FISHES AND THEIR STRUCTURE. 143 
Fishes are described by Cuvier as viviparous. vertebrata, with a double 
circulation, and respiring through the medium of water. For this purpose 
they have, on each side of the neck, branchiw, or gills, consisting of arches 
of bones attached to the os hyotdes, or bone of the tongue; and to these 
arches the filaments of the gills are attached, generally in a row upon each, 
and having their surfaces covered by a tissue of innumerable blood-vessels. 
‘The water taken in by the mouth passes through among the filaments of the 
gills, and escapes by the gill-openings towards the rear. In its progress 
through the filaments of the gills, the water imparts to these the oxygen of 
the air which it contains, and receives carbon in return, the same as in the 
lungs of an air-breathing animal. The gills of a fish do not decompose 
water, so as to derive oxygen from it, but merely separate the oxygen from 
the atmospheric air contained in the water; and hence, if water is deprived 
of this air, or impregnated with deleterious gases, fishes cannot live in it. 
As little can they bear the return of water entering at the gill-openings, 
and escaping by the mouth; for, if a fish is held so that the water is made 
to pass in this direction, it is as speedily drowned as if it were an air-breath- 
ing animal. The blood is brought to the gills by the heart, which thus 
answers to the right ventricle of warm-blooded animals; and from the gills 
it is sent to an arterial trunk, lying immediately upon the under side of the’ 
back bone, which trunk is the left, or systematic, ventricle of the heart, and 
sends the blood throughout the body of the fish, 
Living habitually in water, which is of very nearly the same specific 
eravity as their bodies, fishes have no weight to bear, but merely to propel 
themselves through the water; and their form, and their organs of motion, 
are all adapted to this one purpose, though varying in the species. In 
many there is, under the spine, a membranous air-bladder, which the fish 
can contract or expand, at pleasure ; and this is understood to alter its gray- 
ity, and enable it to suspend itself at any depth in the water. Many fishes, 
wanting this apparatus, have, however, nearly the same habits as others 
which are possessed of it. 
Progressive motion is effected by the tail striking alternately right and 
left against the water, for which purpose the flexure of the spine is lateral, 
whereas, in the other vertebrata, generally, the principal flexure is vertical ; 
and perhaps the jet of water thrown backwards from the gill-openings may 
assist. Thus a fish has but little use for extremities, and the parts analo- 
gous to legs and arms are accordingly very short, terminating in a number 
of rays analogous to fingers and toes; and these, covered by membranes, 
form what are termed fins. The fins, answering to arms, are called pec- 
torals, and those, answering to legs, ventrals ; and, besides these, there are 
often fins on the back called dorsal, behind the yent called anal, and on the 
extremity of the tail called caudal. 
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