Scoliodon: Distribution, Structure and Sense Organs | Dogfishes

In this article we will discuss about Scoliodon:- 1. Introduction to Scoliodon 2. Habit and Habitat of Scoliodon 3. Geographical Distribution 4. External Structures 5. Skin 6. Muscular System 7. Skeletal Structures 8. Locomotion 9. Coelom 10. Digestive System 11. Respiratory System 12. Circulatory System 13. Arterial System 14. Venous System 15. Nervous System 16. Urinogenital System 17. Development.

Contents:

1. Introduction to Scoliodon
2. Habit and Habitat of Scoliodon
3. Geographical Distribution of Scoliodon
4. External Structures of Scoliodon
5. Skin of Scoliodon
6. Muscular System of Scoliodon
7. Skeletal Structures of Scoliodon
8. Locomotion in Scoliodon
9. Coelom of Scoliodon
10. Digestive System of Scoliodon
11. Respiratory System of Scoliodon
12. Circulatory System of Scoliodon
13. Arterial System of Scoliodon
14. Venous System of Scoliodon
15. Nervous System of Scoliodon
16. Urinogenital System of Scoliodon
17. Development of Scoliodon

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1. Introduction to Scoliodon:

The class Elasmobranchii embraces a large variety of cartilaginous fishes. To give a general idea of the class, the common exam­ple of the group, the dogfish, is selected as an introductory type.

There are several genera of dogfishes available in the different parts of the earth. The typical Indian genus, Scoliodon is described below. The genus is represented by about nine species, of which four are very common in the Indian seas.

The genus Scoliodon is distinguished from other dogfishes by having an elongated snout, depressed head and a compressed body. The teeth are similar in both the jaws. The caudal pit and the sub-caudal lobe are prominent and distinct. The four Indian species of Scoliodon are: Scoliodon sorrakowah, S. dumerilii, S. palasorrah and S. walbeehmi.

Systematic Position:

Phylum Chordara

Subphylum Vertebrata

(= Craniata)

Series – Pisces

Class – Elasmobranchii

Subclass – Selachii

Superorder – Selachoidei

(= Pleurotremata)

Order – Lamniformes

Family – Carcharhiniidae 

Scientific Name (Scoliodon sorrakowah Bleeker, 1853) (= Scoliodon walbeehmi, Bleeker, 1856)

Remarks:

In FAO species catalogue, Sharks of the World, Compagno (1984) used Rhizoprionodon (Carcharias) acutus instead of S. sorrakowah as law of priority. Scoliodon sorrokowah is often spelled S. sorrakowa, It is wiled ‘milk shark’ because its flesh is used to promote lactation in women in India.

Vernacular Names

Bengali Hangar

Telugu Nullian-sorrah

Malayam Aul-pidiyan

Marathi Mushi

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2. Habit and Habitat of Scoliodon:

The shark is a marine, carnivorous and predaceous animal. It eats small pelagic schooling and bottom living bony fishes, including anchovies, codlet (Bregmacero-tidae), burrowing gobies (Tripauchenidae) and Bombay ducks (Harpadontidae) as well as shrimps and cuttle fish.

Both sexes mature between 1-2 years old and the males reach largest size at the age of about 5 years and females reach largest size at the age of 6 years.

A common tropical shark of continental and insular shelves close, frequently in rocky areas. The species is abundant in Indian and Pakistani waters.

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3. Geographical Distribution of Scoliodon:

Scoliodon has a wide geographical distri­bution ranging from Zanzibar to Ceylon (Sri Lanka), Ceylon to the Malay Archipelago of the Indian Ocean, Bay of Bengal, Eastern Pacific (from Mexico to Panama), West Indies and eas­tern coasts of South America. Fossils of Scolio­don have been discovered in the geological strata from the lower Eocene to later periods.

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4. External Structures of Scoliodon:

Scoliodon is an elongated spindle-shaped animal. It has a laterally compressed body. A fully-developed specimen of the genus attains a length of about 60 cm. The body is divisible into head, trunk and tail. The head is dorsoventrally flattened and terminates anteriorly into a dorsoventrally compressed snout. The dorsal side of Scoliodon is dark-grey while the under­side is pale white.

The trunk is more or less oval in transverse section. It attains maximum thickness in the middle region and the body gradually tapers posteriorly into a long tail (Fig. 6.2A). The tail is also oval in cross-section (Fig. 6.2B) and bears a heterocercal type of caudal fin, i.e., the posterior end of the vertebral col­umn is bent upwards and lies in the dorsal or epichordal lobe.

The mouth is a very wide crescentic aperture lying on the ventral side of the head near its anterior end. It is bounded by upper and lower jaws, each is beset with one or two rows of sharply pointed and backwardly directed teeth (Fig. 6.2C) to catch the slip­pery prey.

The teeth are replaced if these are broken. The teeth of Scliodon are modified scales. The scales cover its body and extend inside the jaws to serve as teeth (Fig. 6.2D). The transition of the placoid scales into teeth is amply recorded in the jaw regions.

Two prominent circular eyes are present. Each eye is provided with movable upper and lower eyelids. The third eyelid or nictitating membrane can cover the whole eye in emer­gency. The pupil is a vertical slit-like aperture (Fig. 6.2E).

The nostrils are placed one at each angle of the mouth. These are exclusively olfactory in function and have no connection with the mouth cavity. Each nostril is partly covered by a small fold of skin. Posterior to the eyes there are five vertical slits on each side. They are called gill or branchial slits. The branchial slits lead into the gill-pouches which in turn open into the pharyngeal cavity.

The cloaca opens to the exterior by a cloacal aper­ture which is located in between the two pelvic fins. The cloacal aperture is an elongat­ed opening. The cloaca is a common cham­ber, into which anus, urinary and genital aper­tures open. On each side of the cloaca lies the abdominal pores.

The abdominal pores are paired structures and situated on elevated papillae to communicate the coelom to the outside. A faint lateral line is present. Beneath this line a canal is present. The canal opens to the exterior by minute pores at intervals. Many pores, called ampullary pores, are also present on the head.

Fins:

As in other fishes, Scoliodon bears unpaired and paired fins which are actually flap-like integumentary extensions of the body. These are flexible and are stiffened by carti­laginous rods or horny fin-rays. All the fins are directed backwards which is of positive advantage in swift forward movement in water.

Median unpaired fins:

The fins under this category include two dorsals, one caudal and one ventral fin. The dorsal fins are triangular in outline. The anterior dorsal is larger and situ­ated at about the middle of the body. The pos­terior dorsal is comparatively small and occu­pies a median position between the first dorsal and the tip of tail.

