In this article we will discuss about the classification of chondrichthyes.
Class 4. Chondrichthyes (Gr., chondros = cartilage; ichthys = fish):
1. Mostly marine and predaceous.
2. Body fusiform and spindle-shaped.
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3. Fins both median and paired, all supported by fin-rays. Pelvic fins bear claspers in male. Tail heterocercal.
4. Skin covered with placoid scales or naked and bears mucous glands.
5. Endoskeleton entirely cartilaginous often calcified, without true bones. Notochord persistent. Vertebrae complete and separate. Pectoral and pelvic girdles present.
6. Mouth ventral. Jaws present. Teeth are modified placoid seales. Stomach J-shaped. Intestine with spiral valve.
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7. Respiration by gills. Gill-slits 5 to 7 on each side. Operculum or gill-cover absent. No air-bladder and lungs.
8. Heart two chambered (one auricle and one ventricle). Sinus venosus and conus arteriosus present. Both renal and hepatic portal system present.
9. Kidneys opisthonephros (only posterior part of kidney is excretory in function). Excretion ureotelic.
10. Cloaca present. Its aperture serves as a common outlet for rectum, renal and reproductive ducts.
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11. Brain with large olfactory lobes and cerebellum. Cranial nerves ten pairs.
12. Olfactory sacs do not open into pharynx. Membranous labyrinth (internal ear) with three semicircular canals. Lateral line system present.
13. Sexes separate. Male usually possesses claspers. Gonads paired. Gonoducts open into cloaca.
14. Fertilisation internal. Oviparous or ovoviviparous. Eggs large, yolky. Cleavage meroblastic. Development direct, without metamorphosis.
Classification of Chondrichthyes:
The class Chondrichthyes (Gr., chondros = cartilage; ichthys = fish) also called Elasmobranchii (Gr., elasmo = plate; bronchia = gills), including sharks, rays, skates and chimaeras. According to Schultz there are about 600 living species of cartilaginous fishes.
The classification followed here is based after that of Romer (1959):
Subclass 1. Selachii (Gr., selachos = a shark):
1. Paired fins without a median axis.
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2. Base of pectoral fin constricted and pectoral radials jointed.
3. Gill-slits numerous on either side and each is protected by independent skin flaps.
4. Presence of a spiracle behind each eye.
5. Vertebral centra usually differentiated.
6. Cloaca present. Males with claspers.
7. Tail heterocercal with large lobe.
Order 1. Squaliformes or Pleurotremata:
(Gr., pleure = side; trema = opening)
1. Body typically spindle-shaped.
2. Gill-slits lateral, 5-7 pairs. Spiracles small.
3. Pectoral fins moderate, constricted at base. Anterior margin of pectoral fin free.
4. Hyomandibular bearing branchial rays.
5. Right and left halves of pectoral girdle dorsally separated.
6. Tail heterocercal.
Examples:
True sharks. About 250 living species. Dogfishes (Scoliodon, Chiloscyllium, Mustelus, Carcharinus), Spiny dogfish (Squalus), Seven-gilled shark (Heptanchus), Tiger shark or Zebra shark (Stegostoma), Hammer-headed shark (Zygaena or Sphyrna), Whale shark (Rhineodon).
Order 2. Rajiformes or Hypotremata:
(Gr., hypo = below; trema = opening)
1. Body depressed, flattened dorsoventrally.
2. Gill-slits ventral, 5 pairs.
3. Pectoral fins enlarged, fused with sides of the head and body.
4. Spiracles large and functional.
5. Two halves of pectoral girdle united either to each other or to the vertebral column.
6. No branchial rays on hyomandibular.
Examples:
Skates and rays. About 300 species. Skate (Raja), Stingray (Trygon), Electric ray (Torpedo), Eagle ray (Myliobatus), Guitar fish (Rhinobatus), Sawfish (Pristis).
Subclass 2. Holocephali:
(Gr., holos = entire; kephale = head)
1. Single gill opening on either side covered by a fleshy operculum.
2. No scales, spiracles and cloaca.
3. Jaws with tooth plates.
4. Nasal opening single.
5. Lateral line system with open groove.
6. Males with claspers. Cephalic clasper on the head of male.
7. Tail long and thin.
Examples:
Rat fishes or chimaeras. About 25 species. Hydrolagus (= Chimaera)