Module 1 – Chapters 1 & 2

Chapter 1
The Vertebrate Body
 Introduction

Defining characteristics of
chordates: In chordates, four
common features appear at some
point during development: a
notochord, a dorsal hollow nerve cord,
pharyngeal slits, and a post-anal tail.
 Introduction

• These characteristics are only present

during embryonic development in some
chordates.
• The notochord provides skeletal support,
gives the phylum its name, and develops
into the vertebral column in vertebrates.
• The dorsal hollow nerve cord develops
into the central nervous system: the brain
and spine.
 Introduction

• Pharyngeal slits are openings in the

pharynx that develop into gill arches
in bony fish and into the jaw and
inner ear in terrestrial animals.
• The post-anal tail is a skeletal
extension of the posterior end of the
body, being absent in humans and
apes, although present during
embryonic development.
 Learning Objective 1

Identify the key features of

the chordates
 Characteristics of Chordata

Animals in the phylum Chordata

share four key features that
appear at some stage during
their development (often, only
during embryogenesis)
Four Key Characteristics
 Notochord

The chordates are named for the notochord: a

flexible, rod-shaped structure that is found in
the embryonic stage of all chordates and also in
the adult stage of some chordate species. It is
located between the digestive tube and the
nerve cord, providing skeletal support through
the length of the body. In some chordates, the
notochord acts as the primary axial support of
the body throughout the animal’s lifetime.
 Notochord

In vertebrates, the notochord is present during

embryonic development, at which time it
induces the development of the neural tube
which serves as a support for the developing
embryonic body. The notochord, however, is
replaced by the vertebral column (spine) in
most adult vertebrates.
 Dorsal Hollow Nerve Cord

The dorsal hollow nerve cord derives from

ectoderm that rolls into a hollow tube during
development. In chordates, it is located dorsally
(at the top of the animal) to the notochord. In
contrast to the chordates, other animal phyla
are characterized by solid nerve cords that are
located either ventrally or laterally. The nerve
cord found in most chordate embryos develops
into the brain and spinal cord, which comprise
the central nervous system.
 Pharyngeal Slits

Pharyngeal slits are openings in the pharynx (the

region just posterior to the mouth) that extend to
the outside environment. In organisms that live in
aquatic environments, pharyngeal slits allow for the
exit of water that enters the mouth during feeding.
Some invertebrate chordates use the pharyngeal
slits to filter food out of the water that enters the
mouth. In vertebrate fishes, the pharyngeal slits
develop into gill arches, the bony or cartilaginous
gill supports.
 Pharyngeal Slits

In most terrestrial animals, including

mammals and birds, pharyngeal slits
are present only during embryonic
development. In these animals, the
pharyngeal slits develop into the jaw
and inner ear bones.
 Post-anal Tail

The post-anal tail is a posterior elongation of the

body, extending beyond the anus. The tail
contains skeletal elements and muscles, which
provide a source of locomotion in aquatic
species. In some terrestrial vertebrates, the tail
also helps with balance, courting, and signaling
when danger is near. In humans and other
apes, the post-anal tail is present during
embryonic development, but is vestigial as an
adult.
 Review Questions

What is a flexible rodlike structure that forms the

main support of the body in the lowest chordates;
a primitive spine?
a. nerve cord
b. notochord
c. pharyngeal slit
d. post-anal tail
 Review Questions

This is a dorsal tubular cord of nervous tissue

above the notochord of a chordate.
a. nerve cord
b. notochord
c. pharyngeal slit
d. post-anal tail
 Review Questions

The filter-feeding organs found in non-vertebrate

chordates (lancelets and tunicates) and
hemichordates living in aquatic environments is
called
a. nerve chord
b. notochord
c. pharyngeal slit
d. post-anal tail
 Review Questions

The chordates are named for the

a. nerve chord
b. notochord
c. pharyngeal slit
d. post-anal tail
 Review Questions

Pharyngeal slits are openings in the what region

just posterior to the mouth and extend to the
outside environment?
a. liver
b. anus
c. mouth
d. pharynx
 Introduction

• Vertebrata is a subphlyum of Chordata that is

further defined by their bony backbone. As
chordates, vertebrates have the same
common features: a notochord, a dorsal
hollow nerve cord, pharyngeal slits, and a
post-anal tail.
• Vertebrates are further differentiated from
chordates by their vertebral column, which
forms when their notochord develops into the
column of bony vertebrae separated by discs.
 Introduction

