COVENANT UNIVERSITY

COLLEGE OF SCIENCE AND TECHNOLOGY

DEPARTMENT OF BIOLOGICAL SCIENCES

Programme: Applied Biology and Biotechnology

Course Code: BLY 223

Course Title: BASIC Chordate Zoology

Units: 3

Course Lecturer: Ejoh S.A

Course Coordinator: Ayanda O.I

Semester/Session: Omega, 2013/2014

Course Outline

Module I: Introduction to Chordate Zoology

Week 1 General characters and Taxonomical

classification of Phylum Chordata up to orders

- Origin of Chordata.

Week 2 Functional morphology, general diversity and

adaptations of the types included with special

emphasis on the adaptations to their mode of

life and environment.

Module II: Pro-Chordata and Agnatha:

Week 3 General characters, classification and

evolutionary significance - Type study:

Balanoglossus, Amphioxus, Ascidians

Week 4 Affinities and systematic position of Sub

phylum Hemichordata, Cephalochordata and

Urochordata

Week 5 Agnatha :General characters and affinities -

Type study - Petromyzon

Module III: Sub Phylum Vertebrata: Class Pisces:

Week 6 Class Pisces: General characters, classification

and evolutionary significance - Origin of fishes

– Type study: Scoliodon, Tilapia

Week 7 Types of scales and fins - Accessory

respiratory organs - Air bladder - Parental care

- Migration - Economic importance.

INTRODUCTION TO PHYLUM CHORDATA

The Phylum Chordata includes the well-known vertebrates

(fishes, amphibians, reptiles, birds, mammals). The

vertebrates and hagfishes together comprise the taxon

Craniata. The remaining chordates are the tunicates

(Urochordata), lancelets (Cephalochordata), and, possibly,

some odd extinct groups.

With few exceptions, chordates are active animals with

bilaterally symmetric bodies that are longitudinally

differentiated into head, trunk and tail.

Chordates are well represented in marine, freshwater and

terrestrial habitats from the Equator to the high northern and

southern latitudes.

Characteristics

The three most important characteristics of phylum chordate

are:

1. They possess a notochord either throughout or during

early embryonic development.

2. Dorsal hallow nerve chord.

3. Paired gill slits on the sides of pharynx

1. Notochord

The notochord is a flexible rod-like structure, which is

enclosed by a fibrous sheath and extends the length of the

body in larva and/or adult.

It is first part of skeleton to form in embryo

In primitive chordates the notochord persists through life

while in higher chordates, the notochord is replaced by a

vertebral column of bone. The remnants of the notochord

remain as ―intervertebral discs‖

Functions

Provides basic support and serves as main axis for

muscle attachments to permit ―fishlike‖ undulatory

movements

In some non-vertebrate chordates and fishes the

notochord persists as a laterally flexible but

incompressible skeletal rod that prevents telescopic

collapse of the body during swimming.

In most chordates the notochord is replaced by a

vertebral column of bone.

Remnants of the notochord remain as ―intervertebral

discs‖

2. Dorsal hallow nerve cord

A fluid-filled tube of nerve tissue that runs the length nerve

cord is ventral & paired

Present throughout embryonic and adult life

Functions

In chordates the nerve cord is a single dorsal hollow

nerve cord front end usually enlarged to form brain

3. Pharyngeal (gill) slits

Pharyngeal gills are slit-like opening sliding from throat to

outside

In some groups they are only found in embryo and lost as

adults

Functions

It was first evolved as a filter feeding apparatus

It is still used by some organisms to filter water for food

and in others as gills

4. Endostyle or thyroid gland

This is a specific kind of tissue found only in chordates.

