Cam design and manufacture Second

Edition Jensen
Visit to download the full and correct content document:
https://textbookfull.com/product/cam-design-and-manufacture-second-edition-jensen/
More products digital (pdf, epub, mobi) instant
download maybe you interests ...

Engineering textiles : integrating the design and

manufacture of textile products Second Edition
Elmogahzy

https://textbookfull.com/product/engineering-textiles-
integrating-the-design-and-manufacture-of-textile-products-
second-edition-elmogahzy/

Optimum Design and Manufacture of Wood Products Etele

Csanády

https://textbookfull.com/product/optimum-design-and-manufacture-
of-wood-products-etele-csanady/

Transformer-Based Design Techniques for Oscillators and

Frequency Dividers 1st Edition Howard Cam Luong

https://textbookfull.com/product/transformer-based-design-
techniques-for-oscillators-and-frequency-dividers-1st-edition-
howard-cam-luong/

Advances in Gear Design and Manufacture 1st Edition

Stephen P. Radzevich (Editor)

https://textbookfull.com/product/advances-in-gear-design-and-
manufacture-1st-edition-stephen-p-radzevich-editor/
The Stolen Sisters psychological thriller 1st Edition
Louise Jensen [Jensen

https://textbookfull.com/product/the-stolen-sisters-
psychological-thriller-1st-edition-louise-jensen-jensen/

Sustainability and design ethics Second Edition Russ

https://textbookfull.com/product/sustainability-and-design-
ethics-second-edition-russ/

Sampling design and analysis Second Edition Lohr

https://textbookfull.com/product/sampling-design-and-analysis-
second-edition-lohr/

Communicating Science 2nd Edition Roy Jensen

https://textbookfull.com/product/communicating-science-2nd-
edition-roy-jensen/

A Guide to the Manufacture Performance and Potential of

Plastics in Agriculture A volume in Plastics Design
Library Michael Orzolek (Eds.)

https://textbookfull.com/product/a-guide-to-the-manufacture-
performance-and-potential-of-plastics-in-agriculture-a-volume-in-
plastics-design-library-michael-orzolek-eds/
CAM DESIGN
AND MANUFACTURE
 MECHANICAL ENGINEERING

A Series of Textbooks and Reference Books

EDITORS

L. L. FAULKNER S. B. MENK.ES
Columbus Division Department ofMechanical Engineering
Batte/le Memorial Institute The City College of the
City University of New York
and
New York, New York
Department ofMechanical Engineering
The Ohio State University
Columbus, Ohio

1. Spring Designer's Handbook, by Harold Carlson

2. Computer-Aided Graphics and Design, by Daniel L. Ryan
3. Lubrication Fundamentals, by J. George Wills
4. Solar Engineering for Domestic Buildings, by William
A. Himmelman
5. Applied Engineering Mechanics: Statics and Dynamics,
by G. Boothroyd and C. Poli
6. Centrifugal Pump Clinic, by Igor J. Karassik
7. Computer-Aided Kinetics for Machine Design, by
Daniel L. Ryan
8. Plastics Products Design Handbook, Part A: Materials
and Components; Part B: Processes and Design for
Processes, edited by Edward Miller
9. Turbomachinery: Basic Theory and Applications, by
Earl Logan, Jr.
10. Vibrations of Shells and Plates, by Werner Soedel
11 . Flat and Corrugated Diaphragm Design Handbook, by Mario
Di Giovanni
12. Practical Stress Analysis in Engineering Design, by Alexander Blake
13. An Introduction to the Design and Behavior of Bolted Joints, by
John H. Bickford
14. Optimal Engineering Design: Principles and Applications,
by James N. Siddall
15. Spring Manufacturing Handbook, by Harold Carlson-
! 6. Industrial Noise Control: Fundamentals and Applications,
edited by Lewis H. Bell
17. Gears and Their Vibration: A Basic Approach to Understanding
Gear Noise, by J. Derek Smith
18. Chains for Power Transmission and Material Handling: Design
and Applications Handbook, by the American Chain Association
19. Corrosion and Corrosion Protection Handbook, edited by
Philip A. Schweitzer
20. Gear Drive Systems: Design and Application, by Peter Lynwander
21. Controlling In-Plant Airborne Contaminants: Systems Design and
Calculations, by John D. Constance
22. CAD/CAM Systems Planning and Implementation, by Charles S. Knox
23. Probabilistic Engineering Design: Principles and Applications,
by James N. Siddall
24. Traction Drives: Selection and Application, by Frederick W. Heilich Ill
and Eugene E. Shube
25. Finite Element Methods: An Introduction, by Ronald L. Huston
and Chris E. Passerello
26. Mechanical Fastening of Plastics: An Engineering Handbook, by
Brayton Lincoln, Kenneth J. Gomes, and James F. Braden
27. Lubrication in Practice, Second Edition, edited by W. S. Robertson
28. Principles of Automated Drafting, by Daniel L. Ryan
29. Practical Seal Design, edited by Leonard J. Martini
30. Engineering Documentation for CAD/CAM Applications,
by Charles S. Knox
31. Design Dimensioning with Computer Graphics Applications, by
Jerome C. Lange
32. Mechanism Analysis: Simplified Graphical and Analytical Techniques,
by Lyndon 0. Barton
33. CAD/CAM Systems: Justification, Implementation, Productivity
Measurement, by Edward J. Preston, George W. Crawford, and
Mark E. Coticchia
34. Steam Plant Calculations Manual, by V. Ganapathy
35. Design Assurance for Engineers and Managers, by John A. Burgess
36. Heat Transfer Fluids and Systems for Process and Energy Applications,
by Jasbir Singh
37. Potential Flows: Computer Graphic Solutions, by Robert H Kirchhoff
38. Computer-Aided Graphics and Design, Second Edition, by
Daniel L. Ryan
39. Electronically Controlled Proportional Valves: Selection and
Application, by Michael J. Tonyan, edited by Tobi Goldoftas
40. Pressure Gauge Handbook, by AMETEK, U.S. Gauge Division,
edited by Philip W. Harland
41. Fabric Filtration for Combustion Sources: Fundamentals and Basic
Technology, by R. P. Donovan
42. Design of Mechanical Joints, by Alexander Blake
43. CAD/CAM Dictionary, by Edward J. Preston, George W. Crawford, and
Mark E. Coticchia
44. Machinery Adhesives for Locking, Retaining, and Sealing, by Girard S.
Haviland
45. Couplings and Joints: Design, Selection, and Application, by Jon R.
Mancuso
46. Shaft Alignment Handbook, by John Piotrowski
47. BASIC Programs for Steam Plant Engineers: Boilers, Combustion,
Fluid Flow, and Heat Transfer, by V. Ganapathy
48. Solving Mechanical Design Problems with Computer Graphics, by
Jerome C. Lange
49. Plastics Gearing: Selection and Application, by Gifford E. Adams
50. Clutches and Brakes: Design and Selection, by William C. Orthwein
51. Transducers in Mechanical and Electronic Design, by Ha"y L. Trietley
52. Metallurgical Applications of Shock-Wave and High-Strain-Rate
Phenomena, edited by Lawrence E. Mu", Karl P. Staudhammer,
and Marc A. Meyers
53. Magnesium Products Design, by Robert S. Busk
54. How to Integrate CAD/CAM Systems: Management and Technology,
by William D. Engelke
55. Cam Design and Manufacture, Second Edition; with cam design soft-
ware for the IBM PC and compatibles, disk included, by Preben W.
Jensen