• Vertebrata is a subphlyum of
Chordata that is further defined by
their bony backbone. As chordates,
vertebrates have the same common
features: a notochord, a dorsal hollow
nerve cord, pharyngeal slits, and a
post-anal tail.
 Learning Objective 2

Identify the defining

characteristics of vertebrates
 Characteristics of Vertebrates

Vertebrates are members of the subphylum

Vertebrata, under the phylum Chordata and
under the kingdom Animalia. Animals that
possess bilateral symmetry can be divided into
two groups, protostomes and deuterostomes,
based on their patterns of embryonic
development. The deuterostomes, whose name
translates as “second mouth,” consist of two
phyla: Chordata and Echinodermata.
 Characteristics of Vertebrates

Echinoderms are invertebrate marine

animals that have pentaradial symmetry
and a spiny body covering; the phylum
includes sea stars, sea urchins, and sea
cucumbers. The phylum Chordata
contains two groups of invertebrate
chordates, but the most conspicuous and
familiar members of Chordata are the
vertebrates.
 Phylum Chordata

All chordates are deuterostomes, possessing a notochord. Vertebrates are differentiated by

having a vertebral column.
 Phylum Chordata

As chordates, all vertebrates have a similar

anatomy and morphology with the same
qualifying characteristics: a notochord, a
dorsal hollow nerve cord, pharyngeal slits, and
a post-anal tail. However, the subphylum
Vertebrata is distinguished from the phylum
Chordata by the development of the
notochord into a bony backbone. Vertebrates
include the amphibians, reptiles, mammals,
and birds, as well as the jawless fishes, bony
fishes, sharks, and rays.
Diversity of Vertebrates: Animals with
Backbones

The subphylum Vertebrata contains all

animals that possess backbones, gills,
and a central nervous system in at
least one phase of development.
Vertebrates include amphibians,
reptiles, mammals, and birds, as well
as the jawless fishes, bony fishes,
sharks, and rays.
Diversity of Vertebrates: Animals with
Backbones

More than 64,000 species of

vertebrates have been described, but
the extant vertebrate species
represent only a small portion of all
the vertebrates that have existed.
Diversity of Vertebrates: Animals with
Backbones

Vertebrates range in size from the frog

species Paedophryne amauensis (as small
as 7.7 mm (0.3 inch)) to the blue whale
(as large as 33 m (110 ft)). Vertebrates
comprise about 4 percent of all described
animal species; the remainder are
invertebrates, which lack backbones.
 Anatomy and Morphology

All vertebrates are built along the basic chordate

body plan: a stiff rod running through the
length of the animal (vertebral column), with a
hollow tube of nervous tissue (the spinal cord)
above it and the gastrointestinal tract below. In
all vertebrates, there is a mouth at anterior end
of the animal and an anus before the posterior
end of the body. There is a tail posterior to the
anus during at least one phase of the animal’s
development.
 The Vertebral Column

Vertebrates are defined by the presence of the

vertebral column. In vertebrates, the notochord
develops into the vertebral column or spine: a
series of bony vertebrae each separated by
mobile discs. These vertebrae are always found
on the dorsal side of the animal. However, a few
vertebrates have secondarily lost their vertebrae
and, instead, retain the notochord into
adulthood (e.g., the sturgeon fish).
 Vertebral Column

A fossilized skeleton of the dinosaur Diplodocus carnegii shows an extreme example of the
backbone that characterizes vertebrates.
 Central Nervous System

Vertebrates are also the only members of

Chordata to possess a brain. In chordates, the
central nervous system is based on a hollow
nerve tube that runs dorsal to the notochord
along the length of the animal. In vertebrates,
the anterior end of the nerve tube expands and
differentiates into three brain vesicles.
 Vertebrate Classification

Vertebrates are the largest group of

chordates, with more than 62,000 living
species. Vertebrates are grouped based
on anatomical and physiological traits. The
traditional groups include Agnatha,
Chondrichthyes, Osteichthyes, Amphibia,
Reptilia, Aves, and Mammalia.
 Vertebrate Classification
Animals that possess jaws are known as
gnathostomes, meaning “jawed mouth.”
Gnathostomes include fishes and
tetrapods (amphibians, reptiles, birds, and
mammals). Tetrapods can be further
divided into two groups: amphibians and
amniotes.
 Vertebrate Classification

