2 
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 he 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 flbres. 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, flxed 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 j 
to absorb the oxygen with which they are thus : 
brought into contact. In order to carry oft* 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 flsh taking in the water at the 
mouth, ^nd ejecting it on each side, behind the : 
DSI 