ADDITIONAL VOLUMES IN PREPARATION

Fundamentals of Robotics, by David D. Ardayfio
Belt Selection and Application for Engineers, edited by Wallace D. Erickson
Solid-State AC Motor Controls: Selection and Application, by Sylvester
Campbell

Mechanical Engineering Software

Spring Design with an IBM PC, by Al Dietrich

Mechanical Design Failure Analysis: With Failure Analysis System Software
for the IBM PC, by David G. Ullman
CAM DESIGN
AND MANUFACTURE
Second Edition
With Cam Design Software for the IBM PC and Compatibles
Disk Included

Preben W. Jensen
School of Physics, Engineering and Technology
Mankato State University
Mankato, Minnesota

0 ~y~~F~~:~~oop
Boca Raton London New York

CRC Press is an imprint of the

Taylor & Francis Group, an informa business
Library of Congress Cataloging-in-Publication Data

Jensen, Preben W., [date]

Cam design and manufacture.
(Mechanical engineering; 55)
"With cam design software for the IBM PC and
compatibles, disk included."
Bibliography: p.
Includes index.
I. Cams-Design and construction-Data processing.
2. Computer-aided design. 3. IBM Personal Computer-
Programming. I. Title. II. Series.
TJ206.J48 I 987 62 J.8'38 86-327 76
ISBN 0-8247-7512-0

First edition copyright© 1965 by The Industrial Press, New York, NY

COPYRIGHT© 1987 by MARCEL DEKKER, INC. ALL RIGHTS RESERVED

270 Madison Avenue, New York, New York 10016
Neither this book, software, nor any pan thereof may be reproduced or transmitted in any form or by any
means, electronic or mechanical, including photocopying, microfilming, and recording, or by any information
storage and retrieval system, without permission in writing from the publisher, except that one working copy may be
made.
Current printing (last digit):
10 9 8 7 6 5 4 3 2 I
Preface

This book was written to give the practicing engineer a sound grasp of the
methods of solving the problems connected with cams-their design,
application, and manufacture. The above goal is as valid for this new
edition as it was for the first edition. Since that time the most important
change has been the improvement of numerically controlled machine tools
(NC-machines) and the availability of computers in general. Therefore the
emphasis on graphical and analytical methods has shifted toward the latter;
but for a design engineer who has put his creative thoughts into metal it is
of the utmost importance that he can visualize the problems; therefore the
graphical approach has not been neglected. Major changes occur in
Chapters 5 and 12 where analytical expressions that can be programmed on
a home computer have been developed.
The plan of the book is as follows: In Chapter 1 the basic types of cam
and follower systems are described and illustrated. In Chapter 2 the
construction and use of displacement diagrams are explained and formulas
are given for the displacement, velocity, and acceleration curves for various
types of cam motion.
In Chapter 3 displacement diagram synthesis is explained and methods
of combining various curves to obtain a desired motion are given. Chapter
4 outlines the methods of determining cam profiles graphically when
different types of followers are used and formulas for determining the cam
profile using both rectangular and polar coordinates are given.
Chapter 5 takes up the importance of pressure angle, and the procedure
for proportioning a cam with respect to pressure angle limitations is
explained. The importance of avoiding too small a radius of curvature is
also discussed.

iii
iv PREFACE

In Chapter 6 the advantages of circular cams and methods of proportion-

ing these are outlined. Chapter 7 continues with a discussion of circular-arc
and straight-line cams, which have advantages particularly from the
standpoint of ease of production.
Chapter 8 considers the important factors of forces, contact stresses, and
materials. This is followed in Chapter 9 by a discussion of various methods
of cam manufacture.
When cams rotate at high speed, the factors of elasticity and backlash
must be taken into consideration if the desired accuracy of motion is to be
obtained. One way of doing this is to use the polydyne method of cam
design. In Chapter 10 this method is described in detail and the effect of
various members of the cam train are determined.
In Chapter 11 the use of various types of mechanisms with cams to offset
the disadvantages of the latter is illustrated with various examples.
In Chapter 12, formulas to determine velocities, accelerations, and forces
in linkages have been developed. This chapter together with Chapter 8
should enable the reader to find forces in complex mechanisms other than
cam mechanisms.
Chapter 13 includes six computer programs that allow the user to
determine minimum cam size for given maximum pressure angles for eight
different cam displacement diagrams and calculate the maximum com-
pressive stress by both rise and return for translating as well as swinging
roller followers.
My thanks to Mr. A. F. Abou-Ghaledum from Cleveland State University,
who in no time linked the six programs together to make them user-
friendly.
Twenty-eight nomograms are included in order to facilitate computa-
tions.
The bibliography at the end of the book lists more than 1800 titles in
English and German. The list in the first edition comprised "only"
approximately 500 titles. The list is in alphabetical order and is referenced
in eleven groups to facilitate its use.
An author usually puts a little of his heart and convictions into his book
and I am happy to express that the first edition was well accepted.

Preben W. Jensen
Contents

Preface 111

Using the Cam Design Software Vil

Nomenclature IX

List of Nomograms XIII

1. Cam and Follower Systems 1

2. Displacement Diagrams 8
3. Displacement Diagram Synthesis 31
4. Cam Profile Determination 46
5. Pressure Angle and Radius of Curvature 75
6. Circular Cams 106
7. Circular-arc and Straight-line Cams 126
8. Forces, Contact Stresses, and Materials 141
9. Methods of Cam Manufacture 159
10. Dynamics of Cam Mechanisms 170
11. Cam Mechanisms 188
12. Velocities, Accelerations, and Dynamic Forces in Linkages
and Cam Mechanisms 206
13. Computer Programs for Analysis and Synthesis of
Cam Mechanisms 252
Bibliography 321
Index 425

V
Using the Cam Design Software

The following should help the new user in using the cam programs
included on the supplied disk. There are a number of requirements needed
before these programs and the disk can be used. These requirements are
concerned with the type of system you are using, and the operating system
used.
The programs were developed on an IBM-PC running on DOS 2.0, and
using BASICA. However, since the programs do not use any special IBM
formats and system calls they could run virtually on any compatible
running MS-BASIC, CW-BASIC, or similar basic editors.
The following procedure is recommended for proper use of these
programs:

