SHARKS AND RAYS 301 



which are sharp as knives, the edges being in many cases themselves 

 toothed. As a rule, however, the anterior row of teeth only is brought 

 into use at a time. The teeth sit only loosely in the mucous membrane 

 of the mouth, and consequently easily drop out, in which case those of 

 the next row, which hitherto have been lying horizontally in the jaw, 

 with their points directed backwards, are pushed forward and at the 

 same time erected. 



(/) The pharynx and oesophagus are very wide ; the intestine is short 

 (see pike). 



E. Reproduction. 



The blue shark brings forth living young. (The majority of the 

 sharks, however, reproduce themselves by means of eggs which are 

 surrounded by a hard, leather-like, usually square- shaped envelope or 

 " purse," so-called "mermaids' purses." The points of these egg-cases 

 are drawn out into long threads, which are used like tendrils for attaching 

 the eggs to water-plants, stones, etc.) 



Allied Species. 



The Hammer-Headed Sharks (Zygana) are inhabitants of tropical 

 seas. They take their name from the peculiar, hammer-like shape of 

 their head. They are dangerous animals, which often attain to a length 

 of nearly 14 feet. Several species of shark are also found in the British 

 seas. All these are destructive to the fisheries, but never attack man. 



Nearly allied to the sharks are the Rays (Rajce). The most familiar 

 species of our seas is the Thornback Ray (Raja davata). In their mode 

 of life the rays resemble the flat fishes (see p. 293), although in structure 

 they are utterly different. The head and abdomen are compressed 

 from above downwards 'into a broad plate, which laterally is fringed by 

 the large fan- shaped pectoral fins, and which carries posteriorly the 

 smaller ventral fins. The thin, whip-like tail with the two dorsal and 

 the small caudal fin is distinctly marked off from the disc-shaped 

 anterior portion of the body. The back of the animal is covered with 

 numerous nail-like spines (hence name), and is exactly of the colour of 

 the sea-bottom on which the animal rests, or lies in wait for prey (fishes, 

 crustaceans, etc.) ; the under side, on the other hand, is colourless. 

 The transversely placed mouth on the under side of the head and the 

 five similarly placed gill apertures at once distinguish the ray from the 

 flat fishes, and prove its connection with the sharks. The water used 

 for respiratory purposes is not taken in by the mouth, as in teleostean 

 and ganoid fishes, but, as in most sharks (excepting the blue shark 



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