Amniotes are animals whose eggs are adapted

for terrestrial living; this group includes
mammals, reptiles, and birds. Amniotic
embryos, developing in either an externally-
shelled egg or an egg carried by the female,
are provided with a water-retaining
environment and are protected by amniotic
membranes.
 Review Questions

Vertebrates are further differentiated from

chordates by a part, which forms when their
notochord develops into the column of bony
vertebrae separated by discs.
a. nerve cord
b. vertebral column
c. notochord
d. pharyngeal slit
 Review Questions

What are animals whose eggs are adapted for

terrestrial living?
a. amniotes
b. anamiotes
c. terrestrial
d. poikilothermus
 Review Questions

This is a member of the phylum Chordata,

numerous animals having a notochord at some
stage of their development.
a. echinoderm
b. tunicate
c. lancelet
d. chordate
 Review Questions

Gnathostomes include the following, except:

a. milkfish
b. frog
c. snake
d. ostracoderm
 Review Questions

In chordates, the ____ is based on a hollow

nerve tube that runs dorsal to the notochord
along the length of the animal. In vertebrates,
the anterior end of the nerve tube expands and
differentiates into three brain vesicles.
a. sympathetic nervous system
b. central nervous system
c. parasympathetic nervous system
d. Reproductive system
 Introduction

Chordata contains two subphylums of

invertebrates: Urochordata (tunicates) and
Cephalochordata (lancelets).
• Urochordata (tunicates) and Cephalochordata
(lancelets) are invertebrates because they lack a
backone.
• Larval tunicates (Urochordata) posses all four
structures that classify chordates, but adult
tunicates retain only pharyngeal slits.
 Introduction

• Larval tunicates swim for a few days after

hatching, then attach to a marine surface and
undergo metamorphosis into the sessile adult
form.
• Lancelets (Cephalochordata) are marine
organisms that possess all features of
chordates; they are named Cephalochordata
because the notochord extends into the head.
• Lancelets may be the closest-living relatives to
vertebrates.
 Learning Objective 3

Describe the features and

phylogenetic history of
hemichordata,
cephalochordata and
urochordata
Chordates and the Evolution of Vertebrates

The most familiar group of chordates is the

vertebrates. However, in addition to the
subphylum Vertebrata, the phylum Chordata also
contains two subphyla of invertebrates:
Hemichordata, Urochordata and Cephalochordata.
Members of these groups also possess the four
distinctive features of chordates at some point
during their development: a notochord, a dorsal
hollow nerve cord, pharyngeal slits, and a post-
anal tail. Unlike vertebrates, urochordates and
cephalochordates never develop a bony backbone.
 Hemichordata

Hemichordates are distinguished by

a tripartite (threefold) division of the body. At
the forward end of the body is a preoral lobe,
behind this is a collar, and last comes a trunk.
The name "hemichordate" means "half
chordate," and hemichordates share some (but
not all) of the typical chordate characteristics.
The larva is ciliated and is named as tornaria.
 Hemichordata

There are branchial openings, or "gill slits,"

that open into the pharynx; there is a
rudimentary structure in the collar region,
the stomochord, that is similar to
a notochord; and there is a dorsal nerve cord,
in addition to a smaller ventral nerve cord.
However, hemichordates are not classified as
true chordates, although they are quite closely
related.
 Hemichordata
Some DNA-based studies of evolution
suggest that hemichordates are
actually closer to echinoderms than to
true chordates. This is supported by
the fact that the larvae of at least
some hemichordates look very much
like those of some echinoderms.
 Hemichordata

Of the three classes of hemichordates, the

most familiar living ones are
the Enteropneusta, the acorn worms. The
triple division of the body is obvious. Acorn worms also have
multiple branchial openings, as many as 200 in some species.
They are slow burrowers, using the proboscis to burrow
through sediment, and may either deposit feed (consume
sediment and digest the organic matter, rather like
earthworms in soil) or suspension feed (collect suspended
particles from the water). Some of these worms may be small
(10-40 cm) very large; one species may reach a length of 2.5
meters (almost eight feet), although most are much smaller.
Acorn worms, also known
as tongue worms
 Hemichordata
The second living class is
the Pterobranchia, an obscure group
with only about 20 living species.
Pterobranchs as very different from acorn
worms; they form colonies in which the
individuals are interconnected by stems,
or stolons. Individuals, or zooids, are
often less than 1 millimeter long. The
proboscis is not elongated, as it is in
acorn worms, but shield-shaped.
 Hemichordata
The second division of the body bears a pair of
branched tentacles that collect small food
particles from the water. There is only one
branchial opening. Most strikingly, almost all
pterobranch species create and live within a
network of tubes, the coenecium. These
tubes are made up of the protein collagen,
secreted by special glands in the proboscis.
Yet similar larvae and a similar tripartite body
plan unite the enteropneusts and
pterobranchs.
 Pterobranchs