The caudal fin has one ill- developed ventral lobe (hypochordal) which is divided into two parts. Two shallow depres­sions, called caudal pits, are regarded as the diagnostic features of the genus. These are pre­sent at the root of the tail, one at the dorsal sur­face and another on the ventral. The median ventral fin is located in the mid-ventral line and just anterior to the caudal fin.

Lateral paired fins:

Two pectoral and two pelvic fins constitute the lateral paired fins. The pectoral fins are large and are situated posterior to the gill-clefts. The pelvic fins are much smaller. In females, these are simple but in males each of them is connected with a copulatory organ called myxipterygium or clasper. Clasper is rod-like in appearance having a dorsal groove leading to a siphon at its base.

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5. Skin of Scoliodon:

The integument is composed of an outer epidermis and an inner dermis. The epider­mis is composed of epithelial cells inter­mixed with numerous unicellular mucous glands. In the young stage the epithelial cells are ciliated. But in an adult the cilia are lost.

The dermis is composed of three layers:

(i) Stratum laxum,

(ii) Stratum compactum and

(iii) Subcutaneous layer.

The stratum laxum is the outer layer and lacks fibres. The median layer is the stratum compactum which is fibrous in nature. The basal plate of the placoid scale is tied to this layer by fibres. The subcutaneous layer is variable in thick­ness and contains fine fibres arranged in a reticular fashion.

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6. Muscular System of Scoliodon:

Between the endoskeleton and the skin there are closely packed muscles. The muscles are highly developed in the trunk and tail regions and exhibit distinct segmental arrange­ment. There are numerous myotomes which are separated from one another by partitions made up of tough connective tissue called the myocommata.

The muscle fibres run parallel to the length of the body in each myotome. In the head region, the muscles do not exhibit any sign of segmentation and have become specialised to control the movement of the jaws, pharynx and eyes.

In the trunk region, the muscles are greatly thickened on the dor­sal side on each side of the vertebral column, whereas in the tail region the muscles are equally developed round the vertebral column (see Fig. 6.2B).

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7. Skeletal Structures of Scoliodon:

Scoliodon possesses well-developed exoskeletal and endo-skeletal structures. The exoskeleton includes predominantly the scales which are present all over the body. The endoskeleton embraces the axial and appen­dicular skeleton.

Exoskeleton:

The entire surface of the body is covered by oblique rows of placoid scales or odontoids (Fig. 6.3). A typical pla­coid scale has a basal plate made up of calcified tissue which remains embedded in the skin and a backwardly directed spine projecting out of skin.

The basal plate is held in the dermis by Sharpey’s and other fibres (Fig. 6.3E). A perforation is present at the base of the spine which communicates the pulp cavity of the basal plate with the pulp cavity in the spine.

The spine is composed of dentine coated externally with enamel. The nature of enamel is controversial. It is called the fibro dentine, because it is formed as the calcification of the fibrous material between the dentine and the enamel organ.

The scales covering the body extend inside the jaws, where they act as teeth. Placoid scale has dual source of origin and develops partly from epidermis and partly from dermis. The basal plate as well as the dentine of the spine is the derivatives of the mesoderm. The enamel is derived from the ectodermal enamel organ.

Endoskeleton:

The endoskeleton is com­posed exclusively of cartilage which may be hardened by calcification at places.

It can be described under two heads:

(a) The axial skeleton comprising of the skull and the verte­bral column and

(b) The appendicular skeleton consisting of the pectoral and pelvic girdles and the skeleton of the fins.

Axial skeleton:

The skull is a simple carti­laginous casket. It consists of the cranium, four sense capsules enclosing the auditory and olfactory organs and the visceral skeleton which form the jaws and support the pharynx with gills.

These three components of the skull remain intimately fused with each other (Fig. 6.4A, B).

The cranium has four distinct regions:

(a) The occipital region forms the posterior portion of the skull and contains a large foramen magnum through which the spinal cord passes down. An occipital condyle is present on either side of the foramen magnum. Above this foramen there is a median ridge, called the occipital crest.

(b) The auditory region is made of auditory capsules which remain firmly united with the cranium in an adult. An oval depression between the two capsules is called the pari­etal fossa. The roof of the cranium has two large fontanelles on the dorsal side.

(c) The middle region of the skull is composed of orbit. The margin between the roof of the cra­nium and the orbit is marked by the supra­orbital ridge. A slender cartilage, called pre- orbital process, partly encircles the orbit. Similarly a postorbital process emerges from the side forward along the upper margin of the orbit.

(d) The anterior or the ethmoidal region of the cranium is composed of olfactory cap­sules and rostrum. The two olfactory capsules are separated by internasal septum.

The visceral skeleton consists of seven half-hooped cartilaginous structures that encir­cle the buccal cavity and the pharynx. The first pair of the visceral arches give rise to the jaws, the second pair to the hyoid arch and the rest of them support the gills.

The first visceral arch is the mandibular arch. Each half of this arch divides into two parts, the upper part is called palatopterygoquadrate which forms the upper jaw and the lower part is known as Meckel’s cartilage which forms the lower jaw.

The sec­ond arch is called hyoid arch which consists of three parts—a ventral basihyal, a lateral ceratohyal and a dorsal hyomandibular. The sus­pension of the jaws with the cranium is made through the hyomandibular and this type of suspensorium is called hyostylic. The rest of the visceral arches are known as branchial arches which support the pharynx and the gills.

Vertebral column:

The vertebral column is composed of a chain of cartilaginous verte­brae. The vertebrae develop around the notochord which is persistent (Fig. 6.4C). The ver­tebrae differ slightly along the length. A trunk vertebra is taken as the typical one.

Structure of a trunk vertebra:

A trunk ver­tebra has a centrum that encloses the notochord. Above the centrum there is a neural canal through which the spinal cord passes down. The neural canal is enclosed by neural arch which contains a short blunt neural spine. There is a pair of transverse processes which project from the centrum ventrolaterally. The centrum is amphicoelous (i.e., the cen­trum exhibits concavities on both the ends).

The notochord is very narrow within centrum but becomes very much dilated in the inter­vertebral spaces (see Fig. 6.4C). The centra are reinforced by calcified fibrocartilage which forms four wedges and traverse the body of the centrum as a cross. Such centra are generally called asterospondylous types.

The posterior vertebrae contain haemal arch which is present on the ventral side of the centrum. The haemal arch encloses the haemal canal enclosing the caudal artery and vein. The haemal arch gives a haemal spine to support the ventral lobe of the caudal fin. The posterior end of the vertebral column is bent dorsally.

