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 with 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 
