9. FISHES constitute the fifth class of animals. They are all inhabitants
of the water, in which they move by certain organs called _fins_. These,
when situated on the back, are called _dorsal fins_; when on the sides,
behind the gills, they have the name of _pectoral fins_; when on the belly
near the head, they are _ventral_; when behind the vent, they are _anal_;
and that at the posterior extremity of the body is called the _caudal fin_.
Fishes breathe by _gills_, which, in most of the species, are situated at
the sides of the head. In some of the flatfish, however, as the skate and
thornback, they are on the under part of the body. Fish rise and sink in
the water, generally by a kind of bladder in the interior of their body,
called an _air-bladder_. Some of them, as the skate and other flat-fish, do
not possess this organ, and consequently are seldom found but at the bottom
of the water. The bodies of these animals are usually covered with
_scales_, which keep them from injury by the pressure of the water. Several
of them are enveloped with a fat and oily substance to preserve their
bodies from putrefaction, and also to guard them from extreme cold.