II. Serpents.—Having no legs, but crawling on the body.
III. Nantes.—Living in the water, furnished with fins, and breathing by means of gills. These are true Fishes, principally of the group termed Chondropterygii, or Cartilaginous Fishes, by Cuvier.
ORDERS OF FISHES.
Fishes constituted Linnæus’s fourth class of animals. They are all inhabitants of the water, in which they move by certain organs called fins. Those situated on the back are called dorsal fins; those on the sides, behind the gills, pectoral fins; those below the body, near the head, are ventral; those behind the vent are anal; and that which forms the tail is called the caudal fin. Fishes breathe by gills, which, in most species, are situated at the sides of the head. Fishes rise and sink in the water, generally by a kind of bladder in the interior of the body, called an air-bladder. Some of them do not possess this organ, and consequently are seldom found but at the bottom of the sea, from which they can only rise by an effort. The bodies of these animals are usually covered with scales, which keep them from injury by the contact of the water.
The fishes were divided by Linnæus into four Orders:
I. Apodal.—Having no ventral fins, as the eel.
II. Jugular.—Having the ventral fins situated in front of the pectoral fins, as the cod, haddock, and whiting.
III. Thoracic.—Having the ventral fins situated directly under the pectoral fins, as the perch and mackerel.
IV. Abdominal.—Having the ventral fins on the lower part of the body below the pectoral fins, as the salmon, herring, and carp.