Appendicular skeleton:

The supporting skeleton of the fins (both unpaired and paired ones) constitutes the appendicular skeleton. The two dorsal fins and the ventral fin are pro­vided with series of cartilaginous rod-like structures, called the pterygiophores or somactidia.

The distal ends of the somactidia bear double series of the ceratotrichia or horny fin-rays. Somactidia are absent in the caudal fin. Caudal fin is supported by the extensions of the neural and haemal spines.

Pectoral girdle and fin:

The pectoral girdle is situated posterior to the last branchial arch and consists of two semicircular carti­lages united with one another along the mid-ventral line. The dorsal portion of each half is composed of a thick and rod-like scapula and the ventral portion is made up of a thin and flattened corcacoid (Fig. 6.5A).

At the junction of the coracoid and the scapula there are three facets for the articulation of the three basal cartilages of the pectoral fin called the propterygium, mesopterygium and metapterygium. The basal cartilages bear many radial cartilages (radials) supporting the pectoral fin.

Pelvic girdle and fin:

The pelvic girdle comprises of a flattened cartilaginous rod situated along the transverse plane in front of the cloaca. A curved basal cartilage, called basipterygium, supports the radials of the pelvic fin (Fig. 6.5B). The basipterygium is attached anteriorly to the pelvic girdle.

The radials at the distal ends contain small carti­lages bearing the ceratotrichia. In males each clasper is a tubular cartilage and grooved dorsally. Distally the groove ter­minates into a sharp style which is enclosed by two sheathing plates. At the upper end of the style lies a small accessory cartilage.

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8. Locomotion in Scoliodon:

The movement of Scoliodon is caused by the activities of the myotomal longitudinal muscle fibres and is also aided by movement of the fins. In the phylogenetic history of the fishes, the fins were primarily employed to raise the body off the bottom, but these become secondarily used in swimming by producing undulatory movements.

The longitudinal muscle fibres composing the myotomes play the important role in swim­ming. The myotomes are placed on either side of the incompressible vertebral column which acts as a lever upon which the myotomes work.

The contractility of the myotomes thus causes the bending of the body. During for­ward progression the contraction of the myotomes occurs along the anteroposterior direction in such a way that the waves of cur­vature pass down each side of the body alter­nately from the head to the tail.

Such contrac­tion is called the metachronal contraction. It has been calculated that about 54 waves are produced per minute during steady swimming.

The controlling factors governing such con­traction of muscles are not fully known. In Scoliodon, the transaction of the spinal cord behind the medulla oblongata is capable of producing swimming movements for several days. This phenomenon suggests that rhythm of contraction is largely governed by the spinal cord, but the influence of brain on the whole process is yet unsolved.

The body and fins are specially modified to maintain an equilibrium in water during swimming. The dorsal fin is well-developed and helps to restore stability and helps in the restoration of equilibrium, if there is any devi­ation along the vertical axis of the body. The pectoral fins play a very important role in tur­ning of the fish by the unilateral breaking with the pectoral fins.

The pectoral fins also help to maintain stability in the vertical plane. The movable pectoral fins lift the head upwards and this is compensated by the heterocercal tail. The hypochordal lobe is flexible and the epichordal part is rigid. The flexibility of the hypochordal lobe gives a vertical lift of the tail. The pelvic fins have no utility in locomo­tion and usually help in reproductive func­tions, specially in males.

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9. Coelom of Scoliodon:

In Scoliodon, the coelom is spacious and is divided into a smallest pericardial cavity and an extensive abdominal cavity. These two cavities are separated by the septum transversum and communicate with one another through pericardioperitoneal canal situated in the septum.

The abdominal cavity contains the viscera (Fig. 6.6) and opens to the exterior through a pair of abdominal pores. The peri­cardial coelom houses the heart.

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10. Digestive System of Scoliodon:

The digestive system consists of the ali­mentary canal and the digestive glands. The alimentary canal starts with the mouth and ter­minates in the anus. The mouth leads into a spacious buccal cavity which is lined with mucous membrane.

The floor of the buccal cavity becomes folded to form a non-muscular and non-granular ‘tongue’. The mucous membrane is very thick and rough due to the presence of dermal den­ticles or teeth. The teeth are very sharp and are obliquely placed (see Fig. 6.2C). The teeth are homodont (i.e., the teeth are similar in shape) and lyodont (possesses several sets of teeth functioning in succession).

The buccal cavity leads into pharynx. On either side of the pharynx there lie the inter­nal openings of the spiracles and five branchial clefts. The mucous membrane of the pharyngeal wall contains numerous der­mal denticles. The pharynx leads into a nar­row oesophagus. The inner mucous mem­brane of the pharynx is raised to form longi­tudinal folds.

The oesophagus dilates posteriorly to form a large stomach. The stomach is highly muscular and is bent on itself to form a J-shaped configuration. The long limb of the stomach is continuous with the oesophagus and the shorter one passes into the intestine. The entrance of the oesophagus into the stomach is provided with a crescentic fold which serves as the valve.

The long anterior limb is called the cardiac stomach and the short posterior limb is designated as the pyloric stomach. A small outgrowth often called ‘blind sac’ is present at the junction of the cardiac and pyloric limbs.

The inner lin­ing of the cardiac stomach is folded longitu­dinally like that of oesophagus (Fig. 6.7A). The internal lining of the pyloric stomach is mostly smooth though slight foldings are observed at the distal end.

The pyloric valve, at the end of the pylorus, guards the entrance of it into a thick-walled small chamber called the bursa entiana. The bursa entiana is immediately followed by wide tubular intes­tine which becomes narrowed posteriorly as the rectum. The rectum opens into the cloa­ca. A tubular caecal or rectal or digit form gland opens into the rectum.

The inner surface of the intestine becomes folded to form an anticlockwise spiral of approximately two and a half turns. This is called the scroll valve (Fig. 6.7B) which increases the absorptive surface of the intes­tine and also checks the rapid flow of digested food through the intestine.

The major digestive gland is the liver which is a massive yellowish gland and con­sists of two lobes. The lobes are united ante­riorly. A thin-walled V-shaped gall-bladder is present in the anterior part of the right lobe of liver. The bile duct receives a few smaller ducts from the two lobes of the liver and opens into the anterior end of intestine near the commencement of the scroll valve.

The pancreas is a pale compact irregular body and consists of a dorsal lobe situated parallel to the posterior part of cardiac stomach and a ventral lobe which remains closely attached to the pyloric stomach. The pancreatic juice is poured into the intestine by pancreatic duct situated opposite to the aperture of the bile duct.

