AQUATIC VERTEBRATES 369 



side up or upside down. It is just as likely to swim on its back as it 

 is in the more normal position with the back up. The sense of hearing 

 is localized in a row of sense organs contained in a lateral line which 

 runs along both sides of the body. These sense organs pick up vibra- 

 tions in the water. 



With respect to reproduction the sharks seem to be quite advanced, 

 for the young are born alive, a condition that we commonly associate 

 only with the highest class of vertebrates, the mammals. However, upon 

 closer study, it is found that the sharks have eggs just as the other lower 

 vertebrates, but these are retained in the body until they hatch and the 

 young pass from the mother's body alive and active. We say that ani- 

 mals that lay eggs are oviparous and animals that have the young born 

 alive, as the mammals, are viviparous; so this condition where they 

 have eggs and have the young born alive also might be called ovovivi- 

 parous. In any animal in which the young are born alive there must 

 have been copulation and internal fertilization. The median parts of 

 the pelvic fins of the male sharks are modified into claspers which aid 

 in this process. 



The sharks have a bad reputation because they are often blamed for 

 things done by other water animals. There are large tiger sharks found 

 in tropical seas that have been known to attack man, but the cases are 

 not common. Most of the cases of supposed attacks by sharks are actu- 

 ally made by a vicious fish, the barracuda, that has been known to strip 

 the flesh from a person's leg off the southern coast of Florida. The 

 waters of the Atlantic coast contain large numbers of a small shark, the 

 dogfish shark, which is quite a nuisance to fishermen because it gets in 

 their fish nets and may damage them with its sharp teeth. The flesh of 

 the shark is not very palatable, but is sometimes eaten and sometimes 

 canned and sold under a trade name so the person buying it will not know 

 that he is eating shark. The oil from the livers is an important source of 

 vitamins A and D. 



One of the most abundant sharks found along the waters of the 

 Southern states bordering the gulf is a peculiar form called the hammer- 

 head shark. The anterior part of the head spreads out so that it forms 

 a broad extension on either side and the entire animal somewhat resem- 

 bles a hammer with two driving heads. 



The elasmobranchs also include another group of animals called the 

 rays. Their general characteristics are about the same as the sharks, but 

 the body shape has been greatly modified. They are flattened out dorso- 

 ventrally into a broad fan-like body with a long slender tail. One of 

 these is called the sting ray because it has one, or several, sharp spines 

 that project up from its tail. It has a habit of resting on the sand at the 



