5. The second class, or BIRDS, comprises all such animals as have their bodies clad with feathers. Their jaws are elongated, and covered externally with a horny substance called a bill or beak, which is divided into two parts called mandibles. Their eyes are furnished with a thin, whitish, and somewhat transparent membrane, that can at pleasure be drawn over the whole external surface like a curtain. Birds respire by air-vessels, which are extended through their body, and which, in the abdominal cavity, adhere to the under surface of the bones. Their organs of motion are two wings and two legs; and they are destitute of external ears, lips, and many other parts which are important to quadrupeds.
6. Linnæus has divided this class into six ORDERS.
| ORDERS OF BIRDS. | ||
| 1. Land Birds. | ||
| I. | Rapacious Birds (Accipitres) | Have the upper mandible hooked, and an angular projection on each side near the point, as the eagles, hawks, and owls. |
| II. | Pies (Picæ) | Have their bills sharp at the edge, somewhat compressed at the sides, and convex on the top, as the crows. |
| III. | Passerine Birds (Passeres) | Have the bill conical and pointed, and the nostrils oval, open, and naked, as the sparrow and linnet. |
| IV. | Gallinaceous Birds (Gallinæ) | Have the upper mandible arched, and covering the lower one at the edge, and the nostrils arched over with a cartilaginous membrane, as the common poultry. |
| 2. Water Birds. | ||
| V. | Waders (Grallæ) | Have a roundish bill, a fleshy tongue, and the legs naked above the knees, as the herons, plovers, and snipes. |
| VI. | Swimmers (Anseres) | Have their bills broad at the top, and covered with a soft skin; and the feet webbed, as the ducks and geese. |
7. Under the third class, or AMPHIBIA, are arranged such animals as have a cold, and, generally, naked body, a lurid colour, and nauseous smell. They respire chiefly by lungs, but they have the power of suspending respiration for a long time. They are extremely tenacious of life, and can repair certain parts of their bodies which have been lost. They are also able to endure hunger, sometimes even for months, without injury.
The bodies of some of them, as the turtles and tortoises, are protected by a hard and horny shield or covering; those of others are clad with scales, as the serpents, and some of the lizards; whilst others, as the frogs, toads, and most of the water-lizards, are entirely naked, or have their skin covered with warts. Many of the species shed their skins at certain times of the year. Several of them are furnished with a poison, which they eject into wounds that are made by their teeth.
They chiefly live in retired, watery, and morassy places, and, for the most part, feed on other animals; though some of them eat water plants, and many feed on garbage and filth. None of these species chew their food; they swallow it whole, and digest it very slowly.
The offspring of all the tribes are produced from eggs, which, after they have been deposited by the parent animals in a proper place, are hatched by the heat of the sun. The eggs of some of the species are covered with a shell: those of others have a soft and tough skin or covering, not much unlike wet parchment: and the eggs of several are perfectly gelatinous. In those few that produce their offspring alive, as the vipers, and some other serpents, the eggs are regularly formed, but are hatched within the bodies of the females.
8. This class is divided into two ORDERS.
| ORDERS OF AMPHIBIA. | ||
| I. | Reptiles | Have four legs, and walk with a crawling pace, as the tortoises, frogs, and lizards. |
| 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.