Functions

It was originally part of the feeding apparatus

The endostyle secretes mucus and traps food inside the

pharyngeal cavity. Eg. lamprey larva

In most chordates the same tissue has become an

endocrine

This gland in the neck region that helps control

metabolism

5. Post-anal tail

The post- anal tail in aquatic chordates it provides motility, it

was designed for propulsion in water, especially for larval

forms and fish

Fish later gained fins to increase efficiency

In terrestrial chordates it became a tail for leverage and

balance, not for movement

In humans ―tailbones‖ is its remnant

Additional, more variable, Chordate characteristics are also

shared by many or most species:

6. Bilateral symmetry

7. Tube within a tube body plan

8. Segmented muscles called myomeres or myotomes in fish

9. Cephalization

10.Ventral heart

11. Endoskeleton: most members have an internal skeleton of

cartilage and/or bone

<5% of all animals that have ever lived have backbones

CLASSIFICATION OF PHYLUM CHORDATA

The phylum Chordata is sub-divided into three sub- phyla

1. Sub phylum Urochordata or Tunicata

2. Sub-phylum Cephalochordata or Lancelet

3. Sub phylum Vertebrata or Crainiata

Closest Relatives

Hemichordates ("half chordates") have some features similar

to those of chordates: branchial openings that open into

the pharynx and look rather like gill slits; stomochords,

similar in composition to notochords, but running in a circle

round the "collar", which is ahead of the mouth; and a dorsal

nerve cord — but also a smaller ventral nerve cord.

There are two living groups of hemichordates.

The solitary enteropneusts, commonly known as "acorn

worms‖

Pterobranchs are colonial animals

Sub phylum Urochordata or Tunicata

Sub-phylum Urochordata or Tunicata are strictly marine

There are roughly 1,600 species of urochordates; most

are small solitary animals but some are colonial,

organisms. Nearly all are sessile as adults but they have

free-swimming, active larval forms.

The notochord is restricted to the tail region of the larva

and disappears in the adult

Urochordates are unknown as fossils.

Examples – Ascidia(sea squirt), salpa and doliolum

Sub-phylum Cephalochordata or Lancelet

This sub-phylum contains only about 20 species of sand-

burrowing marine creatures.

The notochord extends throughout the length of the

animal dorsally

Cephalochordata include the amphioxus and lancelets.

Sub phylum Vertebrata or Crainiata

Most complex/advanced group of animals-one

explanation for their diversity and dominance is that, in

general, vertebrate species have a lot more genes

o flies

o

o

Fish became the 1st true vertebrates

Notochord is replaced in the adult either wholly or partly

by a cartilaginous or bony segmented vertebral column

HEMICHORDATA

Balanoglossus "acorn worms",

Balanoglossus is an ocean-dwelling acorn worm

(Enteropneusta) genus of great zoological interest because it

is, like Hemichordates, an "evolutionary link" between

invertebrates and vertebrates. Balanoglossus is a

deuterostome, and resembles the Ascidians or sea squirts, in

that it possesses branchial openings, or "gill slits", but no

notochord and no nerve cord. It does have a stomochord,

however, which is gut chord within the collar. Their heads

may be as small as 2.5 mm (1/10 in) or as large as 5 mm (1/5

in).

External Characters

Balanoglossus is a burrowing worm like animal

It is 12-18 inches long

Found in the sandy shores

It is divided into a proboscis, collar and trunk

The mouth lies ventrally in a groove at the junction of

the proboscis and the collar

Feeding

Its feeds on nutrients present in the mud

Gill slits are present and they help in filtering off the

water taken in by the animal along with the food. Thus

they are called filter feeders

They are omnivorous, devoid of biting structures and

swallow nutritive vegetative and animal matter present in

the soil.

They have a ciliated skin

Respiration

Water passes in by the mouth and out by the gill slits and

the respiratory exchange of gases takes place as the water

passes through the gill pouches

Defence Mechanisms

The animal has a characteristic smell of iodoform that

keeps predators away from it

SUB PHYLUM UROCHORDATA OR TUNICATA

Ascidia

External Characters

It is a free living tunicate

The adult has a wrinkled cylinder/sac

It is about 3-4 inches in length

It has a broad base by which it attaches itself to rocks or

substrates

It has a large aperture at its free end called the

mouth/inhalant siphon

A little behind is the atrial/ exhalent aperture

Water goes in through the mouth and out through the

atrial aperture

When disturbed the animals squirts out water through the

atrial aperture and thus called a sea squirt

The body is enclosed in a tough test or tunic composed

of tunicine (similar to cellulose)

The test is attached to the body wall in the mouth region

and the atrial aperture and loose in the other places.

