SHARKS AND RAYS 299 



ORDER III. : SHARKS AND RAYS (SELACHII). 



Skeleton cartilaginous. Opercula absent ; branchial apertures opening 

 on to the surface. Mouth usually ventral. 



The Blue Shark (Carcharias glaucus). 

 (Length up to 15 feet.) 



A. Its Habitat. 



This gigantic animal is an inhabitant of tropical and temperate seas. 

 Its northern range extends to the southern coasts of England ; hence it 

 is not found in the North Sea. As it frequents the 

 open sea, its colour is of a beautiful slaty blue above 

 (see herring), whereas the under surface is white (see 

 carp). The scales are small tooth-like structures 

 consisting of a disc-like root and a spike coated with 

 enamel. Only the spikes of these denticular pro- 00TH ^ L a^^f™ ° F 

 cesses project from the skin, which thereby acquires a 

 granular surface (shagreen). The skin of small sharks is rough, like 

 sand-paper, and is used as a polishing material. 



B. Skeleton. 



The skeleton remains cartilaginous throughout life. A skeleton of 

 this nature would be out of the question in the case of a land animal 

 of the gigantic size of this fish (such an animal would collapse under its 

 own weight). It is sufficient in the case of a fish whose body is sup- 

 ported by the water. (See p. 268, and compare what has been said 

 about the size and skeleton of the Greenland whale.) 



C. Gills and Eyes. 



Gill-covers, like those of teleostean fish, are absent, and the branchial 

 clefts are therefore visible on the neck. The gills lie within the clefts. 

 (For the spiracles of other shark species,' see under ray.) The eyes are 

 provided with upper and lower lids and a nictitating membrane. 



D. Food. 



Like other allied species of shark, the blue shark is of very voracious 

 and predatory habits. It is fond of following ships in order to pick up 

 such kitchen offal as is thrown into the sea (hyaena of the ocean). 



(ft) Its size and strength make it formidable to even the largest fishes, 



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