% FISHES. 



and wholesome food to man, both in a fresh 

 and salted state, and that they afford constant 

 employment for millions of capital, fleets of 

 shipping, and almost the whole population of 

 large and numerous districts, it will be seen 

 that this Class is not devoid of high interest, 

 though, as compared with other animals, little 

 is known of those details of manners and in- 

 stincts, which constitute so large a part of the 

 charm of Natural History. 



Fishes are, for the most part, cold-blooded 

 animals ; their heart consists of but one auricle 

 and one ventricle, which receive the blood from 

 the veins, and send it to the gills for renewal ; 

 it is thence circulated through the body in ar- 

 teries, aided by the contraction of the surround- 

 ing muscular fibres. The gills are organs for 

 respiration analogous to the lungs of terrestrial 

 animals, calculated to extract the oxygen needful 

 for the renewal of the blood from the air con- 

 tained in the water, not, as has been frequently 

 supposed, by the decomposition of water itself. 

 The apparatus is double, placed on each side of 

 the neck, and, in its most common form, consists 

 of several series of membranous plates, fixed on 

 slender arches of bone. Over these plates, in- 

 numerable blood-vessels ramify, whose walls are 

 so thin as to permit the fluid contained in them 

 to absorb the oxygen with which they are thus 

 brought into contact. In order to carry off' the 

 water when deprived of its oxygen, and to bring 

 fresh portions in succession to be respired, a 

 constant current is produced over the surface of 

 the gills, by the fish taking in the water at the 

 mouth, and ejecting it on each side, behind the 



