CHAPTER XXIV. 

 THE SHARKS. 



WE now come to animals with a backbone, a movable 

 lower jaw, a true skull, and with fins in pairs. Such back- 

 boned creatures are the fishes. All fishes agree in having 

 either a gristly or bony skeleton, a lower jaw, and in swim- 

 ming by means of fins. The following is a view of the 



SUB-CLASSES OF FISHES. 



1. Skeleton cartilaginous; 5-7 pairs 



of gill-openings ElasmobrancMi. Sharks, Rays. 



2. Skeleton cartilaginous or bony; 



scales often square, enamelled. Ganoidei. Sturgeon, Garpike, 



and Lung fishes. 



3. Skeleton bony, of numerous sep- 



arate bones; 4 pairs of gills. ..Teleostci. Cod, Gunner, Perch, 



etc. 



The sharks, though fish-like, are very different from 

 ordinary bony fish. Their cartilaginous skeleton, includ- 



FiG. 152. Cestracion, or Australian Shark. 



ing the skull, is so soft that it can be cut with a knife, 

 while the tail is one-sided, the vertebral column ending 

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