Page:Natural History, Fishes.djvu/16

2 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 instincts, 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 arteries, aided by the contraction of the surrounding 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 contained 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, innumerable 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