ZOOLOGY SUPERCLASS- PISCES CLASS - CHONDRICHTHYES

 SUPER CLASS PISCES :-

CLASS CHONDRICHTHYES :- (CARTILAGINOUS FISHES  )  

NATURAL HISTORY 

Skeleton is made of cartilage. Most sharks and rays are marine, but a few live in tropical rivers above salt water. Sharks live in open water, and rays on the bottom. The sharks are predaceous, active swimmers and feed on small fishes. Sharks and rays are used as human food in many countries. Shark liver contains much oil rich in vitamin A. Sharks are also a nuisance to fishermen, because they tear nets and steal captured fishes. Large sharks may capsize small boats and injure or kill fishermen. Living fishes probably exceed 20,000 species, an abundance and diversity unequalled among all other vertebrates. They originated 530 million years ago allowing vast span of time for evolutionary divergence and for origin and extinction of major phyletic lines. 

 SCOLIODON (DOGFISH) :-

CLASSIFICATION 

Phylum .............. Chordata

Group ................ Craniata

 Subphylum ........ Vertebrata 

 Division ............. Gnathostomata 

 Superclass ......... Pisces 

 Class .................. Chondrichthyes (=Elasmobranchi) 

 Sub-class ........... Selachi 

 Order ................ Pleurotremata (=Squali) 

 Family ............... Scyllidae 

 Genus ................ Scoliodon (Dogfish)

Geographical distribution :-Scoliodon has wide distribution. About 4 species are found all along the Indian sea coast. Its presence has been reported from Zanzibar, Srilanka to Malay Archipelago, East Indies, Philippine Islands, Mexico to Panama, Cuba, West Indies and South America. Lower Carboniferous to Recent.  

Habit and habitat :- The natural home of Scoliodon is the sea, but some live in estuaries and even ascend the rivers. They are predacious and voracious feeders attacking their prey with powerful jaws. They are active swimmers. 

CHARACTERS :-

(1) Commonly called dogfish or dogshark. 

(2) Spindle-shaped body, about 60 cm long, is regionated into head, trunk and tail. 

 (3) Dorsal and lateral sides of body are pigmented dark grey or slaty grey, while the ventral side is white. 

 (4) Head is dorso-ventrally compressed and flattened into snout. It contains ventrally situated slit-like mouth, obliquely situated nostrils and laterally situated protuberant eyes. A little behind eyes there are five pairs of lateral gill-clefts. 

 (5) Trunk bears paired fins. A pair of anterior pectoral tins and a pair of posterior pelvic tins. 

 (6) Heterocercal tail turned upwards and caudal tin. 

 (7) A pair of pigmented lateral lines extends from head to tail. 

 (8) Scoliodon exhibits sexual dimorphism. Males are easily recognized by having a pair of intromittent organs, called as claspers. Cloaca is found between 2 pelvic fins.  

Economic importance :- Scoliodon has great educational and experimental value because of its availability and size and is a favourite dissection fish for undergraduate students. 

Special features : Sharks are used as human food in many countries. Shark's fins are dried and then boiled to yield a gelatinous substance favoured for soups. The tanned shark skin shagreen is used to case fine books, jewel boxes and sword handles, etc. Sharks are nuisance to fishermen, because they tear nets, steal captured fishes and take out bait or fish from hooks.