The functional significance of rectal gland is not properly known. The rectal gland has a central cavity lined with cuboidal cells. It is highly vascular and composed of lymphoid tissue. It discharges a fluid into the lumen of the intestine but its actual role is not known.

The buccal cavity possesses no such glands that can be compared with the salivary glands of higher vertebrates. The spleen is located dorsal to the distal end of the body of the stomach. The spleen is functionally associated with the circulatory system, but remains mor­phologically connected with the alimentary canal.

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11. Respiratory System of Scoliodon:

The respiratory organs are the gills which are borne by the gill-pouches. The structure of gill-pouches differs in different dogfishes. Fig. 6.8B shows the organisation of gill-pouch of Brachaelurus, a related genus of Scoliodon. There are five pairs of gill-pouches, each of which communicates with the pharyngeal cavity by a large internal branchial aperture and opens to the outside by exterior gill-slits (Fig. 6.8A).

The mucous membrane lining of the gill- pouches gives a series of horizontal branchial lamellae. The branchial lamellae are highly vascularized structures. Each gill-pouch has an anterior set of branchial lamellae and a pos­terior set of branchial lamellae. The gill- pouches are separated by inter-branchial sep­tum which projects beyond the branchial lamellae (Fig. 6.8C).

The pharyngeal end of each inter-branchial septum is supported by a visceral arch. Each arch supports the anterior set of lamellae of one gill-pouch behind and the posterior set of lamellae of the next gill- pouch. The first gill-pouch lies between the hyoid and the first branchial arches and the last one is present between the fourth and fifth branchial arches.

There are two types of gills:

(i) Holobranch or complete gill when a branchial arch bears two sets of gill lamellae and

(ii) Demi branch or hemi branch or half gill when single set of gill lamellae is present. The hyoid arch sup­ports only a demi branch and the first four branchial arches support holobranchs. The last branchial arch is gill-less.

Mechanism of respiration:

During respiration the floor of the buccal cavity is lowered and the mouth is opened. Then the water rushes in to fill the greatly expanded buccal cavity. The mouth is now closed and the pharynx contracts.

The water then enters the gill-pouches and goes out after gaseous exchange through gill-slits. The spira­cles are occasionally used as accessory path­ways for the entry of water for respiration instead of the mouth when it is otherwise occupied.

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12. Circulatory System of Scoliodon:

The circulatory system consists of:

(a) The circulatory fluid, called blood,

(b) The heart,

(c) The arteries and

(d) The veins.

Blood:

The blood consists of a colourless plasma and corpuscles are suspended in the plasma. Two kinds of corpuscles are encoun­tered; the RBC (or erythrocytes) and the WBC (or leucocytes). The erythrocytes are oval bod­ies containing a nucleus. The haemoglobin is present in the erythrocytes. The leucocytes are amoeboid in structure.

Heart:

The heart is a bent muscular tube and consists of the receiving parts, comprising of a sinus venosus and a dorsally placed auri­cle, and the forwarding parts, consisting of a ventricle and a conus arteriosus (Fig. 6.9A). The heart is situated on the ventral side of the body between two series of gill-pouches.

Receiving parts of the heart:

The sinus venosus is a thin-walled tubular chamber. The sinus venosus is highly contractile and the beating of the heart originates from this part of the heart. Two great veins, the ductus Cuveiri, open into the sinus venosus, one on each late­ral side.

Two hepatic sinuses enter the sinus venosus posteriorly. The sinus venosus opens into the auricle by sinuauricular aperture which is guarded by a pair of valves. The auri­cle is a large, triangular and thin-walled cham­ber situated dorsal to the ventricle but in front of the sinus venosus.

The auricle communi­cates with the ventricle through a slit-like auriculoventricular aperture guarded by two lipped valves. The receiving chambers, (sinus venosus and auricle) receive the venous blood from all parts of the body.

Forwarding parts of the heart:

The ventri­cle has a very thick muscular wall, the inner surface gives many muscular strands, thus giving it a spongy texture (Fig. 6.9B). It is an oval chamber and constitutes the most promi­nent part of the heart. The conus arteriosus is a stout median muscular tube arising from the ventricle.

The lumen of the conus arteriosus is provided with two transverse rows of semi-lunar valves. To keep the valves in position the free ends of the valves are attached to the ventricular wall by fine tendinous threads, called chordae tendinae. The conus arteriosus is continued forward as the ventral aorta.

The function of the heart is to receive the deoxygenated blood from all parts of the body and to pump it for aeration to the gills. Such a type of the heart is designated as the venous or branchial heart, because only the deoxygena­ted blood circulates through its different parts.

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13. Arterial System of Scoliodon:

The arterial system of Scoliodon is divided into two distinct categories of arteries.

These are:

(a) The afferent branchial arteries arising from the ventral aorta which bring the deoxy­genated blood to gills for oxygenation and

(b) The efferent branchial arteries which origi­nate from gills and convey the oxygenated blood to the different parts of the body (Fig. 6.10).

Afferent branchial arteries:

The ventral aorta is situated on the ventral surface of the pharynx and extends up to the posterior bor­der or the hyoid arch. The ventral aorta divides into two branches called innominate arteries, which again bifurcates into the first and sec­ond afferent branchial arteries.

The third, fourth and fifth afferent arteries arise from the ventral aorta. Each afferent branchial artery arises from the ventral aorta by independent opening except the anterior most pairs which arise by a common opening (Fig. 6.10).

Efferent branchial arteries:

The afferent branchial arteries break up into capillaries in the gills. From the gills the blood is collected by efferent branchial arteries (Fig. 6.10). There are nine pairs of efferent branchial arteries and these are equally distributed on each side.

The first eight arteries form a series of four com­plete loops around the first four gill-slits and the ninth efferent branchial artery collects blood from the demi branch of the fifth gill- pouch and from where blood is poured into the fourth loop.

In addition to short longitudi­nal connectives connecting the four loops, these are further connected with each other by a network of longitudinal commissural vessels called the lateral hypobranchial chain.

From each efferent branchial loop arises an epibranchial artery. The four pairs of epi- branchials join in the mid-dorsal line to form the dorsal aorta. The ninth efferent branchial artery has no epibranchial branch but joins with the eighth efferent branchial artery.

Anterior arteries:

The head region gets the blood supply from the first efferent branchial artery and partly from the proximal end of the dorsal aorta.

Arteries from the first efferent branchial (hyoidean efferent) are:

(a) The external carotid,

(b) The afferent spiracular and

(c) The hyodean epibranchial which in turn receives a branch from dorsal aorta.

The exter­nal carotid artery originates from the first col­lector loop and divides into a ventral mandibular artery giving branches to the mus­cles of the lower jaw and a superficial hyoid artery which supplies the second ventral con­strictor muscle, the skin and the subcutaneous tissue beneath the hyoid arch.

