SHARKS AND SKATES. 



73 



occasion offers, do not hesitate to attack man. Many are the cases in history and in 

 fiction whei-e sharks have eaten a human being in one or two mouthfuls. 



In shape the sharks are long and cylindrical, the body terminating in a sharp snout 

 in front and in a long and flattened tail behind. The mouth is transverse, and usually 

 placed on the lower surface of the head some distance behind the tip of the snout ; 

 the gill-clefts vary between five and seven in number and always occupy a lateral 

 position ; the edges of the eyelids are free ; and the shoulder girdle is incomplete. The 

 differences between the typical forms of sharks and skates are very evident, but a 

 series of intermediate forms intergrade between the Squali and the Raias, so th.at 

 almost the only certain external character separating the two is found in the position 

 of the gill slits ; lateral in the sharks, ventral in the rays. 



Almost all the sharks are marine, though many occasionally follow their prey into 

 the mouths of rivers, where the water is brackish, or even fresli, and one species 

 {Etdamia nicaraguensis) occurs in Lake Nicaragua, where it is out off from the ocean 

 by a river a hundred miles in length. Many sjDecies are pelagic, that is, they live in 

 the open sens, following schools of fish or ships for weeks, in the latter case feeding 

 upon tlie refuse from the cook's galley or on anything else that may fall overboard. 

 Other species are found near the shore, living near the bottom, and only rising to the 

 surface in the pursuit of food. The smaller species often form immense schools, which 

 follow the mackerel or other migratory fishes. The majority of species of sharks are 

 found between the tropics. In the temperate regions they are not so numerous, 

 while the sleei^er shark (Somniosics microcephalus), the black dog-fish {Centroscyllium 

 fabricii) and the basking shark 

 {CetorJdnus maxinnis), enter 

 the Arctic seas. Few of them 

 descend into deep water, only 

 Centroscymtnis codolepis being 

 known from a depth of five 

 hundred fathoms. 



The teeth of sharks are es- 

 pecially interesting, from the 

 method of replacement of those 

 worn out or torn out. Li the 

 majority of sharks the teeth 

 are more or less triangular in 

 outline, and are movably artic- 

 ulated with the jaws. They 

 are arranged in several rows, 

 one behind another, the first 

 or outer row alone being used, 

 while the rest are turned back 

 out of the way. When those 

 of the first row are lost, those 

 of the next row rise up to take 

 their place. In some sharks, however, the teeth develoj) a pavement much like that 

 characteristic of the rays. 



Sharks have but a limited economic value. In the eastern countries many of the 

 smaller species are eaten, while the fins, with their abundant cartilage, form the basis of 



Fig. 58.- 



-Jaws and teeth of the blue shark, Carcliarinus gtaucus ; the 

 single teeth are of the natural size. 



