervical vertebroe of
Right Whale (Balama
continuous plate of bone, occasionally with median Greenland
mysticetus) . a. articular surface for
notches or fontanelles. condyle; e. epiphysis of seventh
cervical ; sn. foramen in atlas for
In the skull (Fig. 563) the brain-case is first spinal nerve; 1-7, arches of
rounded, the jaws greatly elongated and often cervical vertebroe. (After Flower.)
unsymmetrical. The parietals (Pa.) in most
Cetacea do not meet in the middle line above. They are generally separated
by the supraoccipital (S. 0.) and an interparietal (/. P.) ; there is thus no
sagittal suture. A large supraorbital plate is developed from the frontal.
 PHYLUM CHORDATA

There are large and stout zygomatic

processes of the squamosal, but the
jugals are extremely small. In all the
recent forms the maxilla (Mx.) is very
large. It extends backwards in the
toothed whales to overlap the frontaL
In the baleen whales the extension is
below the orbital process of the frontal.
Forwards it extends nearly to the ex-
tremity of the snout. The premaxillre
(P.Mx.), which are long narrow bones,
bound only a very small part of the oral
border of the upper jaw. The nasals
(Na.) are relatively small. The tym-
panic bone is very large. In the
Mysticeti two bony pedicels connect it
to the periotic. The anterior pedicel is
absent in the Odontoceti and the pos-
terior support does not involve bony
fusion at the tympano-periotic junction.
The lower jaw is remarkable for the
absence of an ascending ramus. Baleen
is keratinous. It is therefore in no
way related t o true bone (p. 73).
The scapttla in most of the Cetacea
is broad and flat and expanded into the
shape of an open fan. The spine has
practically disappeared. The acromion
is a slightly curved laminar process with
a narrow base of attachment. The
coracoid also is compressed and parallel
with the acromion. In some, both
acromion and coracoid are absent.
There is never any trace of a clavicle.
The humerus is short and very stout.
Its head is freely movable in the glenoid
cavity, and its distal articulating sur-
faces are flat and oblique, meeting at
an angle. The proximal ends of the
radius and ulna are firmly united with
the humerus and therefore allow of
very little movement. At the distal
 ZOOLOGY

end there are no complete synovial membranes. The manus is extremely

modified. In all cetaceans it functions as a single unit with negligible in-
dividual mobility among the component ele-
ments. There are no synovial joints. The
carpus is in some (whalebone whales) almost
entirely cartilaginous, as also are the meta-
carpals and phalanges. These cartilages are
coalescent, or separated by intervals of fibrous
tissues. In some of the carpal elements bone
is deposited. In the toothed whales the car-
pals are completely ossified, and are of
polygonal form. The phalanges are also ossi-
Frc . s6z.- Balrenoptera: fied, with incomplete synovial articulations.
Sternum. {After F lower.)
In the Cetacea there are sometimes five digits,
sometimes only four, of which the second is usually the longest (Fig. 564).
A few species have considerably more than the customary number of

FrG. s63.-Gtobicepllala: Skull. Sagittal section in a dolphin . a. angle of mandible ; an.

external nares ; A. 5. ali-sphenoid; bh. basi-hyal; B . 0 . basi-occipital ; B . S . basi-sphenoid; cd.
condyle of mandible; c. p. coronoid process; Ex. 0 . ex-occipital; Fr. frontal ; I . P. inter-parietal;
M. E. mesethmoid; klx. maxilla; Na. nasal ; Pa. parietal; Per. p eriotic; Pl. palatine; P. mx.
prema x illa; p. n. posterior nares; P .S. pre-sphenoid; Pt. pterygoid; s.h. stylo-hyal; S. 0 .
supraoccipital; Sq. squamosal ; t. h. thyro-hyal ; Vo. vomer. (After F lower.)

phalanges (hyperphalangy) ; sometimes as many as fourteen may occur. In

whales with broad flippers (e.g. Monodon, Delphinapterus and Phoccena) the
number of phalanges is reduced, and in those with long, narrow limbs (e.g.
 PHYLUM CHORDATA

Globicephala, Megaptera), the number is increased with maturity. It is of

interest that whales are the only mammals that retain traces of discrete fourth
and fifth distal carpals.
Vestiges of the pelvis are present in the form of a pair of long narrow bones
(Fig. 561, pelv.), which lie parallel with the spinal column some little distance
below the region where the chevron bones begin.
These are completely isolated and appear to represent
the ischia. A second pair of smaller bones which lie
close to these in the whalebone whales are apparently
vestiges of the femora. There may be additional
vestiges representing the tibire.
Internal Adaptations of Cetacea.-Some whales can
submerge for more than thirty minutes, and their sub- IJ
marine habits (unique as far as homceothermous
animals are concerned) are made possible by a series
of extraordinary morphological adaptations. To some
degree the Cetacea have redeveloped piscine charac-
teristics, e.g. in body-form, the almost total lack of
bristles and absence of external ears; the long snout,
lack of neck, and the broad, powerful tail. These
characters mimic the typical fish form and make for
smooth and unimpeded progress through water.
Even more remarkable adaptations, peculiar to
the group, have developed internally. The so-called
'spout' or 'blow' was traditionally explained as a jet
of water, and is generally held to be the condensation
of warm air expired from the lungs through the dorsal
blow-hole when the whale surfaces. Opinions differ,
however, as to whether the blow is more conspicuous
in polar than in tropical seas. A recent suggestion
is that the 'blow ' is composed of nitrogen-charged Fr G . 564.- Globicephala:
foam. Foam developed from a finely-divided emul- Hyperphalangy. Dorsal sur-
face o f right anterior limb
sion of fat droplets (in mucus) and gas has been found in a dolphin (G. m elas). c.
cuneiform; H. humerus; l.
in the middle ear and trachea of several species of lunar ; R . radius; s. sca-
whales. (In the ear it may subserve a sound- phoid; tdunciform. . trapezoid; U.
(AHer
ulna; t t.
insulating function.) Fat has a high nitrogen absorp- Flower .)
tion capacity : in the respiratory tract the fatty foam
may constitute an auxiliary excretory device. Audition may be improved by
modifications of the Eustachian tubes as capacious, air-filled diverticula in
the pterygoid and palatal regions. The blow-hole leads vertically to the nostrils
which, varying from group to group, have moved backwards with correspond-
ing modification of the dorsal cranial bones. The nostrils are equipped with
790 ZOOLOGY

valves which are closed during diving. The epiglottis is peculiar: it has
become tube-like and intra-narial.
The remarkably prolonged submergence can be maintained only with the
aid of special accessory respiratory arrangements, some of which are still in-
completely understood. The muscles are heavily charged with myoglobin
giving whale-meat its characteristically dark colour, and this device (Fig. 326)
facilitates tissue oxidation during the tremendous dives. There is evidence,
too, that the muscles can function amerobically, thus setting up a large tem-
porary oxygen debt that can be redressed later. The enormous system of retia
mirabilia (Fig. 236, p. 342) also enables the storage of a huge oxygen volume that
can be drawn upon during submersion. Whales avoid the 'diver's paralysis'
('bends', 'caisson sickness') that attacks Man when he ascends quickly from
extreme depths. A submerged human diver continuously breathes com-
pressed air. At the same time the nitrogen dissolves into his blood-stream.
If he comes up too rapidly without 'staging', nitrogen is liberated and bubbles
may block small blood vessels and lead to paralysis, cerebral anremia, and
death. As whales can inhale and submerge with only sufficient air to fill their
lungs, there is a strict limit to the amount of nitrogen that can be dissolved into
the circulation. It has been shown that the species that dive deepest have a
relatively smaller lung capacity than others. Again, there is evidence that
when a whale dives deeply, pressure causes the belly viscera to be thrust against
the immense oblique diaphragm, which then compresses the lungs and forces
air anteriorly into the trachea and forward passages. Meanwhile, the now com-
paratively empty alveoli contract and become thick-walled, and so gaseous
exchanges between the lungs and blood remain at a minimum until the animal
ascends above a certain level.
Whales lack a scrotum, and the testes are intra-abdominal. The elongated
penis, operated by a correspondingly massive ischio-cavernoszts muscle, is curled
when not in erection. There is a bicornuate uterus, but usually only a single
young one is produced at a time. The gestation period is more than a year in
some forms and the new-born calf may be one-third of the mother's size. In
the Blue Whale (Balcenoptera musculus (Fig. 554)), which grows to about 100
feet long, the gestation period is said to be about a year. The neonatus weighs
more than two tons and is some 23 feet long. It is reported to grow at a rate
of about 200 lb. per day while suckling and to reach puberty at about four
years of age. A single pair of teats occur in the inguinal area. The mammary
glands are equipped with a remarkable muscular apparatus that pumps milk
(containing up to 38 per cent fat, cf. p. 583) into the mouth of the calf. For
the composition of other kinds of milk, see p. 849.
 PHYLUM CHORDATA 791

COHORT FERUNGULATA
This large group contains all mammalian groups not previously dealt with.
It is divided into five super-orders. These are: r. the Fer<!! (including only the
Carnivora) ; 2. Protoungulata (four extinct orders and the Tubulidentata) ;
3· P<I!nungulata (four extinct orders and the Hyracoidea, Proboscidea, and
Sirenia); 4· Mesaxonia (order Perissodactyla), and 5. Paraxonia (order Artio-
dactyla). It is possible that the constitution of this major group will be
altered by future study, but convincing pal<!!ontological evidence is available
that the Carnivora and hooved animals of widely differing kinds (the even-toed
artiodactyles, e.g. deer, and the odd-toed perissodactyles, e.g. horses), arose,
together with certain others, from a common Pal<!!ocene ancestry.

SuPER-ORDER FERlE

The clear demarcation of the Carnivora from all other groups even before the
Pala!ocene, their proved antiquity and their peculiar diversity, cannot be
adequately expressed by mere ordinal rank and so they form the sole order of
the present super-order.
ORDER CARNIVORA

The modern carnivores are relatively difficult to characterise. The fusion

of the scaphoid, lunar, and centrale, however, is constant in modern carnivores.
(These elements are said not to be fused in Miacid<!!.) Other characteristic
features are the almost universal retention of the full number of incisors and
the well-developed canines. (Sloth-bears, Melursus, have lost the inner upper
pair of incisors.) The post-canine dentition always shows a certain amount of
reduction and a rather wide range of adaptive variation, as described below.
The pedal digits are usually five, and are never less than four. The toes are
armed with sharp claws which are often retractile. The brain is well-developed.
The present radiation of carnivores may be said to start mainly in the
Oligocene period, but the line can be traced backward through late Eocene
Canida! to the Miacida! of the Pala!ocene, until it becomes merged in the basal
creodont-insectivore stock from which so many orders arose.
The group has been arranged as follows :
Sub-order Creodonta
Families Arctocyonidre (Lower Pala!ocene-Eocene)
Mesonychidre (Pal<!!ocene-Eocene)
Oxyrenidre (Pal<!!ocene-Eocene)
Hyrenodontidre (Eocene-Pliocene)
Sub-order Fissipedia
Super-family Miacoidea
Family Miacidre (Pal<!!ocene-Eocene)
792 ZOOLOGY

Super-family Canoidea
Families Canidre (Eocene-Recent)
Ursidre (Miocene-Recent)
Procyonidre (Miocene-Recent)
Mustelidre (Oligocene-Recent)
Super-family Feloidea
Families Viverridre (Oligocene-Recent)
Hyrenidre (Miocene-Recent)
Felidre (Upper Eocene-Recent)
Sub-order Pinnipedia
Families Otariidre (Miocene-Recent)
Odobenidre (Miocene-Recent)
Phocidre (Miocene-Recent)

SuB-ORDER CREODONTA

Until their final extinction during the Pliocene, the sub-order Creodonta
were the dominant flesh-eating mammals. They radiated into four families.
Of these, the earliest, known from the Lower Palceocene, were the Arcto-
cyonidce. These were small animals primitive in general structure, with low
skulls, small brains, and with a complete dentition. Shearing carnassial
teeth were not yet developed, and the molars were usually of the tubercula-
sectorial pattern. The limbs and body were long and slender, with a long tail,
the feet pentadactyle and plantigrade. From this centralised and unspecialised
assembly arose (mostly during the Palceocene) the Mesonychidce, Oxycenidce
(all of which disappeared about the end of the Eocene), and the Hycenodontidce,
which lived until the Oligocene.
The Mesonychidce, like the Arctocyonoidea, developed no carnassials, but
the cusps of the cheek-teeth became broad and blunt, and in some of the later
forms developed a hypocone. The feet possessed flattened, fissured terminal
phalanges which appeared to have borne small hooves.
The Oxycenidce and Hycenodontidce are characterised by having 'false'
carnassials, in the sense that they are never the corresponding pair of teeth
(fourth upper premolar and first lower molar) as those in the Miacoidea and
modern carnivores. In the Oxycenidce the carnassials are the first upper and
second lower molars, in the Hycenodontidce the second upper and third lower
molars (Fig. 565). The two families in their day were the most successful of
the earlier radiation, and gave rise to some large forms, though Andrew-
sarchus, a mesonychid from the Lower Oligocene of Mongolia, reached the
largest size of all, with a skull three feet in length.
 PHYLUM CHORDATA 793
SuB-ORDER FrssrPEDIA
The first true fissipedes arose in the Palreocene, possibly from the Arcto-
cyonidre. The group expanded during the Eocene, radiating into the modern
families during the Oligocene. As in
other fissipedes, and in contrast with
creodonts, the Miacidre possessed true
carnassials which were developed
from the fourth upper premolar and
the first lower molar. The terminal
phalanges were not fissured. Miacids
differed from other Fissipedia, how-
ever, in that the scaphoid, lunar, and
centrale remained unfused.
The modern Fissipedia are classi-
fied on a number of anatomical
characters such as the structure of
the auditory bulla and the presence
or absence of an alisphenoid canal.
For practical quick identification of
specimens, however, the most useful
guide is the pattern of the molar teeth
and the presence or absence of the
flesh-cutting carnassial teeth (the last
upper premolar and the first lower
molar in all modern forms). In the
classification adopted here we sepa-
rate them into two superfamilies (1)
the Feloidea ( = Aeluroidea) com-
posed of the Viverridre, Hyrenidre,
and Felidre and (2) the Canoidea
(Arctoidea) including the Canidre,

-
Procyonidre, Mustelidre, and Ursidre.
Of the above seven families the
Viverridre, Canidre, and Mustelidre
are perhaps basic groups, possibly FIG. 565.-Carnivora: Comparison of creo-
stemming independently from dont and ftssipede dentition. Upper and lower
teeth of Felis (cat) (top figure); Hyamodon
Miacidre. Within the Canoidea, the (middle figure) and Oxyama (lower figure) show-
ing the different arrangement of the carnassial
history of the Family Canidre (dogs, teeth. The carnassials are shaded.
wolves, and foxes) is illustrated by a
long series of fossils reaching back to the Tertiary. They are almost universally
distributed although the Dingo (Canis dingo) of Australia was introduced
there by colonising aborigines. Canine dentition is usually complete except
 2 I 6 'l
\.0
~

N
7 0
4 3 5 0
t:-'
0

"><

FIG. 566.-Camivora: Dentition in modem forms. Upper jaw of 1. Canis (Dog); 2. Viverra (Civet); 3 Hyama; 4· Pant hera (leopard);
5· Tax idea (American Badger); 6. Procyon (Racoon); 7· Ursus arctos (Brown Bear).
 PHYLUM CHORDATA 795
for the absence of the last upper molar. The carnassial teeth are well-developed
and the upper molars are bluntly triangular in pattern (Fig. 566). There are
only four functional toes on the hind-feet. The claws are non-retractile.
The Viverridre, surviving to-day as the civets, genets, mongooses (of various
kinds), the Madagascar Fossa, and others, probably represents the basal
feloidean stock. Essentially Asiatic and African in distribution, many of
these small active animals are extremely cat-like in appearance and habits.
The otter-civets (Cynogale) of the East Indies are adapted for fishing. The
dentition and certain other features of extant forms show considerable
similarity with those of ancestral miacids.
The Hyrenidre of Africa and Asia, an early offshoot from civet stock, feed
mostly on carrion; the molar dentition is reduced to one in each jaw. The

FIG. 567.- 0rder Camivora, Sub-order Fissipedi. Super-family Feloidea, Family Felidm.
Acinonyx. The Cheetah (A. jubata) is the fastest mammal. Although its speed is often exag-
gerated it can, in fact, reach 6o m.p.h. over very short distances. When travelling at speed over
the veld the cheetah seems to remain- almost arrow-like-nearer the ground than shown in the
above illustration which, however, is drawn in correct relationship from a photograph.

carnassials are well developed. The upper molar is reduced to a functionless

vestige, transversely placed. The teeth as compared with those of the Felidre
are blunter and coarser, an adaptation to bone-crushing. There are never
more than four pedal toes and the claws are non-retractile. The tympanic bulla
has internally a _rudimentary septum which partially divides it. The aberrant
Proteles, the Aard-wolf or Earth-wolf of Africa is insectivorous and has teeth
reduced to simple cones. Like the hyrenas it is obviously a highly specialised
viverrid offshoot and, following recent authors, it is here classified with the
Hyrenidre.
The Felidre are essentially carnivorous nocturnal hunters with large eyes,
short high skulls, and retractable claws. They usually lie in wait for, or creep
upon, their prey, finally securing it with a mighty spring. Unlike dogs, they
are generally individualists in hunting and do not run a great deal. A striking
exception, however, is the Cheetah (Acinonyx ) (Fig. 567), which stalks and then
 ZOOLOGY
pursues its prey and is the fastest living mammal over a very short distance.
In 1937 cheetahs were tested against greyhounds in Britain, and the fastest
were reported to cover a 355-yard lap at 55 m.p.h. The Cheetah moves with a
series of bounds and catches the speediest gazelles. It occurs in Africa and
Southern Asia and has been used as a so-called hunting-' leopard' or hunting-
' dog' for centuries in India and Persia. Its claws are only partly retractile.
The teeth of the Felid::e are highly specialised
for seizing, holding, tearing, and shearing, but not
for grinding, meat. There are very well-developed
carnassials. The Felid::e possess extremely small
molars. The premolars, too, are reduced both in
number and size to three above and two below.
In some extinct Felid::e such as the sabre-toothed
Smilodon remarkable dentitional, and associated
cranial and muscular adaptations arose. The
upper canines became immensely long, flattened,
and blade-like as stabbing and slashing weapons
(Fig. 568). The lower canines were markedly
reduced. The cheek teeth were restricted in
number, and lengthened antero-posteriorly into
a carnassial scissors-like slicing apparatus. The
mastoid process (to which the larger neck muscles
were attached) became massive, suggesting that
the upper canines were speared downwards by a
powerful neck-thrust. Such a movement is quite
different from the action employed by modern
cats, which kill with both upper and lower
canines as the jaws are closed (Fig. 568). The
FIG. 568.-Felidm: Dental relatively reduced coronoid process of sabre-
specialisaUQns, Above: Sabre-tooth
(Smilodon) (Sub-family Machaero- tooths allowed a wide gap, and a shift of the
dontin.:e) qf the Pleistocene (skull-
length abq1~t 12 inches); Below: shearing carnassials (better developed than in
True cat (Metailurus) (Felin.:e) of modern cats) backwards towards the jaw articu-
the Pliocene. (From Romer, after
Zdansky.) lation where they would exert greater power.
Smilodon was extinguished only about 25,000
years ago. There was, incidentally, an approximately parallel evolution among
the marsupials (e.g. the South American Pliocene Thylacosmilus). In the sabre-
tooths the claws were completely retractile. The tympanic bulla was smooth,
inflated externally, and internally divided by a complete septum.
The Mustelid::e (stoats, otters, skunks, martens, wolverines, etc.) form a
large group in which the post-carnassial upper molar, instead of being tri-
angular, is expanded on the inner side (Fig. 566 5 ), sometimes, as in the
European Badger (Metes metes), very much so. The dental formula varies
 PHYLUM CHORDATA 797
somewhat, but is not much reduced, except that there is never more than a
single post-carnassial molar in either jaw. The claws are not retractile and the
tympanic bulla is not inflated and has no internal septum. This group was
well-developed in the Oligocene and one (Megalictis) as big as a bear occurred
in the early Miocene.
The Procyonidre (raccoons, Kinkajou, pandas, etc.) are American in distri-
bution with the exception of the Panda (Ailurus) and the so-called Giant Panda
(Ailuropoda), both of which are Oriental. The dentition, except for the loss of
an anterior premolar and the last molar, shows little reduction. Carnassials
are not developed as shearing-teeth and the upper molars are round in outline
instead of triangular (Fig. 566 6 ). The claws are non-retractile. The Kinkajou
(Potos) is a fruit-eating animal in which the crown surface of the molar teeth
is degraded to a flat surface only obscurely tuberculated. It has a prehensile
tail-a feature almost unique among carnivores. (The arboreal carnivorous
Binturong (Arctictis binturong) of Indo-Malaya also has a prehensile tail.)
The Ursidre (bears) appear to have arisen in the Pliocene. They_. were
probably the last of the Canoidea to appear. They have a fully plantigrade
foot and non-retractile claws. Like the Procyonidre, shearing carnassial teeth
are not formed. The molar teeth are large, elongated, and much tuberculated
(Fig. 566 7 ) ; the dentition, except for the last molar, is unreduced. The
tympanic bulla is flat. With the plantigrade gait of bears there have arisen
pedal and other specialisations that are to some degree convergent with those
of Man. Thus, the ursine femur is long and superficially hominoid and the
foot structure of bear and Man is somewhat alike. Should a person be so
disposed he can sometimes compel himself to confuse the footprints of Man
and bears. Moreover, bears sometimes produce grotesquely over-sized, com-
posite footprints by superimposing the hind-foot partly over the print made
by the adjacent fore-leg. Tracks of the Red Bear (Ursus arctos isabellinus) are
almost certainJy those that keep alive the Yeti ('Abominable Snow Man') legend
of Tibetan shepherd and mountain romantic.
Bears of various species are commonly said to hibernate: certainly some
retreat in winter into prepared dens but whether there is true hibernation
(p. 883), involving a prolonged period of greatly lowered body temperature and
general metabolism, is still uncertain.

SUB-ORDER PINNIPEDIA
Seals (Fig. 569) are in general piscivorous, but many species subsist largely
on crabs, others on squids and some on euphausian plankton. As compared
with the teeth, jaws, and associated muscles of fissipedes (see above) those of
the Pinnipedia are simplified for grasping and tearing rather than chewing.
There are no carnassial teeth. There are never more than two pairs of lower
incisors in the adult; and never more than five pairs of lower post-canines (four
VOL. II. D3
 ZOOLOGY

premolars, one molar). The teeth are basica11y conical, with a single crown,
although specialisations have appeared, ranging from an
anterior and posterior additional cusp to an elaborate
series of notches used for straining euphausians and
other crustaceans by the Crab-eater seal. In the Walrus
(of which there are two sub-species) the teeth are
extremely aberrant: the upper canines may reach a
length of 39! inches along the curve. The primary
function of the tusks in both sexes is to tear clams from
the ocean floor. The facial part of the skull is abbre-
viated, the cranial elongated.
There are three families. The Ota-
riidre (sea-lions and fur seals) and Odo-
benidre (walruses) are gressigrade or
'walking seals'. The members of both
these families can tum the hind-feet for-
ward for progression on land ; the fore-
limbs are the primary swimming organs.
The third family, the Phocidre is com-
posed of reptigrade or ' wriggling seals ' .
These have the hind-limbs (as far as the
ankles) bound up with the short tail with-
in the body covering. Such limbs are
almost useless for terrestrial locomotion.
In this group the hind-limbs are the
primary swimming organs. The Ota-
riidre derive their name from the short,
cartilaginous pinnre which are absent in
the other two families.
As yet no fossils linking land carni-
vores with pinnipedes have been dis-
covered. Nor can it be said with assur-
ance which of the three families is the
oldest. The Pliocene Semantor, a long-
tailed animal which was probably cap-
able of walking, has been often thought
to belong to a primitive pinnipede stock.
Some modem authorities consider it to
be more likely an otter (p. 796).
Skeleton of the Carnivora.- The atlas
is very large, with wing-like lateral processes. The neural spine of the axis is
elongated and compressed; the odontoid process conical. The other cervical
 PHYLUM CHORDATA 799

vertebrre have small spines and large transverse processes. There are twenty or
twenty-one thoraco-lumbar vertebrre. The most anterior thoracics have long,
slender, backwardly-sloping spines. In the posterior thoracics large meta-

FrG. 570.-Canis: Skull. Sph 1

1Jt B.
Dog. Lateral view. C. occ. I
occipital condyle; F. fron-
tal; F . inf. infraorbital
foramen; ]g. jugal; ]m.
premaxilla ; L. lachry mal;
N!. maxilla; Maud. external
auditory meatus; Md.
mandible; N. nasal; P.
parietal ; Pal. palatine;
Pjt. zygomatic process of
squamosal; Pt. pterygoid ;
Sph. ali-sphenoid; Sq.
squamosal; Sq. occ. supra-
occipital; T . tympanic.
(After Wiedersheim.)

pophyses and anapophyses are developed. The transverse processes of the

lumbar vertebrre are extremely long and the spines short. The sternum is
iong and narrow, composed usually of eight or nine pieces. The sternal ribs
are almost uncalcified.
In the skull of the Carnivora vera
(Fig. 570) there are prominent sagit-
tal and lamboidal crests. The tem-
poral fossre are very deep; the orbits
are not separated from them by
bone. The relative development of
the facial region varies in the different
groups: in the bears and their allies,
and in the dogs, it is elongated,
whilst in the cats it is very short.
The zygoma is strong and greatly
arched outwards. The glenoid fossa
is a transverse groove, to the shape
FIG. 571.- Felis: Auditory bulla. Tiger. Left, of which the transversely elongated
in section. * aperture of communica tion between
the two chambers into which the cavity of the condyle is adapted. In the cats
bulla is divided; a. m . external auditory meatus;
B. 0. basi-occipital; e. Eustachian tube ; i. c. the
there is a large rounded tympanic
inner chamber; o. c. the outer chamber; Pt. bulla (Fig. 571), the cavity of which
periotic; s. septum between the two chambers;
Sq. squamosal. (After Flower.) is divided into anterior and posterior
parts by a septum. The anterior
contains the auditory ossicles and the opening of the Eustachian tube. The
bony auditory meatus is short. The paroccipital is closely applied to the
posterior surface of the tympanic bulla. In the dogs the septum of the bulla
8oo ZOOLOGY

is incomplete, the auditory meatus short, and the paroccipital process not
applied to the bulla. In the bears and their allies the bulla (Fig. 572) is
usually less dilated, and the septum is absent or represented by a ridge only,
while the bony auditory meatus is elongated.

FIG. 572.- UTsus: Auditory

bulla. Left, in section. a. m.
external auditory meatus; B . 0.
basi-occipital; Car. carotid canal;
e. Eustachian canal; g. glenoid
canal ('foramen glenoideum which
leads to a venous canal which runs
upwards and backwards into the
lateral sinus'; Sq. squamosal; T.
tympanic; t. tympanic ring. (After
Flower.)

The cranium in the Pinnipedia (Fig. 569) is broad, rounded, and rather
compressed from above downwards. The orbits are large and relatively closely
opposed.
In the Carnivora vera the spine of the scapula is situated at about the middle
of the outer surface of the bone. The acromion is usually well developed, some-
times with a metacromion. The coracoid process is very small. The clavicle is
never complete, sometimes entirely absent.
There is a supra-condyloid foramen in the cats
r~ and some of the other groups, not in the dogs or
bears.
The scaphoid and lunar are united (Fig. 573).
There is no separate centrale. A radial sesamoid
.J1T :m: 11
is usually present. There are five digits, though
FIG. 573.-UTsus: Carpus. c. the pollex may be reduced in size, as in the dog,
cuneiform; m. magnum; p. pisi- and it is vestigial in hyrenas.
form; r. s. radial sesa moid ; s. l.
scapholunar ; td. trapezoid; tm. The pelvis is long and narrow. In the tarsus
trapezium; tt. unciform. (After all the customary elements are developed.
Flower.) The
hallux is fully formed in the bears, etc., but
shorter than the other digits. In the cats and dogs it is represented only by
a vestige of the metatarsal.
In the Pinnipedia (Fig. 569) both acromion and coracoid are short, and
the scapula is curved backwards. There is no clavicle. The bones of the fore-
limb are short and stout. The humerus has a prominent deltoid crest and has
no foramen above its inner condyle. The ulna is greatly expanded at its
proximal, and the radius at its distal, end. The manus is broad and expanded.
 PHYLUM CHORDATA 8or

The scaphoid and lunar are united to form a scapho-lunar. The ungual
phalanges are nearly straight, slender, and pointed. The ilia are short. The
symphysis pubis is short and without firm union of the bones, an advantage
in delivering the large precocious young. The femur is short, thick, and
flattened. The fibula and tibia are commonly ankylosed proximally. The
calcaneum is short and usually without a distinct calcaneal process. The
lateral digits are usually the longest.

SUPER-ORDER PROTOUNGULATA

The term' Ungulate' has been used to cover a multitude of hooved grazing
animals, many of which are but remotely related. Further, modern evidence
from fossils appears to indicate a much closer relationship between the Carni-
vora (p. 791) and the hooved 'ungulates' than was previously surmised.
Hooved animals fall into a variety of widely divergent groups and it is probable
that the even-toed cow is as unrelated to the odd-toed horse as it is to the cat.
Hooves, like so many characters once considered to be fundamental, have
arisen independently in several groups in response to special environmental
conditions. In addition, grazing animals of apparently independent stems
have evolved highly specialised grinding teeth of various kinds (as well as
numerous muscular, articular, and alimentary adaptations), long necks and
legs, and various social tendencies related to self-preservation.
Although Cretaceous strata have so far yielded no hooved grazing mammals,
several unquestionably distinct groups had appeared by the early Tertiary.
Of these, the Condylarthra were the most successful in the Palreocene. Before
their extinction in the Eocene these probably gave rise to several other notable
groups.
The super-order Protoungulata has been arranged as follows:
Super-order Protoungulata
Order Condylarthra
Families Hyopsodontidre (Palreocene-Eocene)
Phenacodontidre Palreocene-Eocene)
Didolodontidre (Palreocene-Eocene)
Periptychidre (Palreocene)
Meniscotheriidre (Palreocene-Eocene)
Order Tubulidentata
Orycteropodidre (Pliocene-Recent)
Order Notoungulata
Sub-orders Notioprogonia (Palreocene-Eocene)
Toxodontia (Palreocene-Pleistocene)
Typotheria (Eocene-Pleistocene)
Hegetotheria (Eocene-Pleistocene)
8oz ZOOLOGY
Order Litopterna
Families Macraucheniidre (Upper Palreocene-Pleistocene)
Proterotheriidre (Upper Palreocene-Lower Pliocene)
Order Astrapotheria
Sub-orders Trigonostylopoidea (Eocene)
Astrapotherioidea (Eocene-Miocene)

ORDER CONDYLARTHRA
This order, an important basal group approaching the early Creodonta in
some respects, is characterised by the foreshadowing of hooves-i.e. they are
on the 'ungulate' line of evolution rather than on the 'unguiculate'. There is
also a tendency for the third toe to become progressively larger than the others,

FIG. 574.- 0rder Con·

dylarthra, Family Byop-
sodontidse. Hyopsodus :
Skull. T his N orth Ameri-
ca n Eocene condylarth
was about the size of a
hedgehog. It had clawed
phala nges and may have
been semi-arboreal. f r.
frontal; i .o.f. infra-
orbital foramen ; j u . jugal ; la.
lachrymal ; la.f. lachrymal fora.
men ; max. maxilla; m .f . mental
fora men ; na. nasal ; oc. occipital ;
pn. parietal ; pa.p. pa roccipital
process; pg.p. postglenoid p ro.
cess; pmx. premaxilla ; sq. squa ·
I
p.yp. pap.
mosal. (After Ma tthews.)

thus producing a mesaxonic (p. 824), as compared with a paraxonic (p. 836), foot.
These two characters suggest that the Perissodactyla arose from some early
members of the group, although no direct ancestor has so far been discovered.
The tooth pattern of such a form as the condylarth Ectocion, itself an Eocene
form too late to be ancestral, could easily be modified into that of Hyracotherium,
the earliest known horse (p. 826).
The chief characters of the early condylarths are mostly negative ones, as is
to be expected in any very primitive group. The omnivorous, non-predaceous
character of the teeth is unlike what was evolving in the allied creodonts, and
there is none of the specialised features, such as the shortening of the jaws and
reduction of the incisors found in the Primates. There is a general resemblance
to the insectivores, but none of the specialisations of that order is observable.
The astragalus, a diagnostically important bone (p. 859, Fig. 623), is of the
creodont-carnivore type and the carpus is normally interlocking with the centrale
retained.
In general, the Condylarthra are to be considered as primitive, and not far
 PHYLUM CHORDATA

removed from the generalised ancestral stock of the Cretaceous period. They
are probably ancestral to a number of later groups.
There are five families: the Hyopsodontidre, Phenacodontidre, Periptychidre,
Meniscotheriidre, and Didolodontidre. This last family contains only South
American animals that appear to be a connecting link with the exclusively
South American Litopterna (p. 8os).
The Hyopsodontidre (e.g. Hyopsodus, Fig. 574) are the more primitive forms,
and have a general resemblance to insectivores and primates. Hyopsodus,
with its sexitubercular molars and hedgehog-like skull, was formerly placed
in one or other of the above orders until the discovery of its typically condylarth
astragalus decided its truer position. The Mioclreninre (e.g. Mioclcenus) have
tritubercular teeth, and both retain, among other primitive features, claws
rather than hooves and an interlocking carpus.

FIG. 575.- 0rder Condylarthra, Family Phenacodontidm. Phenacodus. A North American

(Upper Pal:eocene and Lower Eocene) and European (Lower Eocene) protoungulate which attained
a length of about five feet. (Am. Mus. Nat. Hist.)

The Phenacodontidre comprise several genera, among which are T etraclce-

nodon, which, in having blunt claws rather than hooves, somewhat bridges the
gap between the condylarths and creodonts, and Phenacodus (Fig. 575), its
direct descendant, which has become specialised in having a serially arranged
carpus and by the loss of the clavicle. Ectocion, an Eocene genus, has, as
already mentioned, a dentition approaching that of the early horses. In this
family the feet are, as in the others, pentadactyle, but with a progressive
tendency to tridactylism, to digitigradism, and to a broadening of the ungues
into small fiat hooves.
The Periptychidre (e.g. P eriptychus, Conacodon, Ectoconus) also have narrow
hooves, and are a specialised side-line with enlarged premolars and multicuspid
molars, which prevent them from appearing as ancestral to any later forms.
Their earlier members are closely akin to the Hyopsodontidre.
The Meniscotheriidre (Meniscotherium, Pleurasp~dotherium) retain a very
primitive foot structure, but have become specialised in their very selenodont
 ZOOLOGY
molars. On this ground they have been considered as ancestral to the Hyra-
coidea, though proof is still lacking.
The Didolodontidre, with primitive bunodont molars and simple premolars,
are on the line leading to the early Litopterna, such as Diadiaphorus.

ORDER TUBULIDENTATA

The relationship of this order is problematical. At one time it was grouped

with the Edentata (Xenarthra and Pholidota), but now it appears possible that

Frc. 5i6.--0rder Tubulidentata, Family Orycteropodidre. 01·yderoprcs. Only one species of

Aardvark or' Earth-pig' (0. afer), o f perhaps three geographical races, survives over a wide area
of African savannah. About the size of a small pig, the nocturna l Aardvark lives a lmost exclu-
sively on termites. Mounds are breached with powerful hoof-like claws and the insects a rc
gathered on its sticky tongue. The sub-maxillary glands are relatively enormous and the parotids
highly developed. Orycteropus has remarkable auditory powers and its olfactory turbinals are
perhaps more extensive than in any other mammal. Sensory vibriss<e, too, are highly developed.
The animal lives in a system of subterranean galleries and, if surprised in the open, digs itself to
safety in a few minutes. (From photographs a nd B.M. (N.H .) specimens.)

it is a very early independent offshoot of protoungulate ancestry. The order con-

tains a single modern genus (Orycteropus, the Aardvark of Africa, Fig. 576) and a
problematical Lower Eocene form, Tubulodon, from North America. Orycteropus
is also known from the Pliocene of Europe and Asia and fragments of supposed
orycteropods have been described from the Oligocene and Miocene of Europe.
 PHYLUM CHORDATA 8os

The body of the Aardvark is thickset, with the head produced into a long
muzzle. The fore-limbs are short and stout, with four digits, the palmar
surfaces of which are placed on the ground in walking. The hind-limb has
five digits and the body is covered with thick skin bearing scattered hairs.
Orycteropus lives on termites and has the most strikingly developed turbinals
of all the Mammalia. The tongue is long and the mouth small. Four to five
peg-like teeth of peculiar structure are present in each jaw. They lack enamel,
hut possess a coating of cement and a body of vasodentine which is perforated
by large numbers of fine tubules, whence the order takes its name. The teeth
of Tubulodon also possess them. The structure of the rather primitive hrain
is stated to show some resemblance to that of Anoplotherium, an Eocene artio-
dactyle. The skeleton of the Aardvark is much like that of certain early
condylarths.
SOUTH AMERICAN 'UNGULATA '
In South America there was an evolution of 'ungulates ' of great diversity
of form and of a range in time from the Paheocene to as late a period as the
Pleistocene. They are grouped provisionally in four orders which have no
connexion with one another beyond some remote condylarth ancestor.
Although there are, on occasion, superficial resemblances to animals in other
parts of the world, as, for instance, the curious convergence in foot-structure
of certain Litopterna and the horses, or the proboscidean characters of the
Pyrotheria, these four orders in reality have no special relationships with other
ungulate groups. Each has pursued its own specialised line of evolution.
ORDE R LJTOPTE RNA
This is a small order, consisting of two families, the Macraucheniid<e and
Proterotheriid<e. The genus M acrauchenia had a three-toed foot with well-
developed lateral toes, the neck long
and camel-like, and the skull peculiar
in the position of the nasal openings
far back on the upper surface of the
skull, suggesting the presence of a
proboscis. In this order a series of
genera show an evolution which
closely parallels that of the horses.
N otodiaphorus, an Oligocene form,
had three toes with the lateral ones
stout and still resting on the ground FIG. sn.- Order Litopterna, F amily Protero-
theriidm. Diadiaphorus. South American
- about the stage, that is, of M eso- Miocene- Pliocene. (After Scott.)
hippus among the horses (p. 8z6).
Diadiaphorus, in the Miocene, had the lateral toes much reduced, and can
be compared with M erychippus, while Thoatherium, also Miocene, was
8o6 ZOOLOGY
monodactyle to a greater degree even than the modern horse, in that the splint-
bones are reduced to mere nodules (see Fig. 6r5, p. 858).
The dentition of these animals has the perissodactyle-like feature of
molarisation of the premolars, but unlike the horses, the teeth are usually
low crowned. Although there is a superficial resemblance in the molar pattern
with the earlier horses, there are
differences in detail. The tooth- and
foot-structure combined are never in
phase with that of any genus of horse,
so that there is no real danger of
confusion.

ORDER NOTOUNGULATA
Four sub-orders are recognised :
The first, the Notioprogonia, con-
tains the families Arctostylopidre
FIG. 578.- Sub-order Toxodontia, Family
Toxodontidre. To;r.odon. A South American (occurring m Asia and North
Pleistocene notoungulate about nine feet long. America, the only non-South
(After Scott.)
American notoungulates), Henricos-
borniidre, and the Notostylopidre. This sub-order includes small and primitive
forms confined to the Palreocene and Eocene. Some of them closely resemble
the Condylarthra, and probably reveal the origin of the Notoungulates from
that group. The second sub-order Toxodontia (Homalodotheriidre, Toxo-
dontidre, etc.) reached considerable body size. The genus Toxodon (Fig. 578)

FIG. 579.-0rder Notoungu-

lata, Sub-order Toxodontia,
Family Homalodotheriidre.
HomalodotheTium. A South
American Miocene notoungulate
about six feet long. (After
Riggs.)

of the Pleistocene was as large as a rhinoceros. The typical toxodonts had

large cropping incisors and strongly curved hypsodont molar teeth.
The Family Homalodotheriidre (considered to form a separate sub-order
Entelonychia by some authors) is distinctive. The end forms of this line (e.g.
Homalodotherium, Fig. 579) developed clawed feet resembling those of the
 PHYLUM CHORDATA

chalicotheres of the Northern hemisphere. They became extinct towards the

end of the Miocene or beginning of the Pliocene. The third sub-order, Typo-
theria (Mesotheriidce, Interatheriidce), also developed enlarged incisors and
hypsodont molars. The final sub-order Hegetotheria (like the third) contained
rodent-like forms of which some probably resembled rabbits in appearance
and habits.
ORDER ASTRAPOTHERIA
This order includes two families only which, in spite of a general resemblance,
are so different in basic structure that they are referred to two sub-orders.
These are : (1) the Trigonostylopoidea (with the family Trigonostylopidce)

FIG. sSo.-Order Astrapotheria, Sub. order Astrapotheroidea, Family Astrapotheriidre. Astra-

potherium.. A South American Oligocene and Miocene animal about nine feet long. (From
Scott, after Riggs.)

and (2) the Astrapotheroidea (single family Astrapotheriidce). Of the latter

the typical genus, Astrapotherium (Fig. 580), of Oligocene to Miocene age, was
a large animal with a disproportionately big head, provided with great canine
tusks and with retracted nasals suggesting the presence of a proboscis. The
premolars were reduced and the molars greatly enlarged but not hypsodont.
The whole dentition is suggestive of the amynodont rhinoceroses (p. 835).

SUPER-ORDER PJENUNGULATA (SUBUNGULATA)

This widely diversified 'sub-' or 'near'-ungulate group probably had its
origins in late Cretaceous or early Palceocene 'ungulate' forms, but diverged
in several stems from the basic stock before the condylarth radiation which
allowed the development of the Protoungulates and the Perissodactyla. Thus
it forms a loose, though nevertheless related, assembly containing the peculiar
little Hyrax, the elephants, the dugongs, as well as several extraordinary
extinct orders.
8o8 ZOOLOGY
The super-order is composed as follows:
Orders Hyracoidea (Oligocene-Recent)
Embrithopoda (Oligocene)
Order Proboscidea
Sub-order Mreritherioidea
Family Mreritheriidre (Eocene-Oligocene)
Sub-order Elephantoidea
Families Gomphotheriidre (Oligocene-Pleistocene)
Mammutidre (Miocene-Pleistocene)
Elephantidre (Pliocene-Recent)
Sub-order Deinotherioidea
Family Deinotheriidre (Miocene-Pleistocene)
Sub-order Barytherioidea
Family Barytheriidre (Upper Eocene)
Orders Pantodonta (Palceocene-Oligocene)
Dinocerata (Palceocene-Eocene)
Pyrotheria (?Palceocene, Eocene-Oligocene)
Sirenia
Sub-orders Trichechiformes (Eocene-Recent)
Desmostyliformes (Oligocene-Miocene)

ORDER HYRACOIDEA
The Hyracoidea show a most curious mixture of primitive and specialised
characters, and others, too, wherein they seem to resemble several different
orders of mammals. The living forms of coneys are all small, hooved, rather
rabbit-like animals of Africa and South-west Asia, and are included in a single
family, the Procaviidce ( = Hyracidce) with the genera Procavia ( = Hyrax) and
Dendrohyrax, the latter being semi-arboreal. Hyrax (Fig. 581) should not be
confused with the pikas or conies (Lagomorpha) of North America and Central
Asia (p. 779).
Manus and pes are plantigrade : the former bears four digits and a vestige
of a pollex ; the latter bears three digits, of which the third is the longest. All
digits terminate in small flat hooves, save the second pedal digit which is
finished with a curved claw. The centrale persists in the carpus, and the
astragalus differs greatly in shape from that of any other mammal in that
the malleolus of the tibia has a large flat bearing on it. A clavicle is absent.
The skull shows several peculiar features. The postorbital process arises
largely from the parietal, and the postorbital bar is complete in Dendrohyrax
but not in Procavia (Fig. 582). The jugal is a very stout bone, and extends
backwards to take as large a share in the glenoid cavity as in any marsupial.
The periotic and tympanic are ankylosed together, but not to the squamosal.
 PHYLUM CHORDATA 8og

The tympanic forms a bulla and has a spout-like external auditory meatus.
The seventh cervical vertebra is sometimes pierced by the vertebral artery.
The scapula is triangular, as in Artiodactyla, and lacks an acromion. Dorso-
lumbar vertebrre are as many as hventy-two. The radius and ulna are
partially crossed, as in the Proboscidea.

FIG. sSr.-Order Hyracoidea, Family Procaviidre. P'rocavia. The h yraxes. 'dassies. or

coneys (the 'feeble folk· of the Bible), are robust, tail-less, colonial, and immensely agile animals
about the size of a h a re. Procavia is diurnal and confined to Africa and a few areas of South-
west Asia. It lives in rocky outcrops, often in arid or semi-arid country. A second genus of
rock hyraxes is Heterohyrax. The tree-hyraxes (Dendrohyrax) are adapted to an essentially
arboreal a nd nocturnal life a nd spend much of the day in tree-hollows. Rock a nd tree hyraxes
thus live almost side by side in certain equatorial areas (e.g. Mt. Kenya) without competition.
The manus of Procavia bears four flattened claws, the innermost being stouter and adapted for
gripping laterally on difficult surfaces. The head is protected by prominent tactile vibriss;e and
the body, too, bears elongated bristles on dorsal and lateral surfaces. (From B. M. (N.H.)
specimens.)

The dentition includes a single pair of large curved upper incisors growing
from persistent roots like those of rodents, but triangular in section. Canines
are absent. There are four premolars and three molars which are lophodont
and in pattern look like rhinoceros teeth in miniature. The lower jaw carries
two pairs of incisors, which are spatulate and comb-like, as in some lemurs.
It has a foramen which pierces the coronoid just behind the last molar, as in
the South American typotheres (p. 8o7).
810 ZOOLOGY
There is no gall-bladder. The crecum has a pair of crecal pouches, a feature
unknown in any other mammal. The testes are abdominal. There is a
dorsal gland on the back not unlike that found in the peccaries. This curious
mixture of characters renders it difficult to suggest the affinities or origin of
this order. A remote affinity with the Proboscidea and Sirenia has been
variously suggested.
The order is known as far back as the Oligocene of Egypt, where Sagha-
therium and Megalohyrax have been found. Both are hyracoid in dentition,
but neither appears to be on the direct line of evolution. The latter form was
of considerably larger size than the modern forms. Myohyrax comes from

II

11

13

FIG. 582.-P,.ocavia: Skull and dentition. Male hyrax, in lateral view. I, premaxilla ; 2.
nasal; 3· frontal; 4· maxilla; 5· lachrymal; 6. palatine; 7· parietal; 8. jugal; g. interparietal;
IO. squamosal; 11. supraoccipital; n. mastoid; I3 . paroccipital. (From B.M. (N.H.) specimen.)

the Lower Miocene of Egypt and Pliohyrax from the Lower Pliocene of Greece.
The order appears to have originated, like the Proboscidea, in North or Central
Africa. Rich deposits of hyracoid material from the Miocene of East Africa
have thrown a great deal of light on the evolution of the group. There is great
similarity between it and perissodactyle evolution. This had led Whitworth
to include the order among the Mesaxonia, but these hyracoids do not resemble
early Mesaxonia any more than do some of the South American ungulates,
which certainly are not Mesaxonia. Possibly it is best for the present to
follow Simpson and leave the Hyracoidea among the Prenungulata.

ORDER EMBRITHOPODA
The order Embrithopoda is represented by Arsinoitherittm (Fig. 583), a
heavy animal the size of a rhinoceros from the Lower Oligocene of Egypt. It
 PHYLUM CHORDATA 8rr

has massive limbs and stumpy, five-toed feet. The skull is remarkable for the
presence of a pair of large, forwardly directed nasal horns placed side by side
and united at the bases, and of a pair of quite small frontal horns. The
dentition is complete and without diastemata. The canines are incisiform and

F I G. 583.- 0 rder Embrithopoda, F a mily Arsinoitheriidre. ATsirwithet·iun~. This rem a rkable

genus, solita r y in its order, is sometimes grouped with h y raxes (F ig. 581), p r oboscideans (Figs. 584,
587), a nd sirenians (Fig. 594) as 'subungulate '. A. zitteli (above) lived in t he Egyptian L ower
Oligocene and was about I I feet long a nd la rger than a modern rhinoceros. Rela tionship with
the hyraxes is suggested by its m olar pattern, but nothing is k nown of its ancestry a n d it left no
descendant s. T h e sha r p h o rns, fused at the base, were supported b y bony extensions from t he
nasals. T he cartilaginous nasal septum , too, was ossified in their support in adults. The smaller
p osterior p rojections grew from the front a ls. T h e dentition was complete (44 teeth) a n d points
to a vegetaria n d iet. The a nimal p robably possessed mobile lips adapted t o c ropping. (Redrawn
a fter Andrews, August a .)

the cheek-t eeth hypsodont. The molars are bilophodont. Beyond a faint
suggestion in the molar pattern of some hyracoid affinity, nothing can be said
as t o the relations of this peculiar animal.

