CHAPTER XXIV. 



THE SHARKS. 



We now come to animals with a backbone, a moTable 

 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 lony skeleton, a lower jaw, and in swim- 

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



Stjb-Classbs of Fishes. 



1. Skeleton cartilaginous; 5-7 pairs 



of gill-openings Elasmobranchii. Sharks, Rays. 



2. Skeleton cartilaginous or bony; 



scales often square, enamelled. Oanoidei : Sturgeon, Garpike. 



3. Skeleton bony, of numerous sep- 



arate bones; 4 pairs oi giWs. . .Teleostei: Cod, Gunner, Perch, 



etc. 



4. With one lung or two lungs Dipnoi : Lung-flshes. 



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

 ordinary bony fish. Their cartilaginous skeleton, includ- 



Fio. 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,d;he vertebral column ending 



10 



i,^ne verteoral coin 



tlAtcJl^ 



