THE ALLIES OF THE PERCH: PISCES 317 



Sharks and Rays. The sharks and rays, or Elasmobran'chii 

 (Gr. elasmos, plate ; branchia, gills), are fishes with a cartilagi- 

 nous skeleton and with gills which communicate with the 

 surface by several openings, instead of being covered by an 

 operculum, as in the perch. The skin is roughened by small 

 tubercles, which, when closely set, form shagreen, used in 

 the arts for polishing woods and for ornamental work. The 

 tail is usually unequally lobed, the dorsal division being the 

 larger. 



The sharks (Fig. 156) are, with the exception of one species 

 found in Lake Nicaragua, marine animals, and are developed 



FIG. 156. Photograph of Shark. (American Museum of Natural History) 



to the greatest extent in the tropics. The rays, or skates, are 

 more flattened forms, adapted to a life on the bottom of the 

 sea, which they often resemble in color. Fig. 157 shows a 

 species common on the North Atlantic coast, which grows to 

 be about two feet in length. The most famous of the rays 

 are the torpedoes, so called from their power of giving an 

 electric shock. About fifteen species of torpedoes are known, 

 of which one is sometimes found on the eastern coast of the 

 United States. 



Bony Fishes. The bony fishes, or Teleos'tomi (Gr. teleos, com- 

 plete or perfect ; stoma, mouth), in the higher forms, of which 

 the perch is an example, have the skeleton ossified (converted 

 into bone) and the body usually covered with scales ; in the 