The opening and closing of the mouth and the atrial

opening is regulated by the sphincter muscles

Body wall

The body wall is found underlying the tunic and is

called the mantle

It has an ectoderm which has connective tissue

containing blood vessels

Digestive system

The mouth leads into the stomodeum and then opens into

the pharynx, which opens into the oesophagus followed

by the stomach, intestine and atrium which then opens

out by the anus (does not open directly out through the

anus like the invertebrates, there is an atrium)

There is also a velum with a circlet of tentacles at the

junction of the stomodeum and pharynx

The wall of the pharynx is perforated by numerous gill

slits

These gill slits serve as the mode through which the

pharynx communicates with the atrial cavity

On the ventral side of the pharynx is the endostyle which

secretes mucous.

Food particles are carried into the mouth along with the

current by ciliary action

The tentacles at the entrance of the pharynx serve as

strainer preventing coarse particles from entering into the

pharynx

The mucous secreted by the glandular cells of the

endostyle, entangle the minute organisms into the

oesophagus

Respiratory System

The pharyngeal region has blood vessels so that

exchange of gases takes place as the water passes in

and out

Blood Vascular System

Muscular sac like heart is present close to the stomach.

It is enclosed in a pericardium

No definite vessels are seen

The pulsation in the heart is peculiar, its shows a

periodical reversal of blood flow: ie. Blood is forced

forward during certain beats and backwards during

others

Excretion

This is carried out by certain cells in the intestine

No specialized ducts for removal of waste products are

seen

Nervous system

Single nerve ganglion embedded in the mantle between

the mouth and atrial aperture is seen

Reproductive system

They are hermaphrodites

Testis and ovaries lies in the loop of the intestine

Fertilization is external

Fertilized ovum hatches and the larva is called an

ascidian tadpole

Affinities

The ascidains are similar in their feeding type

Presence of endostyle and atrium complexes

SUB-PHYLUM – CEPHALOCHORDATA

Branchiostoma lanceolatus/ Amphioxus lanceolatus

External Characters

Commonly known as lancelet due to its shape

It is a translucent animal about 1.5 – 2 inches long

Found in shallow seas with sandy bottoms

Look like a fish but differs in its structures and habitat

i.e. it has no distinct head, jaws, paired fins and no scales

Structure

The body is long, pointed at both ends and compressed

It has a dorsal fin, caudal fin and ventral fin

The lateral edges of the body project out as two

longitudinal fin-like folds called the metapleural folds

Below the anterior end of the body, there is a funnel

shaped cavity called the vestibule or stomodaeum

This vestibule is surrounded by a membrane called the

oral hood which has numerous ciliated tentacles called

the buccal cirri

Below the vestibule is the mouth which is fringed by

velar tentacles which are directed backwards into the

pharynx

This movement causes the flow of water towards the

mouth and thus gives it a wheel like movement

appearance thus the tract is called the wheel organ.

These movement allow food to be taken into the mouth

The anus lies a short distance in front of the hind end,

towards the left side of the ventral fin.

It also has an atrium which surrounds the phayrnx, and

thus the coelom is greatly reduced due to the presence of

the atrium

Bodywall

The body wall is covered by the epidermis, below which

is found the fibrous dermis. The two constitute the skin.

Beneath the skin, is a muscular layer arranged

segmentally as muscle blocks called the myotomes.