O imER PROBOSCIDEA
This order includes the elephants (Fig. 584), mastodons, and their allies
which are the largest recent land-animals. E xcept for an aberrant and extinct
812 ZOOLOGY
group of rhinoceroses (p. 835) they are probably the biggest land mammals ever
to have existed. Next to Man, they possess the greatest individual life-span
of all mammals: they may live to an age of 70 compared with the z to 5 years
of life of some of the smaller rodents. Apart from their remote connexion
with the Sirenia and possibly Hyracoidea, the Proboscidea occupy an isolated
position. At an early period of their history they acquired a number of

FrG. 584.- 0rder Proboscidea, Sub.order Elephantoidea, Family Elephantidm. Elephas. The
Hairy Mammoth (E. primigenius), not to be confused with the Mammutid<e (=Mastodons) of the
Miocene-Pleistocene, existed from the Pleistocene until perhaps Ij,ooo years ago (earliest post-
glacial). It was a common animal of the E uropean and Asiatic tundra, and frozen cadavers in a
still edible condition have been uncovered by glacial retreat in Siberia during the present century.
It is not generally known that an appreciable proportion of marketed ivory has come from the
relatively indestructible tusks of E. primigenius. (Redrawn after various authors.)

peculiar and adaptive characters which at once clearly distinguish them from
other mammals, although they have retained to the present day several
persistently primitive features, such as the pentadactyle hand and foot, a
brain in which the cerebellum is not covered by the cerebral lobes (which are,
however, well convoluted), permanently abdominal testes, and paired pre-caval
veins.
The specialised features of the skeleton are perhaps mostly due to the
original trend in trunk development. The progressive and relatively enormous
 PHYLUM CHORDATA 813
hypertrophy of the maxillary incisors necessitated a bulkier, heavier cranium
for their carriage, which in turn requires a shortened cervical region and a
corresponding modification of the muscular and ligamentous apparatus
responsible for the support of skull and tusks. A single tusk of Loxodonta, the
African Elephant, may weigh as much as 226 pounds, or reach a length of rri
feet. At the same time an enlargement of the area of the skull became necessary
for the insertion of cervical muscles and ligaments. This was brought about by
a separation of the inner and outer tables of many of the bones of the skull,
in particular the frontals and parietals (Fig. 585). The diploe between the
tables of bone became filled with air-cells which reduced weight. This caused
an obliteration of most of the sutures between the bones.
The extraordinary elongated trunk has induced a shortening of the facial
region proper, a recession of the nasal opening towards the top of the head, and a
FIG. sBs.-Loa!odonta: Skull
pneumatization. L. africana. Afri-
can Elephant. Weight reducing
adaptations are found in large
Mesozoic reptiles, almost all birds
(Fig. 387, p. 572) and among mam-
mals (especially elephants and
cetaceans). Pneumatic osteolysis
is greater in the African than in the
Asiatic elephant (Elephas indicus),
being correlated " with the rela-
tively greater size and weight of the
tusks-which necessitate a relatively
larger and heavier cranium for both
their support and functional em-
ployment" (Cave). Tusks of E.
indicus are never longer than ro feet
nor the weight apparently more than
r6r lbs (ct. text). To the left of the
mid-line: an. anterior nares; M .E.
mesethmoid; pn. posterior nares;
Vo. vomer. (After Flower.)

reduction in size of the nasal bones similar to the condition seen in the tapirs, but
to a greater degree. The prehensile trunk represents a combination of upper
lip and external nose, involving a functional specialisation of certain of the
facial muscles. To carry the great weight of the head and body, the legs,
simple in their general plan, have become pillar-like, with a loss of angulation.
The carpals and tarsals are serial and compressed and, while the hand and foot
are still pentadactyle, the digits are short, stout, and united by skin, with each toe
ending in a small hoof. Progression is digitigrade, the back of the digits being
supported by an elastic pad of tissue. In the fore-limb the ulna and radius
are permanently crossed in pronation, and the ulna is unique in having its
distal articulation with the carpus larger than that of the radius. The pelvis
has broad ilia and is vertical in direction, an adaptation found in many heavy-
bodied animals. Clavicles are absent.
The dentition is highly characteristic. In more recent elephants the upper
tusks, which are the second pair of incisors, are the only ones to persist, the
VOL. II. E 3
8q ZOOLOGY
lower incisors having disappeared during the mastodont stage of evolution.
The ivory of the tusks is solid, specialised dentine : enamel is absent save for
a small cap on the point of the tusk, which soon becomes worn off. Premolars
have not persisted beyond the mastodont stage. Of the six cheek-teeth, the
three anterior are milk-teeth, which are soon worn down and pushed out by the
three large molars at a very early period in life. Thenceforward the animal

FrG. 586.-0rder Proboscidea, Sub-order Mreritherioidea, Family Mreritheriidre. Mre1·itlterium:

Skull and lower jaws. This animal, the earliest known proboscidean, was an Upper Eocene a nd
Lower Oligocene Egyptian form tha t stood about 2 feet high at the sh oulder. It was of' n orma l,
ra ther heavy mammalian build, with a head remarkable for the forward p osition of the eye a nd
for the h eavy lower ja w. The anter ior t eeth, with an enlarged i 2 both above a nd below, clearly
form a special feeding mech anism, analogous t o that of pigs a nd the hippopotamus, the forwa rdly
directed incisors b eing used for grubbing in the ground ' (Watson). There is some ev idence t hat
M ceritherium is of an ancestry allied to that of the Sirenia (Fig. 594) and , perhaps, the Hyracoidea
(Fig. sSr) (Andrews). (After Osborn.)

possesses only three molars in each half of the upper and lower jaws. Owing
to their size, peculiar structure, and method of growth never more than two of
the molars are in use at one time in each section of the jaws. They consist
of a number of deep plates of enamel-covered dentine bound together by
cement. The developing teeth lie at an angle to those in use and are at a
higher level in the upper and lower jaws. As they slide down into the position
of wear, they push the anterior t eeth forwards until, after being nearly worn
down, these fall out.
 PHYLUM CHORDATA 815
There are perhaps five extinct families, some deserving of sub-ordinal rank
(see p. 8o8): I. Mreritheriidre (e.g. Mceritherium); 2. Gomphotheriidre (e.g.
Gomphotherium, Palaomastodon); 3· Mammutidre (e.g. Mammut =Mastodon);
4· Deinotheriidre (e.g. Deinotherium); and s. Barytheriidre (e.g. Barytherium).
There still survive to-day two species of the family Elephantidre (Loxodonta of
Africa, and Elephas of Asia). This family contained also the mammoths (e.g.
Mammuthus) and other extinct forms. The above are distinguished by
differences in the arrangement of the tusks and in the degree of complexity of
the molar pattern.
The Mreritheriidre (M ceritherium, Upper Eocene and Lower Oligocene of
Egypt) were the most primitive proboscideans (Fig. 586). The trunk was
relatively small. They showed some points of similarity to the Sirenia, but
Fxc. 587. -Sub-order Elephant- pa.
oidea, Family Gomphotheriidre.
Palreoma.'!todon: Skull and denti-
tion. T h is genus. the la rgest member
of w hich st ood about 6 feet high at
the shoulder. w as a cont emporary o f
the a llied P hiomia a nd is so far k nown
o n ly from the African Oligocene. B.
ant. orb. antorbital foramina ; e.a.m.
external a uditory meatus ; exo.
exoccipitol ; Jr. frontal ; ju. jugal ;
lac. lachrvmal ; l . i . lower incisor ;
max. maxilla; n. nasal; nar. exte rnal
na res; pa. parietal ; par. paroccipital
process ; pmx.
premaxilla; sq.
squamosal ; u.i.
upper inc isor.
(After And-
rews.)
l.i.

were probably not directly ancestral to the elephants. The dentition, while
showing the beginning of proboscidean characters, was less highly specialised.
In each upper jaw .there were still three incisors (the second enlarged as a tusk)
a canine, three premolars, and three molars. The lower jaw had only two pairs
of incisors, the second being enlarged as tusks. It lacked the canine. The
molars were bilophodont, except the third, which showed the beginning of a
third ridge.
The Gomphotheriidre existed in great numbers from the Oligocene to the
Pleistocene. This group contained the African Gomphotherium ( = Trilophodon)
and existed in the Pleistocene of Eurasia and North and South America.
In these, the upper tusks were large, downwardly turned, and had an enamel band
on the outer side only. The lower tusks were present in early forms, and were
either down-turned or procumbent with, or without, an enamel band. In later
. forms they disappeared. The molar-teeth varied widely in the number of ridges
8r6 ZOOLOGY
from three to as many as seven (in the third lower molar). The ridges, as
compared with the true elephants, were lower, with the cusps more widely
separated. Examples were Palceomastodon (Fig. 587), an early form from the
Oligocene of Egypt, and Stegomastodon, a Pleistocene animal.

FrG. 588.-0rder Proboscidea, Sub-order Deinotherioidea, F a mily Deinotheriidea. Deino-

tllerittnl. The deinotheres (e.g. D. giganteum of the European Pliocene, above) are emphatically
distinguishable from all other groups, including Palceomastodon (Fig. 587) and Phiomia (Family
Gomphotheriid::c) with which they perhaps share a remote ancestry. The anterior parts of the
lower jaw w ere thrust downwards a lmost at right angles (Fig. 589) and terminated in paired,
d ecurved second incisors of doubtful function. The deinotheres appeared first in the Lower
Miocene of Eurasia and Africa, and persisted to the Middle Pliocene (Europe and India) and the
Middle Pleistocene (only 50o,ooo years ago) in Central Africa. During this long period their
general structure remained extremely uniform , although there was a great increase in size, as in
all proboscidean families. The deinotheres left no descendants. (Partly after Augusta.)

The extinct Mammutidre contains only the single genus Mammut ( = Masto-
don). This family was possibly derived from the gomphotherian genus
Palceomastodon (Fig. 587). Mammut had a few simple molars and huge upper
tusks which converged outward and upward. The genus occurred from the
Lower Miocene to the Pleistocene and lived in Europe, North America, and
Asia.
The Deinotheroidea (Deinotherium, Fig. 588) (Lower Miocene-Middle
Pliocene of Europe and Asia and Lower Miocene and Pleistocene of Africa)
 PHYLUM CHORDATA 8!7
were characterised by the absence of upper tusks and the presence of a pair of
downwardly turned lower tusks. The molar-teeth were partly bilophodont
and partly trilophodont.

FIG. sBg.-Deinotherium: Skull and

dentition. The skull is low and broad,
yet nearly four feet long. The projecting
premaxilla, extending below the root of
the trunk, bore no tusks (cf. Palceoma-
stodon, Fig. 587) although the high position
of the nostrils and other features suggest
a long proboscis (Fig. 588). The cheek·
teeth (insert) were primitive (cf. Fig. 590).
In early (Lower Miocene) deinotheres the
tusks were still only slightly decurved.

(&
(Redrawn after Gaudry and Andrews.)

•
The Barytheriidre, sometimes considered as a separate order, is known by a
single species, Barytherium, from the Upper Eocene of Egypt. Few fragments
have been found beyond a lower jaw
with bilophodont teeth, a trilophodont
third molar, and a procumbent tusk-
like incisor. The dentition was some-
what like that of Deinotherium (Fig.
s8g) but the humerus, almost the only
non-cranial bone discovered, is said to
diverge from the typically probosci-
dean shape.
The Elephantidre have totally lost
the lower tusks and the enamel on the
large upper tusks, except at the tip in
the young stage. The ridges of the
molars deepen and become progres-
sively flattened into plates, the cusps
in each ridge becoming obliterated.
Cement is laid down between the
FrG. sgo.-Elephantidre: Dentition. Left: plates. The ridges, or plates, increase
Elephas (Asian), Right: Lo:rodonta (African)
elephants. (Cf. insert in Fig. 589.) in number to as many as twenty-seven.
The pattern and the number of ridges
are of use in classification, as, for example, in the living Indian Elephant
Elephas and the African Loxodonta (Fig. 590).
8r8 ZOOLOGY

The migration of the Proboscidea is interesting. They appear to have

originated in North Africa, and thereafter to have migrated to Asia, whence a
second radiation took place throughout that continent and throughout
Europe. While some reached North, and later South America, others returned
to Africa.
ORDERS PANTODONTA (AMBLYPODA) AND DINOCERATA
These two orders have been sometimes classed together as a single order
(Amblypoda), but there are differences in dental structure that entitle them to
be separated. Both in all probability arose from some early condylarthrous
stock, but diverged from one another on specialised lines of foot-structure and
dentition.

FIG. 591.- 0rder Pantodonta, Family Coryphodontidre. Coryphodon. A North American

and European (Upper Pal<eocene and L ower Eocene) p.enungulate which grew to a length of
about eight feet. (After Osborn.)

The Pantodonta is composed of the families Pantolambdodontidre, Baryl-

ambdidre, and Coryphodontidre. Pantolambda, a Palreocene genus, represents
a more primitive stage leading to Coryphodon (Fig. 591), the terminal genus of
the Lower Eocene in Europe and North America. The order survived into
the Middle Oligocene of Central Asia (e.g. Hypercoryphodon). Coryphodon
attained the size of a rhinoceros. The feet have the carpals serially arranged,
and the toes tend to become short and stout in adaptation to weight, but there
is considerable variation within the order, some forms possessing tapir-like
hooves (e.g. Coryphodon) while Titanoides possessed digging claws rather like
those of chalicotheres, some creodonts, and notoungulates. The teeth are
present in full series, and the canines large. The upper molars are triangular,
and in Pantolambda have three cusps. In Coryphodon the pattern evolves
into an aberrant type of bilophodont tooth. The order became extinct during
the middle Oligocene.
 PHYLUM CHORDATA

The Dinocerata (uintatheres) were heavy, stump-footed animals reaching a

large size. The skull in typical genera is characterised by the presence of three
pairs of bony horn-cores placed on the nasals, parietals, and on the maxillre
just over the enlarged spear-like upper canines. The molar teeth are bilopho-

FIG. 592.-0rder Dinocerata, Family Uintatheriidre. Uintatlleriu-m. The ponderous, horned

uintatheres, o f unknown origin and without descen dants, were once considered to be amblypoc.is
(Fig. 591) wit h which they share convergent features. The two groups are in fact separated by
several deep-seated differences (e.g . molar structure). The latest (late Eocene) species h ad becom e
a s big as a modern African r hino. The brain was small, yet the d epressed skull was a bout zt feet
long and bore pairs of bony protuberances on nasals , maxill<e, and parietals. It has been sug-
gested t h at these ' horns' may h ave been covered with skin like those of the extant giraffe . The
dent ition was peculiar, with sabre-shaped upper canines, which , like the h orns, were bigger in the
m a le. E ach such tooth was partly protected b y a n expansion of the lower jaw. The inter-
maxillaries were greatly reduced and there were usually no u p per incisors. T h e lower incisors
were small, as were the lower canines. Uintatherium was herbivorous a nd probably ingested great
quantities of aquatic veget ation. Also inhabiting the then lush la n dscape of west ern North
America was the collie-sized browsing horse Orohippus (Fig. 597, p. 827). (After various authors.)

dont superficially like those of Coryphodon, but with the two ridges converging
medially. Any resemblance between the two forms is due to convergence
rather than to any close affinity. The Dinocerata died out at the close of the
Eocene without successors. Examples were Uintatherium (Figs. 592, 593) and
Eobasileus (Fig. 593).
820 ZOOLOGY

ORDER PYROTHERIA
The Pyrotheres are known from a single family (Pyrotheriidre), with a short
history from Eocene (Carolozittelia) to Oligocene (Pyrotherium). Pyrotherium
was a large, elephant-like animal with a proboscis and with two pairs of large
incisor tusks in the upper jaws and one pair
in the lower. Each of the posterior pre-
molars and the molars has two sharp trans-
verse crests, somewhat as in kangaroos,
tapirs, deinotheres, and some other groups.
It was formerly maintained that the pyro-
theres were allied to the mastodons and
elephants, but the resemblance is conver-
4
gent. Pyrotheres, like other South
American ungulates, developed there from
condylarth-like ancestors.

ORDER SIRENIA
The dugongs, manatees (Figs. 594,
595) and their allies are fully aquatic and
present a number of structural features
convergent with those of the Cetacea.
These include the thick, almost hairless,
skin with an underlying layer of blubber,
and the loss of external hind-limbs, exter-
nal ears, and sacrum. The diaphragm,
like the cetacean, is oblique and very
muscular. The fore-limbs, though less
highly modified, are paddle-like, and the
tail is flattened and either rounded or
FrG. 593.-Uintatl&eriidre: Progressive
rhomboidal in outline.
specialisation. The trend culminated in Lack of true affinity with the whales is
extinction during the Upper Eocene. r.
Bathyopsis (U. Palreocene); 2 . Elachoceras shown by various features. Thus, the
(M. Eocene) ; J . Uintatheri1tm (M. Eocene ; skeleton of the arm is more typically
see also Fig. 592); 4· Eobasileus (Middle
and Upper Eocene). (After Scott.) mammalian in its proportions and has the
joints complete and the fingers little modi-
fied, beyond being bound together in a web of skin ; the scapula is long and
narrow, and without cetacean characters. The skull and dentition are
specialised. The tympanic bone is ring-shaped. The cervical vertebrre are not
ankylosed, and the whole skeleton is strong and its bone very dense.
The order is represented at the present day by the manatees (Trichechus =
Manatus), which live in the fresh waters, or along the coasts of America and
 PHYLUM CHORDATA 821

Africa, and the dugongs or sea-cows (Dugong = Halicore) of the Indian Ocean
and South Pacific and Australian coasts. A third genus and species, Steller's
Sea Cow (Hydrodamalis = Rytina), of the North Pacific, only became extinct
during the 18th century. To-day the order is confined to tropical and sub-
tropical waters. Steller's sea-cow, however, was discovered among Kam-
chatka ice-floes. It was exterminated by hunters by the year 1768.

FIG. 594.-0rder Sirenia, Sub-order Trichechiformes, Family Trichechidm. T7'ichechus.

Manatees (e.g. T. manatus, above) and dugongs are strictly vegetarian. The skin is thick and
relatively hairless, but facial vibriss<e are well-developed. The hind-limbs have disappeared.
The arms are converted into flippers, and the tail into a rounded horizontal fluke which enables a
whale-like p rogression. The protruding upper lip is cleft: an adaptation to grazing. The nostrils
are valvular a nd are closed under water. The inconspicuous mammary glands a re thoracic in
position. More active by night, sirenians are probably responsible for the mermaid myth: the
females of at least some manatees nurse their young while browsing. According to Columbus
(who saw three) such mermaids 'were not so beautiful as they had been painted, although to some
extent they were like a man in the face'. (From photographs, New York Zoo!. Soc.)

Sirenians are of especial general interest since it is possible that from these
tropical, vegetarian 'sea-cows' (which of course suckle their young) arose the
traditional sai}ors' stories of mermaids inhabiting distant seas. The possession
of both milk-glands and a superficially fish-like tail might easily have given
rise to such legends. The famous ' song of the sirens', however, came from
fishes, not sea-cows (see p. 346).
The manatees and dugongs are placed in separate families. In the manatees
8zz ZOOLOGY
(Fig. 594) the cheek-teeth are covered with enamel and are bilophodont, some-
what like those of the Proboscidea. There are
up to twenty in each half of the lower jaw and in
each maxilla. They are, however, not all in use
at any one time, but are continuously replaced
from the hind end. The premaxillary region is
small and little deflected. There are six cervical
vertebrce only. The fingers bear the remains of
nails. The dugong, on the other hand, has a
reduced dentition as far as the grinding-teeth are
concerned, never having more than six, and even
these are somewhat degenerate and without
pattern. The premaxillary region is, however,
enlarged into a down-turned rostrum, and bears,
in the male, a pair of stout tusks. The anterior
part of the mandible is covered by a horny pad,
under which can be found traces of the lower
incisors. There are no nails on the fingers, and
the normal seven cervical vertebrce are present.
The origin of the order and of its ancestral
form is unknown. The earliest known genera are
the Eocene Prorastomus of the West Indies, and
Protosiren and Eotheroides ( =Eotherium) of
Egypt. E otheroides is somewhat more advanced
than the other early genera which have a com-
plete eutherian dentition, a rostrum but slightly
enlarged, and small tusks. Although the hind-
limbs have already gone, the pelvis is but little
reduced and these forms are clearly more primi-
tive than, and functionally ancestral to, later
genera.
All the above are united in a sub-order,
the Trichechiformes. A second sub-order, the
Desmostyliformes, is represented by a sole
family and two genera, Cornwallius and Desmo-
stylus, found in Oligocene and Miocene deposits
of the Pacific coasts of North America and ] apan
respectively. The chief characteristics of Desmo-
stylus lie in the peculiar dentition. There are
two procumbent lower incisors, the upper being
small or absent. The molars are columnar and formed of a number of closely
adpressed cylinders covered with a thick layer of enamel.
 PHYLUM CHORDATA 823

Skeleton of Sirenia.-In the Sirenia (Fig. 595) the cervical vertebrre do

not coalesce, with ·the exception of two of them in the manatees. In the
manatees there are six cervical vertebrre only, and the neural arches are some-
times incomplete. In the trunk the thoracic vertebrre are numerous. All
have well-developed facets for the heads of the ribs, and well-developed zyga-
pophyses. The caudal vertebrre are numerous, depressed, with wide transverse
processes. The ribs are numerous, but few are connected with the sternum.
The sternum is a broad bone not composed of distinguishable segments.

FrG. 596.- Trichechus: Skull

and dentition. (T. senegalensis
(i natural size. )) PMx. pre-
maxilla; Vo . vomer; Mx. max-
illa; Fr. frontal; ET. ethmotur-
binal; ME. mesethmoid; F1·.
frontal; Pa. parietal; Sq. squa-
mosal; SO. supraoccipital; E x O.
exoccipital ; Per. periotic; BO.
basioccipital; Ty. tympanic ;
AS. alisphenoid; BS. basi-
sphenoid; PS. presphenoid ; Pt.
pterygoid; Pl. palatine; M :r.
maxilla; cp. coronoid process of
mandible; cd. condyle; a. angle;
s. symphysis; sh. stylohyal; bh.
basihyal ; th. thyrohyal. (After
Flower.)

The skull (Fig. 596) is characterised by its extreme hardness. The cranial
cavity is rather long and narrow as compared with that of the Cetacea.
Although the supraoccipital (S. 0.) is produced forwards on the upper surface
of the skull for a considerable distance, it does not separate the parietals
(Pa.). The frontals develop broad supraorbital plates. The zygoma is stout.
As in the Cetacea, the external nares are very wide, but they are situated
relatively farther forwards. The nasals are vestigial. The tympanic and
periotic are readily separable from the other bones. The premaxillre are
enormous in the dugongs. The mandible has a well-developed ascending ramus
and coronoid process (c. p.).
The scapula of the Sirenia is much more like that of the t errestrial mammals
than the scapula of Cetacea, and is nearer that of the Seals; it is narrow and
curved backwards. The spine is situated about the middle; the acromion is
 ZOOLOGY
directed downwards. The coracoid is fairly well-developed, and of a conical
shape. A clavicle is absent, as in the Cetacea. The skeleton of the arm also
departs less from the ordinary mammalian type than does the cetacean. The
radius and ulna are ankylosed at their extremities. The carpus has seven bones
in the manatees : the pisiform is absent. In the dugongs carpal coalescence
takes place, so that the number of discrete elements is reduced in the adult.
There are five digits, all of which possess the usual number of phalanges.
The pelvis is represented by a pair or more of vestiges widely separated
from the spinal column. They have a vertical position and probably represent
the ilia.

SUPER-ORDER MESAXONIA
In this group is placed alone the Order Perissodactyla which are 'odd-
toed', hooved animals including horses, tapirs, and rhinoceroses, and certain
extinct forms. In the mesaxonic foot, exemplified by the horse, the skeletal
axis passes down the third digit. The other digits are reduced and do not
support any weight. This compares with the paraxonic foot (Super-order
Paraxonia, Order Artiodactyla, p. 837), exemplified by the cow, in which the
skeletal axis passes between the third and fourth digits.
The utmost reduction that can take place in the number of digits in a
perissodactyle is four. (The second and fourth digit are represented in horses
by the splint bones: metacarpals and metatarsals.) In the artiodactyles the
number of digits (as in the cow) can be reduced to two-the third and fourth.
The tarsi also show points of difference, the shape of the astragalus (Fig. 623,
p. 82I) being in each case diagnostic. Apart from the fusion of several of the
tarsals in some artiodactyles (an unknown phenomenon among Perissodactyla)
the artiodactyle cuboid bears always a distinctive articular surface for the
calcaneum (Figs. 6I8, 86o).
Further, whereas the perissodactyle has a third trochanter, twenty-two or
twenty-three dorso-lumbar vertebrre, and a peg-like odontoid process, the
artiodactyle lacks a third trochanter, has uniformly but nineteen dorso-
lumbar vertebrre, and possesses a spout-like odontoid process.
Some, but not all, perissodactyles and artiodactyles have a complete post-
orbital bar, the composition of which differs in the two orders. In the horses
(Fig. 597) a process of the squamosal runs between processes of the frontal and
jugal bones alone, and the squamosal is excluded. In their dentition the
Perissodactyla manifest an almost complete molarisation of the premolar
teeth: in the Artiodactyla all premolars (except occasionally the fourth)
are simpler than the molars. True horns with a bony hom-core never occur in
the Perissodactyla. Artiodactyla usually have a gall-bladder and a more or
less complicated ruminating stomach. Such structures never occur among
Perissodactyla.
 PHYLUM CHORDATA Szs
Both orders appear first in the Lower Eocene. The Perissodactyla largely
replaced the pcenungulates (p. 8o7) in the Oligocene and Miocene, radiating
widely. They in turn were supplanted in the Pliocene and Pleistocene by
the Artiodactyla (p. 837), and, but for the present protection extended to
surviving species and the domestication of others, might well have become
about extinct.
The group is arranged as follows :
ORDER PERISSODACTYLA
Sub-order Hippomorpha
Super-family Equoidea
Families Palreotheriidre (Eocene-Oligocene)
Equidre (Eocene-Recent)
Super-family Brontotheriodea
Family Brontotheriidre (Titanotheridre) (Eocene-Oligocene)
Super-family Chalicotheriodea
Family Chalicotheriidre (Eocene-Pleistocene)
Sub-order Ceratomorpha
Super-family Tapiroidea
Families Isectolophidre (Eocene)
Helaletidre (Eocene-Oligocene)
Lophiodontidre (Eocene)
Tapiridre (Lower Eocene-Recent)
Super-family Rhinocerotoidea
Families Hyrachyidre (Eocene)
Hyracodontidre (Eocene-Oligocene)
Amynodontidre (Eocene-Miocene)
Rhinocerotidre (Eocene-Recent)

ORDER PERISSODACTYLA
The order falls naturally into two divisions which arose from condylarthran
(p. 8oz) ancestry. These are the sub-orders Hippomorpha and Ceratomorpha,
each of which was distinct by the Upper Eocene. Hyracotherium ( = Eohipp~ts)
of the Lower Eocene of Europe and North America belongs to the Hippo-
morpha and shows certain features that suggest that it is not far from the
parent stock. Various species of Hyracotherium were between ro and 20 inches
tall and were not unlike the condylarthran Phenacodus (Fig. 575) in some
respects. Also included among the Hippomorpha are the palceotheres which
arose as an Eocene side-line from an ancestry similar to that of Hyracotherium
and became extinct in the Oligocene after achieving certain notably equine
characters (p. 8z6) before these were developed in the main Oligocene-Miocene
radiation (Fig. 597).
 ZOOLOGY
With the main ordinal features held in common, different groups (the
earliest representatives of which show a good deal of resemblance to one
another) began to evolve on adaptive lines of their own. Thus the terminal
species of each acquired a highly distinctive external appearance. The
different trends of evolution are concerned chiefly with the structures of the
feet and the teeth. If the teeth of Hyracotherium (the earliest known horse),
of Homogalax (a tapir), of Eotitanops (a brontothere), and of Hyrachyus (a
rhinoceros) be compared, it can be seen that all start on the same general
plan, with low crowned teeth, with premolars simpler than the molars, and
with the six original cusps of the upper molars still clearly traceable. Closer
examination of the molars, however, shows that beginnings of differences
which ultimately lead to great divergences in the final result (Fig. 598). In
Hyracotherium, which has the most primitive teeth of any perissodactyle, the
protoconule and metaconule are only just beginning to elongate towards the
paracone and metacone, and these two outside cusps are beginning to elongate
in the anterio-posterior to form the beginning of an ectoloph. In Eotitanops
the intermediate cusps and the hypocone do not progress, but tend to disappear.
The protocone is large and remains bunoid, while the paracone and metacone
form a precociously enlarged ectoloph with a strongly marked W outer border.
Already in the Eocene Homogalax shows the tapiroid structure of a bilophodont
tooth in the joining of protocone, protoconule, and paracone into an anterior
transverse crest and of hypocone, metaconule, and metacone into a corre-
sponding hinder crest. The ectoloph is hardly developed. Hyrachyus, the
earliest rhinoceros, is like the tapir in forming transverse crests, but the ecto-
loph is more strongly developed and is produced into a characteristic metaloph
behind.
The horses gradually evolved very hypsodont teeth, the rhinoceroses rather
less so, but in both cases a deposit of cement was acquired in the deep valleys
between the cusps though to a much less degree in the rhinoceroses. All other
perissodactyles retained more or less brachydont teeth in which cement was not
formed.
Throughout the order the premolars are at first simpler than the molars.
During the course of evolution all the premolars, with the exception of the first,
invariably became molariform. In foot-structure the horses became function-
ally monodactylous. The titanotheres, tapirs, and rhinoceroses show a ]ess
degree of reduction.

FAMILY EQUIDJE
The horses present one of the most completely known phylogenies of all
vertebrates. In a condensed form it is as follows : There was a progressive
increase in size and in height, due to a lengthening of the distal elements of the
limbs (Figs. 597, 598). Simpson has pointed out that as ancestral horses grew
 PHYLUM CHORDATA

'"
"' >II
~·
,,,

0
otp
Pilrahippu$

~(
'It \ lJ Grazing horses

o0oo0o
H---~ ~~~~~ Browsing horses r-- - - - ----l't-----o-+--+------r- -----:,..-------l

Frc . 597.- 0 rder Perissodactyla, Sub-order Hippomorpha, Super-fa mily Equinoidea, Family
Equidm: Phylogeny and distribution of horses. (Redrawn a fter Simpson.)
828 ZOOLOGY

bigger, their proportions had to change and their 'running mechanism had to
become different, stronger and more effective' if they were to maintain even
the same speed as their already fast and agile ancestors. In fact, speed has
increased to a fraction over 40 miles per hour (for about half a mile) in the
selectively bred modern thoroughbred. Starting from a condition with four
digits in the manus (there is no trace of a first digit (pollex) in the manus of
any perissodactyle) and three sub-equal digits in the pes (here with traces of

A 8 c D

A 8 c D

FIG. 598.-Equidm: Progressive

specialisation of limbs and teeth.
~ Upper left: Manus; Upper right:
pes. A. Eohippus (North American
Lower Eocene), with four large toes
retained in front and only three
behind. B. Miohippus (North Ameri-
can Oligocene-Miocene) with only
8 three functional toes. C. M erychippus
(North American Miocene-Pliocene)
with lateral digits reduced and D.
Modern Equus. Lower: A. Left lower cheek teeth of Eohippus (approx. xI) and B. Pleistocene
Equus leidyi (approx. x!). (Redrawn from Romer, after various authors.)

the first and fifth digits, represented by nodules of bone) a gradual change took
place whereby the central digit in both manus and pes became progressively
longer and stouter. The lateral digits became thinner, failed to reach the
ground, and finally became reduced leaving only the metacarpals and meta-
tarsals as splint-bones. The ulna and fibula became reduced to their proximal
halves. Progression, at first sub-digitigrade, became unguligrade. In the skull
the profile of the top of the head became changed from a straight to a convex
line, and the preorbital facial part became elongated. Up to the stage of
Mesohippus (North American Oligocene) the postorbital bar was incomplete;
 PHLYUM CHORDATA

after that stage it became complete and progressively stouter. The teeth
progressed from a brachydont and almost bunodont to a very hypsodont
condition, with an increasing complication of crown pattern. The molars
eventually acquired a coating of cement on their outsides as well as in the
deep hollows between their cusps. The canines became proportionally smaller.
The premolars, at first simpler than the molars, gradually became molariform
and ultimately larger than the molars. A cingulum to the cheek-teeth,
characteristic of the earliest forms, was soon lost.
A series of species can be selected to show these changes (Fig. 597), but
the picture thus presented is only a partial one. Evolution did not of course
proceed by a series of large jumps from the earliest type to the modern horse-
the progress was gradual. Moreover, there was not a single line of evolution of
one genus succeeding another, but a continual branching off of side-lines, the
majority of which, after varying periods of existence, became extinct. It is not
always easy, among these branches, to trace the actual line of descent of the
living horses, though a reasonable approximation to the truth is probable.
Horse-like perissodactyles appeared first in the Lower Eocene. Of these,
the chalicotheres, brontotheres and palceotheres became extinct whereas the
main equine stem (typically represented by Hyracotherium) survived in North
America although it died out in Europe (Fig. 598).
The name Hyracotherium has priority over the more familiar Eohippus
('dawn-horse'). Owen, who described the animal from English material, re-
marked that the skull somewhat resembled that of a 'Hare or other timid
Rodentia'. There was little about it that recalled a modern horse. Owen's
Hyracotlzerium (hyrax-like animal) was later shown to be indistinguishable
from the later described North American Eohippus. In Eurasia, during the
Eocene, there was an expansion of lines from Hyracotlzerimn, all of which became
extinct by the Oligocene at the latest. The reason for this is unknown. No
further horses occurred in Europe and Asia until the Miocene. One such early
Miocene migrant from North America was Anchitherium ( = Kalobathipptts)
which colonised westward when the two continents were re-united. It was
in turn extinguished before the beginning of the Pliocene (Fig. 597).
Meanwhile in North America Hyracotheritt1n (=Eohippus), of which genus a
considerable number of species have been described, evolved by small increments
in height and by a progressive molarisation of the premolars. By the Middle
Eocene a new genus, Orohippus, is recognisable. In the Upper Eocene
Epihippus replaced Orohippus. It is, however, questionable if it was a direct
descendant of Orohippus. On the whole, the American horse genera during the
Eocene formed a more direct line than those of Europe. In the succeeding
Oligocene, horses were represented in America by the genus Mesohippus, which
had increased to the size of a sheep. The feet were still three-toed, with the
lateral elements touching the ground. The median digit, however, had become
VOL. II. F 3
 ZOOLOGY
enlarged. All premolars except the first had now become molariform. To
wards the close of the Oligocene another genus, Miohipptts, arose. This is
found together with the later species of Mesohippus, but persisted into the
Miocene after the extinction of the parent genus. During the Miocene there
was not one line of horses but several existing side by side. All of them were
derived from Miohippus (e.g. Archmohippus, Hypohippus, and Anchitherimn).
As mentioned above, A nchitherium re-established the stock in the Old
World, a process to be repeated in the late Miocene and early Pliocene by
Hypohippus, which in turn became extinct in Europe, as did related deriva-
tives in North America. In fact all such side-lines died out, but Parahippus,
derived directly from Mesohippus, persisted till the end of the Miocene, and
during that time gave rise to the genus Merychippus. In this genus the teeth
were greatly elongated in the sockets and there was formed a heavy deposit
of cement not only between the cusps, but on the outside of the tooth as well.
Cement was absent in the less advanced species of Parahippus, but had
appeared in the later forms. The rapid development of this massive cement
deposit went hand in hand with a changeover from browsing to grazing and
the occupation of an ecological niche that was provided by the evolution of
grasses that began to flourish at that time. Although they were less
dramatic, other changes were manifest. M erychipptts stood over three feet
high and carried its weight on the central digit of each foot. Although con-
siderable differences existed between species, members of the genus (even taking
into account their small hooved lateral digits) probably looked not very unlike
a modern pony.
This stock was unprecedentedly successful and gave rise to perhaps half a
dozen Pliocene lines, including Hipparion which spread widely and successfully
to Eurasia and Africa. (Protohippus, once considered to be a distinct and
important line, is now accorded only sub-generic rank in Merychippus.) One
undeniably valid stem derived from M erychippus was Pliohippus. In the
more primitive species there were still retained jointed small lateral toes; in
advanced forms these were lacking and the pes closely resembled that of
modern horses, as did the lengthened teeth and skull except for relatively
minor, though characteristic, details. From Pliohippus probably arose the
earliest somewhat zebra-like late Pliocene or Pleistocene animals (sub-genus
Plesippus) referable to the genus Equus which contains the horses of to-day.
Although the point of origin of the horses is unknown, there is little doubt
that North America was the principal centre of equine evolution. Immigra-
tions occurred both north-west and south to cross the periodically inundated
land bridges in the region of the Bering Sea and Panama. In South America
at the close of the Pliocene and during the Pleistocene there was a separate
rapid evolution of horses represented by the genera Hippidion, Onohippidium,
and Parahipparion ( = Hyperhippidium), all derived from Pliohippus, and
 PHYLUM CHORDATA

characterised by very long and slender nasal bones. These genera did not
survive the Pleistocene.
A final invasion of South America, however, occurred during the Pleisto-
cene and these spread widely throughout the continent. As in North America,
they survived the Pleistocene Ice Age, but (again as in North America)
mysteriously died out at the beginning of the Recent. Numerous conjectures
have been made to account for the relatively sudden disappearance of their
apparently enormous herds. They were seemingly still present when the
'Indians' arrived and were no doubt eaten-but many ungulates survived.
They may have been attacked by a disease such as trypanosomiasis-but the
above consideration should still apply. The ice-age was not responsible ; and
there is no evidence that any other landscape change was sufficiently wide-
spread to have seriously incommoded them. No new carnivorous enemy is
thought to have assailed them : in fact, the sabre-toothed tiger and other
probable predators died out at about the same time.
The family Equidre continued in the Old World where it was domesticated
by Man and then artificially selected to form the various familiar breeds which
were in turn introduced throughout the world, including North and South
America. At the present time, apart from domestic breeds, wild Equidre are
represented by Equus przeu:alsl<ii of Central Asia and E. asinus of Africa (wild
asses). In addition there are E. hemionus (the Oriental wild ass) and finally four
striped species of African zebra (E. quagga, E. zebra, E. bttrchelli, and E.grevyi).

FAMILY BRONTOTHERIIDiE (TITANOTHERIDiE)

The titanotheres in their day were a large and important group and, although
known chiefly from North America, had a wide distribution. Species have been
found in Mongolia, Burma, and Europe. The family was polymorphic. A
number of lines developed with differences of proportion in the skull, skeleton,
horns, and teeth.
The terminal species attained a great size. Some stood eight feet at the
shoulder. In spite of some specialisation of shape, all remained essentially
undeveloped. The brain remained relatively small, the skull long and low,
and the feet retained four rather unmodified digits in the manus and three in the
pes. The teeth (Fig. 599) were incapable of chewing hard grasses, a condition
that may have played a contributing part in the extinction of the family. In
the upper molars the protocone remained a round but large cusp, the hypocone
round but smaller, the protoconule and metaconule disappeared, while the para-
and metacones became greatly enlarged, and together formed a high, W-shaped
outer wall. The premolars remained rather small and less molarised than in
other Perissodactyla.
The earliest brontotheres appeared in the lower Eocene. These were
Lambdotherittm, a genus that left no successors, and Eotitanops. Through the
 ZOOLOGY

Middle and Upper Eocene there arose a number of genera until the sub-order
culminated in the Lower Oligocene in such large animals as Brontotherium,
Brontops (Fig. 599), Embolotherium, and others. In North America none is
found later than the Lower Oligocene. but some, unearthed in Mongolia, have

FIG. 599.-0rder Perissodactyla, Sub-order Hippomorpha, Super-family Brantotherioidea,

Family Brontotheriidre. Brontops. The vegetarian brontotheres ( = titanotheres) were perisso-
dactyles sharing ancestry with the horses. Best known from North America (though they
occurred also in Eurasia) brontotheres a rose in the Lower Eocene and disappeared during the
Oligocene, leaving no descendants. Brontops (North American Lower Oligocene) attained a
length of a bout 14 feet. Its skull was small and low, yet unusually broad owing to the extension
of the massive zygomatic arches. Outgrowths from t he nasal bones produced the grotesque
'hQrns' tha t were possibly covered with skin like those of extant giraffes. Brontops, representing
the culmination of a trend towards gigantism possessed, unlike the h orses, an 'unprogressive'
dentition which was nevertheless adequate in the humid lush and grassy environment in which it
lived. These animals Jived in a period of orogenic movement-a time of mountain-building that
foreshadowed more stringent conditions to come. A contemporary horse was the three-toed
Mesohippus (Fig. 597) a browsing animal standing about 6 hands (i.e. 24 inches). (Redrawn from
various authors.)

persisted into the Middle Oligocene. Thereafter the brontotheres appear to

have become extinct.
FAMILY CHALICOTHERIIDJE
The members of the former sub-order Chalicotheroidea (or order Ancylo-
poda) are now generally believed to merit family rank only. They show much
 PHYLUM CHORDATA

resemblance to the titanotheres in tooth-structure and tooth pattern, but are

characterised by a foot-structure unusual for a perissodactyle. There are
three functional digits on manus and pes, the manus bearing additionally a
splint of the fifth digit. The digits are subequal, and bear, instead of hooves,
strong claws, a character which may entitle them to be erected into a distinct
sub-order.
Examples are known from the Eocene to the Pleistocene from North
America (e.g. Eomoropus, Moropus) and from Europe and Asia (e.g. Chalico-
therium, M acrotherittm, Schizotherium).

SUB-ORDER CERATOMORPHA
These-the tapirs and rhinoceroses-were once included in the unnatural
assembly of 'Pachydermata' and widely separated from the horses with
which they share an early Eocene ancestry. They form two super-families
of unequal extent.
SUPER-FAMILY TAPIROIDEA
During the Eocene a number of small tapiroid forms evolved, which, like
the European Eocene horse-lines, became extinct. A line, however, can be
traced leading to the living forms, although the number of genera and species
therein is comparatively few. The forest-living habits of tapirs have no doubt
militated against their frequent preservation as fossils. Originally the dis-
tribution of the group was widespread over the Old and New Worlds, but now
it is restricted to one species in the Old World (Tapirus indicus of the Malay
Peninsula and Sumatra), and to four New World species (T. terrestris (Fig.
6oo), and T. roulini, T. bairdi and T. dowi).
The fossil Lophiodontidce were members of a Lower Eocene to Oligocene
radiation with characters intermediate between the tapirs and the rhinoceroses.
The cheek-teeth were bilophodont, like those of the tapirs, but with a metaloph
more like that of the rhinoceroses.
Of all living Perissodactyla, the tapirs are the least specialised. The
feet have stout digits, four in the manus and three in the pes, the third digit
in each case being somewhat enlarged. The skull is devoid of any postorbital
bar. The anterior nasal openings are enlarged and recessed. The nasal
bones are small and pointed upwards in correlation with the proboscis, which
is characteristic of the group. The teeth are persistently brachydont and
without cement. The molar teeth are bilophodont, and devoid of any com-
plication in the way of extra cusps or crests. Modern tapirs do not seem to
have advanced very much beyond the degree of tapiroid development already
attained in the Miocene.
 ZOOLOGY

SUPER-FAMILY RHINOCERATOIDEA
The rhinoceroses were extremely common in the Tertiary but are today
reduced to five species (Family Rhinocerotidce). These are Rhinoceros uni-
cornis (the one-homed Indian Rhinoceros), R. sondaiws (the Javan Rhinoceros,
also one-homed), Dicerorhinus sumatrensis (the Sumatran Rhinoceros, two-
homed and the smallest living species), Diceros bicornis (the African Rhinoceros,

FIG. 6oo.-Order Perissodactyla, Sub-order Ceratomorpha, Super-family Tapiroidea, Family

Tapiridre. Tapirus. The American T. terrestris ranges (as several sub-species) from southern
Mexico to northern Argentine. T h e nearest living allies of tapirs arc the rhinoceroses. Nocturnal,
v egetarian, and amphibious, t apirs a re superficially pig-like and adapted in shape for thrust ing
through undergrowth. T he snout is remarkably flexible and the young (like those of so many
forest-dwelling mammals and cursorial birds) are protectively striped (until t h e fourth or fifth
month). (From photographs by Chapman, and B.M. (N.H.) skins.)

two-homed) , and Ceratotherium simum (the White, or Square-lipped, Rhinoceros

of Africa, also two-homed). In addition there are three extinct families.
During the Oligocene to the Pleistocene period the rhinoceroses formed an
especially large, varied, and important group. The characteristic tooth-
structure has already been mentioned (p. 826). Manus and pes have never
less than three functional toes. Two, one or no horns may be present. These
are peculiar in being formed of a fused keratinous substance allied to hair, and
neither true horn nor bone. From traditional times this substance has been
 PHYLUM CHORDATA

valued in the Far East for its alleged aphrodisiacal properties. It possesses no
such properties. Even to-day it fetches about 6os. per pound in Tanganyika,
where 10-15 lb. are obtainable from a large example of Diceros bicornis.
In certain extinct forms the horns were confined to males. The super-
family, soon after its beginning in the Lower Eocene, divided into four main
lines of evolution: I. Hyrachyidre (the most primitive forms) ; 2. Hyraco-

FrG. 6or.- Order Perissodactyla, Sub-ord er Ceratomorpha, Super-family Rhinocerotoidea,

Family Rhinocerotidre. Balucliitllerium . This giant, aberrant, hornless rhinoceros of the Asiatic
Oligocene and early Miocene wa s the biggest terrestrial ·mammal and, excluding certain dinosaurs
(p. 508) probably the largest of all land animals. The most massive examples of Baluchitherium
were perhaps r8 feet high at the shoulder a nd about 27 feet long . The skull was over four feet
long, yet apparently small by comparison with body bulk. Baluchitherium was a browser with an
essentially grinding dentition and a single pair of massive, blunt incisors. The long post-like
legs were three-toed, with the lateral digits considerably reduced. (After various authors.)

dontidre (light cursorial animals) ; 3· Amynodontidre (about the size of a

hippopotamus) . The above-mentioned families became extinct during the
Oligocene period. The remaining family 4· Rhinocerotidre, began in the
Eocene and spread out into many lines during the Oligocene and subsequent
periods until the Pleistocene. Then the group became much reduced. The
rhinoceroses were a very polymorphic family, and there is still much uncer-
tainty as to their classification and as to the line of evolution of the living
 ZOOLOGY
forms. In certain cases genera can be distinguished by some peculiar character,
as, for instance, Elasmotherium (a large Pleistocene animal with a large, for-
wardly directed, fronto-nasal horn and much folded cheek-teeth enamel),
Balttchitherium (a Miocene rhinoceros with persistently primitive teeth and
procumbent lowerincisor tusks and the largest known land mammal (Fig. 6oi)),
Chilotherium (a Pleistocene genus with an unusually broad mandibular sym-
physis), etc.

SUPER-ORDER PARAXONIA
This, like the Mesaxonia contains but one quite unmistakably defined order
-the Artiodactyla. The even-toed hooved animals that compose it are
characterised essentially by the paraxonic foot (cf. p. 824) but also by other
significant points of similarity. While the Perissodactyla declined, the Artio-
dactyla flourished and radiated so that, despite their hunting by Man and the
carnivores, many remain abundant to-day (e.g. wild pigs, various deer, etc).
The order is arranged as follows :

ORDER ARTIODACTYLA
Sub-Order Suiformes
Infra-order Palmodonta
Families Dlchobunidm (Eocene-Oligocene)-
Chmropotamidm (Eocene-Oligocene)
Cebochmridm (Eocene-Oligocene)
Leptochmridm (Oligocene)
Entelodontidm (Eocene-Miocene)
Infra-order Suina
Families Suidm (Oligocene-Recent)
Tayassuidm (Oligocene-Recent)
Infra-Order Ancodonta
Families Anoplotheriidm (Eocene-Oligocene)
Anthracotheriidm (Eocene-Pleistocene)
Hippopotamidm (Pliocene-Recent)
Cmnotheriidm (Eocene-Miocene)
Infra-order Oreodonta
Families Agriochmridm (Eocene-Miocene)
Merycoidodontidm (Eocene-Pliocene)
Sub-order Tylopoda
Families Xiphodontidm (Eocene-Oligocene)
Camelidm (Eocene-Recent)
Sub-order Ruminantia
Infra-order Tragulina
 PHYLUM CHORDATA

Families Amphimerycidre (Eocene-Oligocene)

Hypertragulidre (Eocene-Miocene)
Protoceratidre (Oligocene-Pliocene)
Geloeidre (Eocene-Oligocene)
Tragulidre (Miocene-Recent)
Infra-order Pecora
Families Cervidre (Oligocene-Recent)
Lagomeryeidre (Miocene)
Giramdre (Miocene-Recent)
Antiloeapridre (Miocene-Recent)
Bovidre (Miocene-Recent)

ORDER ARTIODACTYLA
The 'even-toed' ungulates are represented at the present time by such
animals as pigs, peccaries, hippopotamuses, camels, giraffes, sheep, oxen, deer,
antelopes, etc. In the past there were, in addition, many other remarkable
forms. As a whole, this order is larger and more varied than the Perissodactyla,
and the division between its sub-orders is deeper and more marked. The
division within some of its groups is still in dispute.
The Artiodactyla may be divided into three sub-orders-the Suiformes
(pigs and peccaries), the Tylopoda (camels and llamas), and the Ruminantia
(chevrotains, deer, giraffes, gazelles, cattle, Yak, buffaloes, goats, and sheep.

suB-ORDER SuiFORMEs
Some mammalogists consider that the earliest identifiable Artiodactyla
(e.g. Diacodexis, Bunophorus of the American Lower Eocene) fall into the sub-
order Suiformes whilst others consider them sufficiently distinct to occupy a
sub-order by themselves. These primitive palreodonts have the upper molars
still in the tritubercular condition and seem in several other ways not very
remote from the archaic placental stem. However, the paraxonic feet and
unmistakably diagnostic artiodactyle astragalus (Figs. 624, p. 861) at once
reveal their true taxonomic position.
The Suiforrnes include the peccaries, pigs, wart-hogs, hippopotamuses and
several extinct groups of highly debatable affinity. The teeth are 'bunodont'.
The canines are triangular in section and in some forms become stout tusks.
The molar teeth, while retaining bunoid cusps, exhibit a secondary specialisa-
tion in supernumerary cusps, additional to the original crown pattern. The
limbs are short and carry invariably four digits. Of special interest among
the archaic Suiformes are the giant pig-like entelodonts which were plentiful
during the Eocene and Oligocene. These creatures stood more than five feet
high, were sometimes 12 feet long and possessed small brains in skulls almost
three feet long: the jaws were equipped with lance-like incisors, heavy canines,
 ZOOLOGY
and simple cheek-teeth, a dentition perhaps adapted to the unearthing and
grinding of roots.
The pigs became plentiful in the Old World in the late Tertiary. The
peccaries arose from a common northern stock, and are represented in the
North American Oligocene. They probably reached South America in rela-
tively recent (Pleistocene) times.
The anthracotheres and the hippopotamuses (Ancodonta) (Fig. 602) are
amphibious mammals that were plentiful in the Old World during the Tertiary.
Extinct forms ranged in size from that of a rabbit to the modern Hippopotamus.
Perhaps admissible here, or perhaps under the next sub-order (Tylopoda)
are the Oreodonta, an entirely North American group which had a long range
from the Eocene to the Pliocene. These heavily-built, pig-like, animals had

FIG. 6oz.-Order Artiodactyla, Sub-order Suiformes, Infra-order Ancodonta, Family Hippo-

potamidre. llippopotmnus: Aquatic adaptations. Although the body of H. amphibius may be
q fee t long and weigh more than two tons, the animal can conceal almost the whole of its bulk
under water with little more than the most important exteroceptors exposed. (For comparable
crocodilian adaptations, see Fig. 333, p. soo.)

four toes as well as strongly selenodont teeth which suggest ruminant affinity.
The first lower premolars functioned as canines and the canines themselves
were incisiform. In some forms the last premolar was molariform.