1. Switch your computer ON, insert a DOS disk or a disk containing a

BASIC Editor into drive A.
2. When you receive the A> prompt, type in the command to run basic
(i.e., A> BASICA).
3. Remove the disk from drive A and insert the cam programs disk
in A.
4. Type the following command: RUN "MAIN"

This should get you into the MAIN program, which will then prompt you
with the master MENU. At this point refer to Chapter 13.

vii
Nomenclature

Cam conto"r Offset translaflfig

roller follower

~ Fixed center
of rotation

Swinging roller follower

ix
X NOMENCLATURE

a == !~ !~ == == acceleration of follower, in/sec

2

b == thickness of contacting cam and follower, in.

d, == shaft diameter, in.
e == offset or eccentricity, in.
Fn == normal load, lbs.
g == gravitational constant== 386 in/sec 2 == 32.16 ft./sec 2
h == maximum displacement of follower, in.
L1 == length of oscillating follower arm, in.
N == cam speed, rpm

p == da == pulse of follower, in/sec3

dt
r == radius to trace point, in.
r1 == roller radius, in.
S == distance between cam and oscillating follower centers, in.
t == time for cam to rotate angle 0, sec
T == time for cam to rotate angle p, sec
T 1 == time for cam to rotate angle p1 , sec
T2 == time for cam to rotate angle P2 , sec

v ==!!:}_==velocity of follower, in/sec

dt
y == displacement of follower, in.

y' == dy == follower velocity, in/sec.

dt

y" == !~ == follower acceleration, in/sec2

y'" == !?s == follower pulse or jerk, in/sec 3

Rmax == maximum radius of cam (to center of roller), in.

Rmin == minimum radius of cam (to center of roller), in.
Rb == radius of base circle (to actual cam shape), in.
Rg = radius of cutter or grinder, in.
S, == pressure, lb./in. 2
a == pressure angle, degrees
a 1 == pressure angle by rise, degrees
Oz == pressure angle by return, degrees
a.,,, 1 == max. pressure angle by rise, degrees
a.,,, 2 == max. pressure angle by rise, degrees
NOMENCLATURE xi

~ = cam angle rotation for total rise h, degrees

~ 1 = cam angle rotation for total rise, degrees
~ 2 = cam angle rotation for total return, degrees

0 = cam angle rotation for follower displacement y, degrees

µ = coefficient of friction
µ = transmission angle, degrees
µ,,, µ1 = Poisson's ratio for cam and follower, respectively
R, = radius of curvature of cam, in.
<j> = angle of oscillating follower movement for cam angle 0, degrees
<j>0 = total angle of oscillating follower movement, degrees
w = cam angular velocity, rad/sec
List of Nomograms

Figure
5-5 Pressure angle-constant velocity motion 80
5-6 Pressure angle-parabolic motion 81
5-7 Pressure angle-simple harmonic motion 82
5-8 Pressure angle-cycloidal motion 83
5-9 Pressure angle-3-4-5 polynomial 84
5-10 Pressure angle-modified trapezoidal acceleration 85
5-19 Pressure angle-parabolic, cycloidal and simple
harmonic motions 91
5-23 Minimum radius of curvature-parabolic motion,
translating follower 97
5-24 Minimum radius of curvature-simple harmonic
motion, translating follower 98
5-25 Minimum radius of curvature-cycloidal motion,
translating follower 99
5-26 Minimum radius of curvature-double-harmonic
motion, translating follower 100
6-9 Best μmin 112
a
6-10 d for best μmin 113

b
6-11 d for best μmin 114

d for best
C
6-12 μmin 1I5

xiii
xiv LIST OF NOMOGRAMS

6-13 Best μmin (<!>, = 180°) 116

6-20 Proportioning of a slider-crank, μmin 122

6-21 Proportioning of a slider-crank, ~ 122

s

6-22 Proportioning of a slider-crank, !!_ 123

s
6-23 Proportioning of a slider-crank, 'A. 123
6-24 Proportioning of a slider-crank for given cj>0 best μ min 124
8-3 Acceleration-translating follower; parabolic, cycloidal,
and simple harmonic motions (50 < N < 500) 144
8-4 Acceleration-translating follower; parabolic, cycloidal,
and simple harmonic motions (500 < N < 5000) 145
8-9 Hertz's pressure for convex surface 155
8-10 Hertz's pressure for concave surface 156
8-11 Hertz's pressure for plane surface 157
12-15 Acceleration-inverted crossed slide-crank
(60 <N < 600) 244
12-16 Acceleration-inverted crossed slide-crank
(600 < N < 6000) 245
CAM DESIGN
AND MANUFACTURE
CHAPTER 1

Cam and Follower Systems

Cams are used in a wide variety of machines; such as packaging machines,

can-making machinery, wire-forming machines, engines, computing mecha-
nisms, and mechanical and electronic computers. One important reason
why cam mechanisms are preferred over other types is that the use of cams
makes it possible to obtain an unlimited variety of motions and when
certain basic requirements are followed, cams perform satisfactorily year
after year.
Cams are used to transform a rotary motion into a translating or oscil-
lating motion. In certain cases they are also used to transform a translating
or oscillating motion in to a different translating or oscillating motion.
The requirements which are imposed on cams vary from machine to
machine because the requirements depend not only on the speed of the
cam, but also on the kind of machine in which they are being used. In
certain kinds of wrapping machines, for example, the forces imposed on
the material to be wrapped should be kept as low as possible, but it doesn't
matter if these forces are applied suddenly, whereas in other machines it
is very important for the proper performance of the machinery that the
variation of forces is smooth and gradual. The basic limiting requirements
are: kind of time-displacement diagram, pressure angle, radius of curvature,
and finally, the contact pressure between follower and cam. These re-
quirements will be discussed in subsequent chapters.
The most commonly used cam is the plate cam which is cut out of a
piece of flat metal or plate. Dependent on the kind of follower, various
types of following systems are often employed. A radial translating roller
follower is shown in Fig. I-la and is so called because the center line of the
follower-stem passes through the center of the cam shaft. An offset trans-
2 CHAPTER 1

lating roller follower is shown in Fig. 1-lb; here the center line of the follower-
stem does not pass through the cam shaft center.
Figure 1-lc shows a swinging roller follower which is preferred over the
translating follower because a much higher pressure angle can be allowed,
and hence the overall proportions of the mechanism can be reduced. The
question is often raised as to whether the rotation of the cam should be
away from or toward pivot point M; in the case of Fig. 1-lc this would
mean respectively CCW (counterclockwise) and CW (clockwise) rotation
of the cam. There is a slight advantage in letting the cam rotate away
from the pivot point, but in most cases the advantage is insignificant.

eJ

Fig. 1-1. (a) Radial translating roller follower. (b) Offset translating roller follower.
(c) Swinging roller follower. (d) Flat-faced translating follower . (e) Flat-faced
swinging follower.

