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The below mentioned article provides short notes on Class Chondrichtyes.
1. The fishes of this class bear cartilaginous endoskeleton. They are cold blooded (poikilothermous or ectothermic). There are about 600 species of cartilaginous fishes.
2. The skin is tough, containing minute placoid scales, which are dermal in origin.
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3. Except in the Chimaeras, the gills are not covered by an operculum (gill cover).
4. Jaws are well developed. Mouth is ventrally placed. A spiral valve called scroll valve is usually present in the intestine. The digestive tract leads into the cloaca.
5. External nares are present on ventral side of the head. The internal nares are absent.
6. Paired fins are broad. The caudal fin is mostly heterocercal -asymmetrical (Gr.heteros- different).
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7. Heart is two chambered (one auricle and one ventricle). Heart has a well developed sinus venosus; and conus arteriosus. There is well developed renal portal systems RBCs are oval and nucleated.
8. Kidneys are mesonephric. Urea is chief nitrogenous waste.
9. The brain bears large olfactory lobes and a large cerebellum.
10. There are 10 pairs of cranial nerves.
11. Lateral line system is well developed.
12. Lung or air bladder is absent in these fishes.
13. They have internal ears which help the fish keep its balance. The nictitating membrane in the eye of fish is well developed.
14. Some of them are electric organ (e.g. Torpedo) and some possess poison sting (e.g.Trygon).
15. They have cloaca. In males, pelvic fins may bear 1 or 2 claspers which are used in copulation. The oviducts in cartilaginous fishes are called Mullerian ducts. The fertilization is internal. They are oviparous or ovoviviparous (eggs hatch in mother’s genital tract) Development is direct.
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16. Most of the cartilaginous fishes are marine and most of them are predators.
Examples:
Scoliodon, Torpedo, Trygon and Chimaera. Rhinobatus (guitar fish) Pnstis, Zygaena, Carcharodon (Great White Shark).
Chimaera:
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It is commonly called rabbit fish, rat fish, ghost fish or king of her rings. It is an interesting fish that represents the characters of a shark and a bony fish. Like s ark it has cartilaginous skeleton, a pair of pelvic claspers and placoid scales.
Like bony fish, it possesses operculum (gill cover) on each side, and has distinct anus and urinogenital aperture. Cloaca is absent. Male has a frontal or cephalic clasper on the dorsal surface of the head, its function is unknown.
Scoliodon (Dog Fish):
In India, Scoliodon dumerili is common in the Bay of Bengal, here are present some pores, the ampullary pores on the upper and lower surface of the head: each pore leads into an ampulla(pl. ampullae), called Ampulla of Lorenzini through which the fish receives information of the temperature fluctuations in the surrounding water.
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Behind the head on each side there are present five oblique openings, known as gill slits, which communicate internally with the pharynx. The caudal fin surrounds the tail, showing asymmetry (upper epicaudal lobe and lower hypocaudal lobe). Such a type of tail, where two different kinds of lobes are present, is known as heterocercal tail.
Numerous dermal placoid scales are embedded in the skin, which form the exoskeleton of the fish. The male can be distinguished from the female, since the former has a pair of hard elongated claspers attached to the pelvic fins. The claspers help in the copulation. It is ovoviviparous. Scoliodon is also eaten as food by some persons. It yields liver oil.
Zygaena (Hammer-Headed Shark):
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It is a voracious surface feeder and fast swimmer. The head forms two lateral lobes, bearing eyes at their distal ends. The tail has a heterocercal caudal fin. Oil is extracted from its liver.
Pristis (The Saw Fish):
The head forms a saw like rostrum which bears a series of strong teeth-like denticles along the margin. It uses the denticles for offence and defence.Its body is intermediate between the sharks and the rays. Ventral or anal fin is absent. Caudal fin is heterocercal. The fish is ovoviviparous.
Torpedo (Electric Ray):
It is a bottom-living marine fish, discharging electricity which is sufficient to make unconscious the preys such as small fishes, etc. Both the pectoral fins are fused with the trunk. A pair of electric organs is situated on the dorsal side of the trunk region.
In-fact the electric organs are the modified lateral muscle-plates innervated by the cranial nerves. Torpedo can generate electric current. The water is drawn through the spiracle and not through the mouth as happens in many fishes.
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Trygon (Sting Ray):
It resembles the electric ray in many aspects but does not have electric organs. The body is somewhat kite-shaped. The tail is whip-like bearing a spine which is modified form of the dorsal fin. The spine or sting makes a severe wound on the victim.