SUB-ORDER TYLOPODA
This sub-order consists of camels (extinct forms as well as the modern
llamas, camels and dromedaries) and their allies. Some authorities believe
that the Tylopoda should be included in the Sub-order Ruminantia.
Camels appeared as a distinct stock in the Eocene. They were common
and almost exclusively North American in the Oligocene (e.g. Poebrotherium)
when they were about the size of a small sheep and had already lost the lateral
digits and become didactyle. The development of the manual- and pedal-
digits proceeded at an equal rate. The carpals and tarsals, unlike those of
the Pecora, never fused, but the metacarpals and metatarsals soon fused into
a cannon bone whose distal ends gradually became divergent, with smooth,
unkeeled, articular surfaces for the phalanges.
The dentition of the living species is peculiar. In the upper jaw of the
young there is the full number of incisors, but in the adult the third only is
retained as an isolated recurved tooth similar in pattern to the upper and lower
 PHYLUM CHORDATA

canines. The full number of lower incisors is retained as procumbent spatulate

teeth. The premolars are reduced to the last two, and are small. The molars
are very hypsodont, and their four cusps selenodont. In the neck vertebrre
the vertebral artery takes the unusual course of piercing the anterior part of
the transverse process, instead of traversing its whole length as is usual.
The ruminating stomach is rather simpler than that of the Pecora, and does
not contain special 'water-cells', as was once believed. Experimentally, a
Dromedary can lose fluid equalling 25% of its body weight without being
seriously weakened. Further, it can go for more than 17 days without drinking
even when deprived of succulent plants. It loses relatively little fluid in its
unne. Its body temperature ranges from goo F. (at night) to 105° F.; and it
starts to lose much sweat only near the upper limit. The massive concen-
tration of fat in the hump ensures that heat-loss is little retarded elsewhere
and the inflow of heat is slowed by its wool (Schmidt-Nielsen).
Like the horses, the camels became extinct in North America in the Pleis-
tocene. Migrants from North America gave rise to the modern camels of the Old
World and to the llamas of South America. The sub-order is now reduced to a
few species. Camelus dromedarius, the one-humped Arabian Camel, is no longer
found wild. The other camel is C. bactrianus, the two-humped Bactrian animal
of Turkestan. In South America the representatives are the humpless Llama,
Gnanaco, Vicuna and Alpaca. The sub-order seems never to have been a
large one, but there have been described several genera, some of them (as
Alticamelus, a 'giraffe-necked' form) off the main line of descent.

SUB-ORDER RUMINANTIA
These form an exceedingly large and diverse group in which a few characters
are held in common, as, for instance, the total loss of the upper incisors (p.
872). The upper canine is also usually lost, but, in the rarer cases where it
persists, it is much enlarged. Flat, spatulate lower incisors are all present,
as is the lower canine, which has the same shape as the incisors, and is closely
pressed against them. The premolars are smaller than the molars, and the
latter have four crescentic cusps. The feet have always a cannon bone.
The lateral toes are usually lost and the feet absolutely didactyle. In some
cases the second and fifth digits remain, but are never complete (Fig. 617,
p. 86o). In this group the ruminating stomach attains its highest develop-
ment of four complete chambers (Fig. 639, p. 878). Horns of several types
occur, but are absent in some species. The Ruminantia are sometimes divided
into two principal groups or infra-orders, the Tragulina (chevrotains and their
allies) and the Pecora (all other ruminants).
The Tragulina are the remarkable cud-chewing deerlets of tropical Asia
(Tragulus) and Africa (Hyemoschus). Upper incisors are lacking. Particularly
in males the upper canines are large and adapted to aggression. The molars
 ZOOLOGY
are selenodont. The stomach is three-chambered as compared with the
four-chambered organ of more advanced ruminants. Each limb has four
hooved digits, and a cannon-bone (fused metatarsals) is present in the hind
limbs. The lateral pedal digits are still complete, though slender as is the
fibula, which is reduced in all other ruminants). The navicular, cuboid, and
entocuneiform bones of the tarsus, however, are fused together. The odontoid
process of the axis vertebra is conical, not spout-like. As a rule there are no
horns, but these were present in at least one of the extinct families (Proto-
ceratidre). Superficially the chevrotains resemble large rodents-hence the
name 'mouse-deer'.
The Pecora contains the families Cervidre (deer), Giraffidre (giraffes, okapi),
Antilocapridre (prong-buck) and Bovidre (gazelles, Impala, Eland, Yak, cattle,
buffaloes, goats, and sheep).
The Cervidre date from the Oligocene. They usually have deciduous horns
or antlers confined, with one exception (the Reindeer) to the males. Horns
are absent in only two instances (Moschus and Hydropotes). The lateral toes
are nearly always present, but are never complete. In some species the distal
extremities of the metacarpals are retained (telemetacarpal condition), and in
others, including most of the Old-World deer, the proximal elements alone
persist (plesiometacarpal condition). In both cases the phalanges are much
reduced in size and do not touch the ground. The skull has two orifices for
the lachrymal duct and a very large antorbital vacuity. Except in Moschtts
there is no gall-bladder.
M oschus and Hydropotes are two rather aberrant forms in an otherwise
fairly compact family. M oschus, the hornless Musk Deer, is placed by itself
in a separate subfamily, the Moschinre. The male has large upper canines.
A gall-bladder is present. Hydropotes, the Chinese Water-deer, although
placed in the second subfamily, the Cervinre, differs from them in being hornless
and in having large upper canines like Moschus, but in all other respects is a
true deer. There are many genera of Cervidre still living in the Old and New
Worlds, but none in Africa south of the Sahara.
In the Giraffidre the horns are non-deciduous, always covered with skin.
They are simple and unbranched except in an extinct side-line represented by
a few Pliocene genera such as Vishnutherium, Sivatherium, and Bramatherium,
which had skin-covered palmate antlers. The upper canines are absent, as
are all traces of lateral digits. Not even the lateral hoofs persist. The humerus
has a characteristic double bicipital groove. The group may be traced from
the Miocene, and is now represented by the Giraffe and Okapi.
The Antilocapridre or prong-bucks of North America have been distinct
from Bovidre since at least the Miocene but are often included with them. In
both groups there are neither upper canines nor incisors, and there is only
one orifice to the lachrymal duct. The lateral digits are completely absent
 PHYLUM CHORDATA

except for the persistence of small horny hooves which may contain a nodule
of bone. A gall-bladder is present. Horns are universally present in males
but in many genera of Bovidre the
females are hornless. Horns are never Sr"h
shed except in the prong-bucks, wherein
a soft horny covering is discarded an-
nually. The horns in the Bovidre are
never branched but are of many shapes :
curved, or spiral, or circular. The
Bovidre comprises more than one hun-
dred genera and a large number of sub-
families whose boundaries are not always
satisfactorily definable.

GENERAL ORGANISATION
OF MAMMALIA
Integument and General External
Features.-Nearly all mammals are
covered with hair (Fig. 603) developed
in hair-follicles. Each individual hair
is a slender rod, and is composed of
two parts, a central part or pith (M)
containing air, and an outer more solid
part or cortex (R). Its outermost layer
may form a definite cuticle (0). Com-
monly the cortical part presents trans-
verse ridges so as to appear scaly. In
one case only, i.e. sloths, is the hair
fluted longitudinally. The presence of
processes on the surface, by which the FrG. 6o3.- Mammalia: Hair. Longitu-
hairs when twisted together interlock dinal section (diagrammat ic) . Ap. band of
muscular fibres inserted into the hair-
firmly, gives a special quality to certain follicle, Co. dermis; F. external longitudinal,
and
kinds of hair (wool) used for clothing- follicle; F'. internal circular fibrous layer of
Ft. fatty t issue in the dermis; GH.
the felting quality as it is termed. A hyaline membrane between the root-sheath
and the follicle; HBD. sebaceous gland;
hair is usually cylindrical but there are H P. hair-papilla with vessels in its interior;
many exceptions. In some it is com- M. medullary substance (pith) of the hair;
0. cuticle ; R. cortical layer; Sc. horny
pressed at the extremity, in others it is layer of epidermis; SM. Malpighian layer of
compressed throughout. The latter epidermis; WS. WS' . outer and inner layer
of root-sheath . (After Wiedersheim.)
condition is observable in the hair of
negroes. Fur is usually composed entirely of one kind of hair. In some cases,
however, there are two kinds. The hairs of the one sort may be very
numerous and form the soft under-fur, while those of the other consist of longer
 ZOOLOGY
and coarser hairs scattered over the surface as in Ornitltorlzyncltus, the fur-seal
(Callorhinus), and the mink (genus Putorius =polecats).
The principal function of hair is the part it plays in temperature regulation
but it can be tactile, protective, and possess sexual functions as well. It has
been secondarily reduced or almost lost in widely different animals, e.g. the
almost hairless subterranean rodent, Heterocepltalus glaber, and the Sirenia,
Cetacea, and Man. Most, if not all rodents are born naked; the condition in
the adult H. glaber of tropical East Africa is probably an example of predo-
morphosis. In Man, and certain other animals (cetaceans, sirenians) that have
become relatively hairless, heat loss is retarded by a compensatory layer of
hypodermal fat (blubber) that arises in
•• the superficial fascia. At the same
time, the apparent hairlessness of Man
is illusory. Except for the palmar and
plantar surfaces, the lip borders, parts
of the fingers and toes and external
genitalia, the body of man is clothed
by hair on the scalp, as eye-brows and
-lashes, as secondary sexual characters
in axillary and pubic regions and on
chest and face (in the male); or, on the
other hand, by widespread, short hair-
down or velltts. In the neonatus, vellus
normally replaces the fine lanugo which
almost covers the £cetus from the fifth
FIG. 6o4.-Primates: Tactile vibrissm. or sixth month until (usually) shortly
Arrangement in a typical strepsirrhine monkey.
a. mystacial; 1>. mental; c. inter-ramal; d. car. before birth. Some of this is swallowed
pal; e. genal and f. supraorbital groups. (Re.
drawn after Osman Hill.) by the fcetus and may be voided by the
neonatus in the meconium (freces). It
is of interest that the relatively hairless elephants also possess lanugo.
Hair is also tactile in function and in most mammals (especially arboreal
and nocturnal forms) it occurs also as specialised tactile vibrissaJ on the facial
region and sometimes near carpus and tarsus (Fig. 604). Groups of vibrissre
are richly innervated. These are absent in Man, but it is probable that eye-
brows represent the supraorbital vibrissre of ancestral mammals. The relative
disappearance of hair may be associated with improved tactile acuity of the
human skin.
In animals such as spiny anteaters (Tachyglosstts}, hedgehogs (Erinaceus)
and porcupines (Hystrix) the hair in part assumes the form of defensive spines
or quills. Also protective are the outwardly directed bristles in the nostrils
and auditory meatus of many mammals, including Man, which serve to block
the entrance of foreign substances and insects. Eyelashes have a similarly
 PHYLUM CHORDATA

protective function. Hair too, probably subserves a threat function in com-

bat, and, in specialised areas, assists in the retention of animal odours significant
in courtship.
Hair, with the exception of eyelashes, grows at a slant and is distributed in
tracts (Fig. 6os). Primitively such tracts are directed caudally and ventrally
on the trunk, and distally and postaxially on the limbs (e.g. Myrmecobi1ts,
Dasyurus, and, in addition, many relatively unspecialised eutherians). This

FIG. 6os.-Theria: Hair-tracts. Hairs emerge with a slant in tracts characteristic of the
group. The primitive arrangement appears to be backwards, perhaps related to the reduction
of friction, as is the backward direction of feathers and scales. The primitive condition has been
abandoned in numerous groups: there is sometimes a complete reversal of direction of specific
tracts, seemingly related to toilet adaptations (Figs. 483, p. 6g6; Fig. 607, p. 846). The direction
of hair tracts has been presumed by some authors as evidence of Neo-Lamarckian inheritance.
Left: Marsupial (Wallabia) pouch young; Centre: Prosimii (Loris) juvenile; Right upper and
lower: Head and neck region of Pseudochirus (marsupial possum) and Homo (Man) showing nuchal
hair-whorls. (Redrawn after Wood Jones, Osman Hill.)

primitive distribution, however, is remarkably varied by the development of

whorls and the complete reversal of direction of hair growth over extensive
areas of body surface. Some such arrangements appear to be strikingly
correlated with the combing or scratching habits (Wood Jones).
A hair, like a feather, is formed from the epidermis. The first rudiment of
a developing hair usually takes the form of a slight downwardly projecting
outgrowth, the hair-germ, from the lower layer of the epidermis, beneath which
there forms a condensation of dermal tissue to form the rudiment of a hair
papilla. In some mammals, however, the dermal papilla makes its appearance
 ZOOLOGY

before the hair-germ. The hair-germ, which consists of a solid mass of epi-
dermal cells, elongates, and soon its axial portion becomes condensed and
cornified to form the shaft of the hair, while the more peripheral cells go to
form the lining of the hair-follicle. They become arranged in two layers, the
inner and outer root-sheaths. The epidermal cells in immediate contact with
the hair-papilla retain their protoplasmic character and form the hair-bulb,
by the activity of which the further growth of the hair is effected. Soon the
upper end of the hair-shaft grows out beyond the surface of the epidermis, and
the projecting part eventually becomes much longer than that which lies
embedded in the follicle. At the same time the follicle grows downwards into
the dermis. During its growth the hair is nourished by the blood-vessels in
the dermal hair-papilla, which projects into its base.
Hair is erectile due to the action of unstriated arrector pili muscles which are
innervated by the sympathetic nervous system (p. ug). Thus hair' stands on
end' during extreme fear, 'goose-pimples' are raised in cold and specific emo-
tional states, or hair can be raised (as a bird's plumage is 'fluffed') as a heat
conservation mechanism during cold weather. Hair follicles undergo cyclical
activity by which old shafts are shed and replaced. In some mammals (e.g.
Man, Domestic Cat) each follicle behaves independently of its neighbours, so
that although each body hair (in Man) is replaced at least once, or perhaps
twice, a year, there is no concerted seasonal moult such as occurs in the Arctic
Fox (Alopex lagopus), Arctic Hare (Lepus arctic.us), certain of the Mustelidce
and other forms. In such animals follicle behaviour is more or less synchron-
ised. The white phase of the Arctic fox takes on brown fur during the summer.
The 'blue' (bluish-grey) phase of the same species, on the other hand, does not
change colour. There is some evidence that the 'blue' condition is recessive
and the phasic one dominant. Blue animals remain conspicuous and at an
apparent disadvantage during the winter; they constitute less than 5 per cent
of the population in the rigorous climate of Baffin Land and the Canadian
Arctic. The phases are probably about equal in Western Greenland. There
is some evidence that 'blue' foxes have a lower reproduction rate. Their diet,
too, is different,
Unlike that of many birds and other vertebrates, the mammalian exoskele-
ton is rarely brightly coloured although it is often boldly marked and in this
way achieves an obliterative effect in shadowy situations (e.g. Felis, Tltylacinus,
Equus, and others). No mammal has developed green hair, although the
sloth becomes greenish by special means (p. 768). Certain primates (Cerco-
pithecidce) possess a highly coloured posterior sexual skin, which becomes
brighter in the female under the seasonal action of a=strogens, and which is
no doubt important in sex recognition and stimulation. A few mammals
possess scales. Thus inMan is (Fig. 550, p. 773) the greater part of the surface
is covered with large, rounded, over-lapping horny scales of epidermal origin,
 PHYLUM CHORDATA

similar in their mode of development to those of reptiles. A similar pheno-

menon is seen in the integument of the ventral surface of the tail of Anomalurus,
the African 'flying' squirrel. Here the scales engage the tree-trunk at the end
of each glide and reduce skidding. Armadillos (Fig. 533, p .. 763) are the
only mammals in which there occurs a bony dermal exoskeleton.
Also epidermal in their origin are the horny structures in the form of nails,
claws, or hoofs, with which the terminations of the digits are provided in all
the mammalia except the Cetacea. There seems little doubt that flattened
nails (ttngulce) arose by degeneration and specialisation from claws (falculce)

FIG. 6o6.- Mammalia: Claw structure. The claw (here in sagittal section) is composed of t wo
layers o f which the deep stratum (Deep St.) is the major part, while the superficial stratum (Sup.
St.) m erely forms a protective sh eath. It is the latter stratum which persists in Primates to form
flat nails. The superficial stratum is formed from the basal part of the germinal matrix (Bas. M.)
and the deep stra tum from the terminal part (Ter. M .). The rest of the claw-bed is made up of
the st erile matrix (St. 11!.). (After LeGros Cla rk.)

which are common in the Reptilia and which occur in a few amphibians (e.g.
Xenopus, and the Japanese clawed newt, Onychodactylus).
Strong evidence presented by the form of the terminal phalanx shows that
Palreozoic theromorphs (p. 514) possessed clawed digits. The same is true
of their early mammalian descendants. There is, however, some develop-
mental difference between the claws of at least some reptiles and mammals.
In Chelonia and Crocodilia the young claw develops from the whole of the
claw-bed, whereas in the Mammalia much of the claw-bed is sterile : the ger-
minal matrix is restricted to the proximal segment which is, however, divided
into a basal and terminal matrix (Fig. 6o6). From the terminal matrix arise
oblique horny lamellre which come to form the thick condensed deep stratum
VOL. II. G 3
 ZOOLOGY
or main body of the claw. From the basal matrix there grow lamellre that
become arranged horizontally to form the protective superficial stratum.
It is not improbable that relatively blunt, flattened nails arose with the
broadening of the digital pads (Fig. 6o7) of ancestral primates. The assump-
tion of nails was achieved by the reduction, and next the disappearance, of the
terminal part of the germinal matrix and the deep stratum (Fig. 6o6) until
only the thin superficial element, of changed shape, remained.
Evidence of such a transition is perhaps shown in extant primates. In the
claws of marmosets (Callithrix) the deep
stratum appears to have been greatly re-
duced; but this, on the other hand, may
represent a secondary modification of nails.
In lemurs, nails have arisen on all digits ex-
cept the second toe which has remained
clawed as a toilet digit (p. 843). In tarsiers,
which have developed distal tactile pads of
remarkable size, the nails in turn have been
reduced to vestiges except for ungulre that
have become claw-like toilet-nails on the
second and third toes (Fig. 6o7). Horns,
too, are epidermal in origin, but those of
rhinoceroses are peculiar: they appear to be
formed by the agglutination of hair-like
horny fibres (p. 834).
Cutaneous glands are almost general in
the Mammalia. The most constant are
sebaceous glands, which open into the hair-
FIG. 607.- Tarsius: Pes. Both follicles, and sweat- (sudoriferous) glands.
apical and plantar pads occur, as well
as t oilet claws (not present on manus) Sebaceous glands are lobulated derivatives
on the second and third digits. (After of the Malpighian layer and produce sebum
Osman Hill.)
which lubricates the adjacent hair-shaft
skin. It is bacteriocidal in nature, and in addition probably helps render the
skin impervious to moisture and to desiccation. Sweat-glands are relatively
simple in structure. They are lacking in many mammals and occur chiefly
on the plantar and palmar surfaces of others (e.g. arboreal animals).
In primates in particular sweat ducts emerge at the summit of papillary
ridges which occur, often in association with tactile pads, of varying number
and disposition. These ridges are the basis of finger-printillg. Tactile pads
occur on the palmar or plantar surfaces and sometimes, in addition, on the
digits of the manus. The roughened surface of the touch pads, plus the
secretions from sweat-ducts, probably augments the grasping power of the
limb.
 PHYLUM CHORDATA

In many mammals there occur in various parts of the body multiple or

single glands that secrete odorous material. These are often, but not in-
variably, situated in the genital region. Particularly notable in this respect
are the brachial glands of certain primates (e.g. lemurs). These occur in the

pedal
gland

FIG. 6o8.-Eutheria: Cutaneous glands and olfactory signals. Mammals (and reptiles) that
live on or near the ground make extensive use of olfactory signals. Among the ruminants alone,
pedal, tarsal, metatarsal, carpal, caudal, inguinal, preputial, postcornual, occipital and preorbital
glands have been described, all of which produce highly odorous secretions. Carnivores, rodents,
and many other groups (see Fig. 6og, p. 848) possess cutaneous glands of various kinds which are
significant in courtship, and in the establishment of territory, and perhaps have other functions
as well. For example, wild dogs (and lap-dogs) mark projections with urine mixed with preputial
secretions. A, Inguinal glands of male Bush-buck (Tragelaphus); B, Preorbital gland o£
Four-horned Antelope (Tetraceros) (see also Fig. 6og); C, Anal gland of male civet (Viverra);
D, Pedal gland of Muntjac (Cervulus). (Redrawn from Bourliere, after Pocock.)

proximal region of the arm of the male. Also present in both sexes may occur
an antebrachial or carpal gland in association with an erect horny spur of
variable size and disposition. A variety of odoriferous glands has arisen
in other orders (Figs. 6o8, 6og).
Mammary glands, which secrete milk, are probably specialised sweat-
glands. These arise along the mammary- or milk-lines which run from axilla
to inguinal region on each side of the trunk. They differ considerably in
 ZOOLOGY
number and distribution. From one (e.g. most Primates, Giraffes) to eleven
(Centetes) pairs occur in eutherians, depending, in general, upon the number of
young and their post-natal feeding requirements. Thus, Cavia has only one
pair of mammary glands and produces 3 or 4 well-developed young after an
unusually long period of gestation (67-68 days) for an animal of such size. The
Common Rabbit, on the other hand, generally possesses four pairs of mammary
glands and commonly produces about eight relatively helpless young after an
intra-uterine period of only 30-32 days.
Mammary glands rna y be restricted to the
thoracic region (e.g. Chiroptera, Sirenia,
p. 8zr, and all Primates except lemurs) or to
the inguinal area (e.g. cetaceans, ungulates).
When many young are produced the mam-
mary glands are developed serially and ex-
tensively along thorax and belly. In a few
mammals of arboreal or flying habit, certain
nipples have taken on an accessory anchoring
function (Wood Jones). Thus, in lemurs and
rhinolophid bats, the young cling by means of

r
both arms and legs and, at the same time,
assist in maintaining their grip by grasping
one or other of the otherwise apparently non-
functional inguinal nipples with the mouth.
~lA When the parent comes to rest the young one
[ reverses its position and feeds from a thoracic
organ. The nipples open into the pouch of
FIG. 6og.-Ungulata: Territory
establishment. Male Blackbuck (Anti- marsupials that possess a functional mar-
lope cervicapra) marking a stick with supium. The number varies from two in the
preorbital secretion (see Fig. 6o8B).
(Redrawn from Bourliere, after Hedi- fossorial N otoryctes (p. 714) to about twenty
ger.) in M onodelphis henseli (p. JII). In M onodel-
phis and M armosa the nipples are distributed between axillre and vent.
The complex group of events resulting in (r) the cellular synthesis of milk
from precursors in the blood stream and its passage through the membranes of
the alveolar cells, and (2) its discharge from the mammary gland, is lactation
(see below). The apparatus of lactation is present in both sexes. In Man the
phenomenon of 'witches' -milk' in boy as well as in girl babies exemplifies this
when, occasionally, abnormal amounts of maternal restrogen and prolactin
(p. 849) escape into the fretal circulation just before parturition. Further, milk
production has been experimentally achieved in male rats and other mammals.
In metatherians and eutherians each mammary gland opens to the exterior
by a teat which is guarded by a sphincter apparatus of varying complexity.
In most of the Ruminantia (as well as the Tylopoda, p. 838) the two (e.g.
 PHYLUM CHORDATA

sheep and goats) or four (e.g. cattle) mammary glands are of remarkable size
and are contained in an udder which is partitioned into galactophoric sinuses or
milk-cisterns in which the fluid accumulates before final liberation through the
teats. The careful selective breeding of domestic cattle and goats has resulted
in milk production enormously in excess of the needs of the wild animal.
In the embryo, milk streaks develop into the mammary-lines on each side
of the ventral mid-line. At definite points thereon appear centres of Mal-
pighian-layer proliferation and these at length develop into functional or often
supernumerary mammre. (Supernumerary mammre occasionally appear in
both sexes in Man which may sometimes be functional in females.)
Mammary development and milk-secretion are largely under hormonal con-
trol, but neural influences may be also involved. There is evidence that the
factors initiating the pubertal and subsequent seasonal growth of the mammary
glands differ somewhat among species, but, in general, it can be said that
restrogens (liberated as a result of pituitary gonadotrophic stimulation) are
always involved. In actual lactation, the anterior pituitary hormone, prolac-
tin (see also p. 151) is important, together, probably, with other lactogens
from the same (and possibly other) sources. Certainly the factors governing
the initiation, discharge and inhibition of lactation are more complex than was
formerly realised. Even the apparently well-established principle that a rise
in restrogen inhibits lactation has, perhaps unjustifiably, been called in ques-
tion. As regards neural factors, it seems established that the mechanical
stimulus of sucking causes a reflex discharge of prolactin (and possibly other
anterior pituitary lactogens) and that, in cows at least, stimulation of the teat
as well as other external factors may cause the liberation of posterior pituitary
oxytocin which facilitates 'let-down'. It is undeniably true that fear and
other unfavourable factors impede milk discharge, and so the nervous system
is involved in yet another way. The basic composition of milk is approximately
as follows (cf. c crop-milk', p. 583) :
TABLE IV. Composition of milk (expressed in percentages, modified
after Davies).

Species. Water. Fat. Sugar. Casein. Other

Protein. Ash.

Man 88·50 3'30 6·8o 0·90 0•40 0'20

Rat 68·3 14·8 2·8 4'2 2·6 r·5
Cat 8r·63 3'33 4'91 3'12 5·96 o·58
"---r----'
Porpoise 41'11 48·5o 1'33 11·19 0'57
Elephant 67•85 19'57 8·84 7'11 - 0·46
Ox 87'32 3"75 4'75 3'00 0•40 0 '75
Goat. 82'34 7"57 4'96 3•62
I o·6o o·84

In addition to the above basic constituents, milk also contains both fat-
and water-soluble vitamins, carotenoid pigments, enzymes, mineral salts (such
sso ZOOLOGY
as calcium phosphate and sodium chloride) and other materials. Before, and
briefly after, parturition the maternal mammary glands contain a colostrum
of high nitrogen, globulin and vitamin content. There is some evidence that
maternal antibodies thus continue to be transferred to the offspring for a short
period. The transition from colostrum to milk secretion proper occurs gradu-
ally during the days following birth.
Little is known concerning lactation in the Prototheria. Teats are absent.
The milk glands consist of two groups of very large tubular follicles, the ducts
of which open on the ventral body surface. In the ant-eater Tachyglossus
(p. 695) the two areas on which the ducts open become depressed towards the
breeding season to give rise to a pair of pouches-the mammary pouches. A
large brood-pouch or incubatorium is subsequently formed, and in this the egg is
deposited. When the young animal is hatched it is sheltered in the posterior
deeper part of this incubatorium, while in the shallower anterior part lie the
mammary pouches. In Ornithorhynchus mammary pouches are indicated
only by extremely shallow depressions. No incubatorium is developed.
Endoskeleton.-The spinal column of mammals varies in the number of
vertebrre which it contains, the differences being mainly due to variation in
the length of the tail. The various regions are usually very distinct. In
the cervical region the first two vertebrre are modified to form the atlas and
axis. Owing to the absep.ce of discrete cervical ribs, the posterior cervical
vertebrre are much more sharply marked off from the anterior thoracic than
is the case in reptiles and birds. The cervical vertebrre have double transverse
process (or a transverse process perforated at its base by a foramen which
transmits the vertebral artery) in all except the last. There are certain excep-
tions (such as many marsupials, some rodents, Hyrax and the hippopotamuses)
where the seventh cervical vertebra is also perforated. The ventral portion
of the transverse process in certain cases (e.g., seventh and sometimes some of
the others in Man) arises from a separate ossification, and this is regarded as
evidence that this ventral part, even when not independently ossified, represents
a cervical rib. Seven is the prevailing number of vertebrre in the cervical
region. There are only three exceptions to this-the manatees and two genera
of sloths (p. 764). Thus giraffes have the same number of cervical vertebrre as,
for example, Man. The number of thoracic and lumbar vertebrre is far more
variable. Usually there are between nineteen and twenty-three. Hyrax has
a larger number of thoraco-lumbar vertebrre than any other mammal-from
twenty-nine to thirty-one.
The thoracic vertebrre bear ribs which are connected, either directly or by
intermediate ribs, with the sternal ribs, and through them with the sternum.
Each rib typically articulates with the spinal column by two articulations
-one articular surface being borne on its head and the other on its tubercle.
The tubercle articulates with the transverse process, and the head usually with
 PHYLUM CHORDATA 851
an articular surface furnished partly by the vertebra with which the tubercle
is connected, and partly by that next in front. Thus the head of the first
thoracic rib partly articulates with the vertebral body of the last cervical
vertebra.
In all the Mammalia in which the hind-limbs exist (i.e. all save the Sirenia
and the Cetacea) there is a sacrum consisting of closely united vertebrre, the
number of which varies in the different orders. The caudal region varies
greatly. In many long-tailed mammals there is developed a series of chevron
bones-V-shaped bones which are situated opposite the inter-vertebral spaces.
The body of each vertebra ossifies from three centres-a middle, an
anterior, and a posterior. Usually the two centres of ossification which form
the neural arches also contribute to the formation of the bony body. The
middle centre forms the body proper ; the anterior and posterior form the
epiphyses. The epiphyses are almost entirely absent in the monotremes.
They have not been detected in dugongs (Sirenia). Between successive centra
is formed a series of discs of fibro-cartilage-the inter-vertebral discs-re-
presented in lower vertebrates in crocodiles and birds only. The anterior
and posterior surfaces of the bodies are nearly always flat.
The sternum consists of a number of segments-the presternum in front, the
mesosternum, or corpus sterni (composed of a number of segments or sternebrce)
in the middle, and the xiphisternum behind. The sternum is formed in the
fc:etus in great part by the separating off of the ventral ends of the ribs. Some
of the Cetacea and the Sirenia are exceptional in having a sternum composed
of a single piece of bone. The sternal ribs, by which the vertebral ribs are
connected with the sternum, are usually cartilaginous, but frequently undergo
calcification in old animals. In some cases they soon become completely
converted into bone.
The skull of a mammal (Fig. 6ro) contains the same chief elements and
presents the same general regions as that of the reptiles and birds, but exhibits
certain special modifications. But certain elements present in the skull of
reptiles and birds are not represented (or are not certainly known to be re-
presented) by separate ossifications in the Mammalia. Such are the supra-
orbital, the prefrontal, the postorbital, the ectopterygoid and the quadrato-
jugal. The bones of the skull, with the exception of the auditory ossicles, the
lower jaw, and the hyoid, are all immovably united by means of sutures.
The palatine bones develop palatine plates separating off a posterior nasal
passage from the cavity of the mouth, a condition found among the living
reptiles only in the Crocodilia, and, to a less extent, in the Chelonia and
some lizards.
The zygomatic arch is a strong arch of bone formed partly of the squamosal,
partly of the jugal, and partly of the maxilla; in position it represents the lower
temporal arch of Amphibia, reptiles and birds, but is differently constituted.
 ZOOLOGY

The orbit in the skull of some mammals is completely enclosed by bone, con-
stituting a well-defined cavity. In others it is not completely surrounded
by bone behind, and so communicates freely with the temporal fossa which
lies behind it.
The periotic bones (fused prootic and opisthotic) are not separately repre-
sented in the skull of mammals. Part of the periotic mass sometimes projects
on the exterior at the hinder part of the lateral region of the skull, and is the

••
0

I
'~--------~V.~o ________--J/

:m:

FIG. 6Io.-Mammalia: Skull. Relations of the principal bones. A-S. ali-sphenoid; B-Hy.
basi-hyal; B-0. basi-occipital; B-S. basi-sphenoid; C-Hy. cerato-hyal; E-Hy. epi-hyal; Ex-0.
ex-occipital; Fr. frontal; ju. jugal; La. lachrymal; M. mandible; ME. mesethmoid; Mx.
maxilla; Na. nasal; 0.-S. orbito-sphenoid; Pa. parietal; Per. periotic; Pl. palatine; P.-S. pre-
sphenoid; PMx. premaxilla; Pt. pterygoid; S-Hy. stylo-hyal; S-0. supra-occipital; Sq. squa-
mosal; T-Hy. tympano-hyal; Th-Hy. thyro-hyal; Turb. turbinal; Ty. tympanic; Vo. vomer.
(Capitals-replacing bones; italics-investing bones.) The numbered circles indicate the points
of exit of the cranial nerves; I, olfactory; 2, optic; 3, oculo-motor; 4, trochlear; 5, 5', 5", the
three divisions of the fifth nerve; 6, abducent; 7, facial; 8, auditory; 9, glosso-pharyngeal; 10,
pneumogastric; II, spinal accessory; 12, hypoglossal. (After Flower.)

mastoid portion. The rest is commonly called the petrmts portion of the
periotic, and encloses the parts of the internal ear. The mastoid portion con-
tains only air cells. The tympanic bone, which represents the angular of rep-
tiles and birds, sometimes forms a long tube, sometimes only a mere ring of
bone. In other cases it not only gives rise to a tube-the external auditory
meatus-but also forms the bulla tympani, a dilated bony process containing
a cavity.
The tympanic cavity, in which lie the auditory ossicles, is hollowed out of
 PHYLUM CHORDATA 853
the petrosal which is itself formed by the complete fusion of the pro-otic and
opisthotic bones. In the primitive condition, as can be seen in some
of the marsupials, 'insectivores', edentates, etc., this chamber has no bony
covering, and the tympanic bone (ectotympanic) is an open ring lying loosely
and supporting the ear-drum. When, as in most cases, a bony covering is
formed, this structure is termed the bulla. It is not always formed in the
same way in different groups. In those marsupials in which a bulla is present

INSECTIVORE

PRIMATE
PLATYRRHil'(E

TARSIERS
and
CATARRHINES
FIG. 6n.-Eutheria: Tympanic structure. The tympanic bone is black; the petrosal dotted,
the squamosal horizontally shaded and the basi-occipital vertically shaded. (For 'insectivores'
seep. 726.)

it is always made from the alisphenoid, the tympanic forming the external
auditory meatus. In the Primates it is formed chiefly by an outgrowth of the
petrous bone, which may either grow round the tympanic ring so as to enclose
it (as in the Tupaiidre and Lemuridre (Fig. 6II), where it lies free in the cavity),
or (as in the Lorisidre and platyrrhine monkeys) it joins on to the edge of the
tympanic ring, which thus forms part of the bulla wall and acts as the opening
of the middle ear. Again, in the tarsiers and catarrhine monkeys it is further
produced outwards to form a bony tube-the external auditory meatus. In
other mammals the tympanic, as a rule, forms most of the bulla, but the
VOL. II. G32
854 ZOOLOGY
entotympanic, squamosal, alisphenoid, and basisphenoid may also take some
part either in the formation of the bulla or as supporting elements.
The occipital region presents two condyles for articulation with the atlas.
The mandible consists in the adult of one bone, the equivalent of the dentary
of reptiles and birds, on each side. The two rami, as they are called, are in
most mammals closely united at the symphysis. The mandible articulates
with an articular surface formed for it by the squamosal bone, below the
posterior root of the zygomatic arch.
The hyoid apparatus (Fig. 612) is a bony complex situated between the
larynx and the base of the skull which serves to support the tongue and to
suspend the larynx and trachea. It exhibits
sty. hy. considerable variation in different mammals. It
consists of a transverse, flattened basihyal or
corpus ('body of hyoid') to which are articulated
anterior and posterior cornua (or ' horns'). Each
brief posterior cornu consists of a thyrohyal to
which is attached the thyroid cartilage of the
larynx. On each side of the pharynx extends an
anterior cornu consisting of an articulated
succession of three bones, the ceratohyal, epihyal,
and stylohyal elements. In some mammals (e.g.
Man) the last-named may unite with a tympa-
bas. hy. nohyal as a styloid process.
FrG. 6rz.-Canidre: Hyoid ap- The ratio between cranial capacity and facial
paratus. Extracranial portion (of
dog). Front view. Sty. hy. stylo- skeleton varies greatly in the different orders.
hyal; epi. hy. epihyal; cer. hy. The greater development of the cerebral hemi-
ceratohyal (these three constitute
the 'anterior cornu'); bas. hy. spheres in the higher groups necessitates a more
basihyal or 'body' of hyoid; thy.
hy. thyrohyal or' posterior cornu·. capacious cranium. This is brought about by the
bulging upwards, forwards and backwards of the
cranial roof, resulting in a great modification in the primitive relations of certain
of the great planes and axes of the skull (Fig. 613). Taking as a fixed base line
the basicranial axis-an imaginary median line running through the basioccipital,
basisphenoid and presphenoid bones-it is found that the greatly expanded
cranium in the higher mammals has effected a marked alteration in the relations
to this axis I. of the occipital plane or plane of the foramen magnum ; 2. of the
tentorial plane or plane of the tentorium cerebelli (a transverse fold of the dura
mater between the cerebral hemispheres and the cerebellum), and 3- of the
ethmoidal plane or plane of the cribriform plate of the ethmoid. In the lower
mammals (A) these are nearly at right angles to the basicranial axis. In the
higher groups, by the bulging forwards and backwards of the cranial roof,
the occipital and tentorial planes incline backwards and the ethmoidal for-
wards, until all three may become approximately horizontal. At the same
 PHYLUM CHORDATA sss
time there is produced a change in the relations of the basicranial axis to the
basifacial axis-a line passing along the axis of the face between the meseth-
moid and the vomer. In the lower forms the angle at which the basifacial

axis, when produced, meets the basicranial is an exceedingly open one. In

the higher forms, owing to the downward inclination of the facial region, this
angle decreases in size, though it is never reduced to less than a right angle.
The pectoral arch of the Mammalia has fewer distinct elements than that of
 ZOOLOGY

reptiles and birds. The coracoid, which, in the last two groups, is a large bone,
shares by means of its dorsal end in the bounding of the glenoid cavity. Its
ventral end, articulating with the sternum, is in the adult never present as a
distinct bone. In the young of many mammals it appears to be represented
by a small ossification which enters into the glenoid facet. This very soon
coalesces with the scapula. The coracoid process is a separate ossification in
the young mammal, and, though in most instances it completely fuses with the
scapula and with the smaller coracoid element, it is sometimes recognisable as
a distinct element up to a late period (many marsupials, sloths). It appears
to correspond to the bone called epicoracoid in the Prototheria (see p. 697).
In fretal marsupials the coracoid is represented by a well-developed cartila-
ginous element which extends inwards and meets the rudiment of the sternum.
In the scapula a spine is nearly always developed, and usually ends in a
freely projecting acromion process. Unlike the main body of the scapula it
is developed without any antecedent formation of cartilage, and is perhaps to
be compared with the cleithrum, an investing bone occurring in some Amphibia
and Reptilia (p. 397). A clavicle is well developed in many mammals, but is
incomplete or absent in others. Its presence is characteristic of mammals
in which the fore-limbs are capable of great freedom of movement. In the
mammalian embryo there is, in the position of the clavicular bar, a bar of
cartilage, which coalesces with its fellow in the middle line. The cartilaginous
tract thus formed segments into five portions-a median (which coalesces with
the presternum), two small inner lateral (which unite with the clavicles or are
converted into the sterno-clavicular ligaments), and two long outer lateral
(which give rise to the clavicles). The median and inner lateral portions appear
to correspond to the episternum of reptiles and Prototheria. An additional
small cartilage may represent the inner portion of the precoracoid of Amphibia.
A piece of cartilage at the outer end of the clavicle proper is sometimes dis-
tinguishable-the mesoscapular segment.
The three elements of the pelvic arch unite to form a single bone, the
innominate. The ilia unite by broad surfaces with the sacrum ; the pubes,
and sometimes the ischia, unite in a symphysis. All three may take a share in
the formation of the acetabulum, but the pubis is usually excluded by a small
cotyloid bone. In both monotremes and marsupials (except in Thylacinus
where they are cartilaginous) epipubic bones occur, the so-called 'marsupial'
bones. They occur in both sexes and in unpouched as well as pouched species.
They are large and flat, are only lightly joined to the rest of the pelvis, and
spring upwards and outwards to taper and terminate in the muscles of the
abdominal wall. It would seem that their function is to lend support for the
abdominal wall: there is no evidence of any present or past function in relation
to actual locomotion.
In the shank the inner or tibial element is always the larger ; the fibula
 PHYLUM CHORDATA

may be vestigial. A large sesamoid

bone-the patella-is almost universally
formed in close relation to the knee-
joint.
The most primitive type of ex-
tremity is the plantigrade, with five
sub-equal digits, a flexible carpus and
tarsus, and with the bones' interlocking'
(e.g. Tritemnodon, Fig. 614, and many
primitive mammals). From a general-
ised pattern of this sort many modifica-
tions have been derived. As an adapt-
ation to increasing speed, the wrsorial
patterns proceed through the subdigiti-
grade to the digitigrade, and finally to
the unguligrade condition as the extreme
adaptation along this line. This is ac-
companied by an increase in the length
of the digits, either along the axis of the
third toe- the mesaxonic pattern-or
between the third and fourth toes-the
paraxonic pattern. An early stage of a
mesaxonic foot can be seen in the
condylarth Tetraclcenodon, and its final
stage in horses or in the Litopterna
(Fig. 615) where a functionally one-toed
condition has been reached. Although
these animals are morphologically three-
toed, the second and fourth toes are
still represented by splint bones. The
paraxonic hand and foot , of which an
early stage can be seen in the creodont
M esonyx (Fig. 6r6), has the axis passing
between the third and fourth digits,
which enlarge at an equal rate. The
second and fifth toes, on the other hand,
become equally reduced, and the first FrG. 614.-Camivora: Primitive planti-
toe disappears. The final result can be grade, pentadactyle limbs. Carpus (upper) and
tarsus (lower figure) of Tritemnodon agilis. a.f.
seen in the more advanced Artiodactyla astragalar foramen; ast. astragalus; cal.
cb. cuboid; ce. centra le ; cttn.
(e.g. a cow or sheep) where there is a calcaneum;
cuneiform ; c. I, 2, 3, ento·, meso, and ecto.
complete reduction to two toes and a cuneiforms; lu. lunar ; mg. magnum; nav.
navicular ; sc. scaphoid; td. trapezoid; tm.
fusion of the two metapodials into the trapezium. (After Matthew.)
 ZOOLOGY
characteristic 'cannon bone' (Fig. 617). The bones of the carpus and tarsus,
while retaining the characters of the interlocking type, are arranged to allow

A B c D

FIG. 615.-Eutheria: Convergent adaptations in cursorial mammals. Two widely unrelated

groups-Hippomorpha and Litopterna-illustrating a single-toed trend in fore- and hind-limbs.
A, Merychippus (Miocene horse), three-toed stages; B, Diadiaphorus (Miocene-Pliocene litoptern),
three-toed stages; C, Pliohippus (Pliocene horse), single toed stages; D, Thoatherium (Miocene
litoptern), single-toed stages. (Redrawn after Matthew.)

great flexion in the fore-and-aft direction, but little in the lateral. In many
artiodactyles some of the tarsalia are fused (Fig. 6r8).
In bulky animals such as the elephants, Dinocerata, etc., the carpals and
 PHYLUM CHORDATA 859
tarsals become serially arranged with a great loss of flexibility, and the digits,
while not reduced in number as a rule, are short and stout. Special habits,
such as hopping (the kangaroos ; M acroscelides among in-
sectivores; Dipus and Allactaga among rodents (Fig. 6rg));
swimming (e.g. the Cetacea, Sirenia, seals) ; flying (e.g. the
bats and to some extent the parachuting kinds of mar-
supials and rodents) ; digging (as in the moles and several
other forms) all produce more or less profound modifications
in the limbs, hands, and feet.
The generalised hand and foot has already been men-
tioned (see p. roo, Fig. 71). In the mammals the carpus
contains three proximal bones: the scaphoid, lunar, and
cuneiform, to which is added on the outer side, the pisi-
form, possibly a sesamoid, but equally possibly a true carpal
element. The distal row consists of four bones: the tra-
pezium, trapezoid, magnum, and unciform. The last named
is a fusion of the fourth and fifth original elements of this
row. In primitive mammals another bone is present, the
centrale, which lies between the bones of the first and second
rows. In reptiles there can be as many as three centralia,
but in mammals there is never more than one, which may
disappear either by fusion or loss.
The tarsus (Figs. 6rg, 623, 624) is usually rather more
modified from the generalised pattern than is the carpus.
There are only two proximal bones: the astragalus and
calcaneum, which correspond to the lunar and cuneiform.
The distal row is represented by the ecto-, meso-, and
entocuneiforms, and the cuboid, which is formed by the fusion
of the fourth and fifth cuneiforms and corresponds to the
unciform of the hand. The centrale becomes a large and FIG. 6!6.-Car-
important bone, the navicular. nivora: Paraxonic
carpus and tarsus.
The astragalus is also, in another sense, an important Mesonyx, an Eocene
Creodont. ast. astra-
bone, because its shape differs in the various orders, but galus; C 1 , 2 , 3 , ento-,
itself remains true to its particular type, subject only to meso-, and ectocunei-
forms; cal. calcane-
differences in proportion, however much the rest of the foot um; cb. cuboid; cun.
cuneiform; lu.lunar;
may become modified. It is therefore a very useful guide mg. magnum; nav.
to the affinity of any form, especially in the case of certain navicular; pis. pisi-
form; sc. scaphoid;
extinct mammals. The astragalus consists of a proximal td. trapezoid; tm.
trapezium; unc. un-
grooved surface, the trochlea (which articulates proximally ciform.
with the tibia and fibula), a neck and a head of varying shape
(which articulates with the navicular and sometimes with the cuboid as well).
In many early forms an astragalar foramen is present near the upper border of
86o ZOOLOGY
the trochlea. This foramen usually disappears in later mammals, but still
occurs sporadically, as, for example, in Orycteropus (p. 804).
The main modifications of the astragalus are as follows. In the Condy-
larth-Creodont-Camivore group the tibial trochlea is more or less grooved,
obliquely pitched, with the inner (tibial) crest lower than the outer (fibular),
or even absent; the neck is distinct and the head convex. In the Insectivora
the trochlea is broad and shallow, but with well-defined crests of equal height,
the neck is oblique and oval in section, the head convex. The primate astra-

11-

ch

I
_,

m
IY .J1I
FrG. 6r8.-Cer- FrG. 6zo . -
vus: Tarsus. Right, Tapirus: Manus.
FIG. 61].- dorsal surface. a. T . indicus. c. cunei-
Cervus: Manus. astragalus; c. cal- form; l. lunar; m .
C. elaphus. Red caneum; cb. cuboid; magnum; p. pisi-
1/L
Deer. m2. m 5 • c3 . conjoined ecto- form; R. radius;
vestigial second and meso-cuneiform; FIG. 619.- s. scaphoid; td.
and fifth meta- mill, mlV, third Dipus: Pes. trapezoid; tm. tra-
carpals; R. ra. and fourth metatar- J erboa. Let- pezium; U. ulna;
dius. (After sals; n. navicular. tering as in 11. unciform. (After
Flower.) (After Flower.) Fig. 6r8. Flower.)

galus has a concavo-convex trochlear surface, broader at the neck end than
above, a somewhat oblique neck and a convex head. In the rodents the
general appearance is like that of the insectivores, except that the trochlea
extends backwards to the posterior margin of the bone. In the Perissodactyla
the trochlea is deeply grooved and oblique, the neck short, and the head
flattened. In the earlier forms there is a very small astragalo-cuboid facet.
In more advanced forms, especially in the rhinoceroses and titanotheres, this
becomes larger, though never to the extent that it does in the highly charac-
teristic artiodactyle astragalus. Here the trochlea is deeply grooved in a
straight line with the head and neck, with well-marked facets for both the
 PHYLUM CHORDATA 861

cuboid and navicular. In this type also the sustenacular facet on the medial
aspect (which articulates with the calcaneum) is a single large and flat surface
instead of being divided, as in other astragali.
Other bones of the carpus and tarsus have their distinguishing character-
istics, if rather less obvious than those of the astragalus, as, for example,
the artiodactyle cuboid, which shows a constant difference from the cuboid of
all other mammals in having a facet on its outer border for the calcaneum.

m:
FIG. 62 1 . -
Equus: Manus.
Horse. c. cunei- N DI
~=
form; l. lunar; FIG. 623.- Equus:
m. magnum ; p. FIG. 622. - Tarsus. Horse. Right . FIG. 624 .- Sus:
pisiform; R. Sus: Manus. Pig. a. astragalus; c. calca- tarsus. Pig. Right.
radius; s. sca- c. cuneiform; l. neum; cb. cuboid; c. a. astragalus; c.
phoid; td. trape- lunar; nz. mag- united meso- and ento- calcaneum; cb. cu-
zoid; u. unci- num; R. radius; cuneiform; c 3 • ecto-cunei- boid; c3 • ecto-cu-
form; II, IV, s. scaphoid; td. form; n . navicular; mil, neiform; c2 • meso-
vestigial second trapezoid; U. IV, vestigial second and cuneiform; mii-
and fourth m eta- ulna; u . unci- fourth metatarsals; I I 1, V, metatarsals ; n.
carpals. (After form. (After third m etatarsal. (After navicular. (After
Flower.) Flower.) Flower.) Flower.)

