Questions and their ideal answers

1-Give an account of the digestive system & feeding mechanism of Balanoglossus.

Digestive system:

It is a straight canal comprises the

mouth, buccal cavity, pharynx,

oesophagus, intestine and anus.

The mouth leads to a wide buccal

cavity lying in the collar region.

Its roof sends a blind tubular

diverticulum into the proboscis; the

stomochord. The buccal cavity is

followed by a large pharynx. The

Pharynx is perforated by a seriesof paired

internal u-shaped gill-slits

The oesophagus is the lower part of the pharynx.

It is incompletely separated from the pharynx by

a pair of lateral ridges. The oesophagus leads into a straight

tube called the intestine. The intestine giving off paired hepatic

caeca. Posteriorly, the alimentary canal terminates in an anal

aperture since the tail is absent.

Feeding:

Balanoglossus is a ciliary feeder. The lateral cilia of gill-bars maintain a constant

respiratory current drawing water into the pharynx through the mouth and to the

outside through the outer gill slits. Food particles are caught up in mucus secreted by

epidermal glands of proboscis and passed backwards by cilia to the mouth. The food

particles are collected in the oesophagus and digested in the intestine. Undigested

materials pass through the anus opening to the exterior.

2-Write on the circulatory system of Balanoglossus.

Circulatory system:

The circulatory system consists

of spaces communicating with large

dorsal and ventral blood vessels

(open type).The dorsal blood vessel

dilates to form the venous sinus

and passes into the so called heart.

The heart is non-contractile and

above it lies cardiac sac.

The ventral wall of the cardiac sac

contracts rhythmically, assisting the

circulation of the blood. In addition, the circulation

of the blood is largely maintained by the contraction of dorsal and ventral blood vessels.

The front end of the heart forms a series of glomeruli, covered by a region of proboscis Coelom.

These glomeruli form excretory cells, the nephrocytes. The glomeruli send dorsal and ventral

arteries to the proboscis. The blood is collected from the proboscis by a dorsal vein which connects

the venous sinus. The dorsal vessel is connected to the ventral one through two peribuccal vessels.

The ventral vessel sends small branches to the gill septa and tongue bars in the pharyngeal region.

Collecting vessels from these organs carry the oxygenated blood to the dorsal vessel. The blood

flows forward in the dorsal vessel and backward in the ventral vessel (resembles that in the

invertebrates). The blood is usually colorless and contains a few amoebocytes.

3-Write on body wall, coelomic cavities & stomochord of Balanoglossus.

Body wall:The body wall of is composed of epidermis, nervous layer,

basement membrane and muscles. The epidermis consists of ciliated

columnar cells, sensory cells and gland cells. Below the epidermis is

a nerve plexus receiving the inner processes of receptor cells. The

smooth muscles are arranged in two layers, the outer circular muscle

fibers and the inner longitudinal fibers.

Coelom:

There are five separated cavities

including a single cavity in the

Proboscis (protocoel) , a pair of coelomic

cavities in the collar (mesocoel) and a pair

of coelomic cavities in the trunk (metacoel).

The protocoel and mesocel open to the

exterior by mid-dorsal pores. However, the

metacoels are closed cavities.

Stomochord:

The dorsal wall of the buccal cavity gives off an

anterior diverticulum. The wall of this diverticulum

is thick, composed of vacuolated cells and bears

a certain resemblance to the notochord. However,

the stomochord is tubular, lies anterior to the

alimentary canal and not surrounded by a fibrous

sheath.

4-Discuss the affinities of hemichordates.

1-Affinities with phylum Chordata:

Resemblances Differences

1-external Gill-slits -Gill-slits are dorsally located

-Tail is absent.

2-skin -ciliated epidermis and dermis is absent

3-coelom -open to the exterior

4-stomochord dorsal & contains vacuolated cell hollow, without sheath & lies anterior to gut.

5-circulatory

system

-Dorsal heart.

-Blood flows anteriorly in dorsal vessel

and posteriorly in the ventral one.

6-Nervous

system-dorsal nerve cord

-Collar cord is tubular

-ventral solid nerve cord

-dorsal cord is solid

-collar cord is opened at both ends.

7-reproductive

system-Tornaria larva is transparent, oval and

contains 3 ciliated bands as

echinoderm larvae.

lack cephalization, paired appendages8-vertebrates

features

2-Affinities with phylum Echinodermata:

Resemblances:

-Tail absent. - Ciliated epidermis. -Coelomic cavities open to the exterior.

-dorsal heart and blood flow resembles invertebrates. -Nervous system: solid & poorly developed.

-Tornaria larva of Balanoglossus resembles the echinoderm larvae.

-Both have common habits and possess a remarkable power of regeneration.

5-Give an account of respiratory & excretory systems of Balanoglossus.