Figure 1-ld shows a translating flat-faced follower. The flat does not
necessarily have to be perpendicular to the follower-stem, although it
usually is. Sometimes the center line of the follower-stem is offset as
shown in the right-hand view of Fig. 1-ld. This arrangement will tend to
distribute and reduce wear on the flat because the friction force created
between the cam and the follower will tend to rotate the follower around
its axis. It should be noticed that whether the center line of the follower-
stem passes through the cam shaft center or not has no effect on the cam
CAM AND FOLLOWER SYSTEMS 3

profile. In certain kinds of textile machines, the arrangement shown in

Fig. 1-le with a swinging flat-faced follower is used.
In the systems so far shown, the follower is kept in contact with the
cam with the help of gravity forces. Obviously, this is only possible in
the case of low-speed cams. For moderate- and high-speed cams other
means must be employed. The use of springs is one obvious solution to
keep cam and follower in contact with each other.

R, R,

Fig. 1-2. (a) Closed-track cam. (b) Double closed-track cam. (c) Closed-track
cam with two rollers.

Other possibilities are shown in Figs. 1-2a, b, and c. In Fig. l-2a part
of a closed-track cam is shown. When the roller follower is driven upward,
the roller contacts B, and when driven downward the roller contacts A.
A certain clearance is therefore necessary to permit the roller to roll.
However, this clearance should be kept as small as possible, because the
larger the clearance, the larger the impact will be when the roller changes
contact from one side of the track to the other. This change takes place
when the motion of the roller changes from acceleration to deceleration.
If this change is made gradually, as with a cycloidal motion, the impact
is greatly reduced as compared to the case of parabolic motion where the
acceleration changes suddenly.
It is therefore natural to try to reduce the clearance to a minimum and
it can be done with the help of the arrangement shown in Fig. 1-2b. The
follower arm A carries the two rollers R 1 and R 2 on the same pin B. R 1
rolls on the inner side of the track of cam C and R2 rolls on the outer track.
4 CHAPTER 1

Fig. l-3a. Cylindrical cam with translating roller follower.

Because of its cost, however, this arrangement is seldom used. In Fig. 1-2c
two rollers are placed on different pins. There seems to be no advantage
over that of Fig. l-2b. The track shown in Fig. l-2c is also more difficult
to machine than that of Fig. 1-2b.
A cylindrical cam with translating roller follower, Fig. 1-3a, has the
characteristic that the direction of motion of the follower is parallel to the
cam shaft. Theoretically, the roller has to be cone-shaped with its apex
at the cam shaft center, as shown in Fig. 1-3a, in order that the roller may
roll without sliding. However, a cylindrical roller or one with a slightly
spherical shape will operate satisfactorily in most cases.
Figure 1-3b shows a similar arrangement but two rollers are used instead
of one. The advantage as compared with Fig. l-3a, where one roller

Fig. l-3b. Double-end cam with translating roller follower.

CAM AND FOLLOWER SYSTEMS 5

Fig. 1-4( a). Conical closed-track Fig. 1-4(b). Conical open-track

cam with translating roller fol- cam with translating roller fol-
lower lower

Fig. 1-5. Globoidal closed-track Fig. 1-6. Globoidal closed-track

cam with swinging roller follower cam with swinging roller follower

Fig. 1-7. Globoidal closed-track Fig. 1-8. Spherical closed-track

cam with swinging roller follower cam with swinging roller follower

Figs. 1-4 to 1-8. Special types of cams.

6 CHAPTER 1

Fig. 1-9. Spherical open-track cam with swinging roller follow er.

changes contact from one side of the track to the other whenenr acceler-
ation is changed in direction, is that with both rollers in contact \Yith the
track at the same time there will be no backlash.
A conical cam, Fig. 1-4a, has mu ch the same characteristics as a cy-
lindrical one and is used in cases when the direction of the output motion
is parallel to an element of th e base cone. The conical cam in Fig. l-4 a
has a closed track and that in Fig. l-4b an open track.
The cam and follower systems shO\rn in Figs. 1-;3 to 1-10 have to be
cut by special devices. Because of the cost of making them, th ey are

Fig. 1-10. Kinematic inversion of th e spherical cam; cam is stationary and S\Yinging
roller follow er rotates around cam.
CAM AND FOLLOWER SYSTEMS 7

seldom employed and then only for rather small mechanisms. Figures 1-5,
1-6, and 1-7 show globidal cams with swinging roller followers; the only
difference is in how the roller is placed relative to the input and output
shafts. Figure 1-8 shows a spherical cam with swinging roller follower and
closed-track, and in Fig. 1-9 the mechanism is shown with an open track.
Figure 1-10 is a kinematic inversion of the spherical cam; the cam is
stationary and the swinging roller follower rotates. This kind of mecha-
nism is used in agricultural machinery.

C B A D

Fig. 1-11. Ball runs in groove of closed-track cam, imparting an oscillating motion to
output member A.

A very special kind of cam and follower system is shown in Fig. 1-11.
The cam C is a closed-track cam and the track is formed so that a ball B
can be guided by either side of the track dependent upon the direction of
motion of arm A. A round arm A has a sliding fit in the hole of the ball
and is fastened to shaft D , which is the output shaft . This mechanism is
used in a sewing machine and the cam rotates at 2000 to 3000 rpm. The
advantage of this mechanism becomes clearly obvious when compared with
Fig. 1-9. To cut the cam in Fig. 1-9 would require that t he milling cutter
be moved exactly the same way relative to the spherical cam as the roller
follower, and this requires a special set-up. However, the cam in Fig. 1-11
can be cut with a milling cutter which has the form of the groove, exactly
as if it were a plate cam.
CHAPTER 2

Displacement Diagrams

In Chapter 1 many varieties of cams and followers are illustrated and in

all of them the cam rotates at a constant angular velocity and the follower
moves in a manner prescribed by the functional requirements of the
machine.
The simplest follower motion is a constant velocity rise followed by a
similar return with a dwell in between. A simple graph called a displace-
ment diagram illustrates this sequence of events .

Rise interval Dwell intenal Return interval Dwell interval

- - -------- T,,

_ _ _ _ _ _o_n_e_c ycle = 3 60 °

Fig. 2-1. A simple displacement diagram.

Such a diagram is shown in Fig. 2-1. Here, one cycle is taken to mean one
complete revolution of the cam; that is, one cycle represents 360 degrees.
From this it follows that the horizontal distances, T1 , T2 , T3 , T4 is expressed
in seconds and ~ 1, ~ 2 , ~ 3 , ~ 4 in degrees of rotation. Degrees are mostly used.
The vertical distances represent the motion of the follower dependent on
time. From Fig. 2-2 it can be seen that in place of a rotating cam it is
possible to achieve the identical follower action by means of a translating

8
Another random document with
no related content on Scribd:
Fig. 80.—Dorsal view of male Diastylis
stygia, × 12. A, 2nd antenna; Ab.6, 6th
abdominal appendage. (After Sars.)