The external form of the limbs and the mode of articulation of the bones
vary in the different orders of the Mammalia, in accordance with the mode of
locomotion. In most the habitual attitude is that which is termed the quad-
rupedal wherein the body is supported in a horizontal position by all four limbs.
In quadrupedal mammals the manus and pes sometimes rest on the ventral
surfaces of the entire metacarpal and metatarsal regions as well as on the
phalanges (when the limbs are said to be plantigrade) ; or on the ventral
surfaces of the phalanges only (digitigrade) ; or on the hoofs developed on the
terminal phalanges (unguligrade). Many of the quadrupeds have the ex-
tremities prehensile with the manus and pes converted into grasping organs.
862 ZOOLOGY

This is most marked in quadrupeds that pass the greater part of their life among
the branches of trees. In the sloths the modification goes so far that both
hands and feet are converted into mere hooks by means of which the animal
suspends itself body downwards from the branches of trees (Fig. 539, p. 768).
Certain mammals, again, have their limbs modified for locomotion through
the air. The only truly flying mammals are bats, in which the digits of the
fore-limb are greatly extended so as to support a wide delicate fold of skin
constituting the wing. In the so-called 'flying' squirrels and 'flying'
phalangers and other gliders there is no active flight, and the limbs undergo no
special modification. The 'flying' organ in these cases is merely a parachute
or patagium in the form of lateral flaps of skin extending along the sides of the
body between the fore- and hind-limbs (Fig. 498, p. 717; Fig. 528, p. 754).
Finally, there are several groups of swimming mammals. Most mammals,
without any special modification of the limbs, are able to swim, and some of the
quadrupeds, such as the tapirs and hippopotamuses, spend a great part of their
life in the water. But there are certain mammals in which the limbs are
specially modified to assume the form of flippers or swimming paddles and
for these locomotion on land becomes almost: sometimes quite, impossible.
Such are the whales and porpoises, the dugongs and manatees, and, in a less
degree, the seals and walruses (Fig. 554, p. 780; Fig. 594, p. 821).
Alimentary Canal and Associated Structures.- Teeth are present in nearly all
mammals, but in some they do not occur in the adult condition (whalebone
whales, Ornithorhynchus). In Tachyglossus teeth are absent throughout life.
In some of the ant-eaters teeth are developed in the fretus and are discarded
in utero-the adult anima]. being devoid of them (p. 871).
Teeth, already described in the general account of the Craniata (p. 103),
are developed partly from the epidermis and partly from the underlying dermis.
In the mammals each tooth is lodged in a socket (alveolus) in the jaw. The
part of the tooth developed from the epidermis is the enamel. The remainder
of the tooth--dentine, cement, and pulp-is formed from the subjacent meso-
dermal tissue (Fig. 625).
Along the oral surface of the jaw is formed a ridge-like ingrowth of the
ectoderm-the dental lamina-and from it a bud is given off in the position
to be occupied by each of the teeth. This bud becomes constricted off as a
conical cap of cells-the enamel-organ-which remains in continuity with the
dental lamina by a narrow isthmus. The cap-like form of the enamel-organ
is brought about by its growth over a concentration of dermal tissue-the
dental papilla. The dental papilla has a rich blood supply. On the surface
of this papilla, in contact with the enamel-organ, a layer of cells (odontoblasts)
becomes arranged rather like an epithelium. This layer of odontoblasts is
the dentine-forming layer. The cells of the enamel-organ which are in contact
with the dental papilla become long and cylindrical to form ameloblasts. This
 PHYLUM CHORDATA

layer of ameloblasts forms the inner enamel epithelium. The cells on that
part of the surface of the enamel organ which is not in contact with the dental
papilla form a layer of cubical cells, which is called the outer enamel epithelium.
Between these two layers the remaining cells of the enamel organ become
modified: those in contact with the inner enamel-epithelium form a distinct

Remains of enamel
organ over erupted

regrening stellate
reticulum of
enamel organ

.'n'!II~T--boneot alveolllr
socket

peridental membranE
(fibres)

osteoblast layer of
alveolar periosteum

nerves and vessels

supplying pulp

primordium of
permanent incisor

FIG. 625.-Primates: Tooth structure. Semi-schematic, showing an erupting 'milk' incisor

and primordium of adjacent permanent tooth. In the deciduous tooth the root is not yet fully
formed but the arrangement of its tissues indicates h ow it will b e attached within the alveolar
socket. (Modified after Patten.)

layer (the stratum intermedium), while the remainder become vacuolated and
form a loose tissue known as the stellate reticulum. Around the enamel organ
and dental papilla there develops a layer of connective tissue-the dental
follicle. This contains many blood vessels.
The hard part of the tooth begins to develop by the formation of a cap of
predentine (produced by the odontoblasts); and of a layer of enamel matrix
which is laid down on the surface of this (produced by the ameloblasts). The
 ZOOLOGY
predentine and enamel matrix calcify to form the dentine and enamel. Addi-
tional layers are added until the crown of the tooth is complete. The enamel
organ then degenerates, but the dental papilla remains as the pulp of the fully-
developed tooth. Roots are formed, and the crown of the tooth then erupts
into the mouth cavity. The roots of the teeth consist of dentine over which a
layer of cement is deposited. This is formed by the cells of the dental follicle.
There is no enamel on the roots. (In some mammals cement is also formed
on the surface of the enamel on the crown, e.g. modern horses, rabbits, some
rodents, some artiodactyles, and the elephants.) In mammals the roots of the
teeth generally open at their bases by small foramina through which nerves

FIG. 626.-Canis: Deci-

duous and permanent denti-
tion. Upper(/) and lower (II)
jaws of dog, with the symbols
by which the different teeth-
incisors, canines, premolars
and molars-are commonly
designated. The prefix 'd',
or sometimes 'm', indicates
deciduous or milk teeth.
(After Flower and Lydekker.)

and blood vessels enter the pulp. However, in the teeth of some (e.g. the
molar teeth of modern horses and elephants) the formation of roots is delayed,
and the enamel organ which persists around the sides and within the folds
of the crown continues to add to its height long after the tooth has erupted
and come into use. In other mammalian teeth (e.g. the incisors of rodents
and elephants, and all the teeth of rabbits) roots are never formed and the
crowns of the teeth continue to grow throughout life. Such teeth are said to
have 'persistent pulps'.
Usually mammals have two distinct sets of teeth developed, the deciduous
(milk) and permanent dentitions (Fig. 626). Sometimes there is only one set
present. Accordingly diphyodont and monophyodont dentitions are distin-
guished. In nearly all of the latter, however, another set is developed, though
the teeth early become absorbed or remain as functionless vestiges. The
 PHYLUM CHORDATA s6s
milk-teeth in mammals with typical diphyodont dentition sometimes dis-
appear at an early stage (e.g. seals), and sometimes persist and do not become
replaced by the permanent teeth till long after birth (e.g. Man). Some
mammals have the teeth indefinite in number, e.g. the dolphins and por-
poises. Such teeth are all uniform in appearance (homodont) (Fig. 627) and
not divided into functional varieties (Fig. 628). Where they are divided into
functional varieties they are said to be heterodont.

FIG. 627.-Lagamorllynclms:
Homodont dentition. Adaptation
in dolphins t o fishing. (After Flower
and Lydekker.)

In the typical eutherian dentition there are forty-four teeth, viz. three
incisors on each side, one canine, four premolars and three molars both above
and below. The upper incisors (Fig. 626) are lodged in the premaxillre. The
lower incisors are placed opposite. The upper canine is the most anterior
tooth of the maxilla, situated on or immediately behind the premaxilla-
maxillary suture and has usually a characteristic shape. The lower canine
bites in front of the upper. The premolars are distinguished from the molars
by having milk predecessors. The molars have no teeth preceding them, and
are sometimes looked upon as persistent teeth of the first set. As a rule in
heterodont dentitions the incisors have
cutting edges, the canines are pointed
and conical, the premolars and molars
have broad surfaces with ridges and
tubercles for crushing the· food, and
may have from two to four roots.
The crown surface of the molar
teeth of mammals shows a wide range
FIG. 6z8 .- l'ermne les: Dentition. Mar-
supial ba ndicoot. (After Owen.) of pattern which, used with caution and
due regard to the possibility of con-
vergence, is of great use in classification. With very few exceptions these
patterns can be expressed as modifications of a relatively simple type, and this
type itself can be explained as a modification of a simple ancestral reptilian
cone. This basal type of mammalian molar pattern is termed the tritubercular,
with respect to the upper molar and trituberculo-sectorial with respect to the
lower. The theory of its origin, due originally to Cope, and subsequently
added to and amended by Osborn, is known universally as the Cope-Osborn
theory of trituberculy. The theory in its modified form will be more easily
understood if a brief and condensed history of its origin is first given.
VOL. II. H 3
866 ZOOLOGY
Cope saw that the molar teeth of the earliest eutherian mammals commonly
possessed a triangular crown with three main cusps, and that later forms
tended to have molars with from four to six cusps. These he regarded as
derived from the earlier triangular type. Osborn greatly developed this

.
hypothesis and, on the assumption that the principal cusps are homologous
throughout the Mammalia, gave them names that have been in universal use

COPE-OSBORN
..
,
~ GREGORY
____ .,,.... ...... __

~--
_... -- _.. .... ........... -
- ......
r''

~ $
~~
1
1
:<E3>
I
I
I I
I I
1 I
~ FrG. 629.-Mamm.alia: Den-

¥~ ~
tition. Diagram comparing the
I

original Cope-Osborn theory of

the tri tubercular origin of molar
teeth with its modification by
Gregory. At the top the two
black circles represent an upper
and lower reptilian cone as the
I starting point. The black
circles in the other teeth repre-

~
sent the protocone of the upper
and protoconid of the lower
tooth, respectively.

------
-----------
LABIAL ---------

Metacone~ Paracone
POSTERIOR Metaconule Protoconule ANTERIOR
Hgpocone Protocone

Entoconid ~ Protoconid
Hgpoconulid---Jo ~ .
Entoconulid ~~ Paracontd
Metaconid
BUCCAL

ever smce. Osborn's view as to the method of origin of the individual cusps,
which has since been modified in some important respects, was as follows
(Fig. 627). Starting with a simple reptilian cone, this was assumed to have
elongated in an antero-posterior direction and two subsidiary cones to have
arisen one on the front and one on the hind border, thus producing a triconodont
tooth. These two cusps were then supposed to have rotated in opposite
directions in the upper and lower jaws to form triangular or tritubercular teeth.
The apex of the triangle, or trigon, points inwards in the upper tooth and out-
 PHYLUM CHORDATA

wards in the lower. The cusp at the apex of the triangle, being supposed to
represent the original reptilian cone, was named the protocone in the upper and
protoconid in the lower tooth. 1
At a very early stage criticism was directed against the view that the proto-
cone of the mammalian tritubercular tooth represents the original reptilian
cone largely because in ontogeny it is not the first cusp to be formed, nor is
there any known evidence to support the hypothesis of the rotation of the sub-
sidiary cusps to form the triangles. Within the premolar series, and still more
in their deciduous predecessors, there is usually a transition from behind
forwards from a tooth that resembles a molar in many respects to one that is
more like a canine or an incisor. If the single cusp of the canine is traced
oackwards through the successive teeth, it is seen to correspond to the para-
cone of the molar, and not to the protocone. This is the essence of the 'pre-
molar analogy' theory. The protocone of the molar is represented on the
more anterior teeth as an inner shelf at the base, or is even completely absent,
and it was therefore thought probable that this cusp is secondary in origin
to the paracone, a statement which is consistent with the embryological
evidence. Further evidence for the same theory is found in the fact. that in
some groups the premolars evolve from simple peg-like teeth to a molariform
condition. This process is called the molarisation of the premolars. Further,
in this evolution the original cusp becomes the paracone of the molar rather
than the protocone.
Further study of the dentitions of Mesozoic mammals has led to modi-
fications of Osborn's theory. It is widely held that, whereas in the lower
molar the protoconid does represent the tip of the originally simple reptilian
tooth, this is not true of the protocone of the upper molar. According to some
investigators the original upper molar cusp was the paracone, but others think
it was an 'amphicone ', which split to form the paracone and metacone. How-
ever, the names which Osborn gave to the cusps have become so widely applied
to Cenozoic mammals that it is generally agreed to continue to call the lingual
cusp of the tritubercular tooth the protocone, even though it may not have
arisen as Osborn supposed.
In many early forms of tritubercular teeth, in Cretaceous and Palreocene
mammals, the paracone and metacone were far removed from the buccal edge
of the crown, which was occupied by a wide shelf bearing a number of styles.
This type of upper molar, which is retained by the living opossums, may be

1 The suffix -id designates a cusp on the lower molar. Of the other two cusps, the antero-
external was named the paracone and the postero-external the metacone. A fourth cusp arising
later on the cingulum on the postero-internal side, termed the hypocone, produces a quadri-
tubercular tooth, and by the evolution of two smaller intermediate cusps, an anterior, the prolo-
conule, and a posterior, the metaconule, the six cusped tooth, which underlies the molar pattern of
so many mammals, comes into being. Like the upper, the lower molar consists of a triangle, or
trigonid, and in addition a posterior shelf, the talonid, on which a number of cusps arise to produce
the trituberculo-sectorial pattern, termed the entoconid, hypoconulid, and hypoconulid.
868 ZOOLOGY
called pretritubercular. It is not very different from the dilambdodont teeth
of Soricoidea and Tupaia. It is probable that the more typical tritubercular
tooth has evolved from the pretritubercular condition by reduction of the
buccal shelf to a cingulum, so that the paracone and metacone come to stand
near to the buccal edge of the tooth. At the same time the protocone enlarged.
On the other hand, in the zalambdont teeth of some Lipotyphla and the mar-
supial mole (p. 714), the protocone was reduced and the paracone was displaced
towards the lingual side, so that in extreme cases it occupies the position taken
by the protocone in tritubercular teeth. The upper molars of Dryolestoidea
appear to resemble those of zalambdodont Lipotyphla, but this resemblance
is almost certainly secondary. 1
It is certain that, in whatever manner the tritubercular tooth may have
arisen, the subsequent sexi-tubercular tooth has been derived from it. This is
borne out by an enormous amount of direct fossil evidence, and it is possible
to show that such diverse tooth-patterns as that of an artiodactyle, a perisso-
dactyle, or a carnivore can be derived from the tritubercular type.
Further terms are in use to designate different parts of a tooth. If the cusps
remain separate and rounded, the tooth is termed bunoid, if they join to form
ridges, these ridges are termed lophs, and the tooth is lophodont. A tooth with
crescentic cusps is selenodont. Any tooth may have a combination of these
characters, in which case it is called buno-lophodont, buno-selendont, etc.
Additional pillars such as can be seen in the molar of a horse, or on the outer
border or ectoloph of many teeth, are called styles-e.g. the proto-, meso-, and
metastyle, etc. A tooth with a low crown is termed brachydont, and with a high
crown and deep socket termed hypsodont.
As already stated, the crown pattern of the teeth of mammals is largely
used as a guide to their affinities. Generally speaking, it is a fairly dependable
one, and starting from the more primitive and generalised forms of dentition,
such as is shown by the early insectivores, creodonts, and condylarths, the
major groups of mammals usually have trends of evolution on their own
recognisable lines. Thus, for example, the Carnivora, the Artiodactyla, and
Perissodactyla have all ultimately acquired dental patterns which are suffici-
ently distinctive. Other groups, such as the Proboscidea, Cetacea, and
Xenarthra, have patterns that are both distinctive and diagnostic. At the
same time exceptions occur, due presumably to some unusual and specialised
kind of food, such as in Proteles (an insect- and worm-eating hycena), or Potos
(a fruit-eating procyonid), both of which animals have their dentition modified
away from the usual carnivore type. These two are examples of a dentition
that has degenerated, but there are many instances of tooth resemblance, more
or less close, among unrelated animals due to convergence, the result of similar
1 For a discussion of this question, and a full account of the history of the tritubercular theory,
see Gregory, and for details of the Mesozoic forms, both Simpson and Butler.
 PHYLUM CHORDATA 86g

feeding habits. Thus a permanently growing gnawing incisor is not absolutely

diagnostic of a rodent, but can be found in the primate, Chiromys (Daubentonia)
(p. 737), in Tillotherium and even in the artiodactyle Myotragus. A bilo-
phodont form of molar tooth occurs in many diverse orders of mammals.
The difficulty of assessing the true affinity of some mammals on the evidence of
their teeth alone increases when the earlier and more primitive forms are con-
sidered : and to decide whether a Palreocene or an Eocene tooth is to be
ascribed to an insectivore, a primate, a condylarth or a creodont is difficult,
and sometimes impossible. Precaution therefore must be taken against
regarding as evidence of affinity a similarity of pattern, which may be the result
of convergence due to a similarity of habit.
A comparison of the wide adaptive radiation of the teeth of the marsupials
into insectivorous, carnivorous, rodent, and herbivorous types of dentition
with that obtaining in placental mammals of corresponding habits is
instructive.
The number of the various sets of teeth in
the jaws is conveniently expressed by a
dental formula, in which the kind of tooth
(incisor, canine, premolar, molar) is indicated
by the initial letter (i., c., p., m.), and the
whole formula has the arrangement of four
vulgar fractions, in each of which the
numerator indicates the teeth of the upper,
the denominator those of the lower jaw.
Thus :
· 3'3
z. -- ,c. -I·I ,p. 4'4 3'3
- , m.- = 44..
3'3 I·I 4·4 3'3
or, in a simpler form, since the teeth of the
right and left sides are always numerically
FIG. 63o.- J>IIasco/arctos: Skull.
similar, Koala, in front view, illustrating
diprotodont and herbivorous d enti-
· 3
t. -,
I
c. - , p. 4- , 111. 3- = 44· tion. (After Flower .)
3 I 4 3
Tachyglossus has no teeth at any stage. In Ornithorhyncltus ridged teeth
are present in the young and are functional for a time, but they are replaced by
broad horny plates formed essentially by a down-growth of epidermis. One
of these occurs on each upper, and one on each lower, jaw (Fig. 48I, p. 695).
In marsupials the milk-dentition remains in a degenerate condition. With
the exception of one (the last premolar) the milk teeth remain in an imperfect
state of development, though they persist, as functionless vestiges, to a com-
paratively late stage.
In the adult marsupial dentition the number of upper and lower incisors
 ZOOLOGY
is always dissimilar, except in Phascolomys. With regard to the arrangement
of these teeth, the order falls into two series, termed respectively diprotodont
and polyprotodont. In the former (Figs. 630, 631) the two anterior incisors are
large and prominent, the rest of the incisors and the canines being smaller or
absent. In the latter (Figs. 632, 633), the incisors are numerous and sub-equal

_
and the canines large. There are typically three premolars and four molars .

. __/

FIG. 631.-Mac-ropus: Dentition.

Kangaroo.

FIG. 6]2.-Sa-rcophilus: Skull.

Tasmanian Devil, in front view,
illustrating polyprotodont and carni-
vorous dentition. (After Flower.)

A good example of the diprotodont arrangement is the kangaroo (Macropus,

Fig. 631), which has the dental formula-
. 3 I 2 4
t. - , c. - , p, -, m. - = 34·
I 0 2 4
The canine is very small and lost early. Of the polyprotodont forms the
Australian Dasyurus (Fig. 495, p. 7I3) has the formula-
·4 c. -I , p. -,
t. -,
2
m. -4_- 42 .,
3 I 2 4
and the American opossums (Didelphys) (Fig. 633)-
. 5 c. -I , p. 3-, nt. 4- =
t. - ,
4 I 3 4
so.
 PHYLUM CHORDATA

The Xenarthra, though not by any means all toothless, always have some
peculiarity of dentition. When teeth are present in the adult the anterior series
is absent and the teeth are imperfect, wanting roots and devoid of enamel.
The tooth-characters differ widely in the different groups. In the sloths there
are five teeth above and four below on each side ; no second series is known.
In the American ant-eaters there are no teeth in the adult. In the armadillos,
on the other hand, the teeth are numerous, though simple and rootless, and, in
one genus at least, two series occur. The scaly ant-eaters (Pholidota) lack
teeth. In the Tubulidentata (aardvark) (Fig. 634) again, there are five teeth
on each side which are heterodont and diphyodont, and of peculiar structure,
being perforated by numerous minute, parallel, vertical canals. The pulp of

Frc. 633.-Didelphys: Dentition. Upper

jaw of opossum (D. marsupia/is), showing the
condition typical of marsupials, in all of which
there is no succession except in the last pre.
molar, the place of which is occupied in the
young animal by a molariform tooth repre.
sented in the figure below the line of the other
teeth. (After Flower and Lydekker.)

Frc. 634.-0ryctet'opus: Den-

tition. Section of lower jaw and
teeth of Aardvark. (After Owen.)

each tooth, entire at its base, is divided distally into a number of parallel
columns.
In the Mesaxonia the dentition is heterodont and diphyodont, and the
teeth are very rarely devoid of roots. In the Artiodactyla the premolars and
molars differ from one another in pattern; the first upper premolar is almost
always without a milk predecessor. The Suidre (Fig. 635) are among the very
few recent mammals which possess the primitive and typical dentition, the
formula of which is-
· 3 I
~.-,c. -, p. 4-, m. 3-
= 44·
3 4 I 3
The upper incisors are vertical, the lower greatly inclined forwards. The
canines are greatly developed, especially in the male, and grow from persistent
pulps ; both the upper and lower are bent upwards and outwards and work
against one another in such a manner that the upper wears on its anterior and
external surface, the lower at the extremity of the posterior surface. The
872 ZOOLOGY
premolars are compressed, with longitudinal cutting edges, and the molars are
provided with numerous tubercles or cusps arranged for the most part in
transverse rows (bunodont type). The first permanent premolar has no
predecessor, the formula of the milk dentition being-

i. J, c. ~, m. J = z8.
3 I 3
In the typical ruminants there are no teeth on the premaxillre, the lower
incisors and canines, which resemble them in shape, biting against a thickened
callous pad on the opposed surface of the upper jaw. The upper canines are
also usually absent. There are three premolars and three molars in both upper
and lower series, all characterised by the presence of column-like vertical folds

FIG. 635.-Sus: Dentition. The

slashing canines of the boar possess
persistent pulps. The Melanesians of
the New Hebrides knock out the
upper canine, thus allowing the lower
ones to grow circularly without inter-
ruption. After about eight years the
lower tusk may re-enter the jaw near
its own socket or it may avoid the
bone and form a full circle or more.
(After Flower and Lydckker.)

of enamel. The interstices between these may be filled up with cement-the

worn surface of the tooth presents a pattern of the selenodont type. In the
camels there are a pair of upper incisors and a pair of large canines in each jaw.
In the Perissodactyla the molars and premolars form a continuous series
of large teeth with ridged or complexly-folded crowns, the posterior premolars
often differing little in size and structure from the molars. In the horse (Fig.
636) the formula is-
z.· 3-,c.-,
I 4 m. 3- = 44,
p. -,
3 I 4 3
but the first premolar is a small tooth which is soon lost. A fold of the enamel
dips downwards (i.e. towards the root) from the extremity of the incisor teeth
like the partly inverted finger of a glove ; the canines are small in the female,
and may not appear on the surface. There is a wide interval in both jaws
between the canines and premolars. The premolar and molar teeth present a
 PHYLUM CHORDATA

complicated pattern due to folds of the enamel, which differ in their arrange-
ment in the upper and lower jaws; their roots become completed only at a late
period.
In the Hyracoidea the dental formula is-
. I 0 4 3
z. -, c. -, p. -, m. - = 34·
2 0 4 3
The upper incisors are not unlike the larger pair of the rabbit in shape, though
prismatic and pointed, instead of compressed and chisel-like ; they grow from
persistent pulps. The lower incisors are elongated, inclined forwards, and tri-
lobed at the extremities. The premolars and molars form a continuous series,

FIG. 636.-Equus: Skull and

teeth. Side view in Horse with
the bone removed to expose the
whole of the teeth. c. canine;
Fr. frontal; il. i 2. i 3. incisors;
L . lachrymal ; Ma. jugal; Jllx.
maxilla; mt. m•. m•. molars ; Na.
nasal ; o.c. occipital condyle; Pa.
parietal; p.m.t. situation of the
vestigial first premolar, whic h has
been lost in the lower, but is present
in the upper jaw; pm•. pm•. pm 1 •
remaining premolars; Pm.1:. pre-
maxilla ; p. p. par-occipital pro-
cess; Sq. squamosal. (After
Flower and Lydekker.)

separated by an interval from the incisors, and in pattern closely resemble those
of some of the Perissodactyla.
The elephants (Fig. 584, p. 812) have extremely specialised teeth. There
are no canines and no lower incisors. The single pair of upper incisors are
developed into the enormous tusks (Fig. 588, p. 8r6), which grow continuously
from persistent pulps throughout the life of the animal ; they are of elongat ed
conical form, and usually become curved. The tusks are composed of solid
dentine. Enamel occurs on the apices only, and is early worn away. The
molars (Fig. 590) are very large, and their worn surfaces are marked with
prominent transverse ridges. There are six molars altogether on each side, but
only one or two are functional at once, the more posterior moving forward
and taking the place of the more anterior as these become worn out and shed.
When teeth are developed in the Cetacea they are nearly always numerous,
homodont, and monophyodont. In the sperm-whales they are confined to the
lower jaw. In the whalebone-whales, though teeth are developed in the fcetal
 ZOOLOGY

condition (Fig. 637), they become lost either before or soon after birth, and are
succeeded in the adult by the triangular plates of baleen or whalebone (Fig.
558, p. 785), which hang vertically downwards from the palate.
Of the Sirenia, the dugongs and manatees have a heterodont dentition ; in
Hydrodamalis (Dugongidre) teeth were absent. In the two former sirenians
there are incisors and molars with a wide diastema beween them. In the
manatees there are two rudimentary incisors on each side, in both upper and
lower jaw. These disappear before adulthood. There are altogether eleven
molars on each side above and below, but not more than six of these are in
use at once, the more anterior when worn out being succeeded by the more
posterior. They have enamelled crowns with transverse ridges, and are pre-
ceded by milk-teeth. In the dugongs there are no mandibular incisors in the
adult, and only one tusk-like mandibular pair. These are large in the male
(in which they grow from persistent pulps) and but little developed in the female,
remaining concealed in their sockets. In the young there are rudimentary
mandibular incisors, and also a rudimentary second pair in the maxillre.

FIG. 637.-Balamoptera:
Fmtal jaw and teeth. Right
W ha le. Inner Aspect of left
jaw. Natural size. (After
Julin.)

There are either five or six molars on each side, both above and below. These
are cylindrical teeth devoid of enamel, and with persistent pulps.
In the Fissipedia (Fig. 565) the dentition is complete, heterodont, and
diphyodont, and all the teeth are provided with roots. The incisors are
relatively small, chisel-shaped teeth. There are nearly always three of them
on each side in both jaws. The canines are always large and pointed. The
presence of carnassials, consisting of the last premolar in the upper and the
first molar in the lower jaw, is characteristic. In front of the carnassial the
teeth are compressed and pointed; behind it they have broad surfaces. In
the cat family (FeJidre) the formula is-

. 3
L
I
-,C.-, p. 3-, m.-I = 30.
3 I 2 I

The lower carnassial is thus the last of the series. In the dogs (Canidre) the
formula is usually-
· 3-,c.-,
t.
I
p. 4-, m.-2 = 42,
3 I 4 3
and in the bears (Ursidre) it is the same. (For the pattern of the molar teeth
see Fig. 566, p. 794.)
 PHYLUM CHORDATA

In the Pinnipedia there are always fewer than J

incisors, and carnassials
3
are not developed. The premolars and molars have a compressed, conical,
pointed form. The prevailing dental formula of the seals is-

· 3 I 4 I
z. 2' c. i' p. 4' m. i = 34·

In the Walrus the adult formula is-

. I I 3 0
z. -,c. -, p. -, nt.- = I8.
0 I 3 0

The upper canines take the form of large, nearly straight tusks.
In the large order of the rodents the dentition is remarkably uniform, and,
in all its general characters, resembles what has already been described in the
Rabbit (Lagomorpha) except that in the lagomorphs a second, smaller pair
of incisors is present in the upper jaw. The number of premolars and molars
varies in rodents from-

p. 00-, m. -top. 3 3
I
I
-,
2
m. -, 3
and they may develop roots.
In the two 'insectivoran' orders (see p. 725) the dentition is heterodont,
complete, and diphyodont. All the teeth are rooted. There are never fewer
than two incisors on either side of the lower jaw. The canines are not of large
size. The crowns of the molars are beset with pointed tubercules.
In the Chiroptera (bats) the dentition is complete, and the teeth are all
rooted. There is a milk-series which, if not resorbed, differs entirely from the
permanent teeth (see p. 756). In the insectivorous Chiroptera the molars are
provided with pointed cusps, while in frugivorous forms (e.g. 'flying foxes',
Pteropus) they are longitudinally grooved or excavated.
In the Primates the teeth are heterodont and diphyodont, and always,
with the exception of Daubentonia (p. 737), with roots. (In Daubentonia the
incisors are persistently growing.) There are almost invariably two incisors on
each side of the jaws, and, in all but the marmosets, three molars. The denti-
tion of Man differs from that of the rest of the order in the teeth forming a
continuous series not interrupted by a diastema, and in the comparatively small
size of the canines (Fig. 523, p. 746).
The mouth in mammals is bounded by fleshy lips. On the floor of the
mouth is situated the tongue, which is usually well developed, but varies in size
and shape in different orders. In some herbivorous animals it can be curled
around grass and thus helps pull the 'feed' into the mouth. Its surface is
covered with papillre of different kinds. These are sometimes horny, serving
 ZOOLOGY
either the initial grinding of food, or for the toilet of the hairy coat. Thus the
tongue- and lip-margins in many mammals are equipped with raised processes
which can be moved up and down along the interspaces of the teeth (and on
the surfaces of the teeth) in cleansing action. In association with the papilla:
of the tongue are special end organs of taste ('taste buds') which are often
arranged in zones. The sense of taste differs profoundly among various
groups. The roof of the mouth is formed in front by the hard palate, consisting
of the horizontal palatine plates of the maxillary and palatine bones covered
with mucous membrane bearing palatal rugce. Behind the hard palate there
projects backwards the soft muscular fold of the soft palate which divides the
cavity of the pharynx into two chambers, an upper and a lower. In some
forms the soft palate also bears taste buds. In the Primates a free-hanging
uvula occurs. During deglutition or swallowing both uvula and soft palate are
raised to close off the nasopharynx and prevent the entrance of food thereto.
In front of the opening, leading from the lower division of the pharynx into
the larynx, is a cartilaginous plate (epiglottis) a primitive form of which is
found in certain lower vertebrates (see p. 362). The epiglottis, anatomically
part of the larynx, assists the reflex swallowing mechanism by preventing the
food from entering the trachea and so causing aspiration pneumonia.
It will be recalled that various prominent structures in the oral and pharyn-
geal region are in part derived from the gill-pouches of ancestral vertebrates
(Fig. 73, p. 106). Thus, the tympanum, middle ear, and Eustachian tube are
all partly derived from the first pharyngeal pouch. Again, connective tissue
around the regressing second pharyngeal pouch becomes infiltrated by
lymphoid material which comes to constitute the palatine (faucial) tonsil.
Adjacent and above, the small supratonsillar fossa is a permanent remnant of
the originally extensive second pouch. (The lingual, and pharyngeal tonsils
('adenoids'), are separate and unassociated with gill-clefts.) The thyroid and
parathyroids (p. 152), as well as the thymus (Fig. 638), are also essentially
pharyngeal pouch derivatives.
The msophagus is always a simple, straight tube down which the food is
propelled by peristaltic contractions of the muscular walls, which, like the act of
deglutition, are reflex and involuntary. The opposite action, retroperistalsis,
enables ruminants to regurgitate stomach contents for more leisurely mastica-
tion (see below), and many other animals to expel injurious substances acci-
dentally swallowed. Highly-trained showmen can control the normally in-
voluntary deglutition reflex and make a living as sword-swallowers : actually
they are non-swallowers, for if they allowed the swallow reflex to operate they
would wound their throats internally. Retroperistalsis in Man is sometimes
used by 'spiritualist' fakers to regurgitate in dimly lit seances 'ectoplasm',
1:.e. previously-swallowed filmy cloth, or other material.
The stomach varies greatly in different mammalian orders. In the majority
 PHYLUM CHORDATA

of mammals it is a relatively simple sac, as in the rabbit (p. 669). But in certain
groups it is complicated by the development of internal folds, and may be
divided by constrictions into several functionally different chambers. Such
complication reaches its extreme in the Ruminantia especially, in the Tylopoda
(but not Suiformes) and Cetacea. In a typical ruminant (Fig. 639) such as
sheep or ox, the stomach is divided into four chambers-the rumen (or paunch),
the reticulum, the psalterium, and the abomasum (or rennet stomach). The
epithelium of both reticulum is highly reminiscent of that of the resophagus.
The resophagus opens into the rumen close to its junction with the reticu-
lum. The rumen is much larger than the rest. Its mucous membrane is
beset with numerous short villi. The reticulum, which is much smaller than

mylohyoid
muscle
submaxillary
digastric muscle gland
(anterior belly)

thymus -~"-P-14L­
(right)

F1c. 638.- Primates: Pharyngeal region. Relationships and some cert ain gill-pouc h deriva-
tives (see also Fig. 73). Modified from P a tten, after Bien.

the rumen, has its mucous membrane raised up into a number of anastomosing
ridges. This gives its wall the appearance of a honeycomb with shallow
cells. From the aperture between reticulum and rumen to that by which it
communicates with the psalterium there runs a groove bounded by a pair of
muscular ridges. These are capable of closing together in such a way as to
convert the groove into a canal. The mucous membrane of the psalterium
is raised up into numerous longitudinal leaf-like folds. The abomasum,
smaller than the rumen, but larger than the reticulum, has a smooth vascular
and glandular mucous membrane. The ruminant swallows herbage without
mastication. The habit enables timid herbivores to snatch and swallow at
intervals food that can be digested later in safer circumstances. It has arisen,
with convergent specialisations, in the grazing Quokka Wallaby (Fig. 640).
 ZOOLOGY
In ruminants the food is swallowed, saturated by copious salivation and
passes into the rumen and reticulum, where it lies until, having finished feeding,
the animal begins ruminating or chewing the cud. In this process the sodden
rumen

FIG. 639.-Rumination and digestion. Comparison between ruminant (Bos) and non-ruminant
(Sus) tracts. Above: Cow: Arrowed lines trace the course of foodstuffs in regurgitation (by
retroperista lsis, broken line) and subsequently down through the four compartments. Rumen
and rectic ulum are connect ed by a wide aperture and together they constitute a fermentation
chamber with a capacity of some 40 gallons. Here rr.icro-organisms split cellulose and sugars to
utilisable fatty acids (absorbed through walls of rumen ) as well as carbon dioxide and m ethane
(discharged by belching). Bact eria and protozoa in the rumen a lso synthesise a mino -acids,
proteins and vitamins of the B. complex. This material is pushed peristaltically in a semi-fluid
mass into the osmasum. Here fluid is extracted, and the food passes into the abomasum where
charact eristic gastric digestion occurs. An <esophageal groove (not shown) runs from the cardia
to the reticulo-omasal orifice a nd permits the by-pass of milk in young ruminants. The omasum
is also called the psalterium or m anyplies. Below: Comparison of struct ure, mass and position
of gastric apparatus in pig and cow. (Redrawn after various authors.)

food is returned in rounded boluses from the rumen to the mouth. It now
undergoes mastication. When fully masticated it is swallowed again in a
semi-fluid condition, and passes along the groove into the reticulum, or
over the unmasticated food contained in the latter chamber, to strain
 PHYLUM CHORDATA

between the leaves of the psalterium (omasum) and enter the abomasum. (In
some ruminants the psalterium is absent.) In the rumen and reticulum of deer,
cattle, sheep, etc., there exists a dense population of protozoa and bacteria
which attack and break down the enzyme-resistant cellulose which forms the
major part of the diet. Fermentation produces acetic, butyric and propionic
acids which are neutralised by sodium bicarbonate secreted in the saliva.

OESOPHAGEAL
GROOVE

(a)

COLUMNAR
EPITHELIUM

(b)

FIG. 64o.-Marsupialia: Digestion. A ruminant-like digestion has arisen by parallel evolution

in the grazing marsupials (e.g . the Quokka, Setoni:r brachyurus) a lthough the stomach is not divided
into four compartments like that of eutherian mammals (Fig. 639). In Setonix the cesophagus
leads into a putative rumen (I). An asophageal groove, by-passing the extensive sacculated main
fermentative region (of tubular glands), passes through an area of stratified squamous epithelium
like that of the rumen, reticulum and omasum of sheep. Region III may be analogous to the
abomasum; the functions of II a nd IV are still unknown. These and other complexities are
associated with the possession of a dense bacterial population (demonstrably concerned w ith fatty
acid production) and a very sm all ca:cum. (After Moir, Somers, and W a ring.)

Absorption takes place in the rumen, and liberated gases (e.g. methane and
carbon dioxide) are regurgitated.
Meanwhile, food residues, fluid and micro-organisms descend the alimentary
tract. In the omasum fluid is absorbed. In the abomasum, the protozoa and
probably the bacteria, are destroyed by secreted hydrochloric acid. The
abomasum produces also digestive enzymes, and further absorption takes place
in the small intestine. In ruminants the crecum is relatively small ; in Georgian
times those of sheep were prepared for use as condoms (Huxley). In ruminants
88o ZOOLOGY
the colon too, is comparatively unimportant ; but in non-ruminating herbivores
such as horses both crecum and colon are enormous. All material passing from
ileum to colon enters the crecum which, in the horse, may be some four feet
long and holci as much as eight gallons. In horses the crecum has a fluid-
storing and digestive function. The large colon in horses is principally absorp-
tive in function c.J.though some bacterial (but not enzymatic) digestion occurs
therein.
In the camels the stomach is not so complicated as in the more typical
ruminants. There is no distinct psalterium, and the rumen is devoid of villi.
Both the rumen and the reticulum have connected with them a number of
pouch-like diverticula, the openings of which are capable of being closed by
sphincter muscles. In the Cetacea the stomach is also divided into compart-
ments. In porpoises the cesophagus opens into a spacious paunch, the cardiac
compartment of the stomach, which has a smooth, thick, mucous membrane.
This is followed by a second median chamber of considerably smaller dimen-
sions. This has a glandular mucous membrane, which is thrown into a number
of complex folds. A long and narrow third, or pyloric, compartment follows
upon this, terminating in a constricted pyloric aperture. Beyond this the
beginning of the small intestine is dilated into a bulb.
A crecum (see above), situated at the junction of the large and small in-
testines, is usually present, but varies greatly in extent in the different orders
and even families. In general it is much larger in vegetarian than in carni-
vorous forms. Among the former it is those that have a simple stomach (such
as in the rabbit) that have the largest crecum. The hyraxes (p. 8o8) differ
from the rest of the class in the possession of four definite creca, of which there
is a posterior pair, each carrying a vermiform appendix. The crecum is simple
in monotremes, absent in the sloths, some Cetacea, and a few Carnivora. It
is relatively enormous (about 250 cm.long) in the marsupial Koala, Phascolarctos
(which eats almost only Eucalyptzts leaves), next relatively largest in two other
marsupials (Trichosurus, Didelphys), and big in all phalangers (p. 716). In
Man and a few other animals (civets, some rodents, monkeys) the distal end of
the crecum has degenerated into an appendix vermiformis. The mere pro-
portion of vegetable material ingested is not, however, the only factor governing
the size of the crecum.
The Prototheria resemble reptiles, birds, and the Amphibia, and differ
from most mammals in the retention of a cloaca. Into this not only the rectum
but the urinary and genital ducts open. ln the marsupials a common sphincter
muscle surrounds both anal and urinogenital apertures. In the female there is
a definite cloaca. In nearly all the Eutheria the apertures are distinct, and
separated from one another by a considerable space-the perinceum.
Associated with the alimentary canal is the liver (Fig. 461, p. 66g). It con-
sists of two parts or main divisions (right and left) incompletely separated from
 PHYLUM CHORDATA 881
one another by a fissure termed the umbilical, owing to its marking the position
of the fretal umbilical vein. Typically each of these main divisions is divided
by a fissure into two parts, so that right lateral and right central and left
lateral and left central lobes are clearly distinguishable. When a gall-bladder
is present, as is the case in the majority of mammals, it is attached to, or
embedded in, the right central lobe. A portal fissure crosses the right central
lobe near the anterior border. Through this the portal vein (from the hepatic
portal vessels arising at the small intestine) and hepatic artery pass into the
liver and bile vessels pass out. Hepatic veins drain into the postcaval which
lies in contact with, or embedded in, the right lateral lobe near its anterior
border. Given off from this lobe, between the postcaval and the portal fissure,
is a small Spigelian lobe, of varying extent. The term caudate lobe is applied
to a process of the right lateral lobe (of considerable extent in most mammals),
which has the postcaval vein in intimate relation to it. This is often closely
applied to the kidney. A gall-bladder is absent in the Cetacea, the perisso-
dactyles, the Hyracoidea, and some rodents. It is present in fruit-bats
(Epomophorus).
Blood-vascular System.-The blood of mammals is warm, with a tempera-
ture always of from 35° to 40° C. except in the Prototheria (p. 6go). The red
corpuscles or erythrocytes are non-nucleated : in form they are most usually
biconcave discs, always circular in outline, except in the Camelidre, in which
most of them are elliptical. The relative numbers of red cells differ widely
among various groups and particularly in relation to altitude and therefore
oxygen pressure.
The general statements which have been given with regard to the heart of
the rabbit (p. 670) hold good for the Mammalia in general. The sinus venosus
is never distinct from the right auricle. Of its valves, which are more com-
pletely retained in the Edentata than in the other orders, the right gives rise to
the Eustachian valve. This is a membranous fold, often fenestrated in the
adult, extending from the right wall of the postcaval to the edge of the foramen
ovale (annulus ovalis). The left becomes merged in the auricular septum,
helping to complete the annulus ovalis behind. Each auricle has an auricular
appendix. The right auriculo-ventricular aperture has a three-cusped tricuspid
valve, and the left a two-cusped bicuspid, or mitral, with chordre tendinere
and musculi papillares. In all mammals the openings of the pulmonary artery
and aorta are provided with three-lobed semilunar valves.
The single aortic arch, situated in all mammals on the left side, varies greatly
in the way in which it gives off the main arterial trunks. Sometimes a single
large trunk passes forward from the arch of the aorta and gives rise to both
carotids and both subclavians. Sometimes there are two main trunks-right
and left innominate arteries-each giving rise to the carotid and subclavian of
its own side. Sometimes there is a right innominate giving off right carotid
VOL. II. . 13
882 ZOOLOGY
and right subclavian. The left carotid and left subclavian come off separately
from the arch of the aorta. In the rabbit and many other types an innominate
may give origin to the right subclavian and both carotids, with the left sub-
clavian coming off separately.
In monotremes and marsupials, in most peen ungulates, and in the Rodentia,
'Insectivora' and Chiroptera, both right and left precavals persist; in the others
the left aborts, its vestige giving rise to the coronary sinus. In the monotremes
the openings of all three cavals are provided with valves, only vestiges of which
exist in the other groups. In the monotremes all the pulmonary veins open by
a common trunk. In the Metatheria and Eutheria the four veins sometimes
open separately, sometimes the two veins of each side unite to form a single
lateral trunk. In T achyglossus there is an abdominal vein corresponding to
that of amphibians and reptiles.
The following are some of the principal variations in the structure of the
heart which occur in the different groups of mammals : In the Monotremata
there is a deep fossa representing the fossa ovalis in the auricular septum.
The auriculo-ventricular valves depart from the structure typical of mammals
and approach the corresponding valves in the heart of birds. In the Marsupiala
the fossa ovalis and annulus ovalis are absent. In the uterine fretus of the
kangaroos the auricles communicate by a fissure, but all trace of this becomes
lost before the adult stage is reached.
In the Cetacea, Eustachian and Thebesian valves are both absent. In
some of the Cetacea the apices of the ventricles are separated by a slight
depression. In the Sirenia a corresponding cleft is much deeper and wider, the
apex of the heart being distinctly bifid.
In the ungulata, Eustachian and coronary valves are absent; in some there
occurs a cartilage, or bone (os cordis), often double, at the base of the heart.
(This is a 'visceral' bone of the nature of the os penis (p. 7 4), the diaphragmatic
ossification in camels and the mandible and hyoid.) The Eustachian valve is
absent in most of the Carnivora. In the Pinnipedia an inter-auricular aperture
often persists in the adult.
The Mammalian Heart-beat.-The heart-beat is myogenic in origin even
though the rate and other factors can be controlled by outside influences
(p. n6). In the human fretus, rudimentary heart beats begin after about three
week's gestation and perhaps two weeks before nervous elements arise. (In the
chick embryo the heart rudiment begins rhythmical contractions after about 36
hours, but ganglion cells do not appear until after the sixth day of development.)
In mammals a system of specialised cardiac tissue is responsible for the
initiation and propagation of the heart-beat. The neuromuscular sinu-
auricular node (S.A. node), embedded in the wall of the right auricle with a
special blood supply of its own, is the pace-maker. From this an excitation wave
arises and is transmitted in all directions through auricular cardiac muscle and
 PHYLUM CHORDATA

to the auriculo-ventricular node (A.V. node). Thence the auriculo-ventricular

bundle (of His) and its right and left branches convey the contraction impulse
simultaneously to right and left ventricles. The branches of the A.V. bundle
break up into numerous conducting fibres of the Purkinje network. These
specialised pathways make possible the synchronised contractions that occur in
the healthy animal (see also p. 109).
The cardiac blood supply in mammals comes from right and left coronary
arteries which arise from the aorta near its origin. These terminate in an
unusually rich distribution of capillaries. Anastomotic arrangements of three
kinds feature in the intrinsic circulation of the heart.
Special peripheral or sub-peripheral anastomoses, too, occur as climatic
thermal adaptations in many animals. Such creatures can probably swiftly lose
heat, or ensure its retention, by means of arterio-venous retes at the base of, or
within, the various extremities. Thus, aquatic or other animals that are
heavily insulated by fur or fat appear to avoid serious overheating during
severe exercise by means of heat-loss through relatively uninsulated limbs or
tail into which the blood-flow is reflexly regulated according to the state of the
internal and external environments. Such anastomoses have arisen in whales,
sirenians, seals, certain amphibious rodents, and various strictly terestrial
animals as well (e.g. sloths).
Hibernation.-Man y small mammals (e.g. bats, rodents) undergo hiberna-
tion involving a reduced cardiac and respiratory rate and greatly lowered
general metabolism and temperature (cf. restivation, p. 375). In true hiberna-
tion the animal, deeply hidden in a relatively constant environment, becomes,
except for respiration, almost independent of external conditions. The North
American ground squirrel (Citellus beecheyi) can be experimentally hibernated
irrespective of season. In winter, animals placed in a temperature of 5"5° C.
will hibernate (brain temperature, about 9° C.) within three days. Animals
put into a 7° C. environment during the summer need from 9 to z6 days before
entering deep hibernation. Although during deep hibernation (oral tem-
perature, s·8° C.) there is an average go per cent reduction 'in the amplitude
of general brain wave activity', squirrels are nevertheless susceptible to
external stimuli. They can discriminate between sounds. At brain tem-
peratures of 6·I 0 C. they vocalise when touched. Muscle tone, too, is main-
tained (Strumwasser).
Lymphatic System.-This is very highly developed, ramifying richly through-
out the body. It comprises lymph capillaries, lymph vessels, and adenoid
tissue associated therewith in the form of lymph glands (lymph nodes). Those
lymphatic vessels (lacteals) which occur in the villi of the intestine and absorb
the products of fat metabolism, as also the lymphatic vessels from the hind-
limbs and lower trunk, pour their lymph into a receptacle on the posterior
abdominal wall (receptaculum chyli) from which a tube (thoracic duct) runs
 ZOOLOGY

headwards to open into the great veins of the precaval system by a valvular
aperture. Brief accounts of tissue fluid and lymph are given elsewhere (p. 77).
Respiratory System.-The respiratory organs resemble those of rabbits in
the general features mentioned (p. 676) but in the Cetacea, the epiglottis and
arytenoids are prolonged to form a tube, which extends into the nasal chambers
and is embraced by the soft palate. Thus a continuous passage is formed
leading from the nasal chambers to the larynx, and giving rise to the condition
of an intranarial epiglottis. In the remaining orders a similar condition occasion-
ally occurs. The epiglottis is produced upwards into the respiratory division
of the pharynx behind the nasal chamber. In marsupial pouch-young, in
which the intranarial condition is complete, it is obviously associated with the
passive absorption of the milk while breathing is being carried on continuously
through the nostrils (p. 908). Some Cetacea and Artiodactyla, are ex-
ceptional in possessing a third bronchus, which passes to the right lung anteriorly
to the ordinary bronchus of that side and to the pulmonary artery. In con-
nection with various parts of the respiratory system there are cavities containing
air. The connection of the tympanic cavity with the pharynx by means of the
Eustachian tube has been already mentioned. Air-sinuses, connected with the
nasal chambers, extend into the bones of the skull, especially into the maxillre
and frontals, where they may reach large dimensions, and are known as the
maxillary antra and frontal sinuses. In the howler monkeys (Alottatta, p. 743),
pharyngeal air-sacs occur, and the hyoid has become enormously developed
(and bell-shaped) to house them.
Nervous System.-The brain of mammals (Fig. 467, p. 678) is distinguished
by its relatively large size, and in particular by the large size and complex
structure of the cerebral hemispheres of the fore-brain.
The cerebral hemispheres of opposite sides are connected together across
the middle line in all mammals, except the monotremes and marsupials, by a
band of nerve-tissue termed the corpus callosum-a structure not present in
reptiles and birds. The hemispheres, in all but certain of the lower and smaller
mammals, are not smooth, but marked by a number of grooves (sulci) separating
winding ridges (convolutions). The lateral ventricles in the interior of the
hemispheres are of large size and somewhat complex form.
The optic lobes, which are relatively small, are divided into four parts, and
are hence called the corpora quadrigemina. The pineal body is always small.
Connecting the lateral parts of the cerebellum, which, in the higher mammals,
attains a high degree of development, is a transverse flattened band (pons
V arolii) crossing the hind-brain on its ventral aspect.
In the monotremes and marsupials (Figs. 641, 642) there is no corpus
callosum, while the anterior commissure (ant. com.) is of relatively large size,
and, unlike the corresponding commissure in lower vertebrates, contains fibres
connecting together areas of the non-olfactory regions (neopallium) of the
 PHYLUM CHORDATA 885

hemispheres. The hippocampi extend along the whole length of the lateral
ventricles. The layer of nerve-cells in each hippocampus gives origin, as in
Eutheria, to numerous fibres, \vhich form a layer on the surface, the alveus,
and become arranged in a band-the tcenia hippocampi. In the Eutheria, as
we have seen in the Rabbit, the t~ni~ unite mesially to form the body of the
fornix (seep. 68o). In the monotremes and marsupials, on the other hand,
there is no such union ; the fibres of the t~nia run towards the foramen of
Monro, where they become divided into several sets. Of these one set , con-
stituting the great majority of the fibres, pass into the hippocampus of the
opposite side, giving rise to a hippocampal commissure (hip. com., cf. Figs. 641,

(
m.ed c.ru \
c.m-am. opt\ tu.h.oif
venl:J co{j'orn.
Frc. 64I.-Tat'ltyglo.~su11: Brain. FTc. 642.-Pefl·ogale: Brain. Rock Wallaby,
Spiny Ant-eater, sagittal Sf'ction. ant. sagittal section. aut. com. anterior commissure;
com. antl'rior commissure; cbl. cerebellum; cbl. cerebellum: c. 1liiiJII. corpus mammillare; c. qu.
c. mam. corpus mammillarc; col. forn. corpora qnadrigcmina; crur. crura cerebri; epi.
column of the fornix ; c. qu. corpora epiphysis , with -th e posterior commissure imme-
'JUadrigemina; gang. hab. habcnular gang. diately behind; f. mon. position of foramen of
lion; hip. CO Ill. h ippocampal commissun:; Monro; hip. com. hippocampal commissure, con-
mcd. medulla oblogata ; mid. com. middle sisting here of two layers continuous behind at the
commissure ; olf. olfactory bulb; opt. splenium , somewhat divergent in front where the
optic c hiasma; tub. olf. tuberculum septum lucidum extends between them ; hypo.
ol factorium; vent. 3. third ventricle. hypophysis; med. medulla oblongata; mid. com.
middle commissure; olf. olfactory bulb; opt. optic
chiasma ; vent. 3, third ventricle.

