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. ORDERS OF INSECTS. The fifth class of Linnæus comprised the Insects; and the branch of Zoology which treats of them is called Entomology. Nearly all insects go through certain great changes at different periods of their existence. From the egg is hatched the larva, which is a grub or caterpillar, and destitute of wings; this afterwards changes to a pupa, or chrysalis, wholly covered with a hard shell, or strong skin, from which the perfect or winged insect bursts forth. Spiders and their allies, which were included by Linnæus in the insects, issue from the egg in nearly a perfect state. Linnæus divided his class of insects into seven Orders: I. COLEOPTEROUS.--Having elytra, or crustaceous cases covering the wings; and which, when closed, meet in a straight line along the middle of the back, as the cockchafer. II. HEMIPTEROUS.--Having four wings, the upper ones partly crustaceous, and partly membranous; not divided straight down the middle of the back, but crossed, or incumbent on each other, as the cockroach. III. LEPIDOPTEROUS.--Having four wings covered with fine scales almost like powder, as the butterflies and moths.