222 Shark and Co?npany 



tauriis), is said to have developed a kind of substitute for a swim bladder 

 by swallowing air and keeping an "air pocket" in its stomach. Thus, its 

 stomach is believed to act as a hydrostatic organ similar to the Teleost's 

 swim bladder. 



In its lifelong swim, the shark does not sleep, at least as we humans 

 know sleep. Sharks that spend their lives inshore seem to rest— or perhaps 

 sleep— by swimming into shallow caverns, apparently alighting on rocky 

 ledges, or seemingly resting on the bottom. Divers frequently are able 

 to approach these "sleeping" sharks with ease. Sharks that spend their 

 lives in the open ocean do not appear to rest, for, if they ceased moving, 

 they would sink, often to abyssal depths. Of course, some sharks live 

 in the great deeps permanently. The "sleep" of any shark, at any depth, 

 however, is possibly only a physiological pause in its activity. 



The shark is a creature marvelously adapted to its environment. It 

 achieved this harmony with the sea eons ago, and, from what we know 

 of evolution, the shark's basic structure has remained virtually unchanged 

 mainly because its prehistoric adaptation was so perfect, although much 

 specialization has occurred among different species. 



A tough skin plated with row upon row of teeth; three great muscles 

 flexing nearly the length of each side of its body; a strong, gristly, 

 resilient skeleton— these form the dwelling place of what might be said 

 to be the essence of the shark. In addition, there is a tiny brain and a 

 nervous system perfectly attuned to the animal's activity in its environ- 

 ment. 



The silhouette of a typical shark is unmistakable. Unlike the mouth 

 of the typical Teleost, the mouth of most sharks is curved and lies on 

 the under side of its head. Its tail, or caudal, fin is almost always asym- 

 metrical, with the upper lobe usually the far longer one. Its fins are 

 flipper-like and differ from the Teleost's fins, which are held rigid by a 

 network of rays or spines. Sharks cannot move their side fins freely to 

 swim, as Teleost fishes can. A shark's fin arrangement is also distinctive. 

 The pectoral fins are generally larger than those of the Teleost. The 

 ventral, or pelvic, fins have, in the male, appendages called "claspers," 

 which are intromittent or sexual organs. Aft of the ventral fins, between 

 the vent and the tail, is the anal fin. The caudal itself sweeps upward, 

 forming the two lobes, the upper of which may have a notch, whose 

 purpose is not known. And jutting from the back of most sharks is the 

 familiar dorsal fin that, when seen, is the warning banner of a shark's 

 presence. 



The skeleton of the shark is formed of cartilage, but in some species 

 so much calcium is deposited in the cartilage that it is almost as rigid as 

 bone. Never, however, is true bone developed. This lack of bone does 

 not mean a lack of skeleton; the familiar structural framework of the 