642). The great development of these may lead to its being mistaken for a
corpus callosum. The fibres entering into the formation of this commissure
correspond, however, not to the fibres of the corpus callosum, which is the
commissure of the neo-pallium, but, as proved by their mode of origin, to the
fibres of the fornix, and they connect together only the hippocampi, the fascia:
dentata:, or specialised lower borders of the hippocampi, and an area of the
hemisphere in front of the anterior commissure (precommissural area) : they
thus constitute an olfactory or archipallial commissure, since all these parts
belong to the olfactory region or archipallium of the hemispheres. In the
monotremes (Fig. 643) the hippocampal commissure is only very slightly bent
downwards at its posterior extremity. In most marsupials (Fig. 645) it bends
sharply round posteriorly and runs forward again, becoming thus folded into
886 ZOOLOGY
two layers, dorsal and ventral, continuous with one another at a posterior bend
or splenium, similar to the splenium of the corpus callosum. The dorsal layer
of the hippocampal commissure becomes almost completely replaced in the

FrG. 643--0'NI.itlwrhynclms:
Brain. Platypus. dorsal view Fr G. 64 4.-'l'acllyg los.~lt.'!:
(natural size). cbl. cerebellum; Brain. Spiny Ant-eater, dorsal view.
olf. olfact ory bulbs. Natural size.

Eutheria by the fibres of the corpus callosum, and the ventral part persists in
the shape of the psalterium or lyra.
In Ornithorhynclms (Fig. 643) the hemispheres are smooth; in Tachy-

FrG. 645.- Macropus. Brain. FrG. 646. - Odontoceti: Brain. Whale

Kangaroo, dorsal view. (After (Kogia grayi), dorsal view. (After Haswell.)
Owen.)

glossus (Fig. 644) they are somewhat convoluted. In the lower marsupials
there are no convolutions (Notoryctes, Koala, phalangers) , while in more
advanced forms convolutions are numerous, though the sulci are not deep
 PHYLUM CHORDATA

(Macropus, Fig. 645). Among the Eutheria, from the rodents and lower
insectivores to the higher primates there is a great range in the development of
the fore-brain. In the lower types of mammalian brain the cerebral hemi-
spheres are relatively small, do not over-lap the cerebellum, and have smooth,
or nearly smooth, surfaces. In the higher types the relative development of
the hemispheres is immense. Their backward extension causes them to cover
over all the rest of the brain. The cortex is thrown into numerous complicated
convolutions separated by deep sulci (Fig. 646). This development of the
cerebral hemispheres reaches its maximum in Man.
Organs of Special Sense.-Thesehavethe same general structure and arrange-
ment as in reptiles and birds. Jacobson's organs, however, are developed
only in the 'lower' families of mammals (seep. 139). The olfactory mucosa is
of great extent, owing to the development of the convoluted ethmoturbinal
bones over which it extends. In the toothed Cetacea (alone among mammals)
the nasal chambers lose their sensory functions-the olfactory nerves being
vestigial or absent. Taste-buds occur in the tongue and palate.
In essential structure the eye of the mammal resembles that of the Verte-
brates in general (see p. 139). The sclerotic is composed of condensed fibrous
tissue. The pecten of birds and reptiles is absent. In most mammals there are
three movable eyelids. Two, the upper and lower, are opaque and usually
covered with hair. The third is anterior, translucent, and hairless. This, the
nictitating membrane, is vestigial in higher types-it can be seen in Man as a
small pink fold in the inner canthus of each eye. The secretions of a lachrymal,
a Harderian, and a series of Meibomian glands moisten and lubricate the
conjunctival surfaces of the eye-ball and its lids. In moles, and certain other
burrowing Insectivora and rodents, and in N otoryctes among the marsupials,
the eyes are imperfectly developed and functionless. In whales the eyes are
small, or even vestigial (e.g. Platanista), but often elaborately modified. The
cornea is extremely flattened and the lens rounded in an almost piscine manner.
An unusually thick sclera and specialised lid muscles, derived from the four
recti, provide protection against pressures. The Harderian glands secrete a
fatty material which lubricates the eye-surface. Lachrymal glands and ducts
have disappeared.
The ear of a mammal is more highly developed than that of other verte-
brates, both in respect of the greater complexity of the membranous labyrinth
and in the greater development of the accessory parts. A large external
auditory pinna, supported by cartilage, is almost invariably present, except in
the Monotremata, Cetacea, and Sirenia. This is a widely open funnel, of a
variety of shapes in different groups. It assists the collection of sound waves.
By the action of a system of muscles it is usually capable of being turned
about in different directions. Enclosed by its basal part is the opening of the
external auditory passage (Fig. 93, p. 144). This, the length of which varies,
888 ZOOLOGY

leads inwards to the tympanic membrane, which separates it from the cavity
of the middle ear (tympanic cavity). The wall of the external auditory passage
is sometimes entirely membranous or cartilaginous, but sometimes in part
osseous. In Tachyglossus it is strengthened by a series of incomplete rings
of cartilage. The tympanic cavity, enclosed by the periotic and tympanic
bones, communicates with the upper or respiratory division of the pharynx by a
longer or shorter tubular passage (Eustachian tube). On its inner wall are
the fenestra ovalis and rotunda, and across its cavity, from the tympanic
membrane to the fenestra ovalis, runs the chain of auditory ossicles-the
malleus (hammer), the incus (anvil) and the stapes (stirrup). These, the
smallest bones in the body, vary somewhat in form in different mammals.
The stapes is usually perforated by a considerable foramen and a minute
artery, as in the rabbit, but, in the monotremes, certain marsupials, and Manis
among the Pholidota, it approximates more towards the rod-like shape of the
columella auris of amphibians, reptiles, and birds. The membranous labyrinth
of the internal ear of a mammal is characterised by the special development of
the cochlea which (except in the monotremes) is spirally coiled to a greater
or less extent. In cetaceans the entrance to the long meatus (which often
contains a substantial wax plug) is very small. The tympanic membrane is
thickened and the auditory ossicles are fused into a single rigid column con-
necting tympanum and periotic. The tympanic bulla is held to the periotic
by a pair of thin, flattened bones. The petrosal does not unite with the skull.
The cochlea is not truly spiral, making but one and a half turns.
Endocrine Organs.-See pp. 147-1s4, 889, 9os.
Urinogenital System.-The kidneys of mammals are compact organs of oval
shape. On the inner side is a slit or hilum, by which vessels and ducts enter or
leave the interior (Fig. 99, p. 1S4)· The substance of the kidney consists of two
distinctly marked portions-a central portion or medulla, and an outer part or
cortex. The latter is marked by numerous minute tubules, each of which ends
in an indented expansion (Bowman's capsule). Into this enters an arteriole
which leads to a tiny canalicular knot (glomerulus) of capillaries, the walls of
which are one cell thick. As the blood flows through this tuft of vessels, fluid
and solutions are transferred through its walls and through the capsule into a
proximal convoluted tubule where vital glucose, salts and fluid are selectively
reabsorbed. The concentrated urine now flows into the descending loop oj
Henle, the first section of a hairpin-shaped structure of uncertain function.
The succeeding ascending loop leads into a distal convoluted tubule from which,
possibly, more fluid and dissolved substances may be taken into the tissue
spaces and back into circulation. In any case, the residual urea, sodium
chloride, potassium, uric acid, and traces of a considerable number of other
substances flow down a final, wider collecting tubule (made up from several
renal units) towards the centre of the kidney (Fig. roo, p. ISS). In Procavia
 PHYLUM CHORDATA 88g

(p. 8o8), which inhabits hot, rocky areas, there exists a mechanism that allows
such a high degree of fluid retention that the urine is voided in viscid streaks.
In some mammals (cf. Amphibia, p. 451) the antidiuretic hormone (from the
pars neuralis, p. I5I) operates by increasing the tubule reabsorption of water.
The numerous apertures of the collecting tubules open into the pelvis of
the ureter. They are situated on the summits of papillre, which are the apices
of a series of pyramidal masses into which, in most cases, the substance of the
kidney is completely divided. In many mammals, however, there is no such
division of the kidney substance, and all the ducts open on the surface of a single
papilla. In others again (ox, bears, seals, Cetacea) the division is carried so far
that the kidney is divided externally into a number of distinctly separated
lobules. Each kidney possesses a duct (ureter) which leads, in all the Theria,
into a large median sac, the 11rinary bladder. This is situated in the posterior
or pelvic part of the cavity of the abdomen. From this a median passage, the
urinogenital passage or urethra (into which in the male the vasa deferentia open)
leads to the exterior. Only in the monotremes do the two ureters and the
bladder all have separate openings into the urinogenital division of the cloaca.
The testes only exceptionally retain their original position in the abdominal
cavity (as in cetaceans, elephants, Hyracoidea, some Lipotyphla (p. 726),
Macroscelididre (p. 731)). In the majority of mammals they descend the
inguinal canal (in the posterior abdominal wall) to lie in the perinCBum (space
between the urinogenital and anal apertures) or to be received into a pendulous
pouch of skin, sometimes double-the scrotum.
The testes are oval bodies composed essentially of three elements : I. a
fibrous connective tissue tunica albuginea; 2. seminiferous tubules, and 3. aggre-
gated Leydig or interstitial cells which secrete the male sex hormone, The
two last-named components are under the influence of gonadotrophic hormones
from the anterior pituitary gland (p. 150). The spermatozoa from each testis
are discharged from the convoluted seminiferous tubules into a plexiform rete
testis whence they migrate into the proximal head and next the tail of the
convoluted epididymis. From here they pass to the vas deferens, a narrow
canal which enters the abdomen. Each vas now takes a curved course, ascend-
ing in Man (for example) above the level of the bladder. Passing downward,
the vas expands into an ampulla and, additionally, gives off a complicated
glandular seminal vesicle. This pair of structures, prominent in the prostate
region. do not, in fact, store seminal fluid: they produce fructose, which is
apparently utilised by the descending spermatozoa. (The usual sites of sperm-
storage are the epididymides and the ampullre.) Paired ejaculatory ducts allow
access of spermatozoa into the median unpaired urethra and penis (see below).
A small diverticulum of the proximal part of the urethra (the uterus mascu-
linus) may be the rudiment of the Mullerian duct (p. rsg). Surrounding this
part of the urethra is the large alveolar prostate gland (in close association with
8go ZOOLOGY
the ejaculatory ducts) which is under both nervous and endocrine (sex hormone)
control. Prostatic secretion, together with that of a pair of small racemose
Cowper's or bulbo-urethral glands is added to the spermatozoa. The ducts of
the latter glands open into the urethra near the base of the penis.
The penis, present in all male mammals, varies greatly in form but is
essentially composed internally of sinusoidal erectile tissue arranged in longi-
tudinal columns. In general, two such columns lie in close apposition and
form the dorsal corpora cavernosa penis which is attached proximally to the
ischia (except in monotremes, marsupials, and some edentates). A third column
of erectile tissue, the corpus spongiosum or corpzts
cavernosum urethrce, is perforated by the urethra. It lies
below and is often dilated at its extremity to form the
glans. In some species there develops an osseous element,
the os penis (os priapi), or baculum, in the fibrous septum
between the parallel columns of the corpora cavernosa.
This 'visceral' bone is widespread among rodents, carni-
vores, bats, and primates except Man (p. 75), but it
does not occur in whales, as is often stated. Its homo-
logue may occur in the clitoris where this structure is
well-developed (e.g. Lemur, Fig. 647).
During sexual excitement, nerve impulses cause the
dilation of the arterioles and capillaries from which the
above-mentioned sinuses are filled. At the same time,
the plain muscle of each sinus wall is relaxed and there.
is a contraction of the circular muscles that regulate out-
flow. Thus, blood flow is impeded, resulting in turgidity
FIG, 647.- Lemu1':
Female external geni~ and erection. Many species of several mammalian orders
tali". (After Osman (e.g. Carnivora) possess horny penile spines or copulatory
Hill.)
grapples, and analogous structures have arisen in the
elasmobranch fishes and Reptilia as well (pp. 256, 550).
The ovaries are compressed, oval bodies which retain their primary position
in the abdomen or pass backwards into its posterior (pelvic) region. Each
organ is attached at its hilus to the sttspensory ligaments of the uterus. In the
monotremes, large Graafian follicles project on the surface of the ovary. In
all groups the ovary is wholly, or at least partly, ensheathed in specialised
peritoneum formed of cubical cells-the germinal epithelium. In the mare
(Equus) this epithelium is restricted to an ovulation fossa into which ovulation
inevitably occurs. Within, the ovary is composed of chiefly fibrous connective
tissue ovarian stroma, in which lie the oocytes enclosed in the Graafian follicles
in various stages of maturation and atresia, and, after ovulation, endocrine
corpora lutea (see p. 152). After its regression, each corpus luteum persists as
a scar-like corpus albicans.
 PHYLUM CHORDATA

A
B

Frc. 648.-Metatheria: Urinogenitalia in

females. A, Didelphys dorsigera (you ng oppo-
sum); B, Trichosurus (pha langer); C, Phasco-
lomys (wombat) ; B, urinary bladder; Cl.
cloaca; Finz. fimbri<e; g. clitoris ; N . kidney;
Od. Fallopian tube; Ot. its aperture; Ov.
ovary; r. rectum; r 1 , its opening; Sug, urino-
genital canal; Ur. ureter; Ut . uterus; U/ 1 .
opening of the uterus into the m edian vagina
(Vg. B.); Vg.lateralvagina; Vgl . itsopening
into the urinogenital canal; t•, rectal glands;
t. bend between uterus and vagina. (After
Wiedersheim.)

The ciliated oviducts usually have dilated funnel-like abdominal openings,

the edges of which are generally fimbriated and arranged in a way that allows
direct access of ova from bursting follicles into the adjacent oviducal funnel.
In some mammals (e.g. Pteropus), the ovaries are almost completely encap-
sulated by which means the ovum is extruded into a bursa and is thus compelled
892 ZOOLOGY
to enter the conjoined oviduct. In some bats (Microchiroptera), but not the
megachiropteran Pteropus, only one ovary is functional.
In monotremes the two oviducts are distinct throughout their length, and
open separately into an urinogenital sinus. In nearly all mammals, however,
A B
Od
-~f/F:Yl
\)~;-··
' (/f

J +···-Jji
D FIG. 649.- Eutheria. Urino-
c: genitalia in females. A, B, C,
D, illust rate the different de-
(}~
grees of coalescence of the ovi -
ducts. A, two distinct uteri.
~~
\ lliJ . . l/t
B, bicornuate uterus. C, uterus
with a median partition. D,
complete coalescence. E, female

) ·r--- '9
reproductive organs of one of
the M ustelin.a with embryos (**)
in the uterus. F , female repro-
ductive orga ns o f the H edgehog.
H, urinary bladder; Ce. cervix
u teri (neck of uterus); N, N n ,
E kidneys and adrena l bodies; Od.
Fallopian tube; Ot. ostium tub<e
(abdominal opening of Fallopian
tube); r. rectum ; Sttg. urino-
genita l canal ; Ur. ureter ; Ut.
uterus ; Vg. vagina ; tt. acces-
sory glands. (After Wied er-
sheim.)

' ... Sug

more or less coalescence takes place. In marsupials this coalescence is confined

to the proximal parts of the vagina. In opossums (Fig. 648) the two oviducts
are merely in close apposition at one point behind the uteri, and there is no
actual coalescence. In the rest of the marsupials (B, C) the anterior portions
of the oviduct in the region (vagina) posterior to the uteri unite to form a
median chamber which may send backwards a median diverticulum (median
vagina, Vg., B), and in this way communicate behind with the urinogenital
 PHYLUM CHORDATA

passage. In the Eutheria there is a single median vagina (Fig. 649, Vg.) formed
by the union of the posterior parts of the two oviducts. In some cases the two
uteri (A. ut.) remain distinct; in others their posterior portions coalesce (B, C),
the anterior parts remaining separate, so that there is formed a median corpus
uteri with two horns or corm~a. In Primates and some of the Edentata the

adult ~ pre-pubertal ~

parous i- ~ during
at oestrus lactation
Frc. 6so.-C1·ocuta.: External genitalia. The aberrations depicted occur in the Spotted
species (C. crocuta) but not in other Hy<enid<e. (Redrawn after Harrison Matthews.)

coalescence goes still further. In these there is an undivided uterus (D)

in addition to an undivided vagina. The only parts of the oviducts which
remain distinct from one another are the narrow anterior parts or Fallopian
tubes.
The capacious urinogenital sinus of monotremes extends from the cloaca
anterior to the entire length of the bladder (Fig. 487, p. 700). The paired
Mullerian ducts enter separately the extreme anterior end of the chamber; the
 ZOOLOGY
ureters pierce the dorsal wall a little closer to the cloacal aperture. The
bladder opens independently into the ventral surface of the urinogenital sinus,
being unconnected with the ureters.
In all mammals there is, in the vestibule or urinogenital passage (through
which the vagina communicates with the exterior by the aperture of the
vulva) a small erectile body-the clitoris-the homologue of the penis. This
is only sometimes perforated by the urethral canal (p. 701) . In many mammals,
including Man, the vagina is first almost enclosed by the hymen, a mucosal fold
with a central circular or elliptical aperture. Its function may be to prevent
the reflux of urine into the genital tract of the juvenile.
Among the eutherians the external female genitalia of the Spotted Hyrena
(Crocuta crocuta) and at least one mole (i.e. Talpa europea) are not developed
in the usual way although the internal elements fall into the general pattern.
The external peculiarities of the female Spotted Hyrena gave rise to the ancient
belief that the species changed sex every year.
Both sexes of the Spotted (but not of other
species of) hyrena are externally very similar
and male-like (Fig. 650) : in both the nipples
are surrounded by areolre which in the mature
but non-parous female are practically indis-
tinguishable from those of the male. Both
sexes possess an almost identical phallus and
their external similarity is further emphasised
by posterior perineal swellings in the female
resembling the male scrotal prominences. Al-
though slightly smaller in the female, the glans
FrG. 6 SI.- Crocuta: lntemalgeni- and prepuce in both sexes are almost identical.
talia. (Redrawn after Wood Jones.)
The meat'ttS is similar in position, and is the
only external means of communication possessed by bladder and urethra, uterus,
and vagina (Fig. 651). In both sexes the glans is armed with horny spines
(copulatory grapples) such as occur in several orders of mammals.
At the approach of the breeding season the external meatus of the female
enlarges from a slit 2 mm. long to a longitudinal opening some 15 mm. in the
ventral surface of the phallus. Insemination and parturition occur through
this genito-urinary aperture. Although it has been often claimed that the
urogenital canal extends right to the os uteri (and that therefore the typical
eutherian vagina is absent), Matthews has shown that a vagina does occur as a
slightly expanded part of the tract a little less than half-way down the utero-
vaginal canal. Its entrance is guarded by a hymeneal fold. Thus, the
reproductive tract in the Spotted Hyrena retains the original embryonic con-
nection with the urogenital sinus which in turn finds outlet through the female
phallus. Copulation occurs in essentially the position assumed by other prono-
 PHYLUM CHORDATA

grade mammals, and according to Grimpe 'amid loud laughter, which is

characteristic of hyrenas' (cited by Wood Jones).
In the mole, Talpa europea, both sexes are externally male-like during their
first year. The female phallus is only slightly smaller than, and differently
shaped from, that of the male. Through this phallus discharges the urinary
canal from the bladder. There is no external genital opening. The single
wide internal uterovaginal chamber ends blindly, showing no sign of its
original, bilateral embyronic structure or of the former junction of the Miil-
lerian ducts with the urinary tract. During the first restrus (second season
after birth) an amazingly rapid transformation occurs in the female genitalia.
The perineum becomes hyperremic and a transverse vulval orifice appears
immediately posterior to the base of the phallus. By virtue of its formation
this is necessarily without labia. The internal genital tract shows even more
profound seasonal modification. There develops a vulvovaginal canal, lined
with mucosa, which joins the external orifice and completes the formation of a
reproductive tract that is still, however, in no way connected with the urinary
tract. After parturition the vulval apparatus regresses, loses its connection
with the uterovaginal canal and disappears, sometimes without an obvious
scar. The process is repeated seasonally.
Remarkable temporary (or in some species permanent) genital changes
take place also during the breeding season of marsupials. During early
eutherian development the ureteric elements pass towards the kidneys lateral
to the Mullerian ducts which will give rise to the Fallopian tubes. These ducts
remain separate anteriorly, but unite posteriorly to form a median corpus uteri
to communicate with the vagina. In the Metatheria, on the other hand, the
presumptive ureters pass between the genital elements. This prevents a median
fusion into a single corpus. As development proceeds, the primitive mar-
supials come to possess twin uteri, and corresponding lateral vaginre opening
separately into the urogenital sinus and likewise externally at the cloacal
aperture. The penis, too, is correspondingly bifid at its termination.
Before pregnancy in some marsupials a remarkable and temporary sagging
occurs in the anterior parts of the female ducts so that each forms a vaginal
cul-de-sac between and below the ureters where they enter the bladder.
After high tubal fertilisation the embryos descend into the small twin
uteri. Here they establish a brief placental connection, but soon migrate
posteriorly, not down the lateral ducts, but into the median cul-de-sacs. The
thin double septum separating these pouches disintegrates and the young
are entrapped in a posteriorly closed, greatly dilated chamber distended below
the ureters.
An extraordinary development now occurs. The posterior point of the
elongated brood-chamber ruptures, extravasation of blood occurs and a birth
passage forms through the tissues between chamber and median urinogenital
8g6 ZOOLOGY

sinus. Through this temporary corridor the minute embryos escape after an
extremely short gestation period. The membranes and cords remain in the
passage and are incorporated with the scar tissue on the site of the former
canal. The temporary passage has no epithelium and little definite structure ;
the rent that formed it heals rapidly so that in Dasyurus (for example) the
rupture is repaired within two days of parturition (Hill). In some marsupials
(e.g. Potorous, a genus of rat-kangaroos) parturition is by way of the lateral
vaginre.
In more progressive marsupials the phenomenon differs considerably. The
arboreal phalanger, Trichosurus, has the cul-de-sacs already present in the
22 mm. embryo and in the adult these are separated by only a fragile septum
which ruptures early in pregnancy and remains permanently open during
successive seasons. Nevertheless, there still remains to be formed the tem-
porary median birth-passage during each parturition. In some of the Macro-
podidre the median brood chamber and birth canal have become permanent
structures, the latter with a muscular wall and epithelial lining, but still not
permanently joining the urinogenital sinus. Others of the same family appear
to have gone a step further and retain the final aperture as a permanent feature.
Yet even in these forms (as far as is known) the sperms still ascend the original
lateral ducts.
The spermatozoa of most mammals seem to be extremely short-lived (see
below) but in some temperate zone Microchiroptera (members of the genera
Myotis, Rhinolophus) there has been evolved a mechanism for sperm preserva-
tion in the female tract after copulation in late summer or autumn until the
following spring, when ovulation and fertilisation occur. In addition to
spermatozoa, the male lvfyotis provides also a considerable amount of mucus
from accessory sexual (probably urethral) glands, and this fills and dilates the
vagina as a congealed, sperm-containing plug-the bottchon vaginal. The
encapsulated spermatozoa are freed and make their way to the uterus in time
for a post-winter fertilisation. In some species the bouchon still occludes the
vagina after pregnancy has begun and luteinisation is completed.
Breeding Seasons.-Most mammals, like other vertebrates, breed at more or
less specific times of the year although many (e.g. Man, Baboon, Giraffe,
elephants) have no particular mating season. Those which reproduce at
regular periodic intervals have evolved by natural selection a mechanism
which is influenced by naturally recurring stimuli and inhibitors (e.g. changes
in day-length, temperature, food-supply, psychical factors, etc.) and such
influences, operating via the exteroceptors, central nervous system and anterior
pituitary in conjunction with an internal rhythm of reproduction, ensure that,
in general, the copulation will occur at a period appropriate for the launching
of the young at the time of year most propitious for their survival.
The females undergo an cestrus cycle. Ancestrus, a period of sexual
 PHYLUM CHORDATA

quiescence, is succeeded by procestrus during which there is a progressive

increase in the production of restrogen, endometrial proliferation and, in some
species uterine bleeding superficially like that of menstruation (see below).
Next comes a;strus, or heat, involving further uterine changes, a readiness to
mate, and usually ovulation. Immediately after ovulation, the corpus luteum
develops. Its progesterone operates synergistically with restrogen, further
preparing the endometrium for the reception of the blastocyst and pregnancy.
If fertilisation does not occur, there may be a progesterone-induced pseudo-
pregnancy during which non-pregnant females often exhibit maternal behaviour.
In any case, restrus will be ultimately followed by some degree of endometrial
degeneration and a period of quiescence and rehabilitation varying in duration
among different species.
Mona;strus animals (e.g. dog, Dasyurtts, Koala, bigger kangaroos) have a
single cycle each sexual season, polya;strus animals (e.g. sheep, rat, goat, rat-
kangaroos, p. 719) may have several. In polyrestrus groups a period of
dia;strus intervenes between cycles. Although, in general, parturition is
followed by a lactation ana;strus (during which ovulation and conception rarely
occur), some mammals undergo immediately, or almost so, a post-partum
a;strus and copulation (e.g. the wallaby, Setonix, seals and many rodents). In
the Common Hare (Lepus europceus) an astonishing pre-partum ovulation occurs,
and the female may conceive again before delivery and thus achieve super-
fa;tation.
Attempts to relate precisely the sexual rhythm of the Primates that undergo
true menstruation with the restrous cycle in other animals have not been
conspicuously successful. True menstruation is the post-ovulatory destruction
and discharge of pseudopregnant endometrium and is widely spread only in
'catarrhines' (p. 743), but occurs in certain other Primates and perhaps in
Elephantulus, the African elephant-shrew. It is not impossible that true
menstruation may occur in several other groups but it must be remembered
that different types of uterine hcemorrhage may occur (e.g. cow, bitch, and
intermenstrual hcemorrhage in women). There is, incidentally, no evidence of
the relatively early cessation of female reproductive activity (i.e. menopause)
in any animal other than Man.
In most, but not all (p. 896) mammals the period at which fertilisation can
take place is strictly limited. Thus in the rabbit, in which induced ovulation
occurs ten hours after copulation (p. 898) ova remain fertile for only six hours,
though they live for much longer. In women ovulation usually occurs between
the twelfth and fourteenth day of the cycle : the basis of the possibly 'safe
period' which falls into about the week immediately before menstruation.
The life of spermatozoa in the female tract of most mammals is brief (e.g. from
seventeen to forty hours in most species investigated, including Man, but up
to seven days in horses).
VOL. II. K 3
8g8 ZOOLOGY

Development and Associated Phenomena.-The ova of mammals, like those

of vertebrates in general, are developed from germinal epithelium. Each of
these, surrounded by smaller unmodified cells of the epithelium, sinks into
the stroma of the ovary, in which it becomes embedded, the small cells forming
a Graafian follicle which encloses it. Soon spaces filled with fluid appear
among the follicle cells, and these eventually coalesce to form a single cavity
containing liquor folliculi of high cestrogen content. This cavity, which in
some mammals is crossed by strings of cells, separates an outer layer of the
follicle cells (the membrana granulosa) from the mass (cumulus proligerus)
surrounding the ovum, except on one side where they coalesce. A basement
membrane is formed externally to the follicle cells, and the stroma around this
becomes vascular, and forms a two-layered investment for the follicle. The
cells immediately surrounding the ovum become arranged as a definite layer of
cylindrical cells-the corona radiata. A thick membrane, the zona radiata,
perforated by numerous radially arranged pores, into which project processes
from the cells of the corona, invests the ovum. Beneath this there is a delicate
vitelline membrane. In marsupials the ovum contains yolk which is soon
extruded. The ovum of placental mammals is devoid of yolk.
As the ovum approaches maturity the liquor folliculi in the cavity of the
follicle increases in quantity and cestrogen content. The follicle becomes
greatly distended. The follicle has meanwhile approached the surface of the
ovary, on which it comes to project as a rounded prominence. Eventually the
middle region of the projecting part of the wall of the follicle thins out and
ruptures, setting free the ovum, which passes into the body cavity and then to
the Fallopian tube.
Ovulation may be spontaneous or induced. In the great majority of
mammals (including Man) it occurs as a spontaneous event in the cestrous cycle
(p. 8g6), or it may be induced by sexual excitement, or more usually, copulation
(Mink, Rabbit, Ferret, Domestic Cat, Pteropus, etc.). Some animals that
normally ovulate after copulation may sometimes do so spontaneously.
In a relatively few mammals endometrial implantation of the blastocyst
may be delayed for as long as four months. Like ovovivipary (p. 549) delayed
implantation is adaptive and of little phylogenetic significance. Thus, it has
arisen in widely unrelated animals (e.g. Nine-banded Armadillo, Roe Deer, the
wallaby Setonix, and in Carnivora such as seals, bears, Mink, Weasel, badgers,
Stoat). Although delayed implantation occurs in Roe Deer (Capreolus
capreolus) and Mink (Mustela vison), it is unknown in Red Deer (Cervus elaphus)
and Ferret (Mustela furo). In the Roe Deer delayed implantation allows
individuals to mate during July and August and yet delay parturition until
after the winter and so drop their young in spring pastures after a prolonged
interuterine period of between nine and eleven months. Spring mating indi-
viduals, on the other hand, parturate after only five months.
 PHYLUM CHORDATA

Transuterine migration occurs in numerous species (rodents, Primates,

Chiroptera, Carnivora, horse, cow.) This involves the implantation on the
opposite side (or even cornu) of the uterus from the tube along which the
blastocyst passed.
Multiple pregnancy represents the primitive mammalian condition. This
has been abandoned, however, in many mammals, including particularly flying
or arboreal forms (e.g. Primates). The restriction of the number of ova
extruded during each cestrous cycle is reflected in the reduction of functional
mammary glands (p. 848) and often in the evolution of a simple, fore-shortened
uterus. This principle is well shown in Man and in many other animals. A
fascinating situation in this respect is presented by some, but not all, armadillos.
In the genus Dasypus there has arisen an extraordinary secondary specialisation
in the production of identical quadruplets in Dasypus novemcincta and eight or
even twelve monovular young in D. hybrida. Not all of such fcetuses (the
nidation of which is maintained by a single corpus luteum) survive gestation.
D. hybrida commonly produces from seven to twelve young, however, and these
must be maintained by four mammary glands-one inguinal, and one pectoral
pair. The prolonged period of gestation (eighteen weeks in D. novemcincta)
probably reduces the dependence of the neonatal young (which are protected
in burrows) on lactation. It is probably only in Homo, Bos, and Dasypus
that identical (i.e. monovular) twinning has been shown unequivocally to occur.
The unfertilised mammalian ovarian ovum may naturally undergo several
parthenogenetic divisions (rodents, dog, one record in Man) : ova perhaps
possess an intrinsic capacity to divide. Experimental parthenogenesis has been
achieved in rabbits (Pincus).
Most of the differences between the early stages of development of a higher
mammal (Fig. 652) and those of a reptile or bird are correlated with the absence
of food yolk. One of the most striking of these is in the mode of cleavage.
In the case of the large ovum of birds, as we have seen, the segmentation is
of the incomplete or meroblastic type, being confined to a small disc of proto-
plasm-the germinal disc on one side of the ovum. In the mammals, except in
the monotremes, cleavage is complete or holoblastic, the entire ovum taking
part in the process of segmentation. The cleavage is nearly or quite regular, the
cells into which the ovum divides being of equal, or approximately equal, size.
The result, in the Eutheria, is the formation of a sphere of cells, which soon
become distinguishable into an outer layer the trophoblast, and the inner cell-
mass or embryonal knot. In the marsupials, so far as known, the stage of a
solid cellular sphere or morula does not occur. A central cavity is present from
the outset. In the Eutheria, by imbibition of liquid, a cavity, which is
formed in the interior of the sphere, increases rapidly in size. The stage now
reached is called the blastodermic vesicle. During the growth in size of the
internal cavity the central mass of cells remains in contact with one side only
goo ZOOLOGY
of the trophoblast, where it spreads out as a stratum several cells deep. From
it are derived the embryonal ectoderm and the entire endoderm of the vesicle.
The outer layer is apparently the equivalent of the extra-embryonal ecto-
derm of the bird and reptile, and has been termed the trophoblast or trophoblastic
ectoderm, because of the part which it plays in the nutrition of the £cetus.
Immediately beneath it, throughout its extent, a thin layer of flattened cells
appears-the peripheral endoderm. This iS continuous with a similar layer
formed on the inner surface of the embryonic cell-mass-the embryonic endo-

F I G . 652.-Eutheria: Development. Sections of the embryo at successive stages in t he

segmenta tion and formation o f the layers. A and B , forma tion of enclosing layer (trophoblast )
and inner cell-m ass d estined t o give rise to the embryo; C, bla stodermic vesicle with embryon ic
cell-m ass (enzb .) sepa ra ted fro m trophobla st (tr.) ex cept on one side ; D , blastodermic v esicle in
which peripheral a nd em bry onic p ort ion s of endoderm h ave becom e established : the b reak h ere
r epresented on each side bet ween the two does not occur. E, stage in which the em br yonic ecto-
derm has broken through the t rop h oblast an d b ecom e joined t o it perip herally .

derm- and is formed by outgrowth from it. The rest of the cell-mass gives
rise t o the embryonic ectoderm. The part of the trophoblast lying over this
embryonic ectoderm, known as the covering layer or Rauber's layer, has a widely
different fate in different Eutheria: it may thin out and disappear.
A primitive knot and embryonic shield are formed as in reptiles. The primi-
tive knot has simply the appearance of the somewhat enlarged anterior
ext remity of a primitive streak (Fig. 653, pr.) which is developed very much
in t he same way as in t he bird. Its formation is due to the same cause as in
the bird, vi z. active proliferation of cells leading to the development of the
beginnings of the mesoderm. A dark median streak, the head-process, appears
 PHYLUM CHORDATA gor

in front of the primitive knot, and in some mammals there is an invagination on

the surface of the latter leading to the formation of a neurenteric canal and of a
notochordal canal which gives rise to the rudiment of the posterior part of the
notochord. In the region of the anterior part of the primitive streak, the primi-
tive knot and the head-process, the mesoderm coalesces with the endoderm ;
but there does not appear to be any breaking through into the underlying space
such as occurs in reptiles (p. 551). A medullary groove (rf) and canal are formed
in front of the primitive streak, and a row of mesodermal somites (Fig. 654)
make their appearance on each side of the former. The embryo becomes
folded off from the blastoderm as
in the bird, and at length the body
of the young mammal is con-
stricted off from the r yolk-sac.
so that, ultimately, the two come
to be connected only by a narrow
yolk-stalk (Figs. 655, 656): the
yolk-sac is a thin-walled sac con-
taining a coagulable fluid in place
of yolk. A vascular area early 0
becomes established around the
embryo on the wall of the yolk-sac.
The most important of the
points of difference between a
mammal and a bird, as regards
the latter part of the history of
the development, are connected
with the fate of the fmtal mem-
branes. The amnion is in many FIG. 653.-Eutheria: Development. Embryonic
area of seven days' embryo Rabbit. ag. embryonic
mammals developed in the same area; o. place of future vascular area; pr. primitive
; rf. medullary groove. (From Balfour, after
way as in the bird, i.e. by the streakKolliker.)
formation of a system of folds of
the extra-embryonal somatopleure which arise from the blastoderm around
the embryo, and grow upwards and inwards, eventually meeting in the middle
over the body of the embryo, and uniting in such a way as to form two
layers. Of the two layers thus formed the outer, consisting of trophoblastic
ectoderm and somatic mesoderm, simply constitutes a part of the extra-
embryonic somatopleure which forms a complete investment for the entire
ovum, and is known as the chorion (Fig. 655, 2 and J). In the account of the
development of the bird it has been referred to as the false amnion or serous
membrane. The inner layer or true amnion, as in the bird, forms the wall of
the amniotic cavity (4 and 5, ah.) which becomes tensely filled with fluid (the
liquor amnii) over the body of the embryo. This serves the purpose of
902 ZOOLOGY
protecting the delicate embryo from the effects of shocks. As in the case of the
bird, the folds giving rise to the amnion and serous membrane may consist from
the first (except the head-fold, which, being formed from the pro-amnion,
consists solely of ectoderm and endoderm) of somatic mesoderm as well as
ectoderm (trophoblast: or mesoderm may extend into them later). Thus,
either from the first, or as a result of outgrowth which takes place subsequently,
the chorion contains mesoderm as well as ectoderm. The ectodermal cells-

FrG. 654.-Eutheria: Development. Embryo Rabbit, of about nine days, from the dorsal side.
a. aorta; ab. optic vesicle; af. fold of amnion; ao. area opaca; ap. area pellucida; h. , hz. heart;
h', h'. medullary plate in the region o f the future fore-brain; h". medullary plate in the region of
the future mid-brain; hh and hh'" . hind-brain; mh. mid-brain; ph. pericardia! section of body
cavity; pr. primitive streak; pz. lateral zone; rf. medullary groove; uw. somites; stz. vertebral
zone; vd. pharnyx; vo. vitelline vein. (From Balfour, after Kolliker.)

trophoblast cells- of the chorion may enter into close relationship with the
mucous membrane of the wall of the uterus, and send out processes or primary
villi (Fig. 656, EK) by means of which the ovum becomes intimately attached,
and by means of which perhaps nourishment is absorbed.
In certain mammals the history of the amnion is very different from that
above described. In the hedgehogs and Man, for example, a cavity appears
in the ectoderm of the embryonic area. This is destined to give rise to the
cavity of the amnion. The ectoderm, which forms its roof, is entirely tropho-
blastic or chorionic; that which forms its floor is partly destined to become
 PHYLUM CHORDATA

amniotic ectoderm, partly embryonal ectoderm. After the mesoderm has

begun to become differentiated, the margins of the amniotic part of this ecto-
dermal floor begin to grow upwards, giving rise to a layer which extends over
the roof on the inner side of the chorionic ectoderm and eventually forms a
complete layer-the ectodermal layer of the amnion.

FIG. 6ss.-Eutheria: Fmtal

membranes. Stages in forma-
tion. In I, 2, 3, 4 the embryo
is represented in longitudinal
section. I, Embryo with zona
pellucida, blastodermic vesicle,
and embryonic area; 2, embryo
with commencing formation of
yolk-sac and amnion; 3, embryo
with amnion about to close; 4,
embryo with villous chorion,
larger allantois, and mouth and
anus; 5, embryo in which the
mesoderm of the allantois has
extended round the inner surface
of the chorion and united with
it to form the freta! part of the
placenta; the cavity of the
allantois is aborted. a. ecto-
derm of embryo; a'. ectoderm of
non -embryonic part of the
blastodermic vesicle; ah. am-
niotic cavity; al. allantois; am.
amnion; ch. freta! part of
placenta; chz. placental villi; d.
in I zona radiata, in 2 and 3
chorion; d'. processes of zona
radiata and chorion; dd. em-
bryonic endoderm; df. area
vasculosa; dg. stalk of umbilical
vesicle; ds. cavity of umbilical
vesicle; e. embryo; hh. peri-
cardia! cavity; i. non-embryonic
endoderm; Kh. cavity of blasto-
dermic vesicle; Ks. head-fold of
amnion; m. embryonic meso-
derm; m'. non-embryonic meso-
derm; r. space between chorion
and amnion; sh. subzonal mem-
brane (chorion); ss. tail-fold of
amnion; st. sinus terminalis; sz.
villi of chorion; vl. ventral body
wall. (From Foster and Balfour,
after Kolliker.)

In the moles (Talpa) spaces appear in the layer of ectoderm of the embryonal
area, and these subsequently coalesce to form a single cavity-the primitive
amniotic cavity. This, however, has only a temporary existence, the amnion
arising later by the formation of a series of folds. In M1.ts, Arvicola, and others
the amnion is developed as a cavity in the embryonic knot. In other mammals
the amnion arises in the manner already described. The portion of the
 ZOOLOGY

trophoblast immediately overlying the embryonic part of the ectoderm

eventually disappears.
The allantois has, in all essential respects, the same mode of development
as in the bird, arising in most cases as a hollow outgrowth from the hinder part
of the alimentary canal. This, growing out into the space (extra-embryonic
ccelom) between the chorion and the amnion, becomes in all the Eutheria

FIG. 656.-Eutheria: Development. Rabbit embryo and blastodermic vesicle at the end of
the tenth day. The embryo is represented in surface view from the right side, the course of the
alimentary canal being indicated by the broad dotted line; the blastodermic vesicle is shown in
median longitudinal section. The great part of the tail has been removed. AN'. pro·amnion;
AX. cavity of aminion. C. extra-embryonic portion of crelom; E . ectoderm; E'. thickened
ectoderm by which the vesicle is attached to the uterus and from which the freta! part of the
placenta is derived ; EI. auditory vesicle; EK, ectodermal villi; GF. fore.gut; GH. hind-gut;
GT. mid-gut; H. endoderm; 0, extra-embryonic crelom; OL. lens of eye; R . heart; SI. sinus-
terminalis; TA. allantoic cavity; YS. yolk-sac. (From A. M. Marshall, in part after Van
Beneden and Julin.)

applied to the former, and unites with it to contribute towards the formation
of the placenta.
Chorio-allantoic placentre are also classified into the following five types
according to the degree in which both maternal and fee tal tissues are present:

I. Epitheliochorial (e.g. pig, horse), in which the maternal uterine epithelium

is in simple apposition with the fcetal chorion.
2. Syndesmochorial (e.g. sheep), in which the uterine epithelium disappears
 PHYLUM CHORDATA 905
so that the chorion is in contact with the endometrium or glandular epithelium
of the uterus. Such placentre are usually cotyledonary.
3· Endotheliochorial or V asochorial (e.g. cat, dog), in which the epithelium
and the endometrium of the uterus disappear and the chorion is in intimate
contact with the endothelial wall of the maternal capillaries.
4· Hcemochorial (e.g. Man), in which the maternal epithelium, endometrium
and endothelium disappear, so that the chorionic epithelium is surrounded by
circulating maternal blood.
5. Hcemoendothelial (e.g. many rodents), in which the trophoblastic epi-
thelium disappears, as well as the maternal epithelium, endometrium, and
endothelium, so that only the fcetal endothelium separates the two blood
streams.

Probably in most eutherian mammals gonadotrophic hormones are secreted

perhaps by the cytotrophoblast, but they are not identical with those of the
anterior pituitary (p. 150). Chorionic gonadotrophin itself cannot cause ovarian
luteinisation. It probably does not normally pass through the placenta into
the fcetus except in very small quantities. It is possible that placental gonado-
trophin augments the action of pituitary luteinising hormone in maintaining
the corpus luteum during pregnancy. Its liberation in the urine of pregnant
animals, including Man, makes possible the Aschheim-Zondek and the more
modern tests for pregnancy.
In many mammals the yolk-sac, through the medium of the chorion, enters
into a close relationship with the uterine wall. A connection, the so-called
yolk-sac placenta, is established through which nourishment and antibodies
can be conveyed to the embyro. This rarely persists after the true (allantoic)
placenta has become established.
The stalk of the yolk-sac, with the corresponding narrowed part of the
allantois and the vessels which it contains, forms the umbilical cord by which
the fcetus is connected at the umbilicus with the yolk-sac and placenta. This is
enclosed in a sheath formed by the ventral portion of the amnion. The part
of the allantois which remains within the cavity of the body develops into the
urinary bladder, together with a cord, the urachus, connecting the bladder with
the umbilicus. In some mammals the umbilical cord contains a sphincter
muscle in the umbilical ring (rabbit, Cavy, cow, horse and sheep). In rat,
dog, cat, Man, pangolins and Macaque no such muscle occurs.
In Man, and some other eutherian mammals, the head of the neonatus is
occasionally cloaked with a caul-a fragment of the thin, translucent amniotic
membrane. To be born with, or subsequently to carry on the person, a caul,
betokens (in various communities) good fortune, long life, powers of clair-
voyance, a religious career, safety in mines from fire-damp, and, at sea,
immunity from drowning. In Leeds, in r88g, a miracle was reported whereby
go6 ZOOLOGY
a caul was found to bear the words British and Foreign Bible Society. Great
excitement prevailed, and some still held to a supernatural theory even after it
was shown that after detachment, the caul had
lain for some hours on a Holy Bible, on the cover
of which the above title was deeply indented.
In Napoleonic times the market value of cauls
was sometimes as high as 30 guineas, but by
early in the present century prices had declined
to about £r. During the First World War the
price rose again to from £3 to £5. Cauls are
still occasionally carried by sailors and are pur-
chasable on the water-fronts.
In monotremes the ovum is relatively enor-
FIG. 657.-Metatheria: Apla-
cental arrangement. Embryo and mous (diameter of vitellus about 2·5 mm. in
freta! membranes of Bettongia (a rat-
kangaroo). all. allantoic cavity; Ornithorhynchus and about 4'5 mm. in Tachy-
amn. amnion; amn. c. cavity of glossus); and in marsupials comparatively large.
amnion; uel. extra-embryonic
crelom; ser. serous membrane The ovum is 0·25 mm. in diameter in Dasyurus
(chorion); yk. s. yolk-sac. (After and 0·14 mm. in Didelphys compared with o·o87
Semon.)
mm. in Mus and 0·15 mm. in sheep-the last-
named being the largest recorded for a eutherian.
After fertilisation the marsupial ovum becomes enclosed in a thick shell-
membrane with a layer of albumen. The
first cleavage of the ovum (Didelphis), or
the fourth (Dasyurus), involves a separation
of the embyro-forming part of its substance
from that destined to give rise only to the
trophoblastic ectoderm. Further divisions
take place in such a way as to give rise, not ,y.f.&
to a solid morula as in the Eutheria, but to
a hollow blastodermic vesicle with a wall
composed of a single layer, the cells on one
side of which form the embyronic area. An
elaborate allantoic placenta is not developed _yorw

except in Perameles, Phascolarctos, and FIG. 658.- Metatheria: Placentation.

Embryo and placenta of P erameles (a
Phascolomys (p. 716). The intra-uterine marsupial bandicoot) . Letters as in Fig.
development of the fretus is abbreviated, 658. In addition, all. s. allantoic stalk;
mes. mesenchyme of out er surface of
and birth takes place when the young ani- allantois fused with mesenchyme of serous
membrane; s. t. sinus terminal is; u.t
mal is still relatively very small and incom- uterine wall. (After J. P. Hill.)
pletely developed.
In all marsupials, so far as known, the embyro is covered, except in a
limited area, by the compressed and expanded yolk-sac. In the great majority
(Fig. 657) the allantois (all.) is small, and is completely enclosed with the
 PHYLUM CHORDATA

embryo in the yolk-sac. In native 'cats' (Dasyurus) there is a well-developed

yolk-sac placenta. Only in bandicoots (e.g. Perameles), the Koala and the
wombats, so far as known, is the outgrowth of
the allantois to the chorion followed by the
A
establishment of an intimate relationship be-
tween the chorion and the uterine wall, with the
formation of interlocking ridges and depressions.
The whole constitutes a placenta of the same
essential character as that of the Eutheria,
B
though devoid of actual villi (Fig. 658).
The Prototheria, unlike all the rest of the
Mammalia, are oviparous. In T achyglossus
only a single egg, as a general rule, is laid in a
season. This is placed in a temporary incuba-
torium, formed as already described (p. 695) in
the mammary region of the ventral surface.
The young animal soon emerges from the egg,
and remains enclosed in the marsupium till it
reaches an advanced stage of development. tiveFrG. 659.-Mammalia: Compara-
embryology. A, Blastula stage
Ornithorhynchus develops no marsupium: its two in a eutherian. B, Transition stage
between the morula and blastula in
eggs are deposited in its burrow. In Tachy- a monotreme. Both represented
glossus the eggshell is composed of keratin ; in in diagrammatic section. (After
Semon.)
Ornithorhynchus it contains carbonate of lime.
The ova of the Prototheria (Fig. 659) are very much larger than those of other
mammals, their greater dimensions being due to the presence of a large pro-
portion of food-yolk. The segmentation, unlike that of
all the Theria, is meroblastic, and the blastoderm event-
ually forms a complete investment of two layers, to the
yolk. An embryonic area is differentiated at one pole,
and on it appears a primitive streak with a primitive
knot and head-process. The young of all monotremes
possess a carttncle as well as an egg-tooth (p. 651).
The gestation period of mammals varies widely. It is
about twelve days in Didelphys and Dasyurus, sixteen in
Trichosurus and thirty-eight to forty in the bigger
kangaroos. Among eutherians it varies between about
FrG. 66o.- 1Uacropus:
Mammary fretus. Young three weeks in some smaller rodents to approximately one
kangaroo attached to the year in the whale Physeter and in Equus zebra, thirteen
teat. (Natural size.)
months in Tapirus indicus, fourteen in the Giraffe,
eighteen in Rhinoceros bicornis and nineteen to twenty-two months in Elephas
maximus (see alsop. z8r).
Post-natal Behaviour in Marsupials.-It was often erroneously thought that
goB ZOOLOGY
marsupials are 'born in the pouch' as a growing expansion of the teat (Fig. 66o).
The new-born young of the largest living kangaroo is only 1·3 inches long.
Once the neonatus attaches itself to the nipple the latter expands slightly, so
that if forcible removal is attempted, the neonatal buccal tissues may rupture
and bleed, thus creating an understandable impression of origin in situ. This
is particularly so in the case of the smaller marsupials. For example, the
neonatus (32 hours± 8) of the Yellow-footed Pouched Mouse (Antechinus
jlavipes) has a vertex-rump measurement of less than 5 mm. The head is
3 mm., the fore-limbs 2 mm. and the hind-limbs I mm. in length. The total
weight is a mere 0·0164 gr. (Marlow, unpublished).
Marsupial parturition, first observed by Europeans in 1804, has now been
described in Didelphys, several species of M acropus, in Phascolarctos and
possibly others. Most observers have stated that the neonatus travels unaided
to the pouch. In M acropus, however, the female sometimes licks a 'pathway'
on her fur along which the young one ascends. In captive Great Grey Kan-
garoos, the female has been reported to assist the neonatus with either lips or
paws (Hediger). There is evidence that if the young becomes dislodged
during its ascent she makes no attempt to retrieve it.
The relatively long and powerful fore-limbs of the new-born kangaroo are
already equipped with claws which no doubt assist in the upward 'swimming'
movement over or through the mother's fur. The swelling of the nipple within
the juvenile's mouth helps to keep the mammary Ja:tus in position during its
helpless stage. Milk, expressed from the mammary gland by the contraction
of the cremaster muscle, is thus forced down the resophagus of the young animal
which breathes unobstructedly through its nostrils by virtue of the continuous
passage established between the nasal cavities and the larynx, as already
described (p. 884). An opposing view, however, is that in at least some species
the neonatus extracts milk by its own efforts.
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Watson, D. M. S. (1926) Croonian Lecture. 'The Evolution and Origin of the
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Watson, D. M. S. (1939) 'The Origin of Frogs', Trans. Roy. Soc. Edinb., 60, 195
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 INDEX
All numbers refer to pages: words in italics are names of genera and species: words in thick
type are names of higher divisions: numbers in thick type refer to a definition of the term
or a main section dealing with it.

