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.