Order II. Cumacea.[96]

The Cumacea are a group of small marine animals rarely attaining

an inch in length, which agree with the Mysidacea in the characters
noted above as diagnostic of the Division Peracarida; they possess,
however, in addition a number of peculiar properties, and Sars
believes them to be of a primitive nature showing relationship to
Nebalia, and possibly to an ancestral Zoaea-like form. They follow a
habit similar to that of the Mysidacea, being caught either in the
surface-plankton or in great depths, many of the deep-sea forms
being blind. They are, however, not true plankton forms, and they
appear to attain a greater development both in point of variety and
size in the seas of the northern hemisphere. The thoracic limbs may
be biramous, but there is a tendency among many of the genera to
lose the exopodites of some of the thoracic legs, an exopodite never
being present on the last few thoracic limbs of the female and on the
last in the male. In the Cumidae the four posterior pairs in both sexes
have no exopodites. The first three thoracic appendages following the
maxillae are distinguished as maxillipedes; they are uniramous, and
the first pair carries an epipodite and a large gill upon the basal
joints. Pleopods are only developed in the male sex.
The flagellum of the second antennae in the male may be
enormously elongated, as in the Atlantic deep-sea species shown in
Fig. 80, so as to exceed in length the rest of the body.
Fam. 1. Cumidae.—No sharp demarcation between thorax and
abdomen. Four posterior pairs of legs in both sexes without
exopodites. Male with five well-developed pleopods in addition to the
uropods. Telson wanting. Cuma, Cyclaspis, etc.
Fam. 2. Lampropidae.—Body-form resembles that of Cumidae.
All the thoracic limbs except the last have exopodites. The male has
three pairs of pleopods. Telson present. Lamprops, Platyaspis, etc.
Fam. 3. Leuconidae.—Body-form similar to above. Male has
only two pairs of pleopods. Mouth-parts peculiar, much less setose
than in other families. Telson absent. Leucon, Eudorella.
Fam. 4. Diastylidae.—Anterior part of thorax sharply marked
off from posterior part. Male has two pairs of pleopods. Telson
present. Diastylis (Fig. 80). D. goodsiri from the Arctic ocean
measures over an inch in length.
Fam. 5. Pseudocumidae.—Rather similar to Diastylidae, but
differ in reduced size of telson and presence of exopodites on third
and fourth thoracic legs of female. This family is represented by
three very similar marine forms of the genus Pseudocuma; but, as
Sars has shown,[97] the Caspian Sea contains thirteen peculiar
species, only one of which can be referred to the genus Pseudocuma,
while the rest may be partitioned among four genera, Pterocuma,
Stenocuma, Caspiocuma, Schizorhynchus.

Order III. Isopoda.

The Isopoda and the Amphipoda are frequently classed together as

Arthrostraca or Edriophthalmata, owing to a number of features
which they share in common, as, for instance, the sessile eyes which
distinguish them from the podophthalmatous Schizopoda and
Decapoda, the absence of a carapace, and the thoracic limbs which
are uniramous throughout their whole existence. For the rest, in the
presence of brood-plates and the other diagnostic characters, they
are plainly allied to the other Peracarida, and an easy transition is
effected from the Mysidacea to the Isopoda through the Chelifera or
Anisopoda. Only one thoracic segment is usually fused with the head,
the appendage of this segment being the maxillipede; in the Chelifera
among Isopoda, and the Caprellidae among Amphipoda, two
thoracic segments are fused with the head.
The Isopoda are distinguished from the Amphipoda by the dorso-
ventral flattening of the body, as opposed to the lateral flattening in
the Amphipoda, by the posterior position of the heart, and by the
branchial organs being situated on the abdominal instead of on the
thoracic limbs.
The Isopoda, following Sars’[98] classification, fall into six sub-
orders—the Chelifera, Flabellifera, Valvifera, Asellota, Oniscoida,
and Epicarida,—to which must be added the Phreatoicidea.
 Sub-Order 1. Chelifera.

The Chelifera, including the families (1) Apseudidae and (2)

Tanaidae, are interesting in that they afford a transition between
the ordinary Isopods and the Mysidacea. The important features in
which they resemble the Mysidacea are, first, the fusion of the first
two thoracic segments with the head, with the coincident formation
of a kind of carapace in which the respiratory functions are
discharged by a pair of branchial lamellae attached to the
maxillipedes; and, second, the presence of very small exopodites on
the first two thoracic appendages of the Apseudidae.
The second pair of thoracic limbs, i.e. the pair behind the
maxillipedes, are developed both in the Apseudidae and Tanaidae
into a pair of powerful chelae, and these frequently show marked
sexual differences, being much more highly developed in the males
than in the females. The biramous and flattened pleopods are purely
natatory in function, and the uropods or pleopods of the sixth pair
are terminal in position and slender.
Both families, of which the Apseudidae contain the larger forms,
sometimes attaining to an inch in length, are littoral in habit, or
occur in sand and ooze at considerable depths, many of the genera
being blind. Many Tanaids (e.g. Leptochelia, Tanais, Heterotanais,
etc.) live in the algal growths of the littoral zone, and being highly
heliotropic they are easy to collect if a basinful of algae is placed in a
strong light. The females carry the eggs about with them in a brood-
pouch formed, as is usual in the Peracarida, by lamellae produced
from the bases of the thoracic limbs. The males on coming to
maturity do not appear to grow any more, or to take food, their
mouth-parts frequently degenerating and the alimentary canal being
devoid of food. They are thus in the position of insects which do not
moult after coming to maturity; and, as in Insects, the males are apt
to show a kind of high and low dimorphism—certain of the males
being small with secondary sexual characters little different from
those of the females, while others are large with these characters
highly developed. Fritz Müller, in his Facts for Darwin, observes
that in a Brazilian species of Leptochelia, apparently identical with
the European L. dubia, the males occur under two totally distinct
forms—one in which the chelae are greatly developed, and another in
which the chelae resemble those
of the female, but the antennae in
this form are provided with far
longer and more numerous
sensory hairs than in the first
form. Müller suggested that these
two varieties were produced by
natural selection, the characters
of the one form compensating for
the absence of the characters of
the other. A general consideration
of the sexual dimorphism in the
Tanaidae[99] lends some support
to this view, since the smaller
species with feeble chelae do
appear to be compensated by a
greater development of sensory
hairs on the antennae, but the
specific differences are so difficult
to appreciate in the Tanaidae that
it is possible that the two forms of
the male in Müller’s supposed
single species really belonged to
two separate species.
 Fig. 81.—Apseudes spinosus, ♂, × 15. A,
1st antenna; Ab, 6th abdominal
appendage; T, 2nd thoracic appendage.
(After Sars.)

Sub-Order 2. Flabellifera.