A Adaptation, climatic thermal, Alligator, 499, 502, 543. 552:

883 A. mississippiensis, 502:
AARDVARK, 8o4,87I Adder, Australian Death, 536, Differences from Crocodile,
A bderites, 7 I 6 537. 498: Puff Adder, 537 501
Abdominal cavity, 88 Adduction (muscle), 76 Allotheria, 654. 701
Abdominal pore, 8I Adelochorda, I Allotriognathi, 285, 297
Abducens nerve, I30 Adelochordata, 4, 5: Affinities, Alopex lagopus, 844
Abduction (muscle), 76 :!0: Classification, 5 Alopias, 227
Acanthias, 278 Adelospondyli, 431. See also Alouatta, 743. 884
Acanthodes, 209, 2IO, 2I2 Microsauria Alpaca, 839
Acanthodidm, 2 I o Adenohypophysis, 346 Alternation of generations, 20
Acanthodii, 94, 203, 207, 210: Adhesive papill;:e, 3I Alticamelus, 839
Classification, 207 Adipose bodies, 474 Altricial young, 564
Acanthopegasus, 297 Adipose tissue, 70 Alveoli, 108, 403, 666, 677, 862
A canthop his, 536: A. antarcti- Adrenal cortex, 7I Alytes, 386, 456: A. obstetri-
cus, 537 Adrenalin, I52, I86, 597, 683 cans, 454
Accipiter, 623 Adrenergic nerve-fibre, 683 Ambergris, 783
Accipitridre, 614 JEgothelidre, 6 I 8 Ambiens, 580
Accommodation, visual, 3I5 JEluroidea-see Feloidea A mblotherium, 703
Acetabulum, IOI, 3I8, 470, JEpyornis, 6o9 Amblypoda-see Pantodonta
664 JEpyomithiformes, 565 A mblystoma, 438: A. tigrinum,
Acetylcholine, 683 Aestivation, 375 456
Acinonyx, 795 Aeronautes saxatilis, 559 Amblystornidre, 432
Acipenser, 288: A. ruthenus, Aetosaurus, 499 Ambrein, 784
288 Aftershaft, 568 Ameloblast, 862
Acipenseroidei, 284, 288 Agama, 497 Amia, 282, 283, 287, 292, 322,
Acoustic nerve, 133 Aglypha, 498 329, 330, 33I, 340, 344· 362:
Acoustico-lateral centre, 249 Agnatha, 68, 70, 82, 163, 274: A. calva, I36
Acqueductus cochlearis, 145 Characteristics, 93: Classifi- Arniidm, 293
Acrania, 4, 47, go, 167: Classi- cation, I64: Evolution of, Arnioidea, 284, 292
fication, 4 7 I64 Amnion, 457, 554, 649: Flase,
Acrobates, 7I6: A. pygmams, Agnesiidre, 2 I go I
7I7 Agorophiidm, 783 Amniota, 458
Acrodont teeth, 534 Agoutis, 777. 779 Amniotic cavity, 901
Acromyocli, 62o Agriochteridm, 836 Amniotic fluid, 649
Acrosauria, 461 A iluropoda, 797 Amphibamus, 429
Actinistia-See Ctelacanthini Ailurus, 797 Amphibia, 70, no, n4. I25,
Actinopterygii, 84, 85, 99, 204, Air-sac, 540, 586 135. I39, 381, 485, 532, 856:
205, 282: Alimentary canal Aistopoda, 388, 430 Alimentary canal and as-
and associated structures, Ala spuria, 57I sociated structures, 442:
335: Brain, 344: Blood- Alar membrane, 567 Blood-vascular system, 444:
vascular system, 342: Classi- Alauda, 587 Classification, 388: Endo-
fication, 284: Development, Alaudidw, 620 crine system, 450: Endo-
352: Endocrine organs, 346: Albatross, 6I3, 62I skeleton, 438: Exoskeleton,
Endoskeleton, 330: Electric A lbttla, 342 437: External characters,
organ, 334: Exoskeleton, Albumen, 598, 646 435: General organization,
329: External form, 320: A lea impennes, 556 435: Interrelationship of
Exteroception, 344: General Alcedinidre, 6 I 9 groups, 421: Musculature,
organization, 320: Repro- Alcidre, 6 I 7 442: Nervous system and
duction, 349: Respiratory Alimentary canal, ]I, 102, 336 organs of special sense, 447:
organs, 337: Swim-bladder, Alisphenoid, 659, 72I, 724 Reproduction and develop-
339: Urinogenital organs, Allactaga, 859 ment, 452: Respiratory
347 Allantois, 457. 554. 65I organs and voice, 443:
927
928 INDEX
Urinogenital system and os- Anguilliformes, 285, 296 Apteryx, 623, 63o, 634. 635.
moregulation, 450 Anguimorpha, 497 638, 639, 641: Skull, 63o:
Amphibia-Reptilia relation- Anguis, 497, 519, 547 A. mantelli, 631
ship, 427 Angular, 468 Aptornis, 630, 633
Amphibolurus, 519, 540, 547 Anhimidre, 614 Apus, 559, 623
Amphichelydia, 416 Anhinga, 614, 622 Aqueduct, 126
A mphignathoclon, 385, 442 Anhingidre, 614 Aqueous humour, 141
A mphilestes, 687 Ankylosauria, 462, 511 Arachnoid, r 28
Amphimerycidre, 837 Ankylosaurus, SII Armoxelis, 485, 492
Amphioxididre, 4 7 Ankylosis, 300 Arapaima gig{ls, 320
Amphioxus, 47, 86, 105, 131, Annulus ovalis, 672, 881 Archa;ohippus, 830
r6o: Alimentary canal and Annulus tympani, 395 Archreopterygidre, 599
associated structures, 51: Anrestrus, 896 Archreopterygiiormes, 565, 599
Blood-vascular system, 54: Anolis, 523 A rchmornis, 556: A. siemensi,
Body-wall, 49: Crelom, 54: Anomalopterygidre, 6os 599
Development, 60: Excre- Anomaluridre, 777 Archmopteryx, 427, 556, 599,
tory organs, 56: General Anomalurus, 778, 845 6oo: A. lithographica, 599
anatomy, 51: Ingestion, Anoplotheriidre, 836 Archreornithes, 565, 599, 6zo
digestion, and absorption, Anoplotherium, 8os Archelon, 488
53: Nervous system, 57: Anoptichthys, 251, 345 Arch encephalon, I 26
Organs of special sense, 59: Anseriformes, 565, 614 Archenteron, 12, 418
Pharynx, 51, 54: Reproduc- Ant-eater, 765, 871: Banded, Archeoceti, 779, 782, 781
tive system, 59: Skeleton, 713: Great, 764: Lesser, Archipallium, 885
so : A. lanceolatus, 4 7, 48- 764: Spiny, 688, 842: True, Archipterygium, 99
See also Branchiostoma 726 Archosauria, 461, 484, 498
lanceolatum Antebrachium, 8r, 462 A rctictis binturong, 797
Amphiphnous, 298, 338 Antechinomys, 713 Arctocyonidre, 791, 792
Amphisbrenia, 497 Antechinus jlavipes, go8 Arctoidea-See Canoidea
Amphistyllic jaw-suspension, Antelope, 837 Arctolepida, 2 r r
94 Anterior commissure, 410 Arctolepis, 2II
Amphitherium, 703 Anterior fontanelle, 232 Arctostylopidre, 8o6
Amphiuma, 432, 435, 444• 447 Anthracotheres, 838 Ardeidre, 6 r 4
Amphiumameans, 433 Anthracotheriidre, 836 Area opaca, 551
Amphiamidre, 432 Anthropoidea, 734, 736, 741 Area pellucida, 55 I
Amplexus, 453 Antiarchi, 203, 214 Argentea, 315
Ampulla, 145, 275, 88g: Am- Antilocapridre, 837, 84o Argenteum layer, 327
pull<e of Lorenzini, 25 I Anti-trochanter, 579 Argyropdecus, 327: A. gigas,
Amygdaloid nuclei, 543 Antorbital process, 394 328
Amynodontidre, 825, 835 Anura, 381, 386, 388, 428, 438, Armadillo, 726, 761, 845:
Anablepidre, 351 440, 442, 444· 447· 448, 454· Fairy, 762: Nine-banded,
Anableps, 351: A. dowei, 351: 456, 457 898: Three-banded, 763
A. tetrophthalmus, 322 Anus, 567, 655 Arrector pili, 844
Anabus, 338: A. scandens, 339 Aorta, 373, 475: Dorsal, I II, A rsinoitherium, SIO: A. zitteli,
Anacanthini, 285, 297 241, 477, 673: Ventral, r ro, 8II
Anaconda, 521, 554 241 Artamus, 559
Anagale, 734 Aortic arch, II3, 407, 446 Arterioles, 77, I I I
Anal orifice, 670 Apatornis, 605, 6o6 Arterio-venous anastomosis, 78
Anamniota, 458 Apatornithidre, 6o6 Arteries, 77, II3: Classifica-
Anapophyses, 656 Apatosaurus, 508 tion, 111:
Anapsida, 461, 486 Ape, 726, 741 Afferent branchial, 241, 445
Anaptomorphidre, 736, 739 Aphetohyoidea, 203, 205-See Anterior mesenteric, 241,
Anaptomorphus, 739 also Placoderi 674
Anas, 631 Aplodontia, 777 Branchial, I ro, 241, 674
Anaspida, 70, 99, r64, 170 Aplodontidea, 777 Carotid, III, 446, 674
Anatidre, 614 Aplousobranchia, 2 r Caudal, III
A natosaurus, 509 Appendicula, 278 Creliac, II r, 241, 477
Anchippodus, 760 Appendicularia, 20, 44. 45-See Creliaco-mesenteric, 407, 477
A nchinia-See Doliopsis also Larvacea Coronary, 673, 883
Anchitherium, 829, 830 Apoda,38r, 386,388,434,438, Cutaneous, 407
Ancodonta, 836, 838 443. 444· 448. 455 Dorsal carotid, 241
Ancylopoda, 832 Apodidre, 6r8 Efferent, 117
Andrewsarchus, 792 Apodes, 285, 296 Efferent branchial, 241, 445
Andrias, 432 Apodiformes, 6 r 8 Epibranchial, 373
Angel fish, 321 Aponeuroses, 74 Genital, 477
Angler-fish, 297, 321 Apsidospondyli, 388, 421, 430 Hepatic, II2
Anguilla, I 79, 296: A. anguilla, A ptenodytes forsteri, 560, 6r I Hyoidean, 241, 312
339 Apteria, 569, 626 Iliac, III, 241, 477. 674
Anguillidre, r 7 8 Apterygiformes, 565 Innominate, 591, 674. 881
 INDEX gzg
Intercostal, 477, 674 Atubaria, 5, q, r6, 18: A Batistes, 297
Lienogastric, 241 heteroglypha, IS: A. hetero- Balistidm, 329
Lumbar, 477 lopha, r6 Baluchitherium, S36
Median sacral, 674 Audio-equilibrium system, Bandicoot, 7IS, 907: Pig-
Mesenteric, rrr, 477 145. 3I6 footed, 709
Ovarian, III, 674 Auditory apparatus, 34S Baptornis, 6os
Parietal, 243 Auditory bulla, Felis, 799: Baptornithidre, 6os
Posterior mesenteric, 241 Ursus, Soo Barbel, 230, 322
Pulmonary, 113, 407, 446 Auditory capsules, 9I, 303 Barbet, 6I9
Renal, III, 241, 477, 674 Auditory foramina, 93 Barbicels, 569
Spermatic, r I r, 674 Auditory nerve, 144 Barbs of feather, 56S
Subclavian, 241, 477• 674 Auditory nerves, 13I, 144 Barbules of feather, 568, 569
Systemic, r I r Auditory organ, So Barracudas, 283, 297, 320
Vertebral, 674 Auditory ossicles, 663, S52 Baroceptos, 674
Vitelline, 649 Auditory pinna, SS7 Barylambdidre, Sr8
(See other arteries by name) Auditory range of birds, 642 Barytheriidre, 8oS, 8 I 5, 8 I 7
Arthrodira, 203, 210 Auk, 6I7: Great, ss6, 5S7. Barytherioidea, 8oS
Articular, 306, 468 630: Razor bill, SS6 Barytherium, 8I5, 8I7
Artiodactyla, 726, 791, 8o9, Aulopus, 296 Basalia, 98
824, 82s, 836, 837, 8s7. 868, Auride, rq, 404, 474-See Basibranchial, 307
87I also Atrium Basihyal, 306, 468, Ss4
A rvicola, 903 Auriculo-ventricular node, 590 Basilar membrane, 595
Arytenoid, s8s, 676 Australopithecinre, 745 Basilosauridre, 782, 783
Ascaphus, 429, 44S Autodiastylic jaw suspension, Basilosaurus, 782, 783
Ascidia, 21: Alimentary canal, 94 Basioccipital, 96, 303, 465
25: Blood vascular system, Autonomic nervous system, Basipterygium, 309
25: Body-wall and Atrial 4I2 Basipterygoid process, 467, 576
cavity, 22: Development Autopalatine, 97 Basis cranii, 92, 303
and Metamorphosis, 28: Autopterygoid, 97 Basisphenoid, 96, 3os, 465
Excretion, 27: Larva, 32: Autostylic jaw suspension, 94, Basitemporal, S7S
Neural gland, 27: Nervous 206 Basking Shark, 2I9
system, 27, 28: Pharynx, Autosystylic jaw suspension, Basophils, 7I
23: Reproductive system, 94 Bat, 726, 7S4. 875, SS3: Fruit-,
27: Stigmata, 24: A. mam- Autotomy, 464 756, SSI
milla/a, 32 Aves, 70, 555: Classification, Bathochordmus, 44
Ascidiacea, 20: Classification 565: General organization, Bathytoshia, 265
and general characteristics, 5S5. 598 Batoidea, 22S
21: General organization, Avocet, 6I6, 622 Batrachia, 42S
34 Axis, 655, S5o Batrachoidei, 2S5
Ascidians, composite, 36 Axolotl, 443, 456 Bdellostoma-See Eptatretus
Ascidiidre, 21 Axous, II7 Beak, 620: Specialization, 62 r
Ascopharynx, 709 Aye-aye, 737 Bear, 799, S74, S9S: Red,
Ascophidre, 43S Azygos vein, 670 797
Aspidorhychoidea, 284, 293 Beaver, 777
Aspredo, 3SI Bee-eater, 559, 6I9
Ass, 831 B Belone, 296
Asterixus, 316 Beloniformes, 2S5
Asterolepidre, 215 Baboon, 744, 750, 896 Belonorhynchus, 28S
Asterolepis, 2IS Baculum, 890 Benneviaspis, 174
Asterosteus, 217 Badger, 796, 898 Benthobatis moresbyi, 275
Astragalus, 399, 665, S59 Balcena, 78I, 785: B. mysti- Beryciformes, z85
Astrapotheria, 72s, So2, 807 catus, 78s Berycomorphi, z85, 297
Astrapotheriidre, So7 Balrenicipitidre, 614 Bettongia, 702, 719
Astrapotherioidea, 8o2, So7 Balrenidre, 785 Biceps muscle, 581
Astrapotherium, So7 Balamoptera, 781, 785, 874: B. Bicipital groove, 664
A straspis, r6S musculis, 780, 790 Bidder's organ, 4S2
Astroscopus, So, 335, 339 Balrenopteridre, 785 Bienotherium, 517
Astylosternus, 444 Balanoglossus, 5: Alimentary Bile, I04
Asymmetron, 47 canal, 8: Blood-vascular Bile-duct, I04
A teleaspis, 174 system, 10: Burrow, 7: Binturong, 797
Ateles, 743 External characters and Biogenetic law, 2
Atelopondinre, 452 crelom, 5: Development, r I: Bird-flight, 55S, 582
Atlas, 65s, Sso: Pro-, 525 Locomotion, II: Nervous Birds-of-Paradise, 6zo
Atractosteus, 364 system, 11: Notochord, ro: Bird song, 638
A trial canals, 4 5 Reproductive organs, II: Birgeria, 288
Atriopore, 49 Stomochord, ro Birkenia, 171
Atrium, 109, 3II-See also Baleen, 781 Birkeniidre, I 7 I
Auricle Baleen plate, 785 Bitter lings, 296
930 INDEX
Bittern, 6q, 625: European, Bramatherium, 840 C<ecum, 473. 585, 67o
641: Sun, 615 Branchial arches, 93, 95, 368, CtBnolesfes, 715, 716
Bitis arietans, 537: B. gabon- 370: Nerves, 131 Cmnolestoidea, 710, 715
ica, 537: B. nasicornis, 537 Branchial clefts, 1 Cmnotheriidm, 836
Bladder, Air-, 108: Branchial filaments, I05 Caiman, 459, 50I, 502
Allantoic, 113, 416 Branchial ray, 270 Calamoichthys, 291
Gall-, 402 Branchiogenic organs, 105 Calamus, 568
Gas-, 108 Branchiostegal membrane, Calamus scriptorius, 682
Swim-, Io8, 3II 298, 322 Calcaneum, 399, 533. 665, 859
Urinary, 159, 3I8, 375, 4I5, Branchiostegal ray, 299, 306 Calcar, 400, 755, 759
483, 684. 889 Branchiostoma, 47, 48: B. Callithricidm, 736, 743
Blarina, 728 lanceolatum, 47-See also Callithrix, 846
B!astocrele, I94. 418 Amphioxus lanceolatus Callocalia, 618, 637: C. brevi-
Blastocyst, 686 Branchiostomida, 48 rostris, 637: C. francica, 637
Blastoderm, 55I Branchiostomidm, 4 7. 48 Callorhinus, 842
Blastodermic vesicle, 899 Breeding season, Mammalia, Callorhynchidm, 258
Blastopore, I94 896 Callorhynchus, 258, 259, 260,
Blastozooid, 4I Brevicipitidm, 430 262,265: C. antarcticus, 257,
Blastula, 6o Brill, 297 26I, 263
Blennius, 336 Broad bill, 620 Camarhynchus pallidus, 621
Blind worm, 5I9 Brolga, 6I5 Camel, 837, 838, 88o: Bac-
Blood, 70, II5, 409 Bronchi, 396, 478, 585, 586 trian, 839
Blood platelets, ]I, 4Io Bronchioles, Io8, 677 Camelus bactrianus, 839: C.
Blood sinuses, 78, 243 Bronchus, 540 dromedarius, 839
Blow-hole, Whale, 789 Brontops, 832 Camelidm, 836, 881
Blubber, 78I Brontosaurus, 507 Campanula Halleri, 315
Boa, 497, 389, 52! Brontothere, 826, 829, 83I Camptosaurus, 509
Poar, 872 Brontotheriidm, 825, 83I Camptotrichia, 366
Bobeau, poison lakes of, 535 Brontotheriodea, 825 Canal, Ampullary, 274
Body-wall, 81 Brontotherium, 832 Inguinal, 684, 889
Boidm, 497 Brood-patch, 560 Pericardio-peritoncal, 243
Boltenia, 34 Bubo, 560 Semicircular, 250, 275
Bombinator, 44I Buccal cavity, I02, 666 Temporal, 698
Bone, ]I, 73 Buccal glands, I04 Urinogenital, 685
Bones-See individual entries Bucconidm, 6I9 Vertebrarterial, 655
Bonito, 297 Bucerotidm, 6I9 Canary, 642
Bony labyrinth, 596 Buck, Prong-, 840 Canidm, 791, 792, 793
Booby, 6I4 Buffalo, 837. 84o Canine teeth, 666
Boomslang, South African, 537 Bujo, 387, 403, 406: B. bujo, Canis, 794: C. dingo, 793
Borhyama, I]2 436: B. calamita, 437:. H. Cannon bone, 839
Borhymnoidea, 710, 712 carens, 452: B. marmus, Canoidea, 792, 793
Bos, 899 4I7: B. viridis, 385: B. vul- Caperca, 785
Botaurus stellaris, 625, 64I garis, 447 Capillaries, 77, 108, uo, IIJ:
B othriolepis, 2 I 5 Bufagin, 437 Systemic, III
Bothrops atrox, 536 Bufonidm, 430, 452 Capitonidm, 6I9
Botryllus violaceus, 35 Bufotalin, 437 Capitosaurus, 425
Bovidm, 837, 84o Bulbul, 620 Capitular facet, 463
Bower-bird, 563. 620, 642 Bulbus arteriosus, IIo, 3I 1, 407 Capitulum, 656, 657
Bowfin, 282 Bulla tympani, 66o, 852 Capreolus capreolus, 898
Bowman's capsule, I55. 4I5, Bungarus, 537: B. candidus, Caprimulgidm, 618
888 538: B. multicinctus, 538 Caprimulgiformes, 565, 6I8
BrachtBlurus waddi, 229 Bunophorus, 837 Captorhinomorpha, 461, 486
Brachioradialis muscle, 58I Burhinidm, 616 Captorhinus, 486
Brachiosaurus, 507, 508 Burs;e, 74 Capuchin, 743
Brachium, 8I, 462 Bursa cntiana, 360 Capybara, 774, 777
Brachyacanthidm, 207, 2Io Bursa Fabricii, 585 Carapace, 52I, 526
Brachycephalidm, 430 Bush-baby, 738 Carcharias, 220
Brachyops, 426 Bushmaster, 52I Carcharinidm, 228, 275
Bradyodonti, 204, 220, 222, Bustard, 6I5: Quail, 6I5 Carcharinus, 228, 273
223, 256 Carcharodon, 227, 273
Bradypodoidea, 76I, 764 Carcharodon carcharias, 2I9
Bradypus, 764, 77I, 772: B. c Cardiac muscles, 77
tridactyhts, 768, 769, 770 Cardiac sac, IO
Brain, 344, 4Io, 447. 542, 639. Cadophore, 38 Caretta, 490
678: Flexure of Mid-, I25: Caducibranchiate, 435 Cariama, 6I5
Fore-, I22, I26: Hind-, I22: C<ecal pouch, 81o Cariamidm, 6 I 5
Mid-, I22: -stem, 126: Cmcilia, 43~See also Apoda Carinatm, 6o6, 630
Structure, 125 CtBcoderma, 357 Carina sterni, 577
 INDEX 931
Carnere, 406 Leydig, 255 Chalicotheriodea, 825
Carnivora, 725, 791, 798, 868, Test, 28 Chalicotherium, 833
88o, 882, 890, 899 Cement, I03 Chamreleon, 497, 523, 539:
Carolozittelia, 820 Centetes, 848 Change of colour, 523
Carotid artery, 407 CentetidEe-See Tenrecidm Champsocephalus, 343
Carotid body, I32 Central canal, I22, 682 Charadriidm, 6I6, 6I7
Carotid sinus, I32, 674 Centrale, 400, 469 Charadriiformes, 565, 616, 625,
Carotid trunk, 407 Centralia, IOI 628
Carotid, vental, 24I Centropelina, 557 Chauliodus, 332
Carp, 284, 296 Centropus, 6I7 Chauna, 624
Carpals, IOI, 664 Centrum, 300, 39I Cheetah, 795, 796
Carpo-metacarpus, 578 Cephalaspida, 70, I64, I72- Cheirodus, 288
Carpus, 469, 664 See also Osteostraci Cheirolepis, 285, 288
Cephalaspis, 167, 172
Cartilage, 7 I , 7 2 : Calcified, 73, Chelida!, 489
95: Elastic, 72 Cephalic hook, 350 Chelodina, 526
Cartilage, Antorbital, 270 Cephalochorda, I Chelonia, 459, 461, 476, 486,
Arytenoid, 478, 540 Cephalochordata, 4, 47-See 490, 52I, 522, 523, 524, 525,
Bones, 302 also Acrania 526, 527, 530, 53I, 532, 539,
Coracoid, 270 Cephalodiscidm, 15, IS 540, 541, 549. 845· 85I: c.
Cricoid, 478, 540 Cephalodiscus, 5, I4, 15: mydas, 487
Extra-branchial, 270 Blood-vascular system, I7: Chelonidm, 487, 490
Fibrous, 72 Reproduction, 17: zooid, I6 Chelydrida!, 490
Hyomandibular, 269 Cephalopterus, 174 Chemoreceptors, 4I3
Intercalary, 267, 437 Ceraspis, 2I5 Chevron bones, 464, 85I
lntertemporal, 368 Cerastes, 5 20 Chevrotain, 837. 839
Labial, 269 Ceratias, 350 Chiasma, 130
Meckels', 269 Ceratioidea, 3 2 I. 3 24 Chilotherium, 836
Neural, 367 Ceratobranchial, 307 ChimaJra, 256, 258, 259, 260,
Santorini, 676 Ceratodus, 364, 379 396: C. monstrosa, 257
Scapular, 270 Ceratohyal, 306, 368, 854 Chimmridm, 258
Spiracular, 269 Ceratomorpha, 825, 833 Chimpanzee, 744. 750, 752
Cartilaginous process, 465 Ceratophrys, 442 Chinchilla, 777
Cartilaginous skull, 92 Ceratopsia, 462, 511 Chionididm, 616
Caruncle, 55I, 65I, 907 Ceratotherium simus, 834 Chipmunk, 777
Cassowary, 556, 557. 6o7, 634 Ceratotrichia, 2, 235 Chirocentrus, 337
Castoroidea, 777 Cercaertus, 716 Chiromantis, 436
Castoroides, 774 Cercopithecida!, 736, 844 Chiromys, 737, 869
Casuariiformes, 565, 6o7 Cercopithecoidea, 736, 743 Chironectes, 704, 709, ?II, 712
Casuarius, 607 Cerebellar fossa, 662 Chiroptera, 725, 726, 754, 848,
Cat, 799, 844. 898, 905 Cerebellum, I25, 246, 274, 3I2, 875, 882: Skeleton, 758
Catarthina, 743 480, 592, 68I Chlamydosaurus, 5I8
Catbird, 62I: Tooth-billed, Cerebo-spinal cavity, 88 Chlamydoselachus, 222, 227,
622 Cerebro-spinal fluid, I25 266, 270
Cat fish, 296, 329 Cerebral cortex, I27 Chlamyphorus truncatus, 762
Catharacta skua, 6I2 Cerebral fossa, 662 Choanichthys-See Crossop-
Cathartes aura, 638 Cerebral hemispheres, I26, terygii
Catopterus, 288 274· 478, 884 Chooropotainidm, 836
Cattle, 84o Cerebral nerves, 97 Cholinergic nerve fibre, 683
Caudal region, 300 Cerebrospinal fluid, 68o, 682 Cholinesterase, 683
Caudata, 432-See also Urodela Certhiidm, 62o Cholmpus, 764, 77I: C. hoff-
Caul, 905 Cervidm, 837. 84o manni, 766
Cavia, 778, 848 Cervus elaphus, 86o Chondrichthyes, 94. 99, 203,
Cavum aorticum, 406 Cestracion, 269 219, 274. 409-See also
Cavum arteriosum, 475, 542 Cetacea, 74. 725, 780, 823, 842, Elasmobranchii
Cavum dorsale, 475 85I, 85~ 868, 88o, 88I, 882, Chondrocranium, 97, 303
Cavum pulmo-cutaneum, 406 884, 887: Internal adap- Chondrostei, 204, 284, 287
Cavum pulmonale, 475 tions, 789: Skeleton, 785 Chondrosteus acipenseroides,
Cavum venosum, 475, 542 Cetiosaurus, 5IO 289
Cavum ventrale, 474. 54I Cetorhinus, 2I9, 227, 250, 272, Chorda dorsalis, I
Cavy, 905 274· 275 Chorda-mesoderm, I94
Cebidm, 736, 743. 750 Cetotheriidm, 784 Chordre tendinere, 672
Cebochoorida!, 836 Chtenocephalus, 343 Chorda tympani, 132
Ceboidea, 736, 743 Chceronycteris mexicana, 757 Chordata, I: Classification, 4:
Cebus, 743 ChaJropus, 709, 715 Development of nervous
Cecinus viridis, 621, 636 Chalaza, 647 system, I22
Cells, Cilated, 24 Chalcides, 550 Chorio-allantoic placenta, 278
Follicle-, 28 Chalicotheres, 829 Chorion, 28, 686, 901
Gland-, 24 Chalicotheriidm, 825, 832 Chorionic gonadotrophin, 905
932 INDEX
Chorionic villi, 686 Coccyx, 748 Coraco-scapular, 577
Choristodera, 461 Cochlea, I45, 642, 888 Coracoid, 308, 333, 396, 468,
Choroid, 139, 315 Cochlear duct, 145 577
Choroid fissure, 142 Cochleariidre, 614 Coracoid process, IOI
Choroid plexu~. 125, 246, 68o Cockatoo, 626: Sulphur- Coracoid region, IOI
Chromaffin tissue, 152,347, 597 crested, 560 Corellidre, 2I
Chromatophores, 178, 327 Cod, 282, 297 Corium, 8I
Chrysochloridre, 709, 714, 729, Cc:elacanthini, 204, 354, 356 Cormorant, 557, 614, 623, 644
730 Cwlacanthus, 356 Cornea, I39. 141, 3I5
Chrysochloris, 729 Creliac, artery, I 11 Cornealis muscle, 192
Chrysochloroidea, 726, 727, Cwlogenys, 775 Cornua, 395, 468
729, 730 Cc:elolepida, I 74 Cornual cartilage, 200
Chrysopelea, 520 Cwlolepsis, I74 Cornwallius, 822
Chthone1·peton, 455 Crelom, 87 Corona radiata, 898
Chyle, 675 Crelomic fluid, 4I5 Coronary artery, 673
Cichlidre, 297, 351 Crelomic pouches, 62 Coronary circulation, 115
Cicinnurus regius, 561 Crelomic sacs, 62 Coronary sinus, I 15
Ciconiidre, 6r4 Crenrecium, I9 Coronodontidre, 223
Ciconiiformes, 565, 614 Colies, 6I8 Coronodus, 223
Ciliary muscle, 595 Collide, 6 I 8 Coronoid, 468
Ciliary processes, 140 Coliiformes, 565, 618 Corpora adiposa, 4I7
Cinclidre, 62o Colius, 6I9 Corpera cavernosa, 685, 686
Cingulata, 761, 762 Collagenous fibres, White, 70 Corpora cavernosa penis, 980
Cinixys, 488 Collar-pores, 7 Corpora quadrigemina, 681,
Ciona, 29: Development, 29 Colletoptera, 637 884
Cionidre, 2I Colobus, 743 Corpus albicans, 890
Circumorbitals, 305 Colon, 670 Corpus callosum, 679, 723, 884
Citellusbeecheyi, 883 Colostrum, 850 Corpus cavernosum urethr<e,
Civit, 795, 88o: Otter-, 795 Colubridre, 497, 498 890
Cladistia-See Polypterini Colugo, 726, 742, 753 Corpus geniculatum, 68r
Cladodus, 99, 223 Columba Iivia, 565 Corpusluteum, 152,548,890
Cladoselache, 99, 2I2, 266: C, Columbidre, 6I7 Corpus mammilare, 68o, 68r
fyleri, 223 Columbiformes, 565, 617 Corpus spongiosum, 685, 686,
Cladoselachii, 203, 222 Columella, 146, 395, 467, 482, 890
Clariallabes petricola, 338 577. 595 Corpus striatum, 262, 479, 543,
Clarias, So, 296, 337: C. mos- Columella auris, 36I, 381, 546 593
sambicus, 33 7 Column;e carne<e, 672 Corpus trapezoideum, 682
Claspers, 22I, 230, 255, 259, Colymbus, 613 Corpus uteri, 893, 895
266 Comb, 628 Corpuscles, 70, 675, 88I
Ciavelinidre, z r Commissure of brain, 3I3, 679, Cortex, 125, 593, 684, 84I, 888
Clavellina, 30 68o, 681, 885 Corti, organ of, 146
Clavicle, 101, 309, 396, 469, Conacodon, 803 Corvidre, 62o
75 2, 8 5 6 Conchopoma, 364 Coryphodon, 8r8, 819
Claw, 568, 845: Toilet-, 625 Condor, 560, 614 Coryphodontidre, 8 I 8
Cleithrum, 308, 333, 441, 856 Condylar foramen, 659 Corythosaurus, 509
Climatius, 208, 2IO, 2I2: C. Condylarthra, 724, 725, 8oi; Cosmetornis, 561
reticulatus, 207 802, 8o6 Cosmine layer, 84
Cling-fish, 297 Condyle, 260, 269, 382, 47I, Costeid<e, 297
Clitoris, 686, 894 575. 579. 657. 659. 665 Cottoidei, 285
Cloaca, 8I, I02, 193, 230, 273, Cones of eye, I41, 595 Cotton-tail, 779
366, 376. 585 Coney, 779, 8o8 Cotyloid bone, 665, 856
Cloacal aperture, 389, 463 Conger conger, I79 Cotylosauria, 46I, 484, 486
Club cells, 82, 178 Conger-eel, 432 Coursers, 616
Club-shaped gland, 64 Conilurus, 709 Cow, 878, 905
Clupea, 296 Connective tissue, 70, 74 Cowper's glands, 685
Clupeiformes, 284 Conodonts, I65 Cracidre, 6I5
Cnemial process, 579 Conoryctes, 760 Crane, 6I5, 638
Cnemial ridge, 47I Conus arteriosus, I09, 241, 406 Cranial bones, 96
Cnemiornis, 630, 633 Coot, 615 Cranial cavity, 88
Cobra, 498, 537. 538: Black- Cope-Osborn theory of tri- Craniata, r, 4. 69, 163, 553:
necked, 538: King, 52 I, 536, tuberculy, 865 Blood-vascular system, 108:
538 Copelata, 45 Body, 72: Body-wall and
Coccosteidre, 206, 2II Coprod<eum, 474, 585 internal cavities, 81: Brain,
Coccosteus, zn: C. decipiens, Copulation, Mammalian, 894 118: characteristics, go, 93:
211 Copulatory grapple, 894 Classification, 163: Develop-
Coccygeal. 566 Coraciidre, 6I9 ment, 160: Endodermal de-
Coccygeo-iliacus, 400 Coraciiformes, 565, 619 rivatives, 71; Endocrine
Coccygeo-sacralis, 400 Coraco-humeral, 58I organs, 147: Ectodermal
 INDEX 933
derivatives, 71: External Cyamodus, 493 Dentition, 510, 864, 87o, 871,
characters, 78: Mesodermal CyathaspidOJ, 169 8]2
derivatives, 71: Metamer- Cyathozooid, 41 Dentine, 71, 103
ism, 181: Nervous system, Cyclomyaria, 36-See Doliolida Depressor muscle, 76
118: Respiratory system, Cyclopes, 765, 768, 771, 772 Dermal bones, 302
107: Sensory organs, 135: Cyclopterus, 334 Dermal ray, 323
Tissues, 70: Urinogenital Cyclostomata, 175 Dermal teeth, 103, 229
system, 154: Visceral rela- Cyclotosaurus, 425 DermatemydidOJ, 489
tionship, 87 Cygnus, 638: C. olor, 557 Dermatomes, 161
Cranium, 92, 302 Cylorana, 385, 451 Derm-ethmoid, 305
C!eodonta, 791, 792 Cynocephalidse, 753 Dermis, 71, 81
Cribriform plate, 66o Cynocephalus, 742, 753: C. Dermochelidle, 490
Cricoid, 585, 676 variegatus, 754 Dennochelys, 487, 488, 490,
Cristre, 146 Cynogale, 795 523, 527
Crocodile, 114, 459, 499: Cynognathus, 517, 653 Dermoptera, 725, 726, 753
American, 502: Asiatic Cypriniformes, 285, 296 Derotrematous, 435
marsh, 5oo: Estuarine, 501: Cyprinus, 296, 336 Desmana, 728
Mugger, 500: Nilotic, 502: Cyprbmodontiformes, 285 Desrnadontidle, 757
Orinoco, 502: Salt-water, Cypsiurus, 638 Desmodus, 756, 758: D. ro-
501, 502: Differences from Cystic duct, 104 tundus, 757
Alligator, 501 Desmomyaria, 41-See Salpida
Crocodilia, 75. 139. 459. 461, Desmostyliformes, 8o8, 822
485, 499. 522, 523, 524, 526, Desmostylus, 822
533. 539. 540, 543. 548. 549. D Deuterencephalon, 126
573. 845. 851 Devil-fish, 265
Crocodilidee, 5oo Dactylonax, 707 Diacodexis, 837
Crocodylus, 502, 525, 531, 542: Dactylopsila, 707 Diacrele, 68o
C. aculus, 502: C. inter- Dapedius, 292: D. politus, Diadactyla, 710
medius, 502: C. niloticus, 291 Diadectes, 486
502: C. palustris, 500: C. Darter, 614 Diadectomorpha, 461, 488
porosus, 501, 502, 540 Dasyatidre, 267 Diademonus, 223, 266
Crocuta crocuta, 893, 894 Dasycercus, 713 Diadiaphorus, 804, 858
Crop, 583 Dasypodidee, 762 Diaphragm, 88, 665
Crossbill, 621 Dasypodoidea, 761 Diapsida,486
Crossopterygii, 204, 205, 282, DIJSyurkhe,713, 714 Diastole, 475
290, 322, 353: Alimentary Dasyurinm, 713 Diatryma, 616
canal and associated struc- Dasyuroidea, 710, 712 Diatrymidle, 616
tures, 389: Blood-vascular Dasyuroides, 713 Diatrymiformes, 565, 818
system, 370: Classification, Dasyurus, 721, 870, 843, 896, Diazonidm, 21
354: Development, 377: 897,906 Dicerorhinusinus sumatrensis,
Endoskeleton, 387: General Dasypus hybrida, 899: D. 834
metabolism and excretion, novemcincta, 899: D. sex- Diceros bicornis, 834
375: Nervous system, 374: cinctus, no. 771, 772 Dichobunidm, 836
Reproduction, 378: Re- Daubentonia, 875-See Chi- Dicynodon, 515
spiratory system, 370 romys Dieynodontia, 462, 514
Cro~.498. 547 DaubentoniidOJ, 136, 738 Didelpbia--See llt'letatheria
Crotalus, 498, 520, 522 Deer, 837, 840, 879: Chinese Didelphodon, 712
Crow, 620 water-, 84o: Mouse-, 84o: Didelphoidea, 710, ]II
Crura cerebri, 126, 681 Musk-, 840: Red, 898: Roe, Didelphys, 870, 8]1, 88o, 906,
Crus, 390, 462, 470 898 907,908: D. dorsigera, 891:
Cryptobranchidle, 432 Deinotheriidee, 8o8, 815 D. marmpialis, 871
Cryptobranchus, 436, 444: C. Deinotherioidea, 8o8 Didemnidle, 2 1
alleganiensis, 433 Deinotherium, 815, 816 Didolodontidm, Sox, 8o3, 8o4
Cryptodira, 461, 487, 489 Delphinapterus, 788 Didunculus, 557
ctenacanthidse, 223, 234 Delphinidee, 783 Didus, 630: D. ineptus, 557
Ctenacodon, 701 Deltoid ridge, 664 Didymaspis, 174
Ctenodus, 364 Dendrites, 122 Diencephalon, 126, 274, 312,
Cuboid, 665, 859 Dendroaspis angusticeps, 52 1, 479
Cuckoo,617 537 Digestion, Marsupial, 879: Ru-
Cuculidee, 617 Dendrobates tinctorius, 437 minant, 878
Cuculiformes. 565, 817 Dendrolagus, 704, 719: D. Digits, 81, 389: Toilet, 846
Cumulus proligerus, 898 lumholtzi, 70j. Dimetrodon, 512, 513
Cuneiform, 664, 859 Denisonia, 550 Dimorphodon, 504
Cupola, 595 Dental follicle, 863 Dingo, 793
Cupola terminalis, 250 Dental formula, 869 Dinichthys, 206, 2II
Curassow, 615 Dental lamina, 862 Dinocephalia, 462
Cuscus, 716 Dental papilla, 862 Dinocerata, 724, 726, 8oS, 188,
Cusp, 866 Dentary, 97, 306, 468 819, 858
934 INDEX
Dinomis maximus, 609 Draco. 436, ,5I8 Edriolyclmus, 350
Dinornithidse, 6oS Dragon, East Indian Komodo, Eel, 2S2, 296: Common, 339:
Dinornithiformes, 565, 808 497 Conger, 432: Electric, 296
Dinosauria, 504 Drepanaspis, I70, 212 Effector, II7
Dinosaurs, 504, 5IO Dromadidse, 6I6 Egernia, 550
Diodon, 346 DromtBus, 607 Egg-breaker, 551
Direstrus intervals, S97 Dromedary, 839 Egg-pill Toadlet, 3S6
Diornedea, 6I3: D. exttlans, 621 Dryolestoidea, 653, 654. 703, Egg-tooth, 551
Diomedeidse, 6I3 727, 868 Egret, 614
Diphylla, 756, 75S Dryopithecus, 745 Eland, S4o
Diphyodont, S64 Duck, 6I4 Elapidse, 497, 49S
Diplacanthidse, 2Io Duct, Bile-, 473, 585, 67o Elasmobranchii, 85, 86, 203,
Diblacanthus, 208, 2I2 Cystic, 402, 67o 213: Alimentary canal and
D{plocaulus, 430, 43I Ejaculatory, 889 associated structures, 272:
Diplodocus, 507 Endolymphatic, 250, 275, Audi-equilibration, 275:
Dipneumona, 365 414, 482 Blood-vascular system, 273:
Dipnoi, 94, 107, II4, 204, 353 Genital, 318 Brain, 274: Classification,
361, 384. 393. 40S. 432, 44I, Hepatic, 402, 67o 221: Development, 279:
445, 705, 708: General or- Mullerian, I59. 265, 45I Electric organ, 271: En-
ganization, 379 Pancreatic, 402, 473. 670 docrine organs, 276: Endo-
Dipodidse, 777 Pharyngeo-cutaneous, I99 skeleton, 267: General or-
Dipper, 620 Pneumatic, 3II ganization, 265: Integu-
Diprotodon, 7I9 Pronephric, 158 ment, 267: Muscles, 271:
Diprotodontia, 709, 7Io Thoracic, 592, 670, 676, 883 Organs of special sense, 274:
Dipterus platycephalus, 364: \Volffian, 159, 415, 416, 45I Placentation, 278: Hespira-
D. valenciennesi, 363, 364 Ductus Botalli, 447 tory and associated organs,
Dipus, 859, 86o Ductus Cuvieri, III, 243, 373, 273: Skull, 268: Urino•
Discocephali, 285, 297 447 genital organs and reproduc-
Discoglossidse, 429 Dugongs, 764, 807, 82o, 82I, tion, 276
Dispholidus typus, 537 823, 824, 874 Elasmosaurus, 493
Display, bird, 563 Duodenum, 31 I, 401, 585. 668 Elasmolherium, 836
Distale, 400 Duplicidentata, 774 Electric lobe, 272, 274
Distalia, IOI Dura mater, 128, 478 Electric organ, 334
Distribution of animals, 705, Electrophorus, 296, 334
706 Electroplaxes, 334
Divers, 603, 613 E Elephant, 8o7, 8u: African,
Docodon, 703 813,817: Asian, 815,817
Docodonta, 653, 654. 703 Eagle, 56o, 614: Monl<ey- Elephantidse, 8o8, SI5
Dodo, 557,617,630,633 eating, 6I5: Wedge-tailed, Elephantoidea, So8
Dog, 793. 799, 897, 899, So5: 615 Elephantulus, 733, 742, 897
Wild, 847 Ear, 143, 250, 414, 482, 595, Elephas, 815, 817: E. maxi-
Dogfish, 77, 229: Alimentary 887: Functions, 250: Ex- mus, 907: E. primigenius,
canal and associated struc- ternal, I43, 147: Inner, I43, SI2
tures, 237: Autonomic ner- 4I4, 482: Middle, 143, q6, Eleva or muscle, 76
vous system, 249: Blood- 4I4, 482: Structure, 250 Elopteryx, 614
vascular system, 240: Brain, Ear-drum, 414 Elpistoslege, 423
244: Cranial nerves, 246: Ecdysis, 402, 523 Embolomeri, 3S8, 426
Endocrine organs, 251: Ex- Echeneis, 297, 325 Embolotherimn, 832
cretion and osmoregulation, Echeniformes, 285 Embrithopoda, 724, 726, 8oS,
252: Fins, 229: Nervous Echidna, 688, 694 810
system, 245: Orbit, I43: Echidna pterygoid, 688 Embrvonal knot, S99
Organs of special sense, 249: Echidnidse-See Tachyglossidse Emu,-556, 557, 6o2, 6o6, 6o7,
Reproduction, 254: Respira- Ecllinosorex, 727 630, 634
tion, 239: Skeleton, 231: Echo-location in bats, 756 EmydidtB, 490
Spinal cord, 249 Ectocion, 802, 803 Enaliornis, 6o5
Dolioletta, 36 Ectocomts, So3 Enaliornithidse, 6os
Doliolida, 36 Ectocuneiform, 665 Enamel, of teeth, 103: Epithe-
Dolioloides, 36 Ectoderm, 55I lium, 863
Doliolum, 36, 37: Develop- Ectodermal derh·ath·es, 71 Enamel-organ, S62
ment, 38: General charac- Ectoethmoid, 96 Encephalocrele, 57
teristics, 37: Pharynx, 37 Ectoganus, 76o Endocardium, 474
Doliop$iS, 36 Ectopterygoid, 467 Endochondral ossification, 95
Dolphin, 783, 7S4, S65 Edaphosauria, 462 Endocrine organs, 104, 147
Dorudon, 783 Edaphosaurus, 5I3 Endoderm, peripheral, 900
Dorudontidse, 782, 783 Edentata, 725, 726, 760, 772, Endodermal derivatives, 71
Dove, 617 774, So4, SSI Endolymph, I45
Down, nestling, 626: powder-, Edmontosaurus, 509 Entosphenus, 175
626 Edriolychus schmidti, 350 Endostyle, 23, 24, 32
 INDEX 935
Endothelial lining, 71 Erinaceoidea, 726, 727 Falculre, 845
Endotheliochorial placenta, go 5 Erinaceomorpha, 726, 727 Fallopian tube, 685
Entelodontida!, 836 Erinaceus, 727, 842 Fang, poison-, 534. 53S
Entepicondylar foramen, 771 Eryops, 424 Fascia, 74
Enterocrelic pouches, 62 Erpetoichthys, 282, 290: E. Fascire dentatre, S85
Enterogona, 21 calabricus, 29I Fasciculi, 74
Enteropneusta, 4. 5 Erythrocytes, 70, 115,409,S8I Fat cells, 70
Entopterygoid, 305 Eschrichtida!, 7S5 Feathers, 71, 566, 626: Co-
Endostyle, 105 Eschrichtius, 785 loration, 627: Contour,
Endothiodon, 515 Esocoidei, 2S4 569: Down-, 569, 626:
Eoanthropus, 746 Esoterodon, 5I5 Nomenclature, 571: Rec-
Eoanura, 388, 429 Esox, 296 trice, 566: Structure, 56S
Eobasikus, 819 Esthonyx, 760 Feather-follicle, 569
Eobrasilia, 710 Ethmoid, 66o Feather-germ, 569
Eodelphis, 712 Ethmo-turbinals, 66o Feather-papilla, 56S
Eocephalodiscus, 20 Euarthrodira, 2 I I Feather-pulp, 569
Eocetus, 782 Eubradyodonti, 204, 222 Feather-tract, 569
Eogyrinus, 426 Euchorda, 69 Felidre, 792, 793. 795
Eohippus-See H yracotherium Eunectes, 554 Felis, 799, 844
Eomoropus, 833 Eunectes murinus, 52I Feloidea, 792, 793
Eosinophils, 71 Eunotosauria, 46I, 4S8 Femur, 100, 398, 462, 470,
Eosuchia, 461, 495, 496, 498 Euparkeria, 499 579
Eotheroides, 822 Euphanerida, 70. I 64. I 67 Fenestra ovalis, I46, 381, 468,
Eotherittm-See Eotheroides Euro, 709 482,595.66!, sss
Eotitanops, 826, 83I Eurynotus, 84-See Plectro- Fenestra rotunda, 482, 66I
Ependyma, 122 lepis Ferre, 725, 791
Epibranchial, 307 Eurylrenidre, 62o Fer-de-lance, 536
Epicardium, 404 Eurylremi, 62o Ferret, Sg8
Epicentral, 330 Eurypigidre, 6I5 Ferungulata, 725, 791
Epidermis, 8I Euselachii, 203, 22I, 225, 226 Fibroblast, 70
Epididymis, 255. 483, 685, 889 Eustachian aperture, 661 Fibrous capsules, 74
Epiglottis, 668, 676, 876, 884 Eustachian tube, ]I, 400, 400, Fibula, IOO, 470, 47I, 579, 665,
Epigonal organs, 274 4S2,500,596,66], 888 8 56
Epihippus, 829 Eustachian valve, 672, 88I, 882 Fibulare, IOI
Epihyal, 306, 854 Eustltenopteron, 356 Fila olfactoria, I30
Epimysia! sheaths, 74 Eusuchia, 46I, 499 Filoplume, 569, 626
Epineurals, 330 Eutheria, 654. 704, 706, 723, Filum terminale, 4I2
Epi-otic ossification, g6, 303, 724, 8So, S82, 8S7, Sgg,goo: Fimbria, 68o
466 Classification, 725 Fin, 4S, 308: Development,
Epiotic process, 303 Evolution, Horse, S29: Mosaic, 98: Nomenclature, 8I:
Epipharyngeal groove, 52 427 Structure, 98:
Epiphysis, 74. 102, 245. 274, Excretion, I56 Caudal, 81
480, 655 Exocretus, 296, 32I Dorsal, 8I
Epipterygoid, 467 Exoccipital, g6, 303, 394, 465, Heterocercal, 230
Epipubis, 441, 470 659 Pectoral, 81, g8, 99, 309,
Episternum, 397, 469 Exoskeleton, 88 333. 369
Epithalamus, I88 Exteroception, 344 Pelvic, g8, 333, 369
Epitheliochorial iplacenta, 904 Extension of muscle, 76 Tail-fin, 330, 33I
Epithelial tissue, 70 Extensor carpi radialis, sSr Ventral, 8I
Epithelium, 71: Germinal, 8go Exteroceptive organs, II7 Fin-rays, dermal, 98
Epitrematic bar, 184 Eyes, So, 139, I4I, Igi, 250, Finches, 620
Epoicotherium, 76I 275· 3I5, 4I3. 448. 48I, 544· Fissipedia, 79I, 793, 874
Epomophorus, 88I 595.639.887 Flame cells-See Solenocytes
Eptatreftts, 197. 198, 199: E. Eye-lens, I41, 315 Flamingo, 6I4, 621, 622, 627
stouti, 198 Eye-lids, 413 Flange of feather, 569
Equidre, 825, 826: Limb spe- Eye-muscle, I4I, 142, 143. 250 Flat-fish, 326
cialization, 828 Fleurantia, 364
Equilibration, 143 Flexion muscle, 76
Equoidea, 825 F Flexor perforans, 580
Equus, 830,844,861,873, Sgo: Flight, bird-, 558, 5S2
E. asinus, S3I: E. burchelli, Fabettre, 665 Floccular forsa, 661
83I: E. grevyi, 83I: E. Facet, capitular, 656 Flocculus, 592, 6S1
hemionus, 83I: E. przewal- Facet, tubercular, 656 Flycatcher, 620
.~kii, 83I: E. quagga, S3I: Facial nerve, I30, 132 Flying-fish, 296, 321
E. zebra, S31, 907 Falciform process, 315 Flying-fox, 756
Eremochelys, 490 Falcon, 614 Fretus, 686
Ergosterol, 627 Falconidre, 6I4 Follicles, 254
Erinaceida!, 727, 73I Falconiformes, 565, 614 Fontanelles, ISO, 303, 394, 465
 INDEX
Foot, 81, 101, 390, 462, 824: Galago, 738, 740, 890 Gila l\Ionster, 497, 546
Mesaxonic, 824: Paraxonic, Galatheathauma, 325: G. a:reli, Gills, 105, 299, 379: External-,
824 324 106, 420, 435, 444: Inter-
Foramen, condylar-, 659 Galbulidre, 619 nal-, 444: Spiracular, 273:
Ischiadic-, 579 Galeocerdo, 220 Structure, 106
Magnum, 92, 574, 657 Galeolamna, 220 Gill filaments, 370
Monro, 262, 593 Galeoidea, 222, 227 Gill-pouch, 105
Obturator-, 470 Galeopithecus-See Cynocepha- Gill-rakers, 307
Optic, 659 lus Gill-slits, So, 299
Ovale, 672 Galeus, 228 Giraffe, 837. 84o, 848, 896, 907
Panizza, 501, 542 Gall-bladder, 104, 3II, 402, 670 Giraffi.dre, 837. 84o
Pituitary-, 659 Galliformes, 565, 615 Gizzard, 584
Pulso-ischiac-, 470 Gallinule, 615 Gland, Adrenal, 152, 160, 403,
Triosseum, 578, 581 Galliralltts, 630, 633 474· 596, 597. 684
Fore-arm, 8r Gallus, 585, 623, 64o, 646; G. Anal, 549
Formosan, 538 domesticus, 570, 581 Angle-, 583
Fornix, 68o, 885 Gambusia, 336 Antebrachial, 847
Fossa, Inferior temporal-, 468 Ganglion, 32, 120: Basal, 127: Branchial, 847
Lateral temporal-, 468 Gasserian, 131: Sympa- Buccal, 637
Ovalis, 672 thetic, 314, 683: Semilunar, Bulbo-urethral, 685, 890
Posterior temporal-, 468 131 Carpal, 847
Tympano-eustachian, 468 Gannet, 614 Coccygeal, 626
Fowl, Australian Mallee, 56o: Ganoin, 330 Cowper's, 685, 890
Common, 637: Domestic, Ganoine layer, 84 Crop-, 583, 594
628: Jungle, 623 Garefowl, 556 Crural, 692
Fox, 793: Arctic, 844: Flying, Garfish, 296 Cutaneous, 846
875 Garpike, 282 Gas-, 340
Fraud, scientific, 454 Gasteropelecus, 321 Harderian, 480, 544. 887
Fregatidre, 614 Gasterosteiformes. 285 Infraorbital, 666
Fregata, 636 Gasterosteus, 297; G. aculealtts, Intermaxillary, 400, 442
Frigate-bird, 636 329 Inter-renal, 160, 252
Fringillidre, 620 Gastornis, 636 Lachrymal, 413, 467, 482,
F ritillaria, 4 5 Gastornithidre, 616 544. 848. 887
Frog, 389: Alimentary canal Gastralia, 6oo Leydig's, 253
and associated structures, Gastrocnemiu's muscle, 75 Lingual, 442
400: Blood-vascularsystem, Gastroliths, 539 Mammary, 71, 847
404: Development, 418: Gastrothec{t, 454: G. marsupi- llleibomian, 887
Endocrine glands, 415: En- ata, 453, 454: G. oviferum, Mucous, 471
doskeleton, 391: External 454 Nidamentary, 277
characters, 389: Lymphatic Gastrula, 61 Oil-, 566, 626
system, 410: Metamorpho- Gavia, 6o3, 6r3 Parathyroid, 152, 489, 668
sis, 420: Nervous system, Gavialis, 501: G. gangetictts, 502 Parotid, 437. 666
410: Reproductive organs, Gaviiformes, 565, 613 Perinreal, 655. 685
416: Respiratory system, Gazelle, 837, 840 Pharyngeal, 443
403: Organs of special sense, Geckos, 497, 524, 540 Pituitary, 149, 189, 410, 415,
412: Urinogenital organs, Geese, 614 479, 593: anterior, 402.
415: African hairy-, 444: Gekkota, 497, 519 See also Hypophysis
Marsupial-, 453: :Mink-, 447: Gelocidre, 837 Poison-, 328
South American tree-, 454: Gemttendina, 216: G. sfltrfzi, Preen-, 626
Tree, 430,436,443.448 216 Prostate, 685, 889
Frog-mouth, 621, 626 Genet, 795 Pyloric, 25
Frontal,96,305,467,66o Geniculate body, lateral, 130 Rectal, 685
Frontal lobe, 678 Geniculate ganglion, 132 Renal, 273
Frontal segment, 97 Genital ridges, 7 Salivary, 104, 471, 533
Fronto-parietal, 394 Geological periods and epochs, Sebaceous, 71, 846
Fulcra, 330 3 Shell-, 255
Fulmars, 613 Geomyoidea, 777 Slime-, 198
Fulmarus, 613 Geopsittactts, 617 Sub-lingual, 583
Fundulus, 297, 336 Georychus, 777 Submaxillary, 666
Furcula, 577 Geotria, 175: G. australis, 186 Sudoriferous, 846
Geotrypetes, 455 Suprarenal-See Adrenal
G Germ-layer theory, 72 Thymus, 104, 105, 153. 252,
Germinal disc, 277, 319, 646 273. 402, 589, 671
Gadiformes, 285 Gharials, 459, 501 Thyroid, 104, 148, 150, 197,
Gadus, 297 Gibbon, 744 273. 347. 402, 474. 589,
Galactophoric sinus, 849 Giganlua, 296 668,676
Galreopithecidre-See Cynoce- Giganturiformes, 285 Uropygial, 626
phalidre Giganturoidea, 285, 296 Venom, 535
 INDEX 937
Glans, 890, 894 Gull, 617, 682: Black-headed, Heloderma, 471, 497, 534, 546
Glareolidre, 6r6 562: Herring, 628 Helodus, 256
Glenoid cavity, 397 Gurnards, 297 Hemachatus, 538
Glenoid fossa, 66o Gymnarchus niloticus, 335 Hemibranch, fishes, ro6, 262
Glenoid surface, ror Gymnotus-See Electrophorus H emicentetes, 729
Glider, Pigmy feather-tailed, Gymnophiona, 381, 388, 434- Hemichordata-See Adelochor-
7!6 See also Apoda data
Glires, 725, 774 Gymnopis, 455 Hemicyclaspis, 174
Globicephala, 789: G. me las, Gymnura-See Echinosorex Hemipenes, 483
789 Gyracanthidre, 2ro Hemiprocnidre, 6r8
Glomerulus, ro, 155, 193, 318, Hemiramphidre, 321
408, 415, 888 Hemiscyllium, 230, 246
Glossobalanus, 5, 14 Hemitrypus, 432
Glossopharyngeal nerve, 130, H Henle, loop of, 596, 888
132 Henricosborniidre, 8o6
Glottis, 370, 400, 403, 478 H;:emal canal, 88, go Hepatic c;:eca, 9
Glucocorticoids, 152 H;:ematop;:etic tissue, 74 Hepatic-ducts, 104
Glycogen, 104 Hrematopodidre, 6r6 Hepato-pancreatic duct, 104
Glyptodon, 761, 762 H;:emoendothelial placenta, 905 Hepatic portal system, II2
Glyptodontoidea, 76r H::emoglobin, r 15 Heptranchias, 227, 266, 267,
Glyptodontidre, 762 Hag, 175 269, 270, 397
Gnathostomata, 70, 203: Clas- Hair, 71, 841 Heron, 614, 626
sification, 203 Hair-cells, 595 Herring, 282, 296
Goannas, 497 Hair-follicles, 841 Hesperornis, 6o2, 605, 613, 630,
Goat, 837, 840, 897 Hair-germ, 843 633, 636: H. regalis, 6o4
Gobies, 297 Hair-papilla, 841 Hesperornithidre, 6os
Gobiesocid;:e, 297 Hair-tracts, 843 Hesperornithiformes, 565, 604
Gobiesocoidei, 285 Hake, 297 H eterocephalus glaber, 842
Gobioidei, 285 Halimtus albicilla, 615: H. Heterocercal fin, 230
Gobiomorphi, 285, 297 pelagicus, 615 Heteroccelous vertebr;:e, 573
Gobius, 297 Halicore-See Dugong Heterodontoidea, 222, 227
Goblet-cells, 401, 585 Hallux, ror, 390, 665 Heterodontus, 226, 227, 277
Godwit, Bar-tailed, 625 Halosauriformes, 285 Heterohyrax, Sog
Gomphotheriidre, 8oS, 8r5 Hamadryad, 521 Heteromi, 285, 296
Gomphotherium, 815 Hammer-head, 614 Heterosornata, 285, 297, 326
Gonads, 159, 318 Hamster, 778 Heterostraci, 70, 164, 168
Gonadoids, 152 Hand, 8r, ror-See also Manus Hexanchus, 227, 266, 267, 269,
Gonadotrophic hormone, 148 Haplodoci, 285, 297 270
Gonadotrophins, 150 Haplomi, 284, 296 Hibernation, 883
Goosander, 62r Hare, 779: Artie, 779, 844: Hilus of kidney, 684, 888, 890
Goose, Giant, 630 Common, 897 Hipparion, 830
Gopher, 490: North American, Hargeria, 6o5 Hippidion, 830
548. 777 Harriers, 614 Hippocampal commissure, 410,
Gopherus agassizi, 548 Harrirnanidre, 6 543· 639. 885
Gorgonopsia, 5 r 7 Harriotta, 258, 259: H. raleigh- Hippocampal sulcus, 68o
Gorgosaurus, 507 ana, 257 Hippocampus, 297, 323, 351,
Gorilla, 744, 750, 752 Hatchet-fish, 327 68o, 885
Goura cristata, 617 Hatschek's groove, 49, 59 Hippomorpha, 825, 858
Graafian follicle, 685, Sgo, 8g8 Hatschek's nephridium, 64 Hippopotamus, 707, 837, 838,
Granular cell, 178 Hatteria, 459, 521: H. punc- 850, 862: H. amphibius, 838
Granulocytes, polymorpho- tata, 459-See also Spheno- Hippopotamidre, 836
nuclear, 409 don Hirundinidre, 62o
Graptolita, 20 Haversian canals, 73 Hirundo, 559
Graptolites, 20: Affinities of, Hawk, 623 Histiocyte, 70
20 Hawksbill, 490 Hoatzin, 620, 624
Grebe, 557, 613, 623-See also Head-shield, 522 Hoax, scientific, 454
Divers Hearing, 250 Holoblastic segmentation, 899
Grey matter, 123, 682 Heart, ro8, 273, 3II, 404, 444, Holobranch, fishes, 105, 262
Grouse, 615: Sand-, 617, 623 474, 589, 670, 88r: Res- Holocephali, 94, 204, 222,
Gruidre, 6r5 piratory, r 14: Systemic, II4 223, 256, 323: Alimentary
Gruiformes, 565, 615 Heart-beat, 882 canal, 261: Blood-vascular
Crus antigone, 638 Hedgehog, 726, 727, 842, 892 system, 262: Brain, 262:
Guanaco, 839 Hegetotheria, Sor, 8o7 Endoskeleton, 259: Exter-
Guano, 596 Helaletidre, 825 nal characters, 258: Res-
Guanophores, 390 Heleioporus, 145 piratory organs, 262: Urino-
Guereza, 743, 744 Helichthys, 228 genital organs and reproduc-
Guinea-fowl, 615 Helicotrema, 642 tion, 263
Guinea-pig, 777 Heliornithes, 615 Holostei, 204, 284, 287, 291
VOL. II MJ
 INDEX
Holostylic jaw-suspension, 94 Hymen, 255, 894 Ichthyostegalia, 388, 423
Bomalodotberiida!, 8o6 Bynobiida!, 432 Ichthyotoini-See Pleuracan-
Homalodotherium, 8o6 Hyoid apparatus, 393, 395, tbodii
Bominida!, 736, 744, 745 468, 576, 663, 854 Ichthystegopsis, 423
Hominoidea, 744 Hyoid arch, 93, 98, 368 Icosteiformes, 285
Homo, 72, 138, 676, 745, 751, Hyoid cornu, 94 Ictidosauria, 462, 517
899: H. neanderthalensis, Hyomandibular, 94, 98, 306, Ictops, 734
745: H. sapiens, 734, 745, 368 Iguana, 497
747 Byopsodontidre, Sox, 8o3 lguania, 497
Homwosaurus, 495 Hyopsodus, 803 Iguanidre, 761
Homogalax, 826 Hyostylic jaw-suspension, 94 Iguanodon, soB, 510
Bomostiidre, 212 Hypapophyses, 464, 524, 573, neum, 401, 585, 67o
Homostius, 212 629,656 Iliac process, 270, 101
Honey-eater, 620 Bypercorypbodon, 818 Ilium, 102, 398, 470, 664
Honey-guide, 619 Hyperhippidium-See Para- Illicium, 325
Hoof, 845 hipparion Ilysiidre, 498, 530
Hoopoes, 619 Hyperodapedon, 495 Impala, 840
Hoplodactylus, 548 Hyperphalangy, 788 Impennre, 565, 6o2, 61o, 628,
Hormone, 147 Bypertragulidre, 837 630
Adrenalin, 149 Hypnarce, 270 Incisors, 666
Adrenocorticotrophic, 150, Hypobranchial, 307 Incubatorium, 695, 850
594 Bypobytbiidre, 21 Incus, 146, 663, 888
Follicle-stimulating, 150, 597 Hypocercal tail, 170 Indicator indicator, 619
Gonadotrophic, 148, 594 Hypogeophis, 455 Indicatoridre, 619
Growth, 151 Hypoglossal nerve, 130 Indri, 737, 739
Interstitial cell stimulating, Hypohippus, 830 Indridre, 736
I 50 Hypohyal, 306 Infundibular organ, 58
Lactogenic, 151 Hypoischium, 470, 532 Infundibulum, 245, 274, 479,
Luteinizing, 150, 598 Hypophysial portal system, 677, 681
<Estrogen, 148, 598 593 Indiidre, 783
Placental, 153 Hypophysis, 127, 149, 245, 274, Inioini, 284, 296
Progesterone, 148 410, 479, 592-See also Inscriptiones tendinere, 400
Prolactin, 594 Pituitary body lnsectivora, 725, 726, 882
Somatotrophic, 151 Bypostoinides, 285, 297 Insulin, 153
Steroid, 149 Hypothalamus, 127, 149, 189, Integument, 267
Testosterone, 597 245· 479. 593. 681 Interatberiidre, 8o7
Thyrotrophic, 150, 594 Bypotremata, 222, 226, 265 Interbranchial septa, 105
Thyroxin, 149 Hypsilophodon, 509 Intercalated discs, 77
Tubule-stimulating, 150, 597 H ypsiprymnodon moschatus, Intercentrum, 464
Hornbill, 619 719 Interclavicle, 382, 469, 696
Horse, 824, 826, 904, 905: Hypsiprymnodontidre, 719 Inter-hyal, 306
Phylogeny and distribution, llyracbyidre, 825, 835 Intermedin, 151, 450, 101,
827 Hyrachyus, 826 469
Humerus, xoo, 469, 578, Byracidre-See Procaviida! Intermuscular bones, 88, 94
664 Byracodontidre, 825, 835 Intermuscular septa, 74
Humming-bird, 558, 559, 6x8 Byracoidea, 726, 791, 808, 8x2, Interoceptive organs, II7
Huso huso, 283, 320 873. 881, 889 Interorbital region, 92
Hyarna, 794, 795, 868, 894 Hyracotherium, 8o2, 825, 826, Interorbital septum, 467, 575
Byrenidre, 792, 793, 795 829 Interstitial lamina, 73
Byaenodontida!, 791, 792 Hyrax, 706,707,807, 8oS, 850, Inter-vertebral disks, 655
Hybodont, 226 88o-See Procavia Intestine, 102, 3II, 471, 668
Bybodontida!, 223, 224 Bystricomorpba, 778 Irediparra gallinacea, 624
Hybodus, 227 Hystrix, 842 Iridocytes, 327
Hyemoschus, 839 Iris, 139, 595
Bydrobatida!, 613 Ischia, 398, 664
Hydrochwrus, 774 I Ischiatic symphysis, 470
Hydrodamalis, 821, 874 Ischium, xox, 102, 470, 579
Hydromantes, 447 Ibis, 614 Ischnacanthidre, 2xo
Bydromyinre, 776 Ichthyophis, 455: I. glutinosa, Iscbyromyidre, 777
Bydropbidre, 498, 521 440, 456 Isectolophidre, 825
Hydropotes, 840 Ichthyopterygia, 461, 490 Isoodon, 715
Hydrostatic organ, xo8 Ichthyornis, 6o2, 6o5 Isospondyli,284, 294,296,335,
Hyla, 406, 436, 441: H. Ichthyornithidre, 6os 342
arborea, 447: H. avivoca, Icbthyornithiformes, 565, 805 Isthmus, 125, 299
444: H. jemoralis, 444: H. Ichthyosauria, 461, 486, 490, Istiopboridre, 320
gratiosa, 443 491 Istiophorus, 320
Bylidre, 430 Ichthyosaurus, 81, 491 Isurida!, 227
Hylobates, 744, 751 Ichthyostega, 423 Isuropsis, 220
 INDEX 939
J Labrus, 336 Lateral-line system, 79, 131,
Labyrinth, 145: Membranous, 133. 135. 179. 192, 229, 251,
Jabiru, 614 888 274, 300, 317, 346, 367, 385,
Jacamar, 619 Labyrinthodontia, 382, 388, 414, 448: Bony fish, 136:
Jacanas, 617 422, 469 Cartilaginous fish, 137
Jacanidre, 616, 624, 625 Lacerta, 138, 439, 463, 467, Lateral plate, 64
Jacobson's Organ, 130, 137, Latimeria, 239, 348, 356, 359,
474, 478, 497, 518, 519, 528,
413, 447. 471, 479. 481, 539. 534. 539. 541, 547. 549: L. 360, 370: L. chalumnm, 356,
543. 887 agilis, 549: L. viridis, 546: 358
J agorina, 2 r 7 L. vivipara, 460, 549 Laugia, 357
Jaeger, 617 Lacertilia, 459, 461, 496, 524, Lebistes, 347, 349, 350
jamoytiidce, 167 Leg, 8r
525, 527, 532, 539. 540, 541,
jamoytius kerwoodi, 167 Leiopelma, 705, 706
543. 549. 722
Java Man, 745 Lachesis muta, 521 Leipoa ocel!ata, 56o
Ja,v,g3,97, 205,391,661,746: Lactation, 848 Lemming, 775, 777
Muscles, 662, 775: Origin, Lactation anccstrus, 897 Lemmus lemmus, 775
2o6: Types, 94 Lacteal, 883 Lemur, 726, 733, 737, 752, 847,
Jenny Haniver, 265 Lcemargus, 275 848, 8go: 'Flying', 726,
Jerboa, 733, 777, 86o Lagcenorhynchus, 865
Jugal, 467, 66o 753
Lagena, 145, 414, 482 Lemuridre, 736, 853
Lagomerycidre, 837 Lemuriformes, 734, 736, 737
K Lagomorpha, 654, 725, 774, Lepidosauria, 46r, 484, 495
779 875 Lepidosiren, 361, 362, 365, 379
Kagus, 615 Lagopus, 623: L. mutus, 627 Lepidosirenidre, 365
Kakapo, 557, 630 Lagostomus, 778 Lepidotus, 291, 292
Kakatoe, s6o Lambdotherium, 831 Lepisosteidre, 292
Kalobathippus-See A nchi- Lamina elastica, 77 Lepisosteus, 282, 283, 287, 292,
therium Lamina perpendicularis, 66o 330, 331, 336, 340, 349. 352:
Kangaroo, 709, 717, 859, 870, Lamina terminalis, 127, 128, L. platystomus, 292: L.
897: Great grey, 907: Musk 68r tristcechus, 292
Rat, 719: Rat-, 709, 719, Lamna, 227, 278: L. nasus, Leporidre, 654, 779
8g6, 897: Tree-, 704, 707, 227 Leporillus, 709
719 Lampetra, r6o, 176: L. Lepospondyli, 388, 430, 435
Kannemeyeria, 514 fluviatilis, 176: L. planeri, Leptocephalus, 352
Kentrurosauras, 511 176, 182-See also Lamprey Leptochreridre, 836
Keratin, 82, 85 Lamprey, 148, 175, 176: Ali- Leptodactylus pentadactylus,
Keratinisation, 81 mentary canal and associ- 438
Kestrel, 558 ated structures, 184: Baccal Leptolepidre, 294
Kiceraspis, 174 funnel, 177: Circulatory Leptolepis, 293: L. bronni, 294
Kidney, 154, 193, 252, 263, system, 186: Development, Leptosomatidre, 6rg
276, 318, 347· 403, 415, 482, 194: Endoskeleton, 180: Leptotyphlopidre, 498
596, 684, 888: Develop- External features, 177: Lepus, 72,138: L.articus, 779,
ment, 158: Function, 156 Metamorphosis, 197: Mus- 844: L. europceus, 897
Kin<esthetic receptors, 135 cles, 184: Nervous system, Leucocytes, 70, r 15
Kingfisher, 619: New Guinea, 187: Organs of special Leydig cells, 152, 159, 194,
561 sense, 190: Respiratory 276, 416, 597. 684, 88g
Kinkajou, 797 organs, 185: Skull, 181: Lieberktihn, crypts of, 585
Kinosternidre, 489, 522 Urinogenital system, 193 Ligament, 74
Kirombo, 619 Lampridiformes, 285 Limbs, 8r, roo, 379, 389, 397,
Kite, 614 Lanarkia spinosa, 174 655, 857: Digitigrade, 86r:
Kiwis, 556, 557, 6o6, 630 Lancelot-See Amphioxus Plantigrade, 86r: Special-
Koala, 716, 721, 722, 869, 88o, Laniidre, 620 ization in birds, 623: Unguli-
886, 897. 907 Langerhans, follicles of, r85 grade, 861
Komodo Dragon, 519 Langerhans, islets of, 153, 585 Limb-girdles, ror
Kolbenzelle, 178 Languets, 36, 42 Limnodynastes, 438
Kolliker's pit, 59 Langur, 744 Limnogale, 729
Kotlassia, 428 Lanugo, 842 Limnopithecus, 744
Kowalevskia, 44, 45 Lapillus, 317 Limnoscelis, 467, 486
Krait, 478: Blue, 538 Lapwing, 6r6 Limosa lapponica, 625: L.
Kritosaurus, 509 Laridre, 617, 628 limosa, 625
Kronosaurus, 494 Lark, 620 Linea alba, 400
Kurtus, 350 Larus, 562: L. argentatus, 628 Lingual ridge, 370
Larvacea, 44: Affinities, 46 Liopelma, 429
L Larynx, 443, 478, 540, 585, Liopelrnidre, 429
668, 676 Lipogenys, 296
Labial cartilages, 95 Lasanius, 167 Lipophores, 390
Labia majora, 686 Lasiognathus saccostoma, 324 Lipotyphla, 725, 726, 727, 730,
Labidosaurus, 486 Lateral-ethmoid, 305 868, 889
 INDEX
Liquor amnii, gox M 1\lantle, 22
Liquor folliculi, Sg8 Manubrium sterni, 6S7
Litopterna, 724, 72s, So2, So3, Mabuya, sso Manus, Sx, 3S9, 462, 469, 86o
805, ss7. ass Macaca, IS4. 744 Marmosa, 704, 7II, S48
Liver, 104, JII, 370, 401, 473, Macaque, gos Marmoset, 743
S8o Mackerel, 297 Marrow cavity, 74. 102
Lizard, 459, 462: Alimentary Macrauchenia, 8os Marsupial&, 653, 654. 710, S82:
canal and associated struc- Macraucheniidm, 8o2, 8os Classification, 710; Post-
tures, 471: Australian Jew-, Macrodipteryx, s6x natal beha\'iour, 907
s4o: Barking-, s4o: Blood- Macromeres, 194 Marsupionta, 653
vascular system, 474: Brain, Macropetalichthys, 213 Marsupium, 704
478: Endocrine glands, Macrophages, 409 Marten, 796
481: Endoskeleton, 463: Macropodidm, 707, 717, 719 Mast cells, 70
Exoskeleton, 463: Flying-, Macropodinm, 716 Mastzellen, 70
s18: Mexican bearded-, 497: Macropomia, 3S6 Mastacembeliformes, 285
Respiratory system, 477: Macropus, 719, 720, 870, 887, Mastacembelus, 297
Organs of special sense, go8: M. canguru, 719: M. 1\iastacomys, 709
481: Urinogenital system, rufus, 719 1\Iastodon, 8II, 8xs-See also
482 Macroscelides, 733, 8s9 Mammut
Llama, S37, S39 Macroscelididm, 731 Mastodon, 815
Loach, 33S Macrotlierium, 833 Mastodonsaurus, 422, 426
Lobi inferiores, 24S. 312 Macrotis, 7IS Mastoid process, 661, 749, 852
Loggerhead, 490 Macrarijormes, 28s Maxilla, 97, 305, 395, 467,
Lophiiformes, 28s Macut;e, 146 661
Lophiodontidm, S2s, 833 Magalictis, 797 Maxillo-palatine process, S7S
Lophius, 329: L. piscatorius, Magnum bone, 664, 8sg Mayermys, 776
323 Malacichthyes, 28s, 297 Meatus, 147, 567, 894
Loplwrina supe,ba, s6t Malaclemys, 490 Meckel's cartilage, 93, 97, 233.
Loricata-See Crocodilia Malapterurus, 33S J06, ]68, 395. 468
Loris, 726, 73S: Slender, 73S Malleus, 146, 663, 888 Median organ, So-See also
Lorisidm, 736, SsJ Malpighian body, ISS Pineal body
Lorisiformes, 736, 738 Malpighian layer, S2, s6g, 626 Mediastinum, 670
Lotus-bird, 616, 62S Mamba, 49S: Black, S2I, S37= Medulla, 684, S88: Medulla
Loxia curvirostra, 621 Green, S2I oblongata, 124, 274. 490,
Loxodonta, 813, S1s, 817 Mammalia, 70, 97, II4, 654: 592, 681
Luminescence in fishes, 314, Alimentary canal and asso- Medullary cavity, 74
324, ]28 ciated structures, 862: Medullary folds, 29, 61, 420
Luminous organ in fishes, 272, Blood-vascular system, 881: Medullary groove, 29
324, 327 Breeding seasons, Sg6: Medullary plate, 29, 6r
Lump-fish, 334 Classification, 654: De- llegachiroptera, 755, 756, 758
Lunar bone, 664, 8sg velopment and associated llegadermatidm, 7sS
Lunaspis, 213: L. heroldi, phenomena, 898: Endo- Megaladaspis, 737
213 skeleton, 850: Eye, 139: Megalania, 497
Lungs, IOS, 107, 214, 371, General organization, 841: Megalichthys, 83, JS6
379. 3s4. 403, 444, 540, sB7. Integument and general ex- Megalobatrachus, 387, 436
670, 677: Structure of, ternal features, 841: Lym- Megalohydrax, Sxo
107 phatic system, 883: Nervous llegalonychidm, 761, 764
Lung fishes, 353, 361 system, 884: Organs of Megalonyx, 764
Lycomops, S17 special sense, 887: Respira- Megapode, 623
Lycosaurus, SI7 tory system, 884: Skull, llegapodidm, 615
Lygosoma, sso Ssx: Urinogenital system, Megapodius, 623
Lymph, 71, 78, us. 67S sas Megaptera, 7Bs. 789
Lymph glands, 71 Mammary pouch, Sso llegatheriidm, 764
Lymph valves, 78 Mammoth, hairy, S12 Megatlierium, 764, 76s
Lymph-capillaries, 78, ns, 67S Mammut, Sxs. Sx6 Meiolania, 488
Lymph-hearts, 7S, n6, 410 Mammuthus, Sxs Melanodon, 703
Lymph-sinuses, ns. 410 Mammutidm, SoS, Sxs. Sx6 Melanophores, 390
Lymphatic-nodes, IIS, 67s. Man, I, 4. 726, 741, gas lleleagrididm, 6xs
8S] Manatees, S2o, S23, S74 Metes metes, 796
Lymphatic system, 78, 115, Manatus-See Trichechus lleliphagidm, 62o
477· 67S. 88] Mandible, 97, 46S, S76, 633, Mellivora capensis, 6rg
Lymph-vessels, IIS 662, Ss4-See also Jaw Melomys, 709
Lymphocytes, 71, 78, II6, 409, Mandrill, 744 Melursus, 791
67S Mandrillus, 744 Membrana semilunaris, s86
Lyomeri, 28s Manis, 766, 773. S44: M. Membrana granulosa, SgS
Lyra, 68o, 886 javanica, 773: M. longicau- Membrane, Alar, s67
Lysorophus, 431 data, 773 Internal tympaniform, s86
Lystrosaurus, SIS Manta, 266 Mucous, 102
 INDEX 94I
Nictitating, 27s. 389, 413, Miacoidea, 791 Moropus, 833
463. 482, 59S. 6s4. 887 Microbrachius, 2IS Morula, 686
Serous-, 901 Microchiroptera, 7SS. 756, 7s8 Mosaic evolution, sr7, S56
Tympanic, 389, S9S Microcyprinre, 296 Moschinre, 84o
Membranous labyrinth, 143, Microcyprini, 285, 3so Motmot, 619
482 Microdon, 321 Motor-unit, 76
Meninges, 74. 128 Microgale, 729 Moschus, 840
Meniscotheriidre, 8or, 803 Micromeres, 194 Mouse, Harvest-, 774: Yellow-
llleniscotherium, 803 Micromys minutus, 774 footed pouched-, 908
Meniscus, S73 Microphis boaja, 347 Mouth, So
Menopause, 897 Micropodidre, 637 Mucous cells, 82
Menotyphla, 72s, 726, 731 Micropodiformes, s6s-See Mud-feeders, 643
Menstruation, 742, 897 Apodiformes Miillerian-duct, 451
Mentomeckelian, 97 Micropus, 637: M. apus, SS9 Muller's fibres, 187
lllenura, s6r Micropyli, 319 Multituberculata, 653, 654, 701
Menurre, 62o Microsauria, 388, 431 Murray cod, 297
Mergansers, 622 Micruridre, 498 Mus, 903, 906
Mergus merganser, 621 Miliobatis aquila, 2SO Muscicapidre, 62o
Merluccius, 297 Milk, composition of, 849 Muscle, 75, 271: Function, 76:
Meropidre, 619 Mineralocorticoids, IS2 General features and classi-
Merops, SS9 Minimus digit, 101 fication, 7S: Nomenclature,
Merychippus, 8os. 830, 8s8 Miniopterus, 7S6 76: Types, 86
Merycoidodontidre, 836 Mink, 842, 898 Amphioxine, Ss, 86
Mesacanthidre, 2ro Minnow, 31S Buccinator, 663
Mesaxonia, 726, 791, 810, 824, Miobatrachus, 429 Cremaster, 908
836. 871 Mioclreninre, 8o3 Cardiac, 75, 109, 673
Mesembriomys, 709 Mioclamus, 803 Costopulmonary, s88
Mesencephalon, 68r Miohippus, 83o Cyclostomine, 85
Mesentery, 8, 74. 473. 670: Miripinna, 296, 346: M. esau, End-plate, 123
Gastro-splenic, 67s 346 Geniohyoglossus, 663
Mesethmoid, 92, 96, 66o, 662 Miripinnati, 28s Intrinsic syringeal, 586
Mesochordal canal, ssr Mistichthys luzonensis, 320 Masseter, 662
Mesocoracoid, 308, 333 Moa, ss6, SS7. 609, 631, Pennate, 75
Mesocuneiform bone, 665 633 Piscine, 85, 86
Mesodermal derivatives, 71 Mobulidre, 26s Plain, 75
Mesogaster, 473 Moccullochella, 297 Pterygoids, 662
Mesohippus, 8os, 829, 830, Mmritheriidre, 8o8, Srs Somatic, 75
832 Mmritherioidea, 8o8 Skeletel, 75
Mesomyodi, 62o Mceritherium, 81s Smooth, 71, 75
Mesonephros, IS4. 193 Mola, 334 Sterno-tracheal 586
Mesonychidre, 791, 792 Molars, 666 Striated, 71, 7S
Mesonyx, 857 Mole, 726, 728, 894, 89s: Temporal, 662
Mesopterygium, 236 Marsupial, 713 Unstriated, 7S
Mesorchium, 255, 416, 483 Molgula, 34 Visceral, 7S. 76
Mesorectum, 473 Molgulidre, 2 r Muscle-buds, 281
Mesosauria, 461, 485, 490 Moloch horridus, 518 Muscular layer, Ss
Mesoscapular segment, 8s6 Momotidre, 6r9 Musculature, 86
Mesosternum, 397, 8s1 Mongoose, 79S Musculi papillares, 672
Mesosuchia, 461 Monkey, 726, 741, 88o: Musculi pectinati, 474, 672
Mesotarsal, s8o Howler-, 743, 884: New Musculus tensor chloroidea,
Mesothelium, 74 World, 743: Rhesus, 744: 413
Mesotheriidre, 8o7 Spider, 743 Musophagidre, 617
Mesovariam, 2S4. 417, 483 Monocytes, 71, 409 Mustela juro, 898: M. vison,
Metacarpals, 101 Monodelphia, 6s3-See Eu- 8g8
Metacheiromys, 761 theria Mustelidre, 792, 793, 796, 844
Metamerism, r6r Monodelphis, 704: M. henseli, Mustelina, 892
Metamorphosis, retrogressive, 848 Mustelus, 2so, 27S. 278
20 Monodon, 788 Mutica, 72s, 779
Metanephros, IS4 Monrestrus cycle, 897 Myelencephalon, I2.J.
Metapleural folds, 66 Monophyodont, 864 Myelin, 122
Metapophyses, 6S6 Monopneumona, 364 Myelin sheath, 121
Metapterygium, 236 Monotremata, 6s3. 6s4. 687, M ygale-See Desmana
Metapterygoid, 30S 688, 882, 887 Myliobatidre, 228
Metatarsals, 101, 400, 470, 66s Monro, foramina of, 479, 68o Myliobatis, 274, 27S
Metatheria, 6s4. 704, 882: Morganucodon, 687 Mylodon, 764, 76S
Endoskeleton, 719 Mormyromasts, 33S Mylodontidre, 764
Metencephalon, 12S Mormyrus, 33s: M. kannume, Mylosloma, 2II
Miacidre, 791, 793 33S Mylostomidre, 2u
942 INDEX
Myocardial trabecula, 475 Neopallium, 678, 8S4 Sciatic plexus, 412
Myocrele, 64 Neornithes, 565, 572, 576, 602, Somatic motor, 122, 130
Myocommas, 4g, 85 620: Adoption and geo- Special visceral afferent, I 3 I
Myofibrils, 75 graphical distribution, 612: Special visceral efferent, 131
Myogenic, 10g Alimentary canal and asso- Spinal, 120, 133
Myoglobin, 75 ciated structure, 636: Circu- Spinal accessory, 130, 133
Myohyrax, 810 latory organs, 639: De- Superficial ophthalmic, 247
Myomeres, 4g, 85: Pattern, 86 velopment, 646: Endoskele- Sympathetic, ng
Myomorpha, 777• 778 ton, 628: Excretion, 643: Terminalis, 128
Myotis, 757, 8g6: M. luci- External characters, 620: Trematic, 132
fugus, 757 General characteristics, 620: Trigeminal, 130, 131, 247
Myotome, 64, 161 Nervous system and organs Trochlear, 130, 247
Myotragus, 86g of special sense, 639: Ptery- Vagus, g3, 130, 133, 248
Myriacanthus, 258 losis, 626: Reproduction, Vestibular, 131, 133
Myrmecobius, 713, 843: M. 644: Respiratory and vocal Nerve-cell, II
fasciatus, 704 organs, 638 Nerve-fibres, 124, 6S2: Post-
Myrmecophaga, 764, 766, 770, Nephridiopore, 57 ganglion, 6S2: Somatic, 124:
773 Nephridium, 56 Visceral, 124
Myrmecophagidse, 764, 765 Nephrocytes, 27 Nervous system: Autonomic,
Myrmecophagoidea, 761, 764 Nephron, 155, 156 120, 249, 6S2: Central, n7:
Mysticeti, 7S2, 784 Nerves, abducens, 130, 247 Classification, II 7: Develop-
Mystriosuchus, 4gg Acoustic, 133 ment, 122: Parasympathe-
Myxine, S2, 157, 178, 1g7, 1g8, Acoustico-lateral, 134 tic, 121: Peripheral, u7:
200: M. glutinosa, 17g, 1gS Acousticus, 24S Sympathetic, I 20: Visceral,
Myxinoidea, 70, 164, 1g7, 43g: Auditory, 131, 133, 144, 24S 121
Compared with lamprey, xgS Branchial, 24S Neural arch, go, 3g1
Myxopterygia, 25g, 266 Branchial Arch, 130 Neural crest, 71
Branchial plexus, 412 Neural ganglion, 27
Buccal, 24S Neural keel, 195
Cardiac, 249 Neuromasts, 274
Cochlea, 133 Neural spine, 3g1
N Coronary, xog Neural tissue, 71
Cranial, 133, 246, 6S2: No- Neural tube, go
Naia, 526: N. hannah, 521, menclature, 128: Num- Neurenteric canal, 30, 61
536: N. naia, 535 bering, 130 Neurenteric passage, 280
Nails, 71, 845 Facial, 130, 132 Neurilemma, 121
Naja nigricollis, 53S Facialis, 247 Neurobiotaxis, 3
Nannopterum, 557 Gasserian, 247 Neurocrele, II, 30, 57, 61, 122
Nanygai, 2g7 Gastric, 24g Neurocranium, g7, 303
Narcobatoidea, 22S Glossopharyngeal, 130, 132, Neuroglia, 122
Nare, So, 400, 657: Anterior, 24S Neurohumors, 147
412: Internal, 46S, 4S1: Hypoglossal, 130 Neuromast, 133, 135
Posterior, 412, 468 Hyomandibular, 24S Neuromast-organs, 135
Naris-See Nare Lagina, 133 Neuropil, 187
Narwhal, 7S3 Laryngeal, 133 Neuropore, 30, 5S, 61
Nasals, 96, 395, 467, 661 Lateralis, 24g Neutrophiis, 71
Nasal cavity, 138, 4S1 Lateral-line, 134 Newts, 432, 442
Nasal septum, 412 Lumbo-sacral, 412 Niacin, 343
Nasal spine, 750 Mandibular, 131, 247 Nidation, 6S6
Nasopalatine, 667 Maxillary, 131, 247 Nidiculous, 564
Nasopharynx, 667 Oculomotor, 130 Nidifugous, 564
Nasoturbinal, 661 Oculomotorius, 247 Night jar, 61S: African, 561
Natterjack, 437 Olfactorius, 246 Node, auriculo-ventricular,
Navicular bone, 665, 85g Olfactory, 130 672, 8S3
Nectophrynoides, 386, 452 Opthalmic, 131, 24S Lymph, 675
Nectridia, 388, 430 Optic, 130, 593 Sinu-auricular, 672, SS2
Necturus, 38S, 434, 43g, 441, Palatine, 132, 24S Nodosaurus, 5II
444,447,456: N. Maculosus, Parasympathetic, I xg Nostril, So, 137
433. 435 Petrosal, I 3 2 Notacanthiformes, 285
Neobalama marginata, 785 Pharyngeal, 133 Notaden, 451
Neoceratodus, gg, xoS, 361, 362, pneumogastric, 133, 247 Notch, obturator, 579
364. 37g, 3g6 Pre-olfactory, 247 Notidanoidea, 222, 227
Neodrepanis coruscans, 621 Prespiracular, 132 Notidanus, 227
NeogC!Jornis, 605 Pulmonary, 133 Notechis scuta/us, 536
NeognalhC!J, 556, 565, 602, 610, Ramus opthalmicus pro- Notharctus, 737
6!2 fundus, 131 Nothosauria, 461, 492
Neognathous palate, 6o6 Ramus opthalmicus super- Nothrotherium, 765
Neomorphs, 172 ficialis, 131, 134 Notioprogonia, Sox, 8o6
 INDEX 943