The afferent spiracular artery after originating from the middle of the hyoidean efferent, proceeds for­ward as the spiracular epibranchial artery and enters the cranial cavity. Just before its entry into the cranial cavity it sends a great oph­thalmic artery to the eye ball. Immediately after the entry to the cranium it joins with a branch from the internal carotid to form the cerebral artery.

The cerebral artery immediate­ly divides into an anterior and a posterior cere­bral artery which supply the brain. The hyoidean epibranchial artery runs forwards and inwards to the posterior border of the orbit and gets an anterior branch from the dorsal aorta.

It divides immediately into:

(a) the stapedial artery which gives off the inferior orbital artery and runs forward as the superior orbital artery supplying the six eye muscles and the superficial tissue above the auditory capsule. The superior orbital artery gives a large buccal artery which runs as the maxil­lonasal artery. The maxillonasal gives several arteries to the muscles of the upper jaw, the olfactory sac and the rostrum,

(b) the internal carotid artery passes inward and enters the cranium where it bifurcates into two branches. One of the branches unites with its fellow from the opposite side and other branch unites with the stapedial.

Dorsal aorta and its branches:

The dorsal aorta is formed by the union of epibranchial arteries. It runs posteriorly and is situated ven­tral to the vertebral column. It is continued up to the tip of the tail as the caudal artery.

Along the anteroposterior direction the following arteries have their origin from the dorsal aorta:

(a) Several buccal and vertebral arteries are given off anteriorly.

(b) A pair of small subcla­vian arteries arise from near the origin of the fourth epibranchial arteries.

The subclavian artery gets the epicoracoid artery on its way and divides into:

(i) A branchial artery to the pectoral girdle and pectoral fin,

(ii) An antero­lateral artery to the body musculature and

(iii) A dorsolateral artery to the dorsal muscula­ture.

(c) A large coeliacomesenteric artery ari­ses just behind the origin of fourth epibranchial artery. It divides into a smaller coeliac artery and a larger anterior mesenteric artery.

(d) A lienogastric artery originates posterior to the coeliacomesenteric artery and gives off

(i) An ovarian (in females) or spermatic artery (in males) to gonad,

(ii) A posterior intestinal artery to the posterior part of the intestine,

(iii) A pos­terior gastric to the posterior part of the cardiac stomach and

(iv) A splenic artery to the spleen.

(e) Series of paired parietal arteries emerge out behind the subclavian artery. Each parietal gives a dorsal parietal artery and a ventral pari­etal artery. The dorsal parietal artery supplies the dorsolateral musculature, the vertebral col­umn, the spinal cord and the dorsal fin. The ventral parietal artery supplies the ventral mus­cles and the peritoneum. The ventral parietal gives renal branches to the kidneys.

(f) A pair of iliac arteries extend to the pelvic fin as femoral arteries.

Hypobranchial chain:

A lateral hypo­branchial chain is formed by a network of slender arteries arising from the ventral ends of the loop of the efferent branchial arteries. Four commissural vessels arise from the lateral hypobranchial chain which on the ventral wall of the ventral aorta unites to form a pair of median hypo-branchials which communi­cate with one another by transverse vessels.

Posteriorly the median hypo-branchials unite to form a median coracoid artery which gives rise to the coronary artery and a pericardial artery. The pericardial artery gives off the com­mon epicoracoid artery which in turn divides into left and right epicoracoid arteries each joining one subclavian artery.

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14. Venous System of Scoliodon:

The deoxygenated blood from the differ­ent parts of the body is returned to the heart by veins which form irregular blood sinuses throughout their courses (Fig. 6.11). The exis­tence of extensive blood sinuses is a charac­teristic feature of the venous system of Scoliodon.

The venous system is extremely compli­cated and is described under the following heads:

(A) Cardinal system:

The blood from the anterior region of the body is returned to the heart by paired jugular and anterior cardinal sinuses. The blood from the posterior region is collected by a pair of posterior cardinal sinu­ses. The anterior and posterior cardinals unite on each side to form a transverse sinus called ductus Cuvieri.

(i) Anterior cardinal system:

This system of veins returns blood from the head region and consist of a pair of internal jugular veins. Each internal jugular vein is composed of the olfactory sinus, the orbital sinus, the postorbital sinus and the anterior car­dinal sinus. The blood from the rostral region is drained by the anterior facial vein to the olfac­tory sinus and from there to the orbital sinus.

The orbital sinus opens into the anterior cardi­nal sinus through the postorbital sinus. The anterior cardinal sinus enters the ductus Cuvieri. The anterior cardinal sinus receives the hyoidean sinus and five dorsal nutrient branchial sinuses from the gills.

(ii) Posterior cardinal system:

The caudal vein collects blood from the tail region and proceeds forwards through the haemal canal. In the abdominal cavity, the caudal vein divides into left and right renal portal veins which break up into sinusoid capillaries in the substance of the kid­neys.

Throughout its length, the renal portal vein receives small parietal veins. The renal veins collect blood from the kidneys and unite to form the posterior cardinal sinuses. Two pos­terior cardinal sinuses open into the ductus Cuvieri.

(B) Hepatic portal system:

A large number of small veins carrying blood from the alimen­tary canal and its associated glands unite to form the hepatic portal vein. The hepatic por­tal vein receives the lienogastric vein and anterior and posterior gastric veins.

Actually the hepatic portal vein is formed by the con­fluence of the anterior and posterior intestinal veins. The hepatic portal vein breaks up into capillaries in the liver. From the liver blood is collected by another set of capillaries which unite to form two large hepatic sinuses open­ing into the sinus venosus.

(C) Cutaneous system:

This system con­sists of a dorsal, a ventral and two paired lat­eral cutaneous veins. The inferior lateral cuta­neous vein joins with the lateral cutaneous vein near the anterior end of the pectoral fin. Each lateral cutaneous vein ultimately opens into the brachial vein.

(D) Ventral system:

This system comprises of two sets of veins—the anterior ventral veins pouring blood to the ductus Cuvieri through inferior jugular sinuses and the posterior veins which discharge through the subclavian vein.

Each inferior jugular sinus is formed by the union of the sub-mental sinus from the lower jaw, the hyoidean sinus and the ventral nutri­ents from the gills. Each inferior jugular vein opens to the ductus Cuvieri. The subclavian vein also opens to each side of the ductus Cuvieri.

