TIGERS OF THE DEEP 



lower jaw six rows of renewable teeth, and these can take 

 different motions according to the will of the animal. It 

 has an arsenal of fearsome weapons for the destruction of 

 its victims and enemies, and it uses them economically. 



The rows of teeth are obedient to the muscles round 

 their bases, by means of which the shark can erect or 

 retract any of its rows in accordance with its require- 

 ments. It can even erect a portion of a row, while the rest 

 remain depressed in their beds. Thus the tyrant of the 

 ocean can measure the number and power of its weapons 

 and use the requisite rows or parts of rows. For the 

 destruction of the weak and defenceless one row of teeth 

 may suffice. As it snatches at its prey it may be in such a 

 position that certain rows of teeth are better placed to 

 deal with it. For formidable adversaries it can bring its 

 entire arsenal into play. 



It has other offensive weapons. It can use its skin 

 aggressively and effectively. Its tail is possessed of im- 

 mense power and is capable of breaking a man's arm or 

 leg in one swift lashing stroke. 



The shark again differs from the bony fishes in that it 

 has no air-bladder; while it is distinguished from the 

 rays and skates of its own group by the position of its 

 branchial clefts, which are always lateral, by its fan- 

 shaped pectoral fins, which (with few exceptions) have 

 restricted bases, and by the shape of its body, which is 

 rounded and elongated and which tapers to its tail. This 

 is heterocercal — unequally lobed. 



Sharks breathe by gill-sacs or pouches, which open 

 externally on the neck by gill-slits, of which there are 

 from five to seven pairs. Water may also be admitted 

 through the spiracles : a pair of openings on the upper 

 side of the head which contain a rudimentary gill, and 

 communicate with the mouth. 



The majority of sharks are viviparous, and in some the 

 embryos are nourished in a placenta-like structure. Those 

 that are oviparous, like the dogfish, which is usually 



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