Respiratory system:

It consists of two rows of gill sacs lying inside

the dorsal regions of the lateral walls of

pharynx. The gill sacs communicate with the

pharynx by the u-shaped inner-gill slits and

with the exterior by outer gill-slits. The two

limbs of each internal gill-slit is separated

by a tongue bar and the gill sacs of each row

are separated from each other by gill septa.

The gill septa and tongue bars are ciliated and

are supported by m-shaped chitinous rods.

Respiration takes place by simple diffusion of

oxygen from water to the blood and carbon

dioxide from blood to water.

The front end of the heart forms a series of

glomeruli, covered by a region of proboscis

coelom. These glomeruli form excretory

cells, the nephrocytes. These nephrocytes

collect waste products from blood and

dischage these materials into the proboscis

coelom then to the outside through the

dorsal pore.

6-Describe the structure and function of the body wall of Ascidia.

Test or tunic:

-The test is thick.

-It is composed of a clear &

contains cells, nerve fibres,

blood vessels and calcareous

spicules.

-The matrix is composed of tunicine

(like cellulose), proteins and inorganic materials.

-The blood vessels in the test end by resptratory

ampullae.

Body wall:

It consists of skin and underlying muscles:

1-The skin: consists of two layers:

a-The epidermis consists of a flat ectodermal cells & is surrounded by the test..

b-The dermis consists of a vascular connective tissue layer.

2-The muscle layer:

The muscles are arranged in longitudinal and circular layers.

The Function of the body wall:

1- The test protects the animal.

2- The vascular ampullae scattered in the test act as respiratory surfaces.

7-Give an account of the digestive system ane feeding mechanism of Ascida.

Digestive system:

-The buccal is provided with tentacles forming a sieve which

keeps out the large particles.

-The pharynx is surrounded by the atrial cavity & the lateral

walls are pierced by numerous gill-slits. Between gill-slits there

is a series of papillae in gill-septa provided with cilia.

-A ciliated endostyle extends on ventral side of the pharynx.

-The cilated dorsal lamina extends posteriorly to the oesophagus.

-The oesophagus: is a short tube.

-The stomach: is a muscular sac.

-The intestine: is a U-shaped tube.

-The rectum: opens in the atrial cavity by the anus opening.

Feeding mechanism:

-In pharynx, endostyle glands secrete mucous.

-The mucous is scattered by the cilia on the inner surfaces of

pharyngeal wall.

-As the mucus passes it entangles fine organisms and carries them

to the dorsal lamina.

-The food particles pass back to the oesophagus by cilia of the

dorsal lamina.

-The food materials pass to the stomach where digestion occurs.

-Absorption takes place in the intestine.

-Undigested particles pass through anus.

8-Describe the circulatory system & circulation in Ascida.

Circulatory system:

It consists of heart and blood vessels.

1-The heart: Contractile tube without valves and Surrounded by

pericardial cavity.

2-The blood vessels:

a-The ventral vessel:

Starts from the ventral end of the heart and extends forwards

beneath the endostyle. It sends paired transverse branchial

vessels through the transverse septa.

b-Dorsal vessel: above the dorsal

lamina. It receives the transverse vessels. The dorsal vessel

joins the branchio-visceral

c-Branchio-visceral vessel:

supplies oesophagus, stomach and intestine.

d-Cardio-visceral vessel: originates from dorsal end of heart.

It collects blood from the oesophagus, test, intestine and stomach.

Blood circulation:

Blood circulation differs according to the direction of the

contraction of the heart (periodic reverse of blood flow):

a- When the heart contracts in a dorso-ventrally: the blood flows to the ventral vessel,

transverse vessels (oxygenation), dorsal vessel, viscero-branchial vessel

and finally it flows back to the heart through the cardio-visceral vessel.

b- When the contractions of the heart take the opposite direction: The current of the blood is reversed.

9-Discuss the metamorphosis of tunicate larva.

Metamorphosis:

-The larva is composed of an oval body and a

long tail containing notochord.

-The nervous system is a dorsal tube enlarged

anteriorly to form brain containing otocyst and

eye spot.

-The pharynx contains two pairs of gill-slits and

atrium develops as an ectodermal invagination.

-The endostyle and heart are ventrally located.

-Below the mouth suckers are found.

Retrogressive Metamorphosis:

-The complete disappearance of the tail, nerve cord and notochord.

-The disappearance of the eye and otocyst

-The reduction of the brain to form nerve ganglion and subneural gland.

progressive changes:

- gradual increase of size.in number.

-The gill-slits increased

-The body rotates carrying the mouth to a position opposite to the base of the animal.

-The chief changes involved in the retrogressive

metamorphosis are thecomplete disappearance

of the tail.

Tunicate larva possesses: 1-Long tail.