This diagram shows a colony of pterobranchs in the

genus Rhabdopleura.
 Hemichordata
The third class of hemichordates,
the Graptolithina or graptolites, has a
checkered history. Graptolites are
common fossils in Ordovician and
Silurian rocks, but for a long time no one
was sure what kind of animals they were.
 Hemichordata
Most fossil graptolites look like nothing
so much as tiny sawblades. However,
well-preserved graptolites can be seen
to be tubular in cross-section, with
the "teeth of the saw" formed by
short open branches from the main
tube. Careful study of the microscopic
structure of the tubes of graptolites
showed that they are very similar to
the tubes of pterobranchs.
 Hemichordata
Of the three classes of hemichordates,
the most familiar living ones are
the Enteropneusta, the acorn
worms. The triple division of the body is obvious. Acorn
worms also have multiple branchial openings, as many as 200 in
some species. They are slow burrowers, using the proboscis to
burrow through sediment, and may either deposit feed (consume
sediment and digest the organic matter, rather like earthworms in soil)
or suspension feed (collect suspended particles from the water).
Some of these worms may be very large; one species may reach a
length of 2.5 meters (almost eight feet), although most are much
smaller.
Graptolithina
 Review Questions

In the hemichordates, the process of fertilization

is:
a. External
b. Internal
c. Middle
d. None of the above
 Review Questions

The phylum Hemichordata share characteristics

with:
a. Chordata only
b. Echinodermata
c. Chordata and Echinodermata
d. Annelid
 Review Questions

The larva of hemichordate is ciliated and is

named:
a. tornalia
b. planula
c. trocophore
d. None of the above
 Review Questions

A small size hemichordate is approximately”

a. 2-9 cm
b. 10-40 cm
c. 50-65 cm
d. 20-30 cm
 Review Questions

Acorn worm belongs to sub-phylum:

a. Cephalochordata
b. Urochordata
c. Hemichordata
d. Vertebrata
 Urochordata

Members of Urochordata are also known as

tunicates. The name tunicate derives from
the cellulose-like carbohydrate material,
called the tunic, which covers the outer
body of tunicates. Although tunicates are
classified as chordates, only the larval form
possesses all four common structures.
Adults only maintain pharyngeal slits and
lack a notochord, a dorsal hollow nerve
cord, and a post-anal tail.
 Urochordates

(a) This photograph shows a colony of the tunicate Botrylloides violaceus. (b) The larval stage of
the tunicate possesses all of the features characteristic of chordates: a notochord, a dorsal
hollow nerve cord, pharyngeal slits, and a post-anal tail. (c) In the adult stage, the notochord,
nerve cord, and tail disappear.
Sea Squirt
Sea Squirt
 Urochordates

Most tunicates are hermaphrodites. Tunicate

larvae hatch from eggs inside the adult
tunicate’s body. After hatching, a tunicate larva
swims for a few days until it finds a suitable
surface on which it can attach, usually in a dark
or shaded location. It then attaches via the
head to the surface and undergoes
metamorphosis into the adult form, at which
point the notochord, nerve cord, and tail
disappear.
 Urochordates

Most tunicates live a sessile existence on the

ocean floor and are suspension feeders. The
primary foods of tunicates are plankton and
detritus. Seawater enters the tunicate’s body
through its incurrent siphon. Suspended
material is filtered out of this water by a
mucous net (pharyngeal slits) and is passed
into the intestine via the action of cilia. The
anus empties into the excurrent siphon, which
expels wastes and water. Tunicates are found
in shallow ocean waters around the world.
 Review Questions

The anus of sea tunicate empties into the what

part of its body, which expels wastes and water.
a. Incurrent siphon
b. Excurrent siphon
c. Mouth
d. Mantle
 Review Questions

Tunicates are example of sub-phylum:

a. Hemichordata
b. Cephalochordata
c. Urochordata
d. None of the above
 Review Questions