Myotomes are v-shaped and help during the sinus

movement of the organism

Skeleton

Does not have a bone or cartilage but has notochordal

tissue and geleatinous materials

It has a notochord- which is rod like and lies dorsally to

the alimentary canal and ventrally to the nerve cord

It extends from one end to the other of the body

This notochord is the forerunner of the vertebral column

in vertebrates

Other skeletal structures include

Oral hood- is supported by a ring of small rods of

gelatinous material

Dorsal fin – has a single row of fin rays composed of

gelatinous materials

Ventral fin – has a double row of fin rays

Pharynx- is supported by gill bars which aid in

separating the gill slits

Digestive System

The mouth leads into the pharynx

The wall of the pharynx is perforated by numerous gill

slits on each side

The pharynx then leads into the intestine or midgut

which extends backwards to open to the exterior by the

anus

A blind diverticulum is given off on the right side of the

intestine called the hepatic caecum- which contains the

digestive enzymes

Feeding

Amphioxus don’t move actively around for food but

usually buried in the sand

Its oral end protrudes outside and feeds on small

organisms contained in the currents of water swept into

the mouth by the action of the cilia

The cilia in the wheel organ and the gill bars of the

pharynx maintains the current of water flowing in and

out

The velar tentacles strain off the coarse particles like

sand grains

Small organisms are swept into the endostyle where the

mucous entangles them and the cilia drives them into the

intestine

The water passes out by the gill slits

Vascular System

The blood is colourless and no specialized heart is seen

Respiration

Water enters through the pharynx which also serves

respiratory purpose

The gills have blood vessels, hence as the water passes

between them, there is an exchange of gasses taking

place

Excretory System

Excretory organs are paired nephridia. These are bent

tube with horizontal and vertical limb.

The nephridium bears numerous branches ending blindly

in tufts of peculiar cells known as solenocytes

Each solenocyte is a hollow cell with flagellum

The nitrogenous waste pass through the walls of the

solenocytes

Nervous System

The central nervous system has a nerve cord lying above

the notochord and below the fin rays

No distinct brain is seen

It has an olfactory pit whose function is olfaction

Reproduction

Sexes are separate

Gonads are simple sacs about 26 pairs arranged on each

side of the body wall

Affinities

Similar to the higher chordates

Has notochord

Nerve cord

Gill slits

Differs from higher chordates

No distinct head

No specialized brain or organs associated with the head

like eyes, and auditory organs

No distinct heart

Excretion – nephridia- annelids

Gonads are segmentally arranged with no ducts

Body wall has myotomes

\

Invertebrate chordates provide clues to the origin of

vertebrates

A. Subphylum Urochordata

Species in the subphylum Urochordata are called tunicates.

Entire animal is cloaked in a tunic made of a cellulose-

like carbohydrate.

Most are sessile marine animals which adhere to rocks,

docks and boats.

Some species are planktonic, while others are colonial.

The tunicates are filter feeders.

Seawater enters through an incurrent siphon, passes

through the slits of the pharynx into a chambertcalled the

atrium, and exits via an excurrent siphon, the atriopore.

Food filtered from the water by a mucous net of the

pharynx is moved by cilia into the intestine.

The anus empties into the excurrent siphon.

When disturbed, tunicates eject a jet of water through

the excurrent siphon, so they are commonly called sea

squirts.

Adult tunicates bear little resemblance to other chordates.

They lack a notochord', a nerve cord and tail-

They possess only pharyngeal slits.

Larval tunicates are free swimmers and possess all four

chordate characteristics-

Larva attach by the head on a surface and undergo

metamorphosis to adult form.

B. Subphylum Cephalochordata

Animals in the subphylum Cephalochordata are known as

lancelets due to their bladelike shape. Chordate characteristics

are prominent and persist in the adult. These include:

Notochord

Dorsal nerve cord

Numerous gill slits

Postanal tail

Cephalochordates are marine filter feeders.

They burrow tail first into the sand with only the anterior

exposed.

Water is drawn into mouth by ciliary action and food is

trapped on a mucous net secreted across the pharyngeal

slits.

Water exits through the slits and trapped food passes

down the digestive tube-

Cephalochordates are feeble swimmers with fishlike motions.

Frequently move to new locations

Muscle segments are serially arranged in chevron

like rows, and coordinated contraction flexes the

notochord from side to side in a sinusoidal pattern.

Muscle segments develop from blocks of mesoderm

called somites that are arranged along each side of the

notochord in the embryo.

The serial musculature is evidence of chordate

segmentation which developed independently of that in

annelids and arthropods.

C. Relationship of Invertebrate Chordates to the

Vertebrates

Vertebrates first appear in the fossil records in Cambrian

rocks.

Fossilized invertebrates {about 550 million years old)

resembling cephalochordates are found in Burgess Shale

of British Columbia.