2-Large pharynx perforated with lateral gill-slits.

3-Dorsal hollow nerve cord.

4-Dorsal notochord.

5-ventral heart.

The presence of these chordate characters in their larval stage proves that the tunicates are chordates.

The tunicates lack some vertebrate characters such as:

1- Vertebral column. 2- Paired appendages.

3- Coelom. 4- Head region.

5-Multilayered epidermis.

1-Affinities with cephalochordates:Resemblances:

- Adult with a large pharynx with a ventral endostyle, epipharyngeal groove, gill-slits and atrium.

-The larvae of the two groups having a tail.

Differences:

Tunicates are: 1-sedentary. 2- hermaphrodite, 3- having heart. 4- with a thick test.

Adults without : 1-coelom. 2-nerve cord. 3-notochord.

2-Specialized features:i-They are sedentary. ii-The body is covered with a thick test. iii-Absence of coelom.

iv-periodic reverse of blood flow. v-The retrogressive metamorphosis.

10-Write on the affinities of urochordates.

11-Describe the digestive system & feeding mechanism in Branchiostoma.Digestive system: It comprises:

1-The mouth:

It is surrounded by oral hood and oral

cirri which form a sieve to prevent

the entry of large particles.

2-The buccal cavity:

The lining ectoderm is folded to form the

wheel organ. The Hatschek’s pit secretes

mucus. The vertical velum contains a circular

entrostome which is surrounded by12 velar

tentacles.

3-The pharynx:

The lateral walls of pharynx are perforated by gill-slits.

The primary and secondary gill bars are interconnected

by horizontal synapticula.

The floor of the pharynx contains mucus-secreting endostyle.The frontal

cilia of th gill bars produce an upward current, driving the mucus from the

endostyle into the dorsal epipharyngeal band.

4-The oesophagus: It is a narrow tube.

5-The mid-gut: It is a tubular part from which a large hollow mid-gut diverticulum arises.

6-The hind-gut: It is a tubular organ which opens out by the anus.

Feeding:

Branchiostoma is a ciliary feeder. The water current enters the pharynx via mouth, buccal cavity and entrostome

and passes through the gill-slits into the atrium to leave via the atriopore. The endostyle secrete abundant mucus.

The latter is sent up the gill-bars by their frontal cilia. Food particles move upwards to the epipharyngeal groove

then into the oesophagus. Large food particles left in the pharynx are sent into the epipharyngeal groove by the

cilia of the peripharyngeal bands. The food is carried back through the oesophagus and midgut by ciliary action.

The digestive secretions are produced by the mid-gut diverticulum.

12-Give an account of circulatory system of Branchiostoma.

Circulatory system:

It is a closed system. The heart and respiratory

pigments are absent. The principal blood vessels include:

1-The ventral aorta:

-It lies beneath the endostyle. Blood flows forwards

by rhythmical contraction of the ventral aorta.

-The ventral aorta gives off paired afferent branchial

arteries to the primary gill bars.

-Each afferent branchial artery possesses a contractile

dilation, which assists in the blood circulation.

-The afferent branchial arteries leave the gill bars

asefferent branchial arteries and open into 2 lateral dorsal aortae.

2-The lateral dorsal aortae:

The blood flows backwards in the lateral dorsal aortae. Behind the pharynx the two

lateral dorsal aortae unite to form a median dorsal aorta.The latter sends some

arteries to the intestine and extends in the tail as a caudal artery.

3-The subintestinal vein:

It lies beneath the intestine through which the blood flows forward. Posteriorly, it receives a median caudal

vein from the tail and anteriorly it forms a single vessel, the hepatic portal vein.

4-The hepatic portal vein and hepatic vein:

The hepatic portal vein extends forwards along the ventral surface of the mid-gut diverticulum. It gives

several vessels to the wall of diverticulum. The vessels returning blood from the diverticulum join to form the

hepatic vein which opens into a thin-walled ventral sinus venosus.

5-Cardinal veins:

Blood from myotomes is returned by the anterior and posterior cardinal veins. The two cardinal

veins on each side unite to form a short ductus Cuvieri which opens in the sinus venosus.

Blood of Branchiostoma is colourless and lacks corpuscles. The main function of blood is the transport of

food and it plays very little part in the transport of oxygen.

13-Give an account of the nervous and excretory systems of Branchiostoma.Nervous system:

The nervous system consists of a dorsal hollow nerve

cord lying above the notochord. In cross section, the

nerve cord is triangular in shape. Anteriorly, the nerve cord

terminates at the level of the first myotome, behind the anterior

end of the notochord. The nerve cord encloses

throughout its length a narrow cavity the neurocoel. At the

anterior end, the central canal is usually dilated to form the

cerebral vesicle without any change in the external diameter

of the cord. Thus, no brain is differentiated in Branchiostoma.