The heart of the tunicates lies at the base of:

a. buccal cavity
b. pharynx
c. sinus gland
d. left side of the body
 Review Questions

The blood plasma of tunicates is:

a. red
b. blue
c. brown
d. colorless
 Review Questions

The characteristics of tunicates is having a tail and

a
a. notochord
b. buccal cavity
c. spines
d. backbone
 Cephalochordata

Members of Cephalochordata possess a

notochord, dorsal hollow nerve cord, pharyngeal
slits, and a post-anal tail in the adult stage.
They do not have a true brain, but the
notochord extends into the head, which gives
the subphylum its name ( “cephalo” is Greek for
head). Extinct members of this subphylum
include Pikaia, which is the oldest known
cephalochordate. Pikaia fossils were recovered
from the Burgess shales of Canada and dated to
the middle of the Cambrian age, making them
more than 500 million years old.
 Cephalochordata
Extant members of Cephalochordata are the
lancelets, named for their blade-like shape.
Lancelets are only a few centimeters long and
are usually found buried in sand at the bottom
of warm temperate and tropical seas. Like
tunicates, they are suspension feeders. With
notochord and paired muscle blocks, the
lancelet and Pikaia may belong to the chordate
group of animals from which the vertebrates
have descended.
 Cephalochordates

The lancelet, like all cephalochordates, has a head. Adult lancelets retain the four key features of chordates:
a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail. Water from the mouth
enters the pharyngeal slits, which filter out food particles. The filtered water then collects in the atrium
and exits through the atriopore.
Amphioxus
 Review Questions

Water from the mouth of the lancelet enters

the_________, which filter out food particles.
a. notochord
b. pharyngeal slits
c. buccal cavity
d. post-anal tail
 Review Questions

Cephalochordates lack
a. liver
b. kidney
c. true heart
d. tail
 Review Questions

The cephalochordates have a reduced coelom

compared to other
a. chordates
b. animals
c. annelids
d. platyhelminthes
 Review Questions

The size of a smaller cephalochordate is

a. 2 cm long
b. 5 cm long
c. 7 cm long
d. 9 cm long
 Review Questions

Cephalochordates include
a. 3 genera
b. 2 genera
c. 5 genera
d. 4 genera
 Introduction

Both genomic and fossil evidence suggests that

vertebrates evolved from craniates, which
evolved from invertebrate chordates.
• The clade Craniata includes animals that have a
cranium: a bony, cartilaginous, or fibrous
structure that surrounds the brain, jaw, and
facial bones.
• Members of Craniata include the vertebrates
and hagfish.
 Introduction

• Genomic evidence suggests that vertebrates

diverged from cephalochordates (lancelets),
which had previously diverged from
urochordates (tunicates).
• Fossil evidence suggests that most vertebrate
diversity originated in the Cambrian explosion
540 million years ago.
• Two whole- genome duplications occurred in
early vertebrate history.
 Craniata and Vertebrata

The clade Craniata is a subdivision of Chordata.

Members of Craniata posses a cranium, which is
a bony, cartilaginous, or fibrous structure
surrounding the brain, jaw, and facial bones.
The clade Craniata includes all vertebrates and
the hagfishes (Myxini), which have a cranium
but lack a backbone. Hagfish are the only
known living animals that have a skull, but not a
vertebral column.
 Hagfish

Although it lacks a backbone, the hagfish is a member of the

Craniata clade because it possesses a bony skull
 Clade Craniata

Craniata, including this fish (Dunkleosteus), are characterized by

the presence of a cranium, mandible, and other facial bones
 Vertebrates

Vertebrates are members of the subphylum

Vertebrata, the clade Craniata, and the phylum
Chordata. Vertebrates display the four
characteristic features of chordates, but they
are named for the vertebral column composed
of a series of bony vertebrae joined together as
a backbone. In adult vertebrates, the vertebral
column replaces the embryonic notochord.
 Vertebrates

Vertebrata are characterized by the presence of a backbone,

such as the one that runs through the middle of this fish. All
vertebrates are in the Craniata clade and have a cranium.
 Vertebrate Evolution