This is about 50 million years older than the oldest

known vertebrates.

Most zoologists feel the vertebrate ancestors possessed all

four chordate characteristics and were suspension-feeders.

They may have resembled lance lets but were less

specialized.

Information provided by molecular systematics supports

the idea that cephalochordates are the closest relatives of

vertebrates.

Hemichordata Urochordata Cephalochordata

Non-chordate features

Terminal anus.

Blood flows

forward in

dorsal blood

vessel.

Pelagic larva

similar b

holothurian

echinoderm

larva,

No trace of

notochord n

adult.

No nerve cord

in adult.

Adult are

sessil filter

feeders,

structurally

nothing like a

chordate

Chordate Tripartite body Gill slits in Fish-like animals

Hemichordata Urochordata Cephalochordata

features plan of preoral

proboscis,

collar and trunk

Pharyngeal slits

may have arisen

initially to

dispose of

excess water

created by

feeding

mechanism.

Laterly

developed into

food-collecting

device, e.g.

Balanoglossus

adult

multiplied to

form large

filter-feeding

pharynx.

Larva in

ascidian

tadpole

possesses the

following

features:

notochord,

pharyngeal

slits, dorsal

tubular nerve

cord,

segmental

myotomes,

post-anal tail.

e.g,Ciona

showing all

recognisable

chordate features.

Notochord

extends length of

body in larval and

adult stages.

Large pharynx

with clefts forms

feeding

mechanism.

Ciliated gill bars.

Pharyngeal slits

open into atrium.

Segmental

myotomes.

No head or limbs.

e.g..Amphioxus

The evolution of vertebrate characteristics is associated

with increased size and activity

Vertebrates have retained the chordate features while adding

other specializations.

These features distinguish the vertebrates

from urochordates and cephalochordates.

The unique vertebrate structures probably evolved in

association with increased size and more active foraging for

food. The unique vertebrate adaptations include:

Vertebrates show a much greater degree

of cephalization than cephalochordates.

=> The brain and sense organs are located at the anterior

end which is the portion of the body which is in contact

with the most environmental stimuli.

A skeleton including a cranium and vertebral column is

the main axis of the body, replacing the notochord as the

basic skeleton.

=> The cranium protects the brain.

=> The vertebral column provides support and a strong,

jointed anchor that provides leverage to the segmental

swimming muscles.

=> The axial skeleton made larger size and stronger,

faster movement possible.

=> Most vertebrates also have ribs (anchor muscles and

protect internal organs) and an appendicular skeleton

supporting two pairs of appendages.

The vertebrate skeleton may be composed of bone,

cartilage, or a combination of the two.

=> A majority of the skeleton is a non-living matrix

which is secreted and maintained by living skeletal cells.

=> The living vertebrate endoskeleton can grow with the

animal unlike the exoskeleton of arthropods.

The neural crest, a group of embryonic cells found only

in vertebrates, contributes to the formation of certain

skeletal components and many other structures

distinguishing vertebrates from other chordates.

=> The dorsal, hollow nerve cord develops from

an infolding of the edges of an ectodermal plate on the

surface of the embryo.

=> The neural crest forms near the dorsal margins of the

tube resulting from this

infolding.

=> Cells from the neural crest then migrate to various

specific areas of the embryo and help form a variety of

structures including some of the bones and cartilage of

the head.

Some anatomical adaptations also support the greater

metabolic demands of increased activity .

The generation of A TP by cellular respiration, to replace

the energy used by vertebrates in obtaining food or

escaping predators, consumes oxygen.

=> The respiratory and circulatory systems of vertebrates

show adaptations which support the mitochondria of

muscles and other active tissues.

Vertebrates have a closed circulatory system composed

of a ventral chambered (2 to 4) heart, arteries, capillaries

and veins.

=> The heart pumps the blood through the system.

=>The blood becomes oxygenated as it passes through

the capillaries of the gills or lungs.

The more active the life style, the larger the amounts of

organic molecules necessary to produce energy.

=> Vertebrates have several adaptations for feeding,

digestion and nutrient absorption.

=> For example, muscles in the walls of the digestive

tract move food from organ to organ along the tract.