Two large lateral abdominal veins are formed by a small caudal vein and two iliac veins. The lateral abdominal veins are con­nected posteriorly by a commissural vein. Anteriorly the lateral abdominal vein joins the brachial vein to form the subclavian vein which in turn opens to the ductus Cuvieri.

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15. Nervous System of Scoliodon:

The nervous system of Scoliodon includes:

(i) The central nervous system,

(ii) The periphe­ral nervous system and

(iii) The autonomous nervous system.

Central nervous system:

The central ner­vous system consists of brain and the spinal cord.

Brain:

The brain is highly organised and shows many advancements over that of the agnathans.

The brain is divided into three pri­mary parts:

(a) The forebrain or prosen­cephalon,

(b) The midbrain or mesencephalon and

(c) The hindbrain or rhombencephalon.

The forebrain consists of a massive undivided cerebral hemisphere. The cerebral hemisphere is relatively larger than that of other fishes. From the anterior end of cerebral hemisphere arise two stout olfactory peducles, each termi­nates into a large bilobed olfactory lobe (Fig. 6.12).

The olfactory lobes lie close to the olfactory capsules. Each olfactory nerve is composed of many bundles of nerve fibres. The surface of the cerebrum is smooth and the walls are thick.

A small opening called the neuropore is present on the mid-ventral surface of the cerebrum. The posterior part of fore­brain (diencephalon) is very short. The roof of the diencephalon is thin, non-nervous and contains the anterior choroid plexus. The late­ral walls of the diencephalon form two thick­ened bodies, called thalami.

A long and slen­der tube, the pineal organ or epiphysis cerebri projects from the roof of the diencephalon up to the membrane covering the anterior frontanella. The floor of the diencephalon (or hypothalamus) is well-formed. A hollow infundibulum is given off from the floor of the diencephalon.

The infundibulum is dilated to form two oval thick-walled bodies, called lobi inferiores whose distal ends are produced into two thin-walled glandular sacs, called sacci vasculosi. The lobi inferiors are the centres for gustation and smell.

The sacci vasculosi is believed to be a centre for reception of the pressure of cerebrospinal fluid and at the same time they produce cerebrospinal fluid. The hypophysis is attached to the infundibulum. The optic chiasma lies in front of the infundibulum. The optic chiasma is formed by the decussation of the nerve fibres of two optic nerves (Fig. 6.12B).

The midbrain is large and consists of two round optic lobes. The optic lobes are situated behind the diencephalon. The floor and the side walls are relatively thicker. The midbrain is considered as the centre of co-ordination.

The hindbrain consists of a highly deve­loped cerebellum and a medulla oblongata. The dorsal surface of the cerebellum produces many irregular convolutions. The cerebellum contains a small cavity. The cerebellum is also a centre of co-ordination. The cerebellurp is divided into three lobes by two well-marked transverse furrows.

The medulla oblongata is triangular and the anterior end gives a pair of hollow corpora restiformia with trace of con­volutions in adults. The medulla controls res­piration. Two corpora restiformia are connec­ted by the transverse nerve band. The roof of the medulla oblongata is non-nervous and bears the posterior choroid plexus. The hind- brain controls swimming movements.

The ventricles of the brain are moderately developed (Fig. 6.12C). The cerebral hemi­spheres contain narrow lateral ventricle. The third ventricle is extended forward about half the length of the cerebral hemispheres. The floor of the fourth ventricle is very much thick­ened. The fourth ventricle is large and extends dorsally into the cerebellum and is continuous behind with the cavity of the spinal cord.

The iter (i.e., the communicating duct between the third and the fourth ventricles) is wider. Although the cerebrum is undivided, there are two lateral ventricles which are continued to the rhinocoels (cavity of the olfactory lobes).

Spinal cord:

The spinal cord in Scoliodon shows definite advancement towards the plan of higher vertebrates, The spinal cord is pro­vided with pia mater only. The grey matter is arranged into the dorsal and the ventral horns. The dorsal horns are united to form a single broad region, as a result the grey matter assumes a shape of an inverted ‘T’.

Peripheral nervous system:

The peripher­al nervous system includes the cranial nerves and spinal nerves.

Cranial nerves:

There are ten pairs of cra­nial nerves in all the fishes. An extra pair of anterior terminal or pre-olfactory nerves is pre­sent in Scoliodon. The terminal nerves are des­ignated as cranial nerve no. 0. The first pair of cranial nerves are the olfactory nerves which originate from the olfactory lobes and inner­vate the olfactory sacs.

The terminal nerves are situated between the two olfactory lobes. These nerves emerge from the telencephalon and bear a ganglion, called ganglion termi­nate, near the origin. These nerves supply the nasal septum and the external nostril. The sec­ond pair of cranial nerves are the optic nerves which, after the origin from the optic thalami, form the optic chiasma and supply the eyes.

The third cranial nerve is called oculomotor nerve which originates from the ventral sur­face of the mesencephalon and supplies the anterior, superior and inferior recti and the inferior oblique muscles of each eye ball. The fourth cranial nerve is called trochlear or pathetic nerve which arises from the dorso­lateral surface of the midbrain and supplies the superior oblique eye muscle.

The fifth cranial nerve is the trigeminal which has three branches:

(a) Ophthalmicus superficialis which supplies the skin of the snout;

(b) The maxillaris which is divided into maxillaris superior supplying nerves to the skin of the upper jaw and maxillaris inferior innervating the posterior part of the upper jaw;

(c) The mandibularis innervating the muscles of the lower jaw.

Another nerve called ophthalmicus profundus becomes secondarily associated with the trigeminal to supply nerves to the eye ball and the dorsal surface of the snout (Fig. 6.13). The sixth cranial nerve is the abducens which supplies the posterior rectus muscle of the eye ball.

The seventh cranial nerve is known as facial which divides into two branches:

(i) The ophthalmicus superficialis branch like that of the fifth cranial nerve and

(ii) A bundle of mixed nerves which subdivides into three routes:

(a) A ramus buccalis inner­vating the infraorbital canal of the snout,

(b) A ramus hyomandibularis supplying nerves to the lower jaw and throat and

(c) A ramus palatinus giving nerve supply to the roof of the buc­cal cavity and the pharynx. The eighth cranial nerve is called auditory which gives the vestibular and saccular branches to the inter­nal ear.

The ninth cranial nerve is the glossopha­ryngeal which, in the region of the first gill- cleft, divides into a small pretrematic nerve and a large posttraumatic nerve. These nerves supply branches to the pharynx, pharyngeal muscles and the mucous membrane surroun­ding the first gill-slit. The tenth cranial nerve is the vagus which arises by multiple roots and gives off many branches.

The branches are:

(i) The brachial nerves supplying the gills,

(ii) The lateralis supplying the lateral line sense organs and gives numerous branches along its course.