In the phylum Chordata, the closest

relatives of the vertebrates are the
invertebrate chordates. Based on the
molecular analysis of vertebrate and
invertebrate genomes (genomics),
scientists can determine the
evolutionary history of different
phylogenetic groups.
 Vertebrate Evolution
According to these genomic analyses, vertebrates
appear to be more closely related to the
lancelets (cephalochordates) than to the
tunicates (urochordates). This suggests that the
cephalochordates first diverged from
urochordates, and that vertebrates subsequently
diverged from the cephalochordates. This
hypothesis is further supported by the fossil of a
530 million-year-old organism with a brain and
eyes like a vertebrate, but without the skull
found in a craniate.
 Vertebrate Evolution
A comparison of the genomes of a
lancelet, tunicate, lamprey, fish,
chicken, and human confirmed that
two whole-genome duplications
occurred in the early history of the
Vertebrata subphylum.
 Vertebrate Evolution
Both fossil and genomic evidence suggests that
vertebrates arose during the Cambrian
explosion. The Cambrian explosion was the
relatively brief span of time during the
Cambrian period during which many animal
groups appeared and rapidly diversified. Most
modern animal phyla originated during the
Cambrian explosion.
 Review Questions

Describe organism under clade Craniata.

Why are vertebrates appeared to be more

closely related to the lancelets
(cephalochordates) than to the tunicates
(urochordates)? Explain.

What are the common characteristics of the

members vertebrates and hagfish?
Chapter 1 Quiz
 Chapter 2
Agnathans and
Gnathostomata
 Agnathans vs Gnathostomes

Agnathans
Jawless vertebrates
A= no + Gnatho= jaws
Examples: lamprey & hagfish
They have sucker disc mouths
used to stick to prey to
ingest their insides
Gnathostomes
Modern fish; jawed fish
Gnathos= jaws + Stoma=
mouth
1 appeared in the late Silurian
st
period of the Paleozoic Era
over 400 m.y.a.
Not only acquired jaws, but
also developed paired
nostrils, reduced number of
gill slits, and a series of
pharyngeal arches
 Introduction

The superclass Agnatha describes fish

that lack jaws and includes the extant
species of hagfish and lampreys.
• Most agnathans are now extinct, but
two branches exist today: hagfishes
(not true vertebrates) and lampreys
(true vertebrates).
 Introduction

• The earliest jawless fishes were the

ostracoderms, which had bony scales
as body armor.
• Hagfish are eel-like marine
scavengers in the clade Myxini that
produce slime and can tie themselves
into knots.
 Introduction

• Lampreys are in the clade

Petromyzontidae and appear
morphologically similar to hagfish, but
contain cartilaginous vertebral
elements as an adult; thus, they are
considered true vertebrates.
 Learning Objectives 1 & 2

Differentiate between the taxa of

jawless fishes
Differentiate between jawless and
jawed fishes
 Agnathans: Jawless Fishes

Jawless fishes or agnathans are

craniates that represent an ancient
vertebrate lineage that arose over one
half-billion years ago. “Gnathos” is
Greek for “jaw” and the prefix “a”
means “without,” so agnathans are
“without jaws. ”
 Agnathans: Jawless Fishes
Most agnathans are now extinct, but two
branches still exist today: hagfishes and
lampreys. Hagfishes and lampreys are
recognized as separate clades, primarily
because lampreys are true vertebrates,
whereas hagfishes are not. A defining
feature of agnathans is the lack of paired
lateral appendages or fins.
 Agnathans: Jawless Fishes

Some of the earliest jawless fishes

were the ostracoderms (Greek for
“bone-skin”). Ostracoderms were
vertebrate fishes encased in bony
armor, unlike present-day jawless
fishes, which lack bone in their
scales.
 Ostracoderms

Ostracoderms were some of the earliest jawless fishes and were covered
in bony armor. Present-day jawless fishes lack bone in their scales.
 Myxini: Hagfishes
The clade Myxini includes at least 20
species of hagfishes. Hagfishes are
eel-like scavengers that live on the
ocean floor and feed on dead
invertebrates, other fishes, and
marine mammals.
 Myxini: Hagfishes
Hagfishes are entirely marine and are
found in oceans around the world,
except for the polar regions. Hagfish
have slime glands beneath the skin
that constantly release mucus,
allowing them to escape from the
grip of predators.
 Myxini: Hagfishes

Hagfish can also twist their bodies

into a knot to gain a mechanical
advantage while feeding and are
notorious for eating carcasses
from the inside out.
 Hagfish

Pacific hagfish are scavengers that live on the ocean floor.

