THE FISHES 



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menhaden, herring, and bluefish. The migrations are due 

 to temperature changes, to the seeking after food, and to the 

 spawning instinct. Some fish migrate to shallower water in the 

 summer and to deeper water in the winter; here the reason for 

 the migration is doubtless the change in temperature. 



The herring fisheries have always been a source of wealth to 

 the inhabitants of northern Europe. The banks and shallows of 

 the coast of Newfoundland were undoubtedly known to the 

 Norsemen long before the discovery of this country by Columbus. 



Classification of Fishes. — The animals we recognize as fishes are 

 grouped by naturalists into four groups : 



Sand shark, an elasmobranch. Note the slits leading from the gills. From photograph 

 loaned by the American Museum of Natural History. 



1. The Elasmobranchs. — These fishes have a skeleton formed of carti- 

 lage which has not become hardened with lime. The gills communicate with 

 the surface of the body by separate openings instead of having an operculum. 

 The skin is rough and the eggs few in number. In some members of this 

 group the young are born alive. Sharks, rays, and skates are Elasmobranchs. 



Sturgeon (Acipenser sturio), a ganoid fish. 



2. Ganoids. — Fish in which the body is protected by a series of plate- 

 like scales of considerable strength. These fishes are the only remnant of 