Excretory system:

The excretory system resembles that of invertebrates in having 90 pairs of

nephridia arranged segmentally over the dorso-lateral walls of the pharynx.

Each nephridium consists of a small bent tube, the excretory canal with

vertical and horizontal limbs. These nephridia lie between the atrium and

the dorsal coelomic canals.

The peripheral nervous system consists of two pairs of cerebral nerves and a segmental series of paired spinal

nerves. The cerebral nerves carry sensory impulses to the nerve cord, hence, they are sensory in nature.

In each segment there are a pair of dorsal roots and a pair of ventral roots. The ventral root ispurely motor nerve.

It innervates the adjacent myotomes. The dorsal root is mixed in nature. It divides into two branches; a dorsal

sensory branch from the skin and a ventral mixed one runs to the ventral muscle of the atrial and gut walls.

The two limbs bear flame cells (solenocytes) on its anterior surface.

The vertical limb ends blindly while the horizontal limb has on its ventral

surface a small aperture , the nephridiopore which opens into the

atrium. The solenocytes are connected with body of nephridium by

short tubules. From the cell body arises a flagellum that extends

through the tubule and projects freely into the lumen of the

nephridium. The flame cells collect waste products from coelom.

In addition, there is a pair of excretory brown funnels lying dorsal to the pharynx.

.

14-Give an account of the skeletal system & locomotion in Branchiostoma.

Skeleton:

The Branchiostoma has no exoskeleton. Endoskeleton includes

notochord,Gelatinous rods supporting gill bars and fin rays supporting fins.

a-The notochord:

The notochord is composed of a series of flattened muscular plates

surrounded by a fibrous sheath. Each plate develops as a highly

vacuolated cell, the nuclei being later pushed aside to the dorsal or

ventral sides.

b-Gelatinous rods:

They are formed of agglutinated elastic fibres.

These rods support the gill bars, oral cirri and

the free edge of oral hoods.

c-Fin rays:

The fin rays support the dorsal and ventral fins.

The dorsal fin is supported by a single row while

the ventral fin is supported by two rows.

Locomotion:

Locomotion takes place by lateral undulation of the body.

A wave of contraction starts in the myotomes of the anterior

end on one side and and passes posteriorly. Before this

Contraction reaches the posterior end another one starts

At the front end of the opposite side.

15-Give an account of the coelomic cavities and body wall Branchiostoma.

Body wall:

The skin is composed of two layers; an outer epidermis

and an inner dermis.

1-The epidermis: is a single layer of columnar cells,

which secrete an outer thin layer of cuticle. The

epidermis contains numerous goblet cells.

2- The dermis: is composed of an outer thin compact

connective tissue layer and an inner thick spongy

layer containing blood vessels and nerve fibres.

The muscles are arranged in a series of V-shaped

structures in which their apeces pointed forward.

There are about 60 myotomes on each side

separated by myosepta. The myotomes of one side

alternate with those of the other side.

Coelom:

Brnchiostoma possesses a true

coelom filled with coelomic fluid.

In the pharyngeal region:

1- A pair of dorsal coelomic canals

above the pharynx.

2- A mid-ventral coelomic canal

beneath the endostyle .

3- A double series of coelomic

canals in the primary gill bars.

4-The gut also is surrounded by a

coelomic canal.

1

2

3

4

On the other hand Branchiostoma and urochordates differ in:

i-Urochordates are fixed animals while Branchiostoma is free-swimming animal.

ii-Coelom absent in urochordates and well developed in Branchiostoma.

iii-Blood flow reversal in urochordates whereas it conforms the vertebrate condition in Branchiostoma.

iv-Sexes are united in urochordates and separate in Branchiostoma.

b-Affinities with vertebrates: Cephalochordates resembles vertebrates in:

i-Segmentally arranged myotomes.

ii-True coelom

iii-The tail region.

iv-The blood flows backwards in the dorsal blood vessel and forwards in the ventral blood vessel.

2-Primitive characters of cephalochordates: The Branchiostoma retains many primitive characters.

These are:

i-Ciliary feeding mechanism.

ii-Lack of head and appendages.

iii-Single-cell thick epidermis.

iv- Absence of the endoskeleton.

v-The presence of the endostyle.

vi-The presence of the nephridia.

vii-Persistent of the notochord.

viii-Absence of paired sense organs.

1-Chordate characters of cephalochordates:

Branchiostoma possesses the three diagnostic chordate characters namely: notochord, tubular dorsal nerve

cord and gill-slits.

a-Affinities with urochordates: Branchiostoma resembles urochordates in having:

i-A ciliary mode of feeding.

ii-A large pharynx perforated by gill-slits.

iii-A large atrium surrounding the pharynx.

16-Discuss the affinities of cephalochordates.