These agnathans are classified as Myxini and do not have a
vertebral column.
 Myxini: Hagfishes
The skeleton of a hagfish is composed of
cartilage, which includes a cartilaginous
notochord that runs the length of the
body. This notochord provides support to
the hagfish’s body. Unlike true
vertebrates, hagfishes do not replace the
notochord with a vertebral column during
development. Since they have a
cartilaginous skull, they are classified in
the clade Craniata.
 Petromyzontidae: Lampreys

The clade Petromyzontidae includes

approximately 35–40 or more species of
lampreys. Lampreys are morphologically similar
to hagfishes and also lack paired appendages.
However, lampreys develop some vertebral
elements as an adult. Their notochord is
surrounded by a cartilaginous structure called
an arcualia, which may resemble an
evolutionarily-early form of the vertebral
column.
 Petromyzontidae: Lampreys

As adults, lampreys are

characterized by a toothed,
funnel-like sucking mouth. Many
species have a parasitic stage of
their life cycle during which they
are ectoparasites of fishes.
 Petromyzontidae: Lampreys
Lampreys live primarily in coastal and fresh
waters. They are distributed worldwide,
except for the tropics and polar regions.
Some species are marine, but all species
spawn in fresh water; eggs are fertilized
externally. The larvae differ distinctly from
the adult form, spending 3 to 15 years as
suspension feeders. Once they reach
sexual maturity, the adults die within days
of reproduction.
 Parasitic Lamprey

These parasitic sea lampreys attach to their lake trout host by

suction and use their rough tongues to rasp away flesh in
order to feed on the trout’s blood.
 Review Questions

Differentiate jawless fishes from jawed

fishes.
Describe: hemichordates, urochordates and
cephalochordates.
Illustrate the relationship of hemichordates,
urochordates and cephalochordates to
jawed fishes.
 Gnathostomes - Jawed Fishes

Gnathostomes or “jaw-mouths” are

vertebrates that possess jaws. One of
the most significant developments in
early vertebrate evolution was the
development of the jaw, which is a
hinged structure attached to the
cranium that allows an animal to grasp
and tear its food.
 Gnathostomes - Jawed Fishes
The evolution of jaws allowed early
gnathostomes to exploit food resources
that were unavailable to the jawless
animals. In early evolutionary history,
there were gnathostomes (jawed fishes)
and agnathans (jawless fishes).
 Gnathostomes - Jawed Fishes

In early evolutionary history, there

were gnathostomes (jawed fishes)
and agnathans (jawless fishes).
Gnathostomes later evolved into all
tetrapods (animals with four limbs)
including amphibians, birds, and
mammals.
 Gnathostomes - Jawed Fishes

Early gnathostomes were jawed fishes that

possessed two sets of paired fins, which
increased their ability to maneuver accurately.
These paired fins were pectoral fins, located
on the anterior body, and pelvic fins, on the
posterior. The evolution of the jaw combined
with paired fins permitted gnathostomes to
expand from the sedentary suspension
feeding of jawless fishes and become mobile
predators.
 Placoderms

Dunkleosteous was an enormous placoderm from the Devonian period, 380–360

million years ago. It measured up to 10 meters in length and weighed up to 3.6
tons. As gnathostomes, they were more mobile and could exploit more food
resources than the agnathostomes.
 Review Questions

What are the characteristics of

Gnathostomes?
Compare gnathostomes from
agnathans.
Differentiate placoderm from
ostracoderm.
Chapter 2 Quiz
 References

Bognot, Filipinas L. (2018). Laboratory Manual for Laboratory Guide and Manual for Comparative Vertebrate Anatomy.
Angeles City.

Campbell, Neil and Jackson, Robert. (2015). Biology A Global Approach Volume 1.Singapore: Pearson Education South Asia Pte.
Ltd.

Campbell, Neil and Jackson, Robert. (2015). Biology A Global Approach Volume 2.Singapore: Pearson Education South Asia Pte.
Ltd.

Gacelo, Ester P. (2007). Biological Science with Practical Approach. Manila: C & E Publishing, Inc.

Hill, Richard W., Wyse, Gordon et al. (2016). Animal Physiology. New York: Mc-Graw Hill Education

Kandong, Kenneth V. (2018). Comparative, Anatomy, Function and Evolution. New York: Mc-Graw Hill Education

Kent, George C. (1997). Comparative Anatomy of the Vertebrates. Seattle, US: Golden Rule Book

Kent, George C. & Carr, Robert. (2015). Comparative Anatomy of the Vertebrates. 9th Edition. Seattle, US: Golden Rule Books