Terminal nerve (Cranial nerve ‘0’):

A new cra­nial nerve was first recorded in 1894 which con­nects the anterior end of cerebral hemispheres. It was first identified in Protopterus and had since been recorded in all gnathostomes excepting birds.

To avoid confusion which would result if the long established nomenclature and symbols of other cra­nial nerves were altered, this newly reported cranial nerve was named terminal nerve or cranial nerve ‘0’. Some workers on this line are of the opinion that nerve ‘0’ is a ganglionated remnant of the first branchial nerve. This nerve is apparently unrelated to the olfactory nerve.

The terminal nerve leaves the anteroventral portion of the cerebral hemisphere and passes into the mucous membrane of the olfactory organ. It is a sensory nerve and possesses one or more ganglia. The terminal nerve is best developed in elasmobranchs.

In amphibians, reptiles and mammals the terminal nerve is intimately associated with the vomeronasal organ (Jacobson’s organ). The function­al significance of the terminal nerve is not yet fully ascertained. It has been suggested that it may repre­sent the remnants of an anterior branchial nerve which has lost its significance in course of evolution.

Spinal nerves:

The spinal nerves arise from the spinal cord. Each has one dorsal and one ventral root. The dorsal root bears a ganglionic swelling. After emerging out through the verte­bral column, the dorsal and the ventral roots unite to form a common mixed nerve.

Each spinal nerve gives three branches, such as:

(a) Ramus dorsalis,

(b) Ramus ventralis and

(c) Ramus communicans to join with the autonomous nervous system.

Autonomous nervous system:

This system is made up of a series of paired ganglia arranged irregularly on the dorsal wall of the kidney and the posterior cardinal sinuses. The gastric ganglion is the largest ganglion and sends nerves to the viscera. There is usually one ganglion in each segment. In Scoliodon the successive ganglia are not in distinct con­tinuous chain.

Sense Organs of Scoliodon:

The nervous system is associated with highly developed sense organs, viz., eyes, nose, ear and many others (Fig. 6.14).

Eyes:

The eyes are built on the principle of a photographic camera. The eye ball is com­posed of three layers: the sclera, choroid and retina. The sclera is cartilaginous. The pupil is a vertical slit and it cannot be dilated or con­tracted. The retinal layer consists of rods and cones.

A large number of guanine plates are present on the inner surface of the choroid layer which is called tapetum lucidum. It acts as reflector. The crystalline lens is spherical. The lens is kept in position by suspensory lig­ament which extends from the margins of the lens to the ciliary processes.

The ciliary pro­cesses are longitudinal folds of the choroid layer. The eye is kept in its position in the orbit by six extrinsic eye muscles. Besides the eye muscles there are an optic nerve and a carti­laginous optic pedicle (Figs. 6.15 and 6.16).

These eye muscles are attached with the eye ball in two groups.

The first group include:

(i) Superior rectus,

(ii) Inferior rectus,

(iii) Ante­rior rectus and

(iv) Posterior rectus.

The sec­ond group include:

(i) Superior oblique and

(ii) Inferior oblique.

The superior rectus mus­cle runs outwards and upwards and is inserted on the dorsal side of the eye ball. The inferior rectus muscle extends outwards and down­wards to be inserted on the ventral side of the eye ball.

The anterior rectus muscle runs out­wards and downwards and is attached to the ventral surface of the eye ball. The posterior rectus muscle runs backwards and is attached with the posterior surface of the eye ball.

The superior oblique muscle is attached on the dorsal surface of the orbit just in front of the insertion region of the superior rectus muscle. The inferior oblique muscle is attached to the ventral surface of the eye ball just in front of the inferior rectus muscle.

The eyes of Scoliodon are quite prominent and are proportionately larger in size. The eyes are laterally placed and each eye has its own range of vision, i.e., Scoliodon has monocular vision. Scoliodon is possibly colour-blind (Fig. 6.16).

Olfactory organs:

There are two blind sac-like olfactory organs situated in front of the mouth. Each olfactory sac is placed in a carti­laginous capsule and does not communicate with the buccal cavity. The mucous membrane of the olfactory sac is thrown into two series of folds called Schneiderian folds which are placed by the median raphe (see Fig. 6.14B).

The Schneiderian folds are composed of olfactory sense cells and supporting cells. The olfactory sense organs are greatly developed in Scoliodon. Physiologically each nasal opening is divided by three muscular nasal valves into a median ex-current siphon and a lateral incurrent siphon.

Internal ear:

The internal ear or the mem­branous labyrinth of Scoliodon is a closed ectodermal sac. It consists of three semicircu­lar canals and a central body differentiated into an anterior utriculus and a ventral sacculus (see Fig. 6.14A). The three semicircular canals are arranged in three planes of the body and open into the central body by both ends. But one end of each canal dilates to form ampulla.

The utriculus gives off an invagina­tion called recessus utriculi beneath the ampullae of the anterior vertical and horizon­tal semicircular canals. The sacculus gives a posterior outgrowth called the lagena. The membranous labyrinth is placed in a cartilagi­nous auditory capsule. The inner cavity is filled with a fluid called the endolymph into which many calcareous bodies (otoliths) are present.

The space between the membranous labyrinth and the auditory capsule is filled with the perilymph. A long tube called the endo-lymphatic duct communicates the cavity of sacculus to the exterior through a small opening.

The membranous labyrinth of Scoliodon performs three functions:

(a) It helps in orientation in relation to gravity,

(b) It accelerates in changing the direction during swimming and

(c) It helps in hearing.

The utriculus together with the semicircular canals is responsible for the orientation and acce­leration, while the sacculus is meant for hea­ring. The internal ears are the statoacoustic organs in Scoliodon.

Neuromast organs:

These organs com­prise of:

(i) The lateral line receptors and

(ii) The pit organs.

The lateral line sense organs lie inside the lateral line canal. These sense organs are the epidermal derivatives and are called neuromasts which remain embedded in the wall of the lateral line canal. This canal communicates to the exterior by minute pores.

The pit organs occur on the lateral and dorsal sides of the head. They are actually ectoder­mal pits connected by groups of sense-cells. The neuromast organs help to orient the body in relation to currents and waves and are designated as rheoreceptors.

Ampullae of Lorenzini:

Enumerable pores on the dorsal and the ventral sides of the head lead into a long tube which terminates into radially arranged ampullary sacs (see Fig. 6.14C). The ampullae lie in clusters and each of these consists of eight or nine chambers arranged radially around a central core called centrum (see Fig. 6.14D).

