538 



THE OCEAN WORLD. 



muzzle, is elongated ; its jaws are cartilaginous, and furnished with 

 many rows of hooked and pointed teeth ; the nostrils are placed in 

 front of the mouth. The eyes, which open in the upper part of the 

 head, are half projecting, and protected in part by a continuation of 

 the soft, elastic, and retractile skin which covers the head. Imme- 

 diately behind the eyes are two spiracles, which communicate with the 



Fig. 359.— The Thornback (Raia clavata). 



interior of the mouth. The animal is able to open and close these 

 holes at pleasure, by means of an extensible membrane, which acts 

 as a sort of valve. Through these holes it ejects the- superabundant 

 water beyond that which is necessary for respiration. In its general 

 colour the animal is ashy grey on its upper surface ; white vrith rows 

 of black spots below. 



Its tail is long, flexible, and slender, acting at once as a rudder 

 and a weapon of offence or defence. When lying in ambush, nearly 

 buried in mud at the bottom of the sea — and it has no desire to 