The Flabellifera include a number of rather heterogeneous families

which resemble one another, however, in the uropods being lateral
and not terminal, and being expanded together with the telson to
form a caudal fan for swimming. The pleopods are sometimes
natatory and sometimes branchial in function. Some of the families
are parasitic or semi-parasitic in habit.
Fam. 1. Anthuridae.—These are elongated cylindrical creatures
found in mud and among weeds upon the sea-bottom; their mouth-
parts are evidently intended for piercing and sucking, but whether
they are parasitic at certain periods on other animals is not exactly
known. Anthura, Paranthura, Cruregens.
Fam. 2. Gnathiidae.[100]—These forms appear to be related to
the Anthuridae; they are ectoparasitic on various kinds of fish during
larval life, but on assuming the adult state they do not feed any more,
subsisting merely on the nourishment amassed during the larval
periods. The larvae themselves are continually leaving their hosts,
and can be taken in great numbers living freely among weeds on the
sea-bottom. The larvae, together with the adults of Gnathia
maxillaris, are extremely abundant among the roots of the sea-weed
Poseidonia cavolinii in the Bay of Naples. The young larvae hatch
out from the body of the female in the state shown in Fig. 82, A. This
minute larva fixes upon a fish, and after a time it is transformed into
the so-called Praniza larva (B), in which the gut is so distended with
the fluid sucked from the host that the segmentation in the hind part
of the thorax is entirely lost. When this larva moults it may, however,
reacquire temporarily its segmentation. After a certain period of this
parasitic mode of life the Praniza finally abandons its host, and
becomes transformed into the adult male or female. This may take
place at very different stages in the growth of the larva, the range of
variation in size of the adults being 1–8 mm., and it must be
 remembered that when once the
adult condition is assumed
growth entirely ceases. What it is
that determines the stage of
growth in each individual when it
shall be transformed into the
adult is not known. The males
and females differ from one
another so extraordinarily that it
was for long denied that they
were both derived from the
Praniza larvae. This is
nevertheless the case. The change
from the Praniza to the female
(Fig. 82, C) is not very great. The
ovary absorbs all the
nourishment in the gut and
comes to occupy the whole of the
body, all the other organs
degenerating, including the
alimentary canal and mouth-
Fig. 82.—Gnathia maxillaris. A, parts. Indeed, only the limbs with
Segmented larva, × 10; B, Praniza their muscles and the nervous
larva, × 5; C, gravid female, × 5; D, system remain. The change to the
male, × 5.
male (D) is more radical. The
food is here stored in the liver,
which increases in the male just as the ovary does in the female. The
segmentation is reacquired, and the massive square head is formed
from the hinder part of the head in the Praniza, the anterior portion
with its stylet-like appendages being thrown away. The powerful
nippers of the male are not formed inside the cases of the old
styliform mandibles, but are independent and possibly not
homologous organs. The meaning of the marked sexual dimorphism
and the use of the males’ nippers are not in the least known, though
the animals are easy to keep under observation. In captivity the
males never take the slightest notice of either larval or adult females.
Fam. 3. Cymothoidae.[101]—This is a group of parasites more
completely parasitic than the foregoing, but their outer organisation
does not differ greatly from an ordinary Isopodan form. A great
many very similar species are known which infest the gill-chambers,
mouths, and skin of various fishes. The chief interest that attaches to
them is found in the fact that a number of them, and perhaps all, are
hermaphrodite, each individual acting as a male when free-
swimming and young, and then subsequently settling down and
becoming female. This condition is exactly the same as that
occurring universally in the great group of parasitic Isopoda, the
Epicarida, to be considered later. There is no evidence that the
Cymothoidae are phyletically related to the Epicarida, so that the
similar sexual organisation appears to be due to convergence
resulting from similar conditions of life. The general question of
hermaphroditism in the Crustacea has been shortly discussed on pp.
105–106. Cymothoa.
Fam. 4. Cirolanidae.—In this family is placed the largest Isopod
known—the deep-sea Bathynomus giganteus, found in the Gulf of
Mexico and the Indian Ocean, sometimes measuring a foot long by
four inches broad. A common small littoral form is Cirolana.
Fam. 5. Serolidae.[102]—The genus Serolis comprises flattened
forms bearing a curious resemblance to Trilobites, which Milne
Edwards considered more than superficial. The genus is confined to
the littoral and deep waters of the southern hemisphere.
Fam. 6. Sphaeromidae.[103]—These are flattened, broad-bodied
forms, most commonly met with in the Mediterranean and warmer
seas. Without being actually parasitic, they are frequently found as
scavengers in decaying material, and they show some relationship to
the parasitic Cymothoidae. In some of the genera, e.g. Cymodoce, the
ovigerous female shows a degenerate condition of the mouth-parts,
while the maxillipedes undergo an enlargement, and are used for
causing a current through the brood-chamber.

Sub-Order 3. Valvifera.

The Valvifera, illustrated by the Idotheidae and Arcturidae, are

characterised by the uropods being turned back and expanded to
form folding doors covering up the delicate pleopods, which are
mostly respiratory in function, though the anterior pairs may serve
as swimming organs. Arcturus is a typically deep sea genus, many
species, remarkably furnished
with spiny processes, having been
taken by the Challenger in the
southern hemisphere. The
Idotheidae are more littoral
forms, several species of Idothea
being commonly met with off the
British coasts, occasionally
penetrating into brackish or even
fresh water.

Fig. 83.—Munnopsis typica

(Munnopsidae), ♂ , × 2. A, 2nd
antenna; Ab, abdomen; T, 5th thoracic
appendage or 4th leg. (After Sars.)

Sub-Order 4. Asellota.

In this group the abdominal segments are fused dorsally to form a

shield-like caudal region; the pleopods are respiratory in function
and reduced in numbers, the first pair being often expanded and
produced backwards to form an operculum covering the rest. Several
of the Asellota are fresh-water, Asellus aquaticus (Asellidae) being
extremely abundant all over Europe in weed-grown ditches, the mud
of slowly-moving streams, and even on the shores of large lakes.
They are mostly sluggish in habit, but the marine Munnopsidae
(Fig. 83, Munnopsis) are expert swimmers, the swimming organs
being fashioned by the expansion and elongation of the thoracic legs.

Sub-Order 5. Oniscoida.
 The Oniscoida[104] are terrestrial forms in which the abdomen is
fully segmented, the pleopods are respiratory, their endopodites
being delicate branchiae, while their exopodites are plate-like and
form protective opercula for the gills, and the uropods are biramous
and not expanded. The epimera of the segments are greatly
produced. The terrestrial Isopods, although air-breathers,[105] are
dependent on moisture, and are only found in damp situations. It
seems probable that they have been derived from marine Isopods,
since the more generalised of them, e.g., Ligia (Fig. 84), common on
the English coasts, are only found in damp caves and crannies in the
rocks.

Fig. 84.—Ligia oceanica, ventral and dorsal views, × 1. (From

original drawings prepared for Professor Weldon.)

