Extranal character of scoliodon
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(i) Head:
The head is strongly compressed dorso-ventrally and is produced in front into a wedge-shaped snout or rostrum.
(ii) Trunk:
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The trunk is almost elliptical in transverse section. Its thickest part lying in front of the middle of the body. The trunk gradually tapers behind into the tail.
(iii) Tail:
The tail is laterally compressed and is bent upwards at a small angle and fringed with a caudal fin. Such a tail is known as heterocercal tail.
Fins:
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Dogfish (Scoliodon) is provided with two sets of fins which are flattened expansions of the skin supported by cartilaginous rods and horny fin rays- these are unpaired or median fins and paired lateral fins.
(i) Median Fins:
The median fins are two dorsal fins, a ventral or anal fin and a caudal fin. The first dorsal fin is large and triangular in shape and is situated a little in front of the middle of the body. It has a basal lobe. The second dorsal fin is also triangular in outline but is very small and is situated midway between the first dorsal and the tip of the tail. The caudal fin extends along the dorsal and ventral surfaces of the tail in the median line and forms a dorsal and ventral lobe.
The dorsal lobe is very much reduced and forms only a low ridge along the greater part of the upper surface but the ventral lobe is well developed and is divided into two parts, the anterior part being much larger and more extensive than the posterior. The ventral or anal fin is situated in the mid-ventral line about 5.0 cm in front of the caudal fin, more or less opposite the second dorsal. Each of the two dorsals and the ventral fin are produced behind into long and narrow fleshy processes called basal lobes.
(ii) Lateral Fins:
The lateral fins are paired pectoral and pelvic fins. The large pectoral fins originate from the ventro-lateral margins of the body immediately behind the gill-clefts. The pelvic fins are much smaller than the pectoral fins and arise close together from the ventral surface at the junction of the trunk and tail. In the male Scoliodon, the medial part of each pelvic fins is produced into a dorsally grooved, stiff rod-like clasper (intromittent organ) used during copulation. All the fins are directed backwards a feature which is advantageous in forward progression.
(iii) Eyes:
On each side of the head is a large circular eye. Each eye has two poorly-formed immovable eye-lids and nictitating membrane located antero-ventrally. The pupil is narrow and vertical.
(i) Mouth:
On the ventral side of the head is a wide crescentic mouth. It is bounded by upper and lower jaws. Each jaw is armed with one or two rows of sharp backwardly directed teeth, having smooth not-serrated edges for holding and tearing the prey. In catching its prey the shark brings its jaws into play by raising the snout and thrusting the mouth forward.
(ii) Nostrils:
On the anterior side of the mouth are present two crescentic apertures the nostrils leading into olfactory sacs which do not open into the mouth cavity. A small fold of skin from the anterior edge partially covers each nostril. The nostrils are only olfactory and have no respiratory function.
(iii) Gill-Clefts:
Behind the eyes are situated a series of vertical slits, five on each side, called branchial or gill-clefts. These apertures lead into the gill-pouches and thence into pharynx. These are respiratory in function.
(iv) Cloacal Aperture:
The cloacal aperture is an elongated median opening at the root of the tail between two pelvic fins. It leads into the cloaca into which the intestine and urinary and genital ducts open. Anus lies anteriorly and in the cloaca and a cone-shaped papilla bearing urinary pore in female and urino-genital pore in male located behind the anus.
(v) Abdominal Pores:
There is a pair of openings situated on the elevated papillae on either side of the cloaca. These are the abdominal pores. Through these pores the coelom communicates with the exterior.
(vi) Caudal Pit:
At the root of the tail, just in front of the caudal fins, is a small dorsal and a ventral shallow depressions, the caudal pits. The caudal pits are the characteristic feature of the genus Dogfish (Scoliodon).
(vii) Lateral Line and Pores:
A faint line runs on either side of the body extending from the head to the posterior end of the tail, this is called the lateral line. It marks the position of an underlying canal which runs along each side of the body and contains special receptor organs. The lateral line canal extends anteriorly into the head where it branches into several canals; at intervals these canals open to the exterior through minute pores.
(viii) Ampullary Pores:
On the head and snout open several groups of minute ampullary pores of the receptors called ampullae of Lorenzini. When pressed they exude mucus.