Notochor d, 1, 10, 30, 45, 50, Olfactory sense, Birds, 641 Ornithosuchus, 499
62, 89 Olfactory signal, 847 Oro/estes, 715
Notochor dal canal, 551 Olfactory stalk, 478 Orohippus, 829
Notodiaphorus, 8os Olfactory tracts, 344, 679 Orycteropodidre, 8or
Notomys, 709 Olivary body, 682 Orycteropus, 86o: Dentition ,
Omentum , Duodeno -heptatic , 871: 0. ajer, 804
Notomis, 630,633: N. mantelli,
473: Gastro-he patic, 473 Oryctolagus, 654: Skull, 658,
557 659-See under Rabbit
Notoryctes, 709, 713, 714, 730, Omostern um, 397
848, 886, 887: N. typhlops, Ompax spatuloides, 364 Oscines, 62o
Oncorhynchus, 318: 0. nerka, Os cloac<e, 470
715 Os cordis, 74, 94, 882
Notoryctidre, 715 345
Os marsupiu m, 694
Notostylopidre, 8o6 Onohippi dium, 830
Nototherium, 719 Onychodactylus, 438, 845 Os palpebr<e , 75
Notoungulata, 724, 725, 8o1, Onychodectes, 760 Os penis, 75, 94, 882, 890
Oozooid, 41 Os planum, 739
806 Os priapi, 75, 94, 890
Numbat, 714: Australia n, 704 Openbill, 614
Opercula r, 298, 306 Os uteri, 894
Numididre, 615 Osmoreg ulation, 156
Nuptial pad, 390, 416 Operculu m, 81, 258, 298, 322,
366, 421, 448 Osprey, 614
Nursery-f ish, 350
Nuthatch , European , 560 Ophiacodontia, 462 Ossa innomina ta, 470
Ophiderpeton, 430 Ossificati on, 95
Nyctibiidre, 6 r 8 Ostariophysi, 285, 296
Ophidia, 459, 461, 497, 528,
532, 539. 540, 541, 543. 549 Osteichthyes, 70, I08, 204,
0 Ophthalmosauridre, 490 282
Oar-fishe s, 297 Ophthalmosaurus, 490, 491 Osteoclas ts, 74
Obliquus externus, 400 Opisthocc elous vertebr<e, 330 Osteocyte , 73
Obliquus internus, 400 Opisthocomi, 615 Osteoden tine, 103
Obstetric toad, 386 Opisthocomus, 615, 620 Osteogen etic layer, 74
Obturato r foramen, 665 Opisthoglypha, 498 Osteolmmus, 502
Occipital condyle, 303, 394, Opisthomi, 285, 297 Osteolepidoti, 204, 354
Opisthoti c, 303, 466 Osteolepis, 285, 355. 356: 0.
465 macrolepidotus, 354
Occipital crest, 303 Opisthoti c ossificatio n, 96
Occipital lobe, 678 Opossum , 711, 867, 871, 891: Osteostraci, 70, 164, 171,
Occipital region, 92, 303 'Water, 712 1 74