Two types of cells are encountered in the ampullae—glandular cells and sensory cells. The ampullae get the names according to their location. These sense organs are the thermo receptors and are also the pressure receptors.

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16. Urinogenital System of Scoliodon:

The excretory organ consists of a pair of elongated kidneys. The functional adult kid­neys are called opisthonephros type according to Graham Kerr. The anterior portion of the kidney is non-functional and the posterior por­tion becomes greatly developed.

The kidneys are composed of coiled glandular uriniferous tubules or nephrons. Each tubule consists of a double-walled cup (or Bowman’s capsule) enclosing glomerulus and a much coiled renal tubule. A few renal tubules open into a com­mon collecting tubule. The kidney tubules have the peculiar property of reabsorbing urea.

The collecting tubules of the anterior non-renal portion of the kidney open to the Wolffian duct and the posterior tubules open into the ureter which in turn opens into the urinogenital sinus. The mesonephric duct divides into two.

The dorsal one is named as Wolffian duct and the ventral one is the Mullerian duct. The Wolffian duct in males, becomes the vas deferens which is connected with the vasa efferentia from the testis. The Mullerian duct becomes the oviduct in females.

The kidneys are extremely elongated structures extending from the root of the liver to the cloacal region. The posterior portion is designated as the kidney proper and it is greatly developed. The anterior portion becomes narrower and takes part in conveying the genital products. This part is designated as the organ of Leydig or epididymis.

Male reproductive system:

The testes are paired elongated organs (Fig. 6.17A). Each testis is attached to the dorsal body wall by peritoneal membrane called mesorchium and posteriorly attached by ordinary tissue with the caecal gland. The sperm cells escape by vasa efferentia into the vas deferens which becomes extremely coiled in the anterior por­tion of the kidney.

Posteriorly the vas deferens becomes very much dilated to form the semi­nal vesicle. The seminal vesicles open into the urinogenital sinus which in turn opens into the cloaca. The wall of the urinogenital sinus has become evaginated to form a sperm sac. A pair of sacs designated as siphons are present. The siphons are located under the skin on the ven­tral aspect of the body.

In posterior of the sacs these are continued as the siphon tubes which open to the groove of the clasper of the respec­tive sides (Fig. 6.17C). The siphons are not connected with the genital system. They con­tain sea-water and help in the expulsion of the sperms through the clasper groove.

Female reproductive system:

In females there is no connection between the kidneys and the genital organs. The kidneys are typical excepting that the ureters unite posteriorly and open by a single urinary aperture into the uri­nary sinus. The ovaries are two in number and are kept in position by peritoneal folds called mesovarium.

The shape, size and colour of the ovaries vary greatly according to the age of the individual. A pair of epigonial organs is pre­sent between the ovary and the rectal gland. The oviducts are very long tubes and remain united both posteriorly and anteriorly (Fig. 6.17D). Posteriorly, two oviducts unite to form the vagina which opens into the cloaca.

Anteriorly the oviducts converge and open into the coelomic cavity by a longitudinal slit-­like opening designated as oviducal funnel. At the anterior portion of each oviduct there is a dilated shell gland which has but little signifi­cance in Scoliodon. As Scoliodon is viviparous the posterior portion of the oviduct becomes dilated to form the uterus for the development of the young.

The number of embryos present inside one uterus varies greatly in different species of Scoliodon. In S. sorrakowah about seven embryos have been observed. The mucous membrane of the uterus becomes divided into a number of compartments each of which houses an embryo. The number of compartments varies directly with the number of embryos.

The eggs are large and heavily yolked. Each egg gets a coating of albumen during its transit down the oviduct. The egg, particularly in oviparous forms, is enclosed by horny shell which is oblong in shape. The angles of the shell are prolonged into four coiled elongated filamentous processes (Fig. 6.17B). This condi­tion is not observed in Scoliodon but is observed in case of oviparous sharks.

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17. Development of Scoliodon:

Scoliodon is viviparous and gives birth to living youngs. Fertilization is internal. It is claimed that during copulation the claspers are introduced into the cloacal aperture of the female for the transmission of spermatozoa. The egg of Scoliodon is strongly telolecithal. The cleavage is restricted to a small germinal disc which floats on the top of the yolk mass (Fig. 6.18A).

The cleavages are meroblastic and the first cleavage plane is simply a furrow in the surface of the germinal disc. The second cleavage occurs at right angle to the first one (Fig. 6.18B). After this stage, the cleavage plane is irregular. By this way of cleavage, a blastodisc is separated from a layer of periblast cells. The periblast cells form a syncitial layer over the yolk mass.

A blastocoel is formed between the blastodisc and the central region of the periblast. With the expansion of the blastocoel the blastodisc becomes multi- layered. The posterior region of the blastodisc grows faster than the other regions and is raised from the yolk mass to become a double- layered germ ring. This germ ring is raised pos­teriorly to form a blastopore (Fig. 6.18D).

At this stage an upper layer (epiblast) and a lower hypoblast is separated. The blastocoel lies between the ectoderm (epiblast) and the inner hypoblast (mesendoderm). The process of gastrulation actually starts with the formation of the dorsal lip of the blastopore. The endoderm is differentiated by cellular proliferation from the underside of the hypoblast. The mesoderm produces a prechordal plate.

As the embryo elongates along the antero­posterior axis, a head fold is marked off from the blastodisc and it becomes raised up to form the neural folds. The body of the embryo is constricted from the blastodisc and the tail fold is differentiated and extended backward.

The blastopore is separated into two (embry­onic and vitelline blastopores) by the fusion of the edges of the yolk sac. The circulatory sys­tem appears in the mesoderm in the form of blood islets which unite to form vascular net­works. As the embryo grows, the mouth is formed from the stomodeal invagination and the anus is formed anew from the proctodeal invagination because the embryonic blasto­pore becomes closed.

The developing embryo is provided with a tubular yolk stalk. This yolk stalk connects the intestine of the embryo with the yolk sac. The yolk sac contains yolk material which pro­vides nutrition for the developing embryo. When the yolk is fully exhausted the yolk sac becomes folded and becomes anchored with the uterine wall of the mother as the yolk-sac placenta.

With the formation of placenta, the yolk stalk is lost and the blood vessels of the yolk stalk form the placental cord. The pla­cental cord connects the embryo with the yolk-sac placenta (Fig. 6.18E). The placental cord develops many finger-like processes called appendicula.

Each appendicularium is made up of a central core of loose connective tissue surrounded by several layers of epithe­lial cells. The distribution of the appendicula in the placental cord varies in different regions. The appendicularia help in the absorption of nutrients secreted from the uter­ine wall of the mother.