CHAPTER XI. 

 BRANCH CHORDATA. 



CLASS PISCES (Continued). 

 The Elasmobranchs. 



THE sharks and rays are the chief representatives of the 

 subclass designated Elasmobranchii. They are nearly all 

 marine. One of the best representatives is the shark 

 known as the "dogfish," which is caught in large numbers 

 along the New England coast for the sake of the oil ob- 

 tained from the livers. It differs from the " true fishes " 

 in the following points : 



1. The skeleton is cartilaginous, never bony. 



2. There is no gill cover, the gill slits (usually five) 

 opening separately. 



3. The mouth and nostrils" open ventrally. 



4. The scales are small and separate, making the skin 

 rough. 



5. The tail is heterocercal, that is, the spinal column 

 extends up into the upper lobe. 



6. The eggs are few and large, inclosed in a tough case, 

 the walls being strengthened by chitin. In some sharks, 

 as the dogfish, the young are brought forth alive. 



7. There is no air bladder. 



The Sharks. The typical shark has a spindle-shaped 

 body, and is exceedingly active. The mouth is far back 

 under the head instead of in front, as in most bony fishes. 



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