The related Ligidium is found far inland, but always in the

neighbourhood of water. These two genera may be distinguished by
the numerous joints in the flagellum of the second antennae, the
flagellum being in all cases the portion of the antenna succeeding the
long fifth joint. Philoscia muscorum occurs usually near the coast,
but it is also found inland in England under trees in damp moss. This
genus and the common Oniscus, found in woods, are distinguished
by the presence of three joints in the flagellum of the second
antenna. Philoscia can be distinguished from Oniscus by its
narrower body and the pretty marbled appearance of its back. The
genus Trichoniscus has four joints in the flagellum; various species
are found in woods. In Porcellio and Armadillidium there are only
two joints in the flagellum, while Armadillidium, the common
garden wood-louse, can be distinguished from all others by the
flattened shape of the uropods, and the habit of rolling up into a ball
like an Armadillo.
There is also a very peculiar species, Platyarthrus hoffmannseggii,
which occurs in England and Northern Europe, and always lives in
ants’ nests. It is supposed that they serve as scavengers for the ants,
which tend them carefully, and evidently treat them as domestic
animals of some kind. The small creature is quite white and blind,
and has exceedingly short antennae.

Sub-Order 6. Epicarida.

The Epicarida include an immense number of Isopods, parasitic

upon other Crustacea. In the adult state they become greatly
deformed, and offer very few characters of classificatory value, but
they all pass through certain highly characteristic larval stages which
are essentially similar in the different families. All the species are
protandric hermaphrodites, each individual being male while in a
larval state, and then losing its male organisation and becoming
female as the parasitic habit is assumed.
Two series of families are recognised according to the larval stages
passed through, the Cryptoniscina, in which the adult male
organisation is assumed in the Cryptoniscus stage, and the female
condition is imposed directly upon this form, and the Bopyrina, in
which the Cryptoniscus passes into a further larval stage, the
Bopyrus, which performs the function of the male, and upon which
the female organisation is imposed as the parasitic habit is assumed.
The following is a list of the Epicarida with the Crustacea which
serve as their hosts[106]:—
 Microniscidae on Copepoda.
Cryptoniscidae on Ostracoda.
Liriopsidae on Rhizocephala.
Cryptoniscina Hemioniscidae on Cirripedia.
Cabiropsidae on Isopoda.
Podasconidae on Amphipoda.
Asconiscidae on Schizopoda.

Dajidae
Phryxidae
Bopyrina on Decapoda
Bopyridae
Entoniscidae
In all cases the first larval form
which hatches out from the
maternal brood-pouch is called
the Epicaridian larva (Fig. 85).
This little larva has two pairs of
antennae, a pair of curious frontal
processes, and a pair of
mandibles. The other mouth-
parts are missing; there are only
six thoracic limbs, but the full
complement of six biramous
pleopods are present, and at the
end of the body there may be a
long tube of unknown function.
Fig. 85.—Epicaridian larva, probably As a type of the
belonging to one of the Cryptoniscina. Cryptoniscina we may take the
A, 2nd antenna; Ab, abdominal Liriopsidae,[107] parasitic on the
appendages; T, thoracic appendages. Rhizocephala, which are, of
(From Bonnier, after Hansen.)
course, themselves parasitic on
the Decapoda, the whole
association forming a very remarkable study in Carcinology.
Almost every species of the Rhizocephala is subject to the attacks
of Liriopsids, the latter fixing either on the Rhizocephala themselves,
or else on the Decapod host at a point near the fixation of the
Rhizocephalous parasite. An exceedingly common Liriopsid is
Danalia curvata, parasitic on Sacculina neglecta, which is itself
parasitic on the spider-crab, Inachus mauritanicus, at Naples. The
adult Danalia is a mere curved bag full of eggs or developing
embryos, and without any other recognisable organs except two pairs
of spermathecae upon the ventral surface where the spermatozoa
derived from the larval males are stored.
In Fig. 86 is represented a
female of Inachus mauritanicus
which carried upon it two
Sacculinae and a Danalia
curvata, and upon the latter are
seen two minute larval males in
the act of fertilising the adult
Danalia. The eggs develop into
the Epicaridian stage, after which
the larva passes into the
Cryptoniscus stage (Fig. 87). In Fig. 86.—Inachus mauritanicus, ♀, × 1,
this larval form the segments are carrying two Sacculina neglecta (a, b),
clearly delimited; the only mouth- and a Danalia curvata (c), the latter
bearing two dwarf males.
parts present are the mandibles,
but there are seven pairs of
thoracic limbs and the full number of pleopods. This Cryptoniscus
stage is found in all the Epicarida, and only differs in detail in the
various families.
In the Cryptoniscina the Cryptoniscus larva is the male, and at this
stage possesses a pair of large testes in the thorax. The ovaries are
also present at this stage as very small bodies applied to the anterior
ends of the testes. The larval males in this state seek out adult fixed
Danaliae and fertilise them; and, when this is accomplished, they
themselves become fixed to the host and begin to develop into the
adult female condition. The limbs are all lost, and out of the mouth
grows a long proboscis (Fig. 88, P), which penetrates the tissues of
the host. The ovaries begin to grow, and a remarkable process of
absorption in the testes takes place. These organs, when fixation
occurs, are never empty of spermatozoa, and are frequently crammed
with them. After fixation some large cells at the interior borders of
the testes begin to feed upon the remains of these organs and to grow
enormously in size and to multiply by amitosis. These phagocytes, as
they really are, attain an enormous size, but they are doomed to
 degeneration, the chromatin
becoming dispersed through the
cytoplasm, and the nuclei
dividing first by amitosis and
then breaking up and
disappearing. As the parasite
grows, the heart at the posterior
end of the body ceases to beat;
the ovaries increase enormously
at the expense of the alimentary
canal, and on the ventral surface
two pairs of spermathecae are
invaginated ready to receive the
spermatozoa of a larval male. In
the adult condition, after
fertilisation has taken place and
the ovaries occupy almost the
whole of the body, the remains of
the phagocytic cells can be seen
on the dorsal surface in a
degenerate state. They evidently
are not used as food, and their
sole function is to make away
with the male organisation when
it has become useless.[108]
In the series Bopyrina, after
Fig. 87.—Ventral view of Cryptoniscus the free-living Epicaridian and
larva of Danalia curvata, ♂, × 25. Cryptoniscus stages, a further
larval state is assumed, called the
Bopyrus, which is the functional
male, and, after performing this function, passes on to the adult
female condition.
The family Bopyridae is parasitic in the branchial chamber of
Decapoda, especially Macrura and Anomura. When one of these
Decapods is infested with an adult Bopyrid the gill-chamber in which
it is situated is greatly swollen, as shown in Fig. 90. A very common
Bopyrid is Bopyrus fougerouxi, parasitic in the gill-chambers of
Palaemon serratus. The Bopyrus larva or functional male has the
 appearance shown in Fig. 91. It
differs from the Cryptoniscus
stage in possessing a rudimentary
pair of anterior thoracic limbs
and seven pairs normally
developed, while the abdominal
limbs are plate-like and branchial
in function. The male can often be
found attached to the female
beneath the last pair of
incubatory lamellae.
Fig. 88.—Side view of Danalia curvata, The adult female condition,
× 15, shortly after fixation and loss of which is assumed after the
larval appendages. A, Alimentary canal; Bopyrid stage is passed through,
E, eye; H, heart; N, phagocytic cells; O,
ovary; P, proboscis.
is illustrated in Fig. 92. The body
acquires a remarkable
asymmetry, due to the unequal
pressure exerted by the walls of
the gill-chamber. The antennae
and mandibles (Fig. 92, B) are
entirely covered up by the largely
expanded maxillipedes; maxillae
are, as usual, entirely absent.
Very large lamellae grow out from
the bases of the thoracic limbs to
form a brood-pouch, and in this
manner the adult condition is
attained.
The final complication in the
life-histories of these Isopoda is
reached by the family
Entoniscidae, which are
parasitic when adult inside the Fig. 89.—Optical section (dorsal view)
thoracic cavity of Brachyura and of Danalia curvata, in the same stage
Paguridae. The cephalothorax of as Fig. 88. A, Alimentary canal; Ec,
ectoderm; H, heart; N, phagocytic cells;
a Carcinus maenas, which O, ovaries; P, proboscis.
contains an adult Portunion
maenadis (P), is shown in Fig. 93.
 The parasite is of a reddish colour
when alive.