Occipital segment, 97 Opsanus tau, 347 Ostium tub<e, 417

Oceanites, 613 Optic capsules, 91 Ostrich, 556, 557, 6oz, 6o6,
Ochotona, 779 Optic chiasma, 245 6o7, 629, 645
Ochotonidre, 654, 779 Optic cup, 142 Otariidre, 792, 798
Oculomo tor nerve, 130 Optic disc, 141 Otic capsules, 180
Optic foramina, 93, 659 Otic placode, 145
Odobenidre, 792, 798 Otic process, 393, 575
Odontaspidre, 227 Optic lobes, 125, 246, 312, 480,
Odontobl ast, 862 681, 884 Otic vesicle, 145
Odontoceti, 782, 783 Optic pedicel, 275 Otidre, 615
Odontognathre, 565, 6oz, 6o3 Optic tract, 681 Otocyst, 32, 45
Odontoid process, 269, 439, Optic ventricles , 126 Otolith, 146, 316
Optic vesicle, 142 Otolithic membran e, 146
464. 655
Odontopteryx, 614, 636 Oral fimbri<e, 177 Otter, 796
OEsophag us, 102,309, 401,471, Ora serrata, 140 Ova, 28, 29, r6o,255. 276,277,
668, 876 Orang-ut an, 744· 750, 752 417, 452, 456, 549, 656, 685,
CEstrus, 897: Cycle, 896: Post- Orbits, 92, 143, 852 693, 891, 898
partum-, 897 Orbital process, 575 Ovarian stroma, 890
Oikopleura, 4S: 0. dioica, 46 Orbitals, 467 Ovary, 27, 153, 253, 254, 277,
Oikopleuridre, 44. 45 Orbito-sp henoids, 305, 659 318, 376, 417, 450, 483, 548,
Oil-bird, 618 Orectolobidre, 2 z 7 598, 685, 7oo, 89o
Okapi, 84o Orectolobus, 228 Oviduct, 29, 253, 255, 277,
Olecrano n pr9cess, 469, 664 Oreodonta, 836, 838 349. 376, 417, 450, 483, 598,
Olfactory bulbs, 126, 312, 478, Oreopithecus, 745 685, 7oo, 891
Orientati on of vertebrat es, 250 Oviposito r, 350
594. 641, 679
Olfactory capsule, 91, 181 Origma rubricata, 612 Ovulatio n fossa, 890
Olfactory foramina , 93 Ornithischia, 462, 504, 505, 507 Ovulation , pre-partu m, 897
Olfactory mucosa, 887 Ornithodesmus, 504 Owl, 618: Barn-, 618, 625:
Ornitholestes, 506 Eagle-, 560
Olfactory organ, 137, 249, 275,
412, 481, 594 Ornithopoda, 462, so8 Oxen, 837
Olfactory peduncle , 126, 262, Ornithorhynchidre, 688 Oxyrenidre, 791, 792
Ornithorhynchus, 688, 691, 692, Oxytocin , 151, 849
344
Olfactory pit, 315 695, 696, 697, 698, 699, 7 oo, Oxyuranu s, 536: 0. scutellatus,
Olfactory region, 92 842, 85o, 862, 869, 886, 906, 521
Olfactory sacs, 44 7 907: 0. anatimts, 690 Oyster-ca tcher, 616
944 INDEX
p Parameles, 907 Peltephilidre, 762
Paramyxine, 197 Pelvic arch (girdle), ror, 270,
Pacas, 775 Paraphysis, I28 333. 398, 441, 470, 578. 664,
Pace-maker, 590 Parapineal body, I88 856
Pachyornis, 633 Parapineal eye, I28 Pelvis, 752, 789, 88o, 824
Pachystomias, 327 Parapithecidre, 736, 744 Pelvi-sternum, 442
Padymelon, 709 Parapophyses, 301 Pelycosauria, 462, 513
Prenungulata, 726, 791, 807 Parapsida, 484 Penguins, 556, 6oz, 6ro, 628,
Palreanodonta, 761 Parasaurolophus, 508 630, 644: Emperor, s6o
Palamodon, 761 Parasaurus, 486 Penial-tube, 176
Palmochiropteryx, 758 Parasphenoid,g6,305,395,465 Penis, Sg, 655
Palreodonta, 836 Parasympathetic system, 682 Penn<e, 626
Palreognathre, 565, 6o2, 606 Parathormone, I52, 668 Pentadactyle limb, 8r, roo
Palmogyrinus, 426 Parathyroid, I04, 402, 589 Pentlandia, 364
Palmomastodon, 8I5, 8r6 Paraxonia, 726, 791, 824, 836 Pepsin, 237
Palreoniscoidea, 284, 285 Paridre, 62o Peralesles, 687
Palmoniscus, 286, 288, 354 Parietals, g6, 305, 467, 66o Perameles, 708, 715, go6
Palmophis, 497 Parietal foramen, 467 Peramelidre, 704
Palreotheriidre, 825 Parietal lobe, 678 Perameloidea, 7ro, 715
Pal<eotheres, 829 Parietal organ-See Pineal eye Peramys, 848
Palreospondylia, 203, 217 Parietal segment, 97 Peratherium, 712
Palmospondylus, 217: P. Paroccipital, 659 Perea, 297, 86: P. ftuviatilis,
gunni, 218 Parotia sefilata, 561 136
Palatal rug<e, 876 Parotic process, 467 Perch, 297,312: Climbing, 338
Palate, .i'Egithognathous, 632: Parotid, 535 Perciformes, 2 85
Desmognathous, 632: Dro- Parrot, 626 Percomorphi, 285, 297
m<eognathous, 632: Hard, Pars ciliaris, r 40 Percopsiformes, 285
667: Neognathous, 612: Pars distalis, 150, 415, 593, 68r Percopsis, 296
New, 613: Pal<egnathous, Pars intermedia, I5I Perennibranchiate, 435
612: Schizognathous, 632: Pars iridica, I 40 Pericardia! cavity, 88, 404
Soft, 667 Pars nervosa, 347 Pericardia! fluid, 672
Palatine, 305, 395, 467, 66I Pars optica, qo Pericardium, 88, 404, 474. 670
Palatine plates, 85I Pars tuberalis, 4I5 Perichondrium, 73, 74, 95
Palatine region, 393 Partridge, 6r5 Perichordal tube, go
Palatine tonsil, 876 Passer, 642: P. domesticus, Perikargon, I22
Palatopterygoid, 368 628 Perilymph, 482
Palatoquadrate, 30 5: Carti- Passeres, 62o Perilymphatic space, I45
lage, 93 Passeriformes, 565, 620 Perin<eum, 655. 88o, 88g
Paleaspidre, I6g Passerine-bird, 563 Perineural sheath, 74
Pallium, 127 Patagium, 753, 862 Periophthalmus, So, 322, 325,
Pan, 744 Patella, 579. 665, 857 339
Pancreas, 104, 152, 370, 402, Paurodon, 703 Periosteum, 73
473, sss. 67o Pavo, 561 Periotic bone, 66o, 852
Pancreatic juice, 104, 473 Peacock, Indian, 56r Peripatus, 278
Pancreozymin, 147 Peccary, 837 Peripharyngeal band, 44, 52
Panda, 797: Giant, 797 Pecora, 837. 838, 839, 84o Periptychidre, So I
Pangolin, 726,905 Pecten, 481, 546, 595 Periptychus, 803
Panizza, foramen of, 501 Pectocaulus, r8 Perissodactyla, 726, 791, 8o2,
Panniculus adiposus, 74 Pectoral arch (girdle), ror, 297, 8o7, 824, 825, 831, 868, 873
Panthera, 794 369, 468, 577. 663, 731, sss Peritoneum, 71, 88
Pantodon, 321 Pectoralis major, sSo Perivascular sheath, 74
Pantodonta, 724, 726, 8oS, 818 Pedetes, 775 Peri-vitelline membrane, 59
Pantolambda, 8r8 Pedicle, 391, 393 Perleidus, 288
Pantolambdodontidre, 8 r 8 Pediculati, 285, 297 Perodicticus, 738: P. potto, 739
Pantotheria, 654, 703 Pedionomidre, 6I5 Peroneus medius, 58o
Papilla basilaris, 482 Pegasiformes, 285 Perophoridre, 2I
Papilla, urinary, 252: Urino- Pegasus, 297 lJes, 8r, 390, 462, 470, 722:
genital, 253 Pekin man, 745 Mesaxonic, 8oz, 824, 836:
Papio, 744 Pelagomithidre, 6I4 Paraxonic, 802, 836
Pappocetus, 782 Pelecanidre, 6 r 4 Pessulus, 586
Parachordals, 91 Pelecaniformes, 565, 614, 628 Petalichthyida, 203, 212
Paracyclotosaurus, 425: P. Pelecanus, 603, 614: P. ery- Petaurus, 7I6
dairdi, 425 throrhynchus. 62 I Petrels. 6 I 3
Paradiseidre, 561, 62o Pelican, 560, 6o3, 6I4, 621 Petrogale, 885
Paradisea apoda, 561 Pelobatidre, 429 Pet1·omyzon, 86, I59. 172, 175,
Paradisornis rudolphi, 561 Pelodytes, 447 176, 178, 204, 245. 251, 276,
Parahipparion, 830 Pelomedusidre, 489 343, 348: P. marinus, 176,
Parahippus, 830 Pelonaia, 34 179, r8r-See also Lamprey
 INDEX 945
Petromyzontia, 70,164,175,439 vascular system, 589: Car- Plesiosauria, 461. 493
Petromyzonidre, I 75 rier-, 565: Common-, 565: Plethodontidre, 432
Petrous, 66o Excretory and reproductive Pleura, 587
Pezophaps, 630: P. solitarius, organs, and adrenal glands, Pleuracanthodii, 203, 221, 224
557 596: Exoskeleton, 567: Ex- Pleuracanthus, 99: P. decheni,
Phrethontidre, 6 I 4 ternalcharacters, 566: Loco- 224
Phalacrocoracidre, 6I4 motion, 582: Lymphatic Pleural membrane, 677
Phalacrocorax, 623, 644 system, 592: Muscular sys- Pleural sac, 670, 677
Phalamoptilus nuttalli, 559 tem, 580: Nervous system Pleuraspidotherium, 803
Phalanger, 7I6, 886, 89I and hypophysis, 592: Or- Pleurodeles, 439
Phalangeroidea, 707, 710, 716 gans of special sense, 594: Pleurodira, 461, 489
Phalanges, IOI, 400, 470, 664 Respiratory and Vocal Pleurodont teeth, 534
Phalaropes, 6I6 organs, 585: Rock-, 565: Pleurogona, 2 1
Phalaropodidre, 616 Thyroid, parathyroid and Pleuronectiformes, 285
Phalaropus, 6I7 thymus glands, 589: Tooth- Pleuropterygii-See Clado-
Pharyngeal region, 106 billed, 557 selachii
Pharyngobranchial, 307 Pigeon's milk, 583, 617 Pleurotremata, 222, 227, 265
Pharyngohyal, 260 Pigment layer, 140 Plesiadapidre, 736
Pharynx, 71, I02, 309, 40I, Pigment-spot, 59 Plesiadapis, 737
667 Pika, 779, 8oS Plesiametacarpal condition,
P hascogale, 7 I 3 Pike, 284, 296 840
Phascolarctidre, 7 I 6 Pilosa, 76I, 762, 764 Plesippus, 830
Phascolarctos, 708, 7I6, 88o, Piltdown fraud, 747 Plexus, choroid, 262
906, 908 Pineal apparatus, 480 Plica semilunaris, 413
Phascolomyidre, 716 Pineal body, 262, 3I3 Pliohippus, 83o, 858
Phascolomys, 708, 7I6, 720, Pineal eye, I88, 480, 547 Pliopithecus, 745
775. 87o, 89I, 906 Pineal organ, I28, 245. 480 Pliohyrax, 8ro
Phasianidre, 6 I 5 Pineal pillow, 375 Pliotrema, 266
Pheasant, 6I5 Pinkus' organ, 379 Ploceidre, 62o
Phenacodontidre, 8o1, 8o3 Pinna, 147 Plotosus, 275
Phenacodus, 8o3, 825 Pinn<e, 654 Plover, 6r6: New Zealand
Phlebobranchiata, 21 Pinnipedia, 792, 797, 8oo, 875 Wrybilled, 622
Phlebolepis, I 75 Pipa, 442, 456: P. americana, Plumage, Coloration, 627:
Phocama, 788: P. phocama, 454 Epigamic, 561
787 Pipidre, 429 Plumes, semi-, 626
Phoca vitulina, 798 Piranha, 283, 296 Plumul<e, 626
Phocidre, 792, 798 Pisiform bone, 469, 664, 859 Pneumatic bones, 572
Phrenicopteri, 62 2 Pith, 841 Pneumatic foramina, 572
Phrenicopteridre, 6 r 4 Pithecanthropus, 745 Pneumogastric, 670
Phreniculidre, 6 I 9 Pithecophaga, 6I 5 Podargidre, 6r8
Phcenicopterus anliqu01·um, Pituitary body, 8g, 91, 127, Podargus, 618, 626, 641: P.
621,622: P. minor, 622: P. 245. 263 strigoides, 62 I
ruber, 627 Pituitary foramen, 659 Podica, 623
Pholidophoroidea, 284, 293 Pituitary fossa, 659 Podieipitiformes, 565, 613
Pholidota, 725, 726, 772, 8o4, Placenta, 686, 904: Diffusion, Podicipitidre, 613
87! 43 Poebrotherium, 838
Phorozooid, 39 Placentalia, 653-See Eutheria Preciliidre, 350
Photoreceptor organs, 192 Placentation, pseudo-, 455 Poison, 328
Photokinesis, Ig6 Placodermi, 70, 203, 205 Poison apparatus of Ornithor-
Phoxinus, 3I5, 345: P. phoxi- Placodontia, 46r, 493 hynchus, 692
nus, 329 Placodus, 493 Poison lakes of Bobeau, 535
Phyllolepida, 2 I I Placoid scale, 103 Polecat, 842
Phyllolepis, 2I2 Placula, 29 Policis, 582
Phyllomedusa, 437, 454 Plagiaulax, 701 Pollex, IOI, 470, 752
Phyllospondyli, 431 Plantain-eater, 617 Polycitoridre, 2 1
Phyllostomatidre, 757 Plasma, 70, 115 Polyclinidre, 2 r
Physeter, 783, 907 Plastron, 521, 526 Polydolops, 715
Physeteridre, 78 3 Platanista, 887 Polymorphs, 71
Physiculus bachus, 345 Platanistidre, 783 Polyodon, 288, 320, 321, 329,
Phytosauria, 461 Plateosaurus, 507 364
Phytosaurus, 499 Platynota, 459, 497 Polycesti'us cycle, 897
Pia mater, r28 Platypus, 688 Polyprotodontia, 709, 7Io
Picidre, 6Ig Platyrrhina-See Ceboidea Polypteridre, 108
Piciformes, 565. 619 Platysomus, 288 Polypterini, 99, 283, 284, 290
Pig, 837, 904 Platysternidm, 489 Polypterus, So, ro8, 282, 287,
Pigeon, 6I7, 626, 640: Ali- Plectognathi 285, 297, 329 288, 290, 322, 326, 330, 331,
mentary canal and associ- Plectrolepis, 84 333. 336, 340, 352, 359. 371,
ated structures, 583: Blood- Plerodactyla-See Pterosauria 382, 441: P. bichir, 290
 INDEX
Pongidre, 736, 744, 750 Proconsul, 744 Pseudis paradoxa, 454
Pongo, 744 Procoracoid, 396 Pseudobranchia, 311
Pons Varolii, 681 Proctodceum, 102, 420, 474, Pseudobranchus, 434
Poor-will, 559 585 Pseudochmnichthyes, 343
Porcupine, 778, 842 Procyon, 794 Pseudocheirus, 716
Porcupine fish, 329 Procyonidre, 792, 793, 797 Pseudoccele, 68o
Pore, Abdominal, 267: Geni- Proechidna, 694 Pseudohymenochirus, 429
tal, 319 Progalago, 738 Pseudomys, 709
Porichthys, 297 Progesterone, 148, 152, 598, Pseudosuchia, 461, 499
Porpoise, 783, 865 897 Psittacidre, 617
Portheus, 294 Prolactin, 151, 849 Psittaciformes, 565. 617
Possum, 716: Honey-, 716: Pronation (muscle), 76, 663 Psittacotherium, 76o
Pigmy, 716 Pronephros, 154 Psophiidre, 615
Post-acetabular, 578 Pro-otic ossification, 96, 303, Ptarmigan, 615, 623, 627
Post-anal gut, 102 466 Pteranodon, 503, 504
Post-temporal, 309 Propliopithecus, 744 pteranodontidre, 503
Potamogale, 727, 729, 731 Proprioceptors, 117 pteraspida, 70, 164, 168, r69-
Potoo, 618 Propterygium, 236 See also Heterostraci
Potorous, 896 Prorastomus, 822 Pteraspis, r68: P. rostra/a, 169
Palos, 797, 868 Prosencephalon, 312, 678 Pterichthyodes, 215
Potto, 738 Prosimmi, 736 Pterichthyomorphi, 203, 214-
Pouch, Monotreme, 695 Proteidre, 432 See Antiarchi
Pratincoles, 616 Proteles, 79 5, 868 Pteridophora alberti, 561
Pre-acetabular, 578 Proterotheriidre, So2 Pterobranchia, 4, 5, 14
Pre-adaptation, 383 Proteus, 434, 435, 439, 447 pteroclididre, 6 r 7
Pre-anal plate, 463 Prothylacinus, 712 Pterodactyloidea, 462, 503, 504
Precavals, 373 Protobatrachus, 381, 428 Pterodactylus, 503, 504
Precocial, 564 Protoceratidre, 837, 84o Pterois, 323
Pre-coracoid, 101 Protocetidre, 782 Pterophyllum, 321, 323
Predentine, 863 Protocetus, 782 Pteroplatcea valencienni, 137
Preen, 566 Protochordata, 4 Pteropus, 756, 875, 891, 898:
Prefrontal, 467 Protocone cusp, 867 P. giganteus, 755
Preganglionic fibres, 121 Protoglossus, 6 Pterosauria, 462, 502
Prehallux, 400, 418 Protohippus, 830 Pterosphenoids, 305
Premaxilla, 97, 305, 395, 467 Protopterus, 8o, 361, 362, 364, Pterotlc process, 303
Pre-oral pit, 64 369, 371, 379: Alimentary Pterygoid process, 66o
Pre-patagium, 567-See also canal and associated struc- Pterygoid region, 393
Alar membrane ture, 369: Blood-vascular Pterygiophores, 98, 235, 270
Prepollex, 397 system, 370: Dentition, 368: Pterygium, 99, 226, 270
Prepuce, 685, 894 Development, 377: Endo- Pterygoid, 395, 467, 661, 688
Pre-sphenoid, 96, 659 skeleton, 367: General meta- Pterylce, 569, 626
Pre-sternum, 657, 851 bolism and excretion, 375: Pterylosis, 571, 626
Prevomers, 368 Nervous system, 374: Re- Ptiloceras, 733
Priacodon, 687 production, 376: Respira- Ptilonorhynchidre, 453, 62o
Primary cranium, 97 tory system, 370: P. mthio- Ptilonorhynchus violaceus, 562
Primates, 725, 726, 734, 848, picus, 365, 370, 377: P. Ptychodera, s: Branchial
853, 875, 897, 899: Classi- amphibius, 365, 379: P. region, 9
fication, 736: Endoskeleton, annectens, 365, 375, 377, Pubic symphysis, 470
751: Pes, 753: Skeleton, 379: P. dolloi, 365 Pubis, 101, 398, 470, 579, 664
748 Protorosauria, 461, 492 Pubo-ischial region, 101
Pristidre, 228 Protoselachii, 203, 221, 225 Puff-birds, 619
Pristiophorus, 266 Protosiren, 822 Puffer fish, 329
Pristis, 254 Protosuchia, 461, 499 Puffin, 617
Proamnion, 648 Prototheria, 654. 687, 8so. 856, Pufinus, 583: P. tenuirostris,
Proanura, 388 88o, 907: Skeleton, 696: 6!3
Proboscidea, 726, 791, 8o8, 8o9, Visceral anatomy, 700 Pulmo-cutaneous trunk, 407
8ro, 811, 818, 822, 868 Protoungulata, 725, 791, 801: Pulmonale, 541
Proboscis, 14, 16, 17 Classification, 801 Pulmonary aponeurosis-See
Proboscis-ccelom, 5, 12 Protoungulates, 8o7 Pleura
Proboscis diverticulum, 10, 19 Protovertebra, 64 Pulmonary artery, 474
Proboscis-pore, 5 Protraction (muscle), 76 Pulp, dental, 864
Proboscis-skeleton, 10 Protractor lentis, 413 Pupil, 140
Procavia, 8o8, 888 Proventriculus, 584 Purkinje fibres, 591
Procaviidre, 8o8 Prozeuglodon, 783 Purkinje network, 672, 883
Procellariidre, 6 r 3 Psalterium, 68o, 877, 886 Putorius, 842
Procellariiformes, 565, 613, 641 Psephurus, 288 Pycnodontoidre, 284, 293
Proccelus vertebrce, 391 Psetta, 297 Pycnonotidre, 62o
Procolophon, 486 Psettus seba;, 21 Pyctodonta, 2 11
 INDEX 947
Pygochord, 9 pipiens, 387, 389, 417: R. Rhinal process, 394
Pygopus, 520 ridibunda, 390: R. septen- Rhinarium, 741
Pygoscelis adelie, 6II trional is, 447: R. temporaria, Rhinatrema, 435
Pygosteuspungitius, 329 389, 390, 39I, 394. 443 Rhinobatidre, 228
Pygostyle, 574. 599 Ranidre, 389, 430, 6I7 Rhinobatus, 277
Pyloric c;:eca, 337 Rat, 897, 905: J\lole-, 777: Rhinoceros, 824, 826, 833, 834,
Pyloric sphincter, 237. 401, Water-, 776 846: African, 834: R. bi-
471, 668 Ratitre, 6o6, 63o cornis, 907: Indian, 834:
Pyloric valve, 3II Ratite palate, 6o6 Javan, 834: R. sondaicus,
Pyramidal tract, 679 Rattlesnake, 497: American, 834: Sumatran, 834: R.
Pyrosoma, 39; Development, 522 unicorn is, 834: White, 834
4I: Pharynx, 40 Rattus, I54 Rhinocerotidre, 825, 835
Pyrosomida, 36, 39 Rauber's layer, goo Rhinocerotoidea, 825, 834
Pyrosomidre, 40 Ray fish, 228, 265: Sting, 267 Rhinochimmra, 258
Pyrostremma, 39 Receptaculum chyli, 883 Rhinochimreridre, 258
Pyrotheria, 724, 726, 8os. 8oS, Rectrices, 6oo Rhincodon, 2I9
820 Rectum, 401, 47I, 585, 670 Rhinocceles, 274
Pyrotheriidre, 82o Rectus abdominus, 400, 442 Rhinoderma darwini, 454
Pyrotherium, 8zo Recurvirostridre, 6 I 6 Rhinolophus, 756, 8g6
Python, 497, 498: African Red Flier, 7I9 Rhipidistia, 204, 354
Rock-, 529: Reticulated, Refection, 670 Rhizodus, 356
52 I: P. seba:, 529 Regalecus, 258, 207, 334 Rhodeus, 296, 349
Pythoninre, 498 Reindeer, 840 Rhombencephalon, 681
Pyuridre, 2 I Hemige, 57I, 6or Rhynchocephalia, 459, 461,
Remigolepis, 2I5 495, 498, 521, 524
Remora, 297 Rhynchocyon, 733
Q Reptilia, 70. 437, 445, 457, 595, Rhynchodipterus, 364
8go: Alimentary canal and Rhyncholepsis parvulus, 171
Quadrate,93,97, 305,467,633 associated structures, 533:
Quadrato-jugal Arch, 395, 468
Rhyncholestes, 715
Blood vascular system, 541: Rhynchopidre, 6I7
Quail, 615 Classification, 46i: Develop- Rhynchosaurus, 495
Quelca, 623 ment, 551: Endocrine glands, Rhynochetidre, 615
548: Endoskeleton, 524: Ribbon-fish, 334
R Excretion, 548: External Ribs, gr, 629,657: Abdominal,
featnres, 5I8: General or- 44I: Cervical, 464: Occipi-
Rabbit, 654. 779, 862, 875, ganization of, 518: Heart tal, 368: Pleural, 330, 368:
885, 897, 899, 905: Alimen- structure, 476: Integument Sacral, 464: Sternal, 91,
tary canal and associated and Exoskeleton, 522: Inter- 464: Vertebral, 91
structures, 666: Autonomic relationship of groups, 484: Roach, 312
nervous system, 668: Blood Nervous system, 542: Or- Roc, 6og
vascular system, 670: Cce- gans of special sense, 543: Rodentia, 654, 725, 774, 882:
lom, 666: Development, Reproduction, 549: Respira- Skeleton, 778
686: External characters, tory system and voice, 540: Rods of eye, 141, 595
654: Lymphatic system, Temporal openings, 485 Roller, 6rg
675: Nervous system, 678: l(espiration, organs of, 105 Rorqual, 785
Respiratory system, 676: Respiratory system, 107, 884 Rostral tubes, 358
Skeleton, 655: Spinal cord, Restiform body, 262 Rostratulidre, 6 r 6
cranial and spinal nen·es, Rete mirabile, 273, 315, 337 Rostrum, 92, 303, 574
682: Urinogenital organs, Retia mirabilia, 34I, 78I Rotunda, 888
684 Reticular cell, 67 5 R ousettus, 757
Raccoon, 797 Reticular fibres, 70 Rumen, 877
Radialia, g8 Reticulo-endothelial system, Ruminantia, 837, 839, 848, 877
Radiale, IOI, 469, 578 Reticulum. 877 Ruminant stomach, 840
Radio-ulna, 397 Retina, I40 Rumination, 878
Radius, Ioo, 469, 578, 663 Retraction of muscles, 76
Raiidre, 228 Rhabdopleura, 4. 5, 14, IS, 19
Rail, 6I5 Rhabdopleuridre, 18 s
Raja, 276: R. blanda, 83 Rhachitomi, 388, 424
Rajidre, 265 Rhacophoridre, 430 Saarolepis, I71
Rallidre, 6 I 5 Rhacophortts, 436: R. mala- Sabre, tooth, 796
Ramphastidre, 6Ig baricus, 436 Saccoglossus, 5, 8, I I: 5. horsti,
Ramus communicantes, 249, Rhamphodopsis, 212 12, 13
3I4, 4I2, 682 Rhamphorlzynchus, 504, 599 Saccopharyngi.formes, 285
Ramus lateralis, 4I4 Rhamphorhynchoidea, 462,504 Saccopharyn:r, 296
Rana, I94, 430, 438, 44I, 445: Rhamphotheca, 563, 566 Saccule, 145, 250
R. catesbiana, 389, 398: R. Rhea, 556, 557, 6o6, 607, 635 Sacculus, 414, 482
cancrivora, 385: R. esculen- Rheiformes, 565, 6o7 Sacculus rotundus, 670
ta, 389, 390, 39I, 42I: R. Rhenanida, 203, 216 Saccus vasculosus, 128,245,314
 INDEX
Sagenodus, 364 Sclerotic, gi, I3g, 3I5 Sinanthropus-See Pithecan-
Saghatherium, 810 Sclerotic plates, 5g5 thropus
Sagitta, 3I6 Sclerotomes, I6I Sinuo-auricular node, 5go
Sagittariidre, 6 I 4 Scoliodon, 278: S. sarrakowah, Sinuses, III
Sail-fish, 320 278 Anterior cardinal, 243
Salamander, 432, 444: Spotted, Scolopacidre, 6I6 Blood-, 243
437: Tiger, 456 Scomber, 343 Genital-, 244
Salamandra, 438, 445: S. atra, Scombridre, 2g7 Inferior jugular, 243
452: S. maculosa, 436 Scopeliformes, 284 Posterior cardinal, 243
Salamandrt:em, 432 Scopidle, 6I 4 Precaval, 243
Salamandridre, 432 Scrotal sac, 655, 684 Rhomboidalis, 5g4
Salamandrine, 435 Scrotum, 88g Urinary, 252, 263, 276
Salienta, 428--See also Anura Screamer, 6I4: Crested, 624 Urinogenital, 255, 276
Salmo, 72, 2g6: S. trutta, 2g6, Scutes, 32g Venosus, Iog, 3II
298: S. salar, 2g8 Scyliorhinidre, 228 Siphon, 23, 256
Salmon, 2g8: Cranial develop- Scyliorhinus, 228, 229, 230, Siren, 434, 435, 444: S. lacer-
ment, go 24I, 244· 255. 256, 262, 273· tina, 433
Salmonidre, 2g6 274, 276, 303, 3I 2: S. Sirenia, 726, 764, 7gi, 8o8, 8Io,
Salmopercre, 285, 2g6 Caniculus, 28I 8I2, 82o, 842, 848, 85I, 859,
Salpa, 4I, 278: Alimentary Scyllium, I38, 22g 874, 882, 887: Skeleton, 823
Canal, 42: Development, Scymnorhinus, 262, 274 Sirenidre, 434
43: Nervous system, 43: S. Sea-cow, 82 I Sistrurus, 522
democratica, 4I Sea-horses, 2g7, 35I Sivapithecus, 745
Salpida, 36, 41 Seal, 7g7, 865, 883, 8g7, 8g8: Sivatherium, 840
Salpidae, 42 Crab-eater, 7g8: Fur, 7g8, Shag, 6I4
Salvelinus jontinalis, 2g8 842: 'Wriggling', 7g8 Shagreen, 267
Sandpiper, 6I6 Sea-lion, 7g8 Shank, 8I, 390, 462
Santorini, cartilage of, 676 Sea-squirt, 21 Shark, 99, 265: Abyssal, 275:
Sarcolemma, 75, I2I Sebecosuchia, 46I Basking, 227, 272, 275:
Sarcophilus, 7I2, 87o: Skull, Sebum, 846 Black whaler, 220: Blue,
870 Secretary-bird, 6I4 228: Blue pointer, 220:
Sarcoplasm, 75 Secretin, I47 Great white, 2Ig: Grey
Sauria, 4g6-See Lacertilia Segmentation, 202 nurse, 220, 227: Hammer-
Saurichthys, 288 Selachii, 203, 220, 22I, 222 head, 266; Mackerel, 227:
Sauripterus, 382 Seleucidis melanoleucus, 56I Mako, 220: Port Jackson,
Saurischia, 462, 504, 505 Selientia, 388 227: Thresher, 227: Tiger,
Sauropleura, 43I Sella turcica, g6, I4g, 5g4, 220
Sauropoda, 462, 507 65g Shearwaters, 6I3
Sauropterygia, 46 I. 492 Semantor, 7g8 Sheath-bill, 6J7
Sawfish, 228, 266 Semicircular canals, I45. Ig2, Sheep, 837, 840, 8g7, go4,
Sawshark, 266 414· 482 905
Scales, 300, 32g: Cosmoid, 83, Seminal vesicle, 88g Shrew, 726: Elephant, 726,
84: Ctenoid, 32g: Cycloid, Seminiferous tubules, 4I6, 5g7, 731: Tree, 726, 731, 742
32g: Ganoid, 83, 84: Pla- 684, 88g Shrike, 620
coid, 83, I03, 22g, 267 Semionotidre, 2gi Shoebill, 614
Scaphirhynchus, 288 Semionotoidea, 284, 2gi, 292 Shoulder-girdle, 308, 333, 440
Scaphoid bone, 664, 85g Semionotus, 2g2 Silver-eyes, 620
Scaphognathus, 504 Semnopithecus, 744 Sinus venosus, 474
Scapula, IOI, 308, 333, 3g6, Sense-vesicle, 3 I Sitla, 560
46g, 577, 752, 75g, 7'd7, Boo, Sensory tubes, 274 Skate, 228
823, 856 Septum lucidum, 68o Skeletogenous layer, go
Scapular-region, IOI Septum transversum, 404 Skeleton, 88
Scapula-numeral, 58I Serinus, 642 Skimmer, 617
Scaumenacia, 364 Serpentes, 45g, 4g7-See Skin, 81, 83, 3go
Scelidosaurus, 5II Ophidia Skua, 6I7
Scenopretes dentirostris, 62I Serranidre, 34g Skull, gi, 30I, 39I, 439, 465,
Schistometopum, 455 Serranus, 34g 574: Armadillo, 767: Birds,
Schizocrele, I6o Serrasalmus, 283, 2g6 63o: Bony, 9S: Canis, 7g9:
Schizotherium, 833 Serous membrane, 64g, 686 Carnivora, 799: Cartilagin-
Schlegelia wilsoni, 56I Sesamoid, bones, 74 ous, g2: Cetacea, 785: De-
Schneiderian membrane, I37 Setifer, 729 inotherium, 8I7: Develop-
Schoinobates volans, 7I7 Setonix, 708, 897, 8g8: S. ment of, gi: Equus, 873:
Schwann cell, I2I brachyurus, 708, 879 Homo, 748, 74g: Lemur,
Scit:enidae, 346 Sewellel, 777 738: Mammalia, 657, 85I:
Scincomomorpha, 4g7 Seymouria, 427, 467, 556 Metatheria, 72I: Moerither-
Sciuroidea, 77 Seymouriamorpha, 388, 427 ium, 8I4: Monotreme, 697:
Sciuromorpha, 777 Silurodea, 32g Palt:eomastodon, 8I5: Rab-
Scleroparei, 297, 285 Simplicidentata, 774 bit, 657: Rodentia, 778:
 INDEX 949
Sal'cophilus, 870: Triche- Splenial, 468 Struthio camelo, 607
chus, 823 Splenic pulp, 675 Subarachnoid space, 128, 145
Skunk,796 Splenium, 679, 886 Subatrial ridge, 66
Sloth, 726, 841, 883: -Bear, Spoonbill, 614 Subcutaneous sinuses, 410
791: Ground-, 761, 764: Squab, 583 Sub-mucosa, 102
Three-toed, 764: Two-toed, Squalodontidse, 783 Substantia propria, 142
764. 766 Squaloidea, 222, 227 Subungulata-See Psenungu-
Slow-worm, 497 Squaloraja, 258 lata
Smilodon, 796 Squalus, 86, 228, 229, 230, 241, Subvertebral sinus, 410
Srninthopsis, 713 2jj, 246. 274 Succus entericus, 670
Smooth muscle, 102 Squamata, 459, 461, 489, 495, Sucker, 420
Snakes, 459: Australian black, 496, 498, 522 Sucker-fish, 297
498: l\lovement of, 520: Squamosal, 97, 395, 467 Suidse, 836, 871
Tiger-, 498, 536, 538 Squamous suture, 657 Suiformes, 836, 837
Snake-bird, 614: Tropical, 622 Squatina, 228, 274 Suina, 836
Snipe, 616 Squatinidse, 228 Sulcus, 678, 884
Sole, 282, 297 Squirrel, 777: African 'flying', Suncus etruscus, 726
Solea, 297 845: 'Flying'-, 777,778,862 Sunfish, 334
Solenichthyes, 285, 297 Stannias, corpuscles of, 347 Sun-grebe, 615
Solenocytes, 57 Stapes, q6, 663, R88 Superior pharyngeal bones, 332
Solenodon, 726: S. paradoxus, Stargazer, 328, 339 Superior temporal arch, 468
728 Starling, 620, 628 Supination (muscle), 76
Solenodontidse, 728 Statocyst, 45 Supracoracoideus, 581
Solitaire, 557. 630, 633 Steatomithes, 618 Supramaxilla, 306
Somatic layer, 64 Stegomastodon, 816 Supra-occipital, 96, 303, 465
Somites, 62, 160 Stegosauria, 402, 510 Supraorbital process, 66o
Somniosus, 273, 277 Stegostoma, .Z28 Suprascapula, 396
Sorex minutus, 726 Stellate reticulum, 863 Surangular, 306
Soricidse, 727, 728, 731 Stercorariidae, 617 Sus, 861, 878: Dentition, 872
Soricoidea, 726, 727, 728, 868 Stereospondyli, 388, 425, 524 Suspensorium, 306, 393
Soricomorpha, 726, 729, 731 Sternebrse, 657 Suture, Coronal, 467, 66o
Spalacotherium, 687 Sternotherus odoratus, 522 Frontal, 467, 66o
Spalax, 777 Sternum, 91, 440, 465, 527, Lambdoidal, 66o
Sparidse, 349 577. 629, 6 5 7. 786, 85 r Sagittal, 66o
Sparrow, 62o, 628 Stickleback, 297, 327, 329 Skull, 657, 66o
Spear-fish, 320 Stigmata, 23, 32, 45 Swallow, 558, 559, 62o: Aus-
Spermatophores, 263, 452 Stilt, 616 tralian wood-, 559
Sperm-sac, 255 Stingaree, Captain Cook's, 265 Swan, 614, 628: Mute, 557:
Sphargis, 490 Stoat, 796, 898 Whooper, 638
Sphenacodontia, 462 Stolidobranchiata, 21 Swift, 618, 623: Palm-, 637:
Sphenethmoid, 394 Stolon, 38 White-throated, 559
Spheniscidse, 6II, 612 Stomach, 102, 309, 336, 401, Swim-bladder, 3II, 339
Sphenisciformes, 565, 61o 471, 584, 668, 876: Rumin- Sword-fish, 320
Spheniscus, 644 ant, 84o, 877 Sylvilagus, 779
Sphenodon, 464, 459. 477, 491, Stomata, 115 Symmetrodonta, 653, 654, 687,
495. 496, 521, 524, 525, 526, Struthiomirnus, 506 703
527, 530, 531, 532, 539. 540, Struthioniformes, 565, 607 Symmorium, 223
541, 544· 547· 548. 551, 554. Sturgeon, 282, 288 Sympathetic nervous system,
573. 6oo, 705, 706 Sturnidse, 62o 412, 682
Sphenodontidse, 496 Sturnus vulgaris, 628 Symphysis, 398, 662, 664, 856
Sphenoidal fissure, 66o Styelidse, 21, 35 Symplectic, 306
Sphenotic, 303 Stylinodon, 760 Synance_ia, 297: S. horrida, 329
Sphincter, ileo-colic, 670 Styloglossus, 659 Synapmda,462, 486,513,652
Sphincter, pyloric, 668 Stylohyal, 659, 854 Synapticuhe, 9
Sphyrsenidse, 297 Styloid process, 750, 854 Synaptic junction, 121
Sphyrama, 283 Stylomastoid, 661 Synaptosauria, 461, 492
Sphyrna, 266 Stomias, 328 Synbranchiformes, 285
Spigelian lobe of liver, 881 Stomochord, 10, 17 Synbranchii, 285
Spilocuscus, 716 Stomodreum, 23, 102 Syndactyla, 710
Spinal cord, 122,249,481,594, Stone-fish, 329 Syndesmochorial placenta, 904
682 Stork, 614 Synentognathi, 285, 296
Spinax molleri, 272 Storm-petrel, 613 Syngnathidse, 82, 351
Spiracles, 322 Stratum corneum, 81, 455 Syngnathiformes, 285
Spiral valve, 237 Stratum intermedium, 863 Synsacrum, 574
Splanchnic layer, 64 Strigidse, 61 8 Syrinx, 585, 638
Splanchnotome, 64 Strigiformes, 565, 618 Syrrhaptes, 623
Spleen, 71, 104, 239, 3II, 402, Strigops, 630: S. habroptilus 557 Systemic trunk, 407
474. 477. 675 Stroma, 159 Systole, 475
950 INDEX
T Molarisation, 867 Thoracopterus, 321
Palatine, 261, 298 Thoracostei, 297
Tachyglossidre, 688 Polyprotodont, 870 Threskiornithidre, 614
Tachyglossus, 694, 696, 697, Pre-molars, 666, 687, 743 Thrinaxodon, 516
698, 699, 7oo, 842, 85o, 862, Pretritubercular molar, 868 Thunnus, 343
869, 882, 885, 886, 888, 89o, Selenodont, 868 Thursus, 356
906, 907 Tritubercular, 865 Thyestes, 174
Tactile Organs, 643 Trituberculo-sectorial, 865 Thyrohydal bone, 854
Tadpoles, 420 Vomerine, 261, 298, 370 Thylacine, 712
T<Enia hippocampi, 68o, 885 Zalambdont, 868 Thylacininm, 714
Treniodonta, 724, 725, 760 Tegmentum, 126 Thy/acinus, 709, 712, 844, 856:
Treniolabididre, 702 Telemetacarpal condition, 840 T. cynocephalus, 714
T t:Bniolabis, 702 Telencephalon, 126 Thylacoleo, 719, 721
Tt:Bniparadisea meyeri, 561 Teleost, 99 Thylacomys, 715
Taipan, 521, 536, 538 Teleostei, 204, 284, 293, 335: Thylacosmilus, 712, 796
Talpa, 72, 903: T. europea, Intermediate orders, 296: Thyroxin, 149, 151, 589, 668
894. 895 Soft-rayed, 296: Spiny- Tibia, 1oo, 470, 471, 665, 856
Talpidre, 728 rayed, 296, 297 Tibiale, 101
Tamandua, 764 Telolecithal, 194 Tibio-fibula, 399, 471
Tanysiptera, 561 Telotroch, 12, 14 Tibio-tarsus, 579
Tapetum lucidum 141, 250, 275 Temnospondyly, 524 Tilapia, 297, 35: T. gall ilea,
Tapir, 813, 824, 826, 833, 862 Temporal lobe, 678 352: T. nilotica, 352
Tapiridm, 825 Temporal openings, 485 Tiliqua, 550
Tapiroidea, 825, 833 Tench, 82 Tillodontia, 724, 725, 760
Tapirus americanus, 833: T. Tenrec, 727 Tillotherium, 76o, 869
bairdi, 833: T. dowi, 833: Tenrecidre, 729 Tinea, 82, 179
T. indicus, 833, 86o, 907: T. Tenrecoidea, 726, 727,728, 729, Tinamidm, 610
roulini, 833 731 Tinamiformes, 565, 609
Taricha, 452 Tensores patagii, 581 Tinamu, 6o6
Tarsals, IOI, 390, 470 Teratornis incredibilis, 615 Tissue, 70
Tarsier, 726, 846 Tern, 617, 634 Tissue fluid, 77, 410
Tarsiidre, 736 Terrapin, 489: Diamond black, Tissue-spaces, 410
Tarsiiformes, 736, 739 490: Stinkpot, 522 Tit, 62o
Tarsipes, 706, 716, 722 Testis, 255, 263, 318, 376, 416, Titanichthys, 211
Tarsius, 739, 740, 742: T. 483, 597, 655, 684, 889 Titanoides, 818
spectrum, 740 Testosterone, 152, 255, 597, Titanotheres, 826, 831, 833
Tarso-metatarsus, 579 684 Titanotheridre- See Bronto-
Tarsus, 471, 665, 86o Testudinata, 461 theriidm
Tasmanian Devil, 712, 870 Testudinidm, 490 Toads, 436: Common, 442:
Tasmanian Marsupial-'wolf', Testudo, 490, 549: T. grt:Bca, Obstetric, 454: Surinam,
712, 714 460, 548: T. loveridgei, 526: 454
Taste, 192 T. tornieri, 488 Toad-fish, 297
Taste buds, 250, 315, 642, 667, Tetonius, 739 Todidm, 619
876, 887 Tetracht:Bnodon, 803, 857 Todypeutes, 763
Taxidea, 794 Tetraonidm, 615 Tomistoma, 501
Tayassuidre, 836 Tetrapturus, 320 Tone (muscle), 76
Teat, 655 Tetrodontiformes, 285 Tongue, 385, 400, 442, 583,
Tectum, 125, 480 Thalamus, 127, 245, 479, 681 667, 875
Teeth, 103, 272, 335, 400, 422, Thalassidromidre-See Bydro- Tonsil, 876
666, 862 batidm Tooth, Affinities, 103: Struc-
Brachydont, 868 Thaliacea, 20, 36: Classifica- ture, 103
Bunoid, 868 tion, 36 Tooth-pulp, 104
Carnassial, 792 Thebesius, valve of, 672 Tornaria, 13
Canines, 666, 733, 738, 796, Thecodontia, 461, 495, 499 Torpedo, 271, 276, 278
829, 865, 869, 873 Thelodus, 174 Tortoises, 459, 488, 489: Euro-
Dilambdodont, 868 Therapsida, 462, 514 pean, 548: Giant, 490
Diprotodont, 870 Theria, 654, 703, 889 Toucan, 619
Heterodont, 865 Theriodontia, 462, 514 Toxodontia,8o1, 8o6
Homodont, 865 Theocephalia, 517 Toxodontidre, 8o6
Hypsodont, 868 Theropods, 462, 505 Toxodon, 8o6
Incisor, 654, 666, 733, 738, Thigh, 81, 462 Toxotes, 336
746, 775· 796, 813, 864, Thigmotaxis, 196 Trabecula'!, 24, 91: Myocardial,
86g, 873 Thinocoridm, 616 475
Lophodont, 868 Thoatherium, 805, 858 Trachea, 478, 638, 668
Lophs, 868 Thoracostei, 285 Tracheal 'lung', 540
Mandibular, 261 Thorax, 88 Trachodon, 509
Molars, 666, 687, 702, 733, Thread cells, 198 Trachypterus, 258, 334
777.795.796,864,867 Thrombocytes, 71, 410 Trt:Bichthodes, 297
 INDEX 95I
Tragulidre, 837 Tuber cinercum, 68I Ungulate, 8oi
Tragulina, 836, 839 Tuberculum olfactorium, 679 Upper-arm, 8I
Tragulus, 839 Tubulidentata, 725, 79I, 8oi, Upupidre, 6Ig
Tragus, 756 804, 87I Urachus, 905
Transverse processes, go, 464 Tubulodon, 8o4 Uranoscopus, 3I4, 328, 335
Trapezium, 664, 859 Tunic, 22 Ureotelic mechanism, 254
Trapezoid, 664, 859 Tunica albuginea, 889 Ureter, 483, 596, 684, 889
Tree-creeper, 620 Tunica externa, 77, 340 Urethra, 685, 889
Tree-frog, 386 Tunica interna, 340 Uricotelic, 548
Tremataspidre, I 72 Tunica media, 77 Urinary bladder, 263, 347
Trematosaurus, 425 Tunicata, I, 4, 20, I05: Classi- Urinogenital aperture, 655
Triceps, 58I fication, 20 Urinogenital organs, 154
Triceratops, 5 I 2 Tunicin, 22 Urinogenital sinus, 892
Trichechiformes, 8o8, 822 Tunny, 297 Uroiietus, 583: U. audax, 6I5
Trichechus, 820: T. senegal- Tupaia, 733, 742: T. belangeri, Urochord, 45
ensis, 823 732 Urochordata-See Tunicata
Trichosurus, 72, 716, 88o, 8gi, Tupaiidre, 73I, 733, 754, 853 Urod<eum, 474, 483, 585
8g6, 907 Turacin, 627 Urodela, 38I, 386, 388, 432,
Triconodon, 687 Turaco, 6I7 438, 439. 440, 442, 444. 445.
Tricondonta, 653, 654, 687, 703 Turacoverdin, 627 446, 448. 456
Tricuspid valve, 672 Turbinals, 48I, 595, 662 Urohyal, 307
Trigeminal foramina. 93 Turbot, 297 Uromys, 709
Trigger-fish, 297, 329 Turkey, 615 Uronemus, 364
Trig/a, 3I5 Turnicidre, 6I5 Uropeltidre, 498
Triglidre, 297 Turtle, 459. 488, 489: Green-, Urophus, 268
Trigoniceps occipitalis, 621 490: Leathery, 490, 523, Uropsilus, 728
Trigonostylopidre, 8o7 527: Musk-, 522: Soup-, Uropygium, 566
Trigonostylopoidea, 8o2, 8o7 490 Urostyle, 30I, 39I, 438
Trilophodon. 8I5 Tytonidre, 6 I 8 Ursidre, 792, 793, 797, 874
Trionychidre, 490 Tylopoda, 836, 837, 838, 848, Ursus, 8oo: U. arctos, 794: U.
Tristychiidre, 22 5 877 arctos isabellinus, 797
Tritemnodon, 857: T. agilis, Tylototriton, 439 Utaiitus, 76I
857 Tympanic bone, 66o Uterine-chamber, 255
Tritor, 261 Tympanic bulla, 724 Uterine-crypts, 686
Trituberculata - See Panto- Tympanic cavity, 482 Uterine-milk, 278
theria Tympanic membrane, 38I, 414, Uterus, 277
Triturus, 436, 442, 452: T. 463. 888 Uterus duplex, 723, 738
alpestris, 456: T. crista/us, Tympanohyal bone, 854 Uterus masculinus, 685, 889
437 Tympanum, I46, 585 Uterus simplex, 723
Tritylodon, 5 I 7 Tympanunchus cupido, 638 Utricle, I45, 250, 4I4
Trochanter, 47I, 579, 665 Typhlomolge rathbuni, 433 Utriculus, 482
Trochilidre, 6 I 8 Typhlonectas, 434, 444. 455 Uvula, 667, 876
Trochlea, 664, 859 Typhlopidre, 498
Trogoniformes, 565, 619 Typhlosole, 23, 25, I85
Trogontherium, 774 Typotheria, 8oi, 8o7 v
TroGdon, 510 Tyranni, 6zo
Trophoblast, 899, goo Tyrannidre, 62o Vagina, 277, 685, 892
Trout, I05, 296, 393; Alimen- Tyrannosaurus, 506 Vagus foramina, 93
tary canal and associated Tyto alba, 625 Valve, Eustachian, 88I, 882
structures, 309: American Semilunar, 672
brook-, 298: Autonomic Spiral, 336
nervous system, 314: Blood: u Thebesian, 672, 882
vascular system, 311: Vicussens, 682
Brown-, 298: Crelom, 309: Udder, 849 Valvula cerebelli, 3I4
Development, 319: Endo- Uintatherium, 8Ig Vampire, 756
skeleton, 300: External Ulna, Ioo, 469, 572, 663 Vampyrum, 758
characters, 298: Lake, 298: Ulnare, IOI, 469, 578 Vanadocytes, 26
Muscles, 309: Nervous Ultino-branchial cells, 367 Vane, 568
system, 312: Organs of Umbilical cord, 905 Varanidre, 497
special sense, 315: Osmo- Umbilicus, inferior, 568 Varanus, 47I, 5I9, 544, 547,
regulation and excretion, Umbilicus, superior, 568 548. 694
317: Respiratory system, Unciform, 664, 859 Varanosauras, 5I3
311: Reproduction, 318: Uncinus, 573 Vasa efferentia, I59, 4I6
Skin and Exoskeleton, 300 Undina, 356 Vas deferens, 255, 483, 597,
Trumpeter, 6I5 Unguiculata, 725, 726 685, 889
Trygon, 254. 278 Ungul<e, 845 Vasochorial placenta, 905
Trygonorhina, 270, 277 'Ungulata', South American, Vasodentine, I03, 26I
Tuatara, 459, 595, 521 805 Vasopressin, I5I
952 INDEX
Veins, 78: Classification, 111 Opisthocrelus, 429, 438 Hump-back, 785; Killer,
Abdominal, 409, 447. 477 Procrelus, 430, 463 783: Pigmy Right, 785:
Anterior cardinal, 373: Sacral, 39I, 655 Pilot, 783, 784: Sperm, 783,
Jugular, III Thoracic, 655 784: Whalebone, 862,
Azygos, 447 Vertebral Column, 8g, go, 300, White, 783
Brachial, 407 438, 463, 572 Wheel-organ, 49
Cardiac, 409, 497 Vertebrata, 4, 6g-See Craniata White matter, 123, 682
Caudal, II2, 224, 447. 477 Vesicles, I22 Wing, 623
Caval, 674 Vesicles of Savi, 275 Wolf, 793: Aard-, 795
Coccygeo-mesenteric, 591 Vesicula seminalis, 255, 263, Wolffian body, 159, 596-See
Duodenal, 675 377. 4I6, 598 also Mesonephros
Efferent renal, 477 V espertilio, 756 Wolffian duct, 4I5, 4I6, 45I
Epigastric, 477, 591, 674 Vespertilionidm, 757 Wolverine, 796
External jugular, 407 Vestibule, 145, 250, 685, 894 Wombat, 716, 8gi, 907
Femoral, 374, 408, 477 Vexillum, 568 Woodcock, 616
Genital, 374 Vibrissce, 654, 842 Woodpecker, 6Ig, 621
Hepatic, 374. 477. 675 Vicuna, 839 Wryneck, 6Ig
Hepatic portal, II2, 244, Viper, Gaboon, 537, 538: Pit-, Wundernetz, 341
409, 477 52I, 547; Rhinoceros, 537:
Iliac, II2, 674 Russell's, 537
Ilio-lumbar, 675 Vipera berus, 460, 549, 550:
Inferior jugular, 373 V. russelli, 537 X
Intercostal, 674 Viperidm, 497, 498, 534
Internal jugular, 407 Viperinm, 54 7 Xenarthra, 781, 8o4, 868, 871:
Internal mammary, 674 Viscacha, 777, 778 skeleton, 765
Intestinal, 374 Visceral arch, 93 Xenicidm, 62o
Jugular, 243, 447, 477, 674 Visceral bars, 93 Xenopeltidm, 498
Lateral, II2, 244 Visceral relationships, 87 Xenopeltis, 498
Lieno-gastric, 675 Visceral skeleton, 93 Xenopterygii, 285, 297
Mesenteric, 675 Visceroceptors, II7 Xenopus, 406, 4I5, 429, 438,
Musculo-cutaneous, 408 Vishnutherium, 840 44I, 442, 448, 450, 845
Ovarian, II2 Vitelline membrane, 59 Xiphias, 320
Pelvic, 408, 477 Vitamin D, 627 Xiphisternum, 397, 657, 851
Portal, 244. 408, 477, 675 Vitreous body, I4I Xiphodontidm, 836
Postcaval,4o8,447.477.672 Vitrodentine, 84
Posterior cardinal, III Viverra, 794
Precaval, III, 407, 672 Viverridm, 792, 793, 795 y
Pulmonary, II3, 374 Vocal cords, 403,676
Renal, 374 Vocal sac, 390,403 Yak, 837, 840
Renal portal, II2, 249, 408, Vole, 777 Y aleosaurus, 507
447· 477 Vomer, g6, 305, 395, 66o Yeti, 797
Sciatic, 408, 477 Vomeronasal organ, I30, I37. Yolk-endoderm, 55 I: -plug,
Segmental, 374 4I3, 481 4I8: -sac, 28I, 457
Spermatic, II2, 675 Vultur, 560 Youngina, 495
Subclavian, II2, 243, 373, Vulture, 6I4, 62I: Turkey, 638 Younginiformes, 461
477· 674 Vulva, 655, 686, 894
Sub-intestinal, II2
Vesical, 409
Vitelline, 649 w z
Vellus, 842
Velum, 5I, Ig6 Wader, 617 Zaglossus, 694
Velum interpositum, 68o Wallaby, 709, 720: Quokka, Zanycteris, 758
Venom, 535. 536 7o8, 8 77 Zebra, 831
Ventricle of heart, Iog, 3II, Wallaroo, 709 Zeiformes, 285
404, 474 Walrus, 798, 875 Zeomorphi, 285, 297
Ventricles of brain, 68o: Wandering cells, 82 Z euglodon-See Basilosaurus
Lateral, 126: Third, I27 Wanderzellen, 82 Zeus, 297
Venules, 78, III Warbler, Sydney Rock, 6I2 Ziphiidm, 783
Vermiform appendix, 88o Warning colours, 437 Zona radiata, 3I9, 8g8
Vermis, 681 Wart-hog, 837 Zooid, 16, I8, 38
Vertebrce, go, 300, 850 Weasel, 8g8 Zosteropidm, 62o
Atlas, 464 Weaver, 623 Zygantra, 524
Axis, 464 Weever, 329 Zygapophyses, 300, 391
Caudal, 655 Weberian apparatus, 342: Zygodactylous foot, 6I7
Cervical, 391, 655 ossides, I46 Zygomatic Arch, 851: process,
Embolomerous, 426 Whales, 820, 883: Baleen, 784: 66o
Lepospondylous, 430 Blue, 780: Bottle-nosed, Zygosphenes, 524
Lumbar, 655 783: Californian Grey, 785: Zymogen cells, I85