The head is strongly compressed dorso-ventrally and is produced in front into a wedge-shaped snout or rostrum.
(ii) Trunk:
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The trunk is almost elliptical in transverse section. Its thickest part lying in front of the middle of the body. The trunk gradually tapers behind into the tail.
(iii) Tail:
The tail is laterally compressed and is bent upwards at a small angle and fringed with a caudal fin. Such a tail is known as heterocercal tail.
Fins:
ADVERTISEMENTS:
Dogfish (Scoliodon) is provided with two sets of fins which are flattened expansions of the skin supported by cartilaginous rods and horny fin rays- these are unpaired or median fins and paired lateral fins.
(i) Median Fins:
The median fins are two dorsal fins, a ventral or anal fin and a caudal fin. The first dorsal fin is large and triangular in shape and is situated a little in front of the middle of the body. It has a basal lobe. The second dorsal fin is also triangular in outline but is very small and is situated midway between the first dorsal and the tip of the tail. The caudal fin extends along the dorsal and ventral surfaces of the tail in the median line and forms a dorsal and ventral lobe.
The dorsal lobe is very much reduced and forms only a low ridge along the greater part of the upper surface but the ventral lobe is well developed and is divided into two parts, the anterior part being much larger and more extensive than the posterior. The ventral or anal fin is situated in the mid-ventral line about 5.0 cm in front of the caudal fin, more or less opposite the second dorsal. Each of the two dorsals and the ventral fin are produced behind into long and narrow fleshy processes called basal lobes.
(ii) Lateral Fins:
The lateral fins are paired pectoral and pelvic fins. The large pectoral fins originate from the ventro-lateral margins of the body immediately behind the gill-clefts. The pelvic fins are much smaller than the pectoral fins and arise close together from the ventral surface at the junction of the trunk and tail. In the male Scoliodon, the medial part of each pelvic fins is produced into a dorsally grooved, stiff rod-like clasper (intromittent organ) used during copulation. All the fins are directed backwards a feature which is advantageous in forward progression.
(iii) Eyes:
On each side of the head is a large circular eye. Each eye has two poorly-formed immovable eye-lids and nictitating membrane located antero-ventrally. The pupil is narrow and vertical.
(i) Mouth:
On the ventral side of the head is a wide crescentic mouth. It is bounded by upper and lower jaws. Each jaw is armed with one or two rows of sharp backwardly directed teeth, having smooth not-serrated edges for holding and tearing the prey. In catching its prey the shark brings its jaws into play by raising the snout and thrusting the mouth forward.
(ii) Nostrils:
On the anterior side of the mouth are present two crescentic apertures the nostrils leading into olfactory sacs which do not open into the mouth cavity. A small fold of skin from the anterior edge partially covers each nostril. The nostrils are only olfactory and have no respiratory function.
(iii) Gill-Clefts:
Behind the eyes are situated a series of vertical slits, five on each side, called branchial or gill-clefts. These apertures lead into the gill-pouches and thence into pharynx. These are respiratory in function.
(iv) Cloacal Aperture:
The cloacal aperture is an elongated median opening at the root of the tail between two pelvic fins. It leads into the cloaca into which the intestine and urinary and genital ducts open. Anus lies anteriorly and in the cloaca and a cone-shaped papilla bearing urinary pore in female and urino-genital pore in male located behind the anus.
(v) Abdominal Pores:
There is a pair of openings situated on the elevated papillae on either side of the cloaca. These are the abdominal pores. Through these pores the coelom communicates with the exterior.
(vi) Caudal Pit:
At the root of the tail, just in front of the caudal fins, is a small dorsal and a ventral shallow depressions, the caudal pits. The caudal pits are the characteristic feature of the genus Dogfish (Scoliodon).
(vii) Lateral Line and Pores:
A faint line runs on either side of the body extending from the head to the posterior end of the tail, this is called the lateral line. It marks the position of an underlying canal which runs along each side of the body and contains special receptor organs. The lateral line canal extends anteriorly into the head where it branches into several canals; at intervals these canals open to the exterior through minute pores.
(viii) Ampullary Pores:
On the head and snout open several groups of minute ampullary pores of the receptors called ampullae of Lorenzini. When pressed they exude mucus.
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