Fig. 90.—Galathea intermedia, with a

Pleurocrypta microbranchiata under
its left branchiostegite (B), × 1. (After
Sars.)
Fig. 91.—Ventral view of male Bopyrus
fougerouxi, × 30. A, 1st and 2nd
antennae; T, 8th (last) thoracic
appendage. (After Bonnier.)
 Fig. 92.—Bopyrus fougerouxi. A, Ventral
view of female carrying a male (M)
between her abdominal appendages, × 8;
B, ventral view of part of head of female,
the maxillipedes and the left mandible
having been removed. A.1, A.2, 1st and
2nd antennae; M, male; Mn, right
mandible; Mx, left maxillipede; O,
oostegite; T, left 4th thoracic appendage
or 3rd leg. (After Bonnier.)

The Entoniscidae pass through a free living Epicaridian and

Cryptoniscus stage, and become adult males in the Bopyrus stage. It
is stated, however, by Giard and Bonnier[109] that these individuals,
which actually function as males, never grow up into adult females,
though all the adult females have
passed through a male stage in
which the male genital ducts are
not formed. The
hermaphroditism, therefore, in
these animals at any rate is
absolutely useless from a
reproductive point of view, and
this justifies our looking for some
other explanation of it, such as
was suggested on p. 105.

Fig. 93.—Cephalothorax of Carcinus

maenas, seen from the ventral side,
containing a parasitic Portunion
maenadis (P), × ½. (After Bonnier.)

Fig. 94.—Portunion maenadis, ♀:—A, Young, × 10; B, older, × 5;

C, adult, before the eggs are laid, × 3. A, 2nd antenna; Ab,
abdomen; B, anterior lobe of brood-pouch; B′, its lateral lobe; H,
head; 1, 2, 1st and 2nd incubatory lamellae (oostegites). (After
Giard and Bonnier.)
 The Bopyrus fixes in the gill-chamber of the host and becomes
converted into the adult female by a series of transformations. As
these changes take place it invaginates the wall of the gill-chamber
and pushes its way into the thoracic cavity of the crab, though it lies
all the time enveloped in the invaginated wall of the gill-chamber,
and not free in the body-cavity of the crab. The transformations
which it undergoes are shown in Fig. 94. The body first assumes a
grub-like appearance (A), and two pairs of incubatory lamellae (1, 2)
grow out from the first and second thoracic segments. In the next
stage (B) these lamellae assume gigantic proportions, and four pairs
of branchiae grow out from the abdominal segments (Ab). In the
final stage (C) the incubatory lamellae have further increased in size,
and constitute the main bulk of the body; the enormous mass of eggs
is passed into the incubatory pouch, and all that remains of the rest
of the body is the small head (H) and the abdomen (Ab), furnished
with its branchiae. Communication with the external world is kept
up through an aperture which leads from the brood-pouch into the
gill-chamber of the host, and through this aperture the young are
hatched out when they are developed sufficiently.
The presence of these parasites, although they are never in actual
contact with the internal organs of the crab, calls forth the same
phenomenon of parasitic castration as was observed in the
Rhizocephala. A remarkable association is also found to exist
between the Entoniscidae and Rhizocephala, of such a kind that, on
the whole, a crab infested with a Rhizocephalan is more likely to
harbour an Entoniscid than one without. The explanation of this
association is probably that a crab with a Sacculina inside it is
prevented from moulting as often as an uninfected crab, and, in
consequence, the larval stages of the Entoniscid in the crab’s gill-
chamber are more safely passed through.

Sub-Order 7. Phreatoicidea.[110]

The members of this sub-order, although agreeing with the

Isopoda in the essentials of their anatomy, resemble the Amphipoda
in being rather laterally compressed, and in having the hand of the
first free thoracic limb enlarged and subchelate. The abdomen is
greatly produced laterally by expansions of the segments. In fact, the
shape of the body and of the limbs is very Amphipodan.—
Phreatoicus from New Zealand, Southern Australia, and Tasmania.
Phreatoicopsis,[111] a very large form from Gippsland, Victoria. Only
one family exists, Phreatoicidae.

Order IV. Amphipoda.

In this order the body is flattened laterally, the heart is anterior in

position, and the branchial organs are attached to the thoracic limbs.
There are three well defined sub-orders, (i.) the Crevettina,
including a vast assemblage of very similar animals, of which the
common Gammarus and Orchestia may serve as examples; (ii.) the
Laemodipoda or Caprellids, and (iii.) the Hyperina.
We cannot do more than touch on the organisation of these sub-
orders.

Sub-Order 1. Crevettina.

In this sub-order only one thoracic segment is fused with the head;
the basal joints of the thoracic limbs are expanded to form broad
lateral plates, and the abdomen is well developed, with six pairs of
pleopods, the last three pairs being always turned backwards, and
stiffened to act as uropods.
This group has numerous fresh-water representatives, e.g.
Gammarus of several species, the blind well-shrimp Niphargus, and
the S. American Hyalella; but the vast majority of the species are
marine, and are found especially in the littoral zone wherever the
rocks are covered with a rich growth of algae, Polyzoa, etc. The
Talitridae or “Sand-hoppers” have deserted the waters and live
entirely in the sand and under rocks on the shore, and one common
European species, Orchestia gammarellus, penetrates far inland,
and may be found in gardens where the soil is moist many miles
from the sea.
The Rev. T. R. R. Stebbing, in his standard work[112] on this group,
recognises forty-one families, and more than 1000 species, so that