PISCES 



205 



have spines on the tail. The thresher-shark {Alo'pias vulpes) 

 has its pUable tail prolonged into a terrible weapon, with which, 

 it is said, it can kill a whale. This lashing tail serves a second 

 purpose by so frightening the small fishes that they crowd to- 

 gether and are thus easily obtained for food. The "devil-fish" 

 strikes terrible blows with its broad pectoral fins. The saw-fish 



Fig. 165. — Sword-fish (Tetraptu'rus), yellow-fin tuna, and yellow tail, 

 caught with rod and reel at Santa Catalina Island. (Bulletin of B. of F., 

 vol. xxviii, 1908.) 



(Fig. 164), sword-fish (Fig. 165), and the hke use their long 

 strong jaws as frightful weapons. The torpedo and other 

 electrical fishes surprise and stun their victims by an electric 

 shock. 



Influence of Temperature. — Species differ in their ability to 

 endure cold or heat. The brook trout loves the cool water of 



