Chapter II: SERPENTS Have no legs, but crawl on their belly

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.

10. The fishes are divided into six ORDERS.

_ORDERS OF FISHES._

I. APODAL Have bony gills; and no ventral fins, as the _eel_.

II. JUGULAR Have bony gills; and the ventral fins situated in front of
the pectoral fins, as the _cod_, _haddock_, and _whiting_.

III. THORAIC Have bony gills; and the ventral fins situated directly
under the pectoral fins, as the _perch_ and _mackerel_.

IV. ABDOMINAL Have bony gills; and the ventral fins on the belly behind
the pectoral fins, as the _salmon_, _herrings_, and _carp_.

V. BRANCHIOSTEGOUS Have their gills destitute of bony rays.