ZOOLOGY, constitutes that branch of natural history which relates to animals. Various methods of arrangement have, by different naturalists, been devised to render this branch of study easy of com prehension, and familiar to the minds of those who wish for a general view of ani mated nature. We shall, in this article, give an outline of the Linnxan system, which has, in the various departments of the British Encyclopedia, been adopted, as most generally approved by philoso phers of all countries.
Linnwus divides the whole animal king dom into six classes, the characters of which are taken from the internal struc ture of the being treated of. It may be observed, that a considerable portion of the bulk of animals is composed of tubu lar vessels, which originate in a heart : the heart propels through the arteries, with the assistance of their own muscu lar powers, either a colourless transpa rent fluid, or a red blood, into the ex tremities of the veins ; through which it again returns to the origin of motion. Insects and worms have their circulating fluids a little warmer than the surround ing medium, and in general it is colour less ; but insects have legs furnished with joints, and worms have nothing but sim ple tentactila at most, in place of legs. Fishes have cold red blood, which is ex posed to air contained in water by means of their gills. Amphibia receive the air into their lungs, but their blood is likewise cold, and in both fishes and um phibia the heart has only two regular ca vities, while that of animals with warm blood has four. Of the latter, the ovipar ous are birds, and are generally covered with feathers ; the viviparous are either quadrupeds or cetaceou s animals, and are furnished with organs for suckling their young. See PHYSIOLOGY.
Each of the classes of animals is sub divided by Linnmus into different orders : for a scientific account of these orders, and also of the classes from whence they spring, the reader is referred to the seve ral heads of the Dictionary in the alpha betical order : and here we shall take a cursory view of the subject, in order to give, in a short compass, a sort of outline of the science.
The first class, denominated Mamma lia, from the female's suckling its young, comprehends all viviparous animals with warm blood. These, with very few ex ceptions, have teeth fixed in their jaw bones ; and from the form and number of these teeth, the orders are distinguished, except that of cetaceous fishes, which is known by the fins that are found in the place of feet. The distinctions of the
teeth are somewhat minute, but they ap pear to be connected with the mode of life of the animal, and they are tolerably natural. The first order, Primates, con tains man, monkeys, and bats : the second, Bruta ; among others, the elephant, the rhinoceros, the ant-eater,• and the orni thorynchus, an extraordinary quadruped, lately discovered in New Holland, with a bill like a duck, and sometimes teeth in serted behind it ; hut there are some sus picions that the animal is oviparous. The order Fern contains the seal, the dog, the cat, the lion, the tiger, the weasel, and the mole, most of them beasts of prey ; the opossum and the kangaroo also belong to this order, and the kangaroo feeds on ve getables, although its teeth are like those of carnivorous animals. The fourth or, der, Glires, comprehends beavers, mice, squirrels, and hares : the fifth, Pecora, camels, goats, sheep, and horned cattle. The sixth order, Bellux, contains the horse, the hippopotamus, and the hog. The cetaceous fishes, or whales, form the se venth and last order ; they reside in the water, enveloped in a thick clothing of fat, that is, of oily matter, deposited in cells, which enables their blood to retain its temperature, notwithstanding the ex ternal contact of a dense medium consi derably colder.
Birds are distinguished from quadru peds by their laying eggs ; they are also generally feathered, although some few are rather hairy, and instead of hands or ibre.legs, they have wings. Their eggs are covered by a calcareous shell ; and they consist of a white, or albumen, which nourishes the chick during incuba tion, and a yolk, which is so suspended within it, as to preserve the side on which the little rudiment of a chicken is situa ted, continually uppermost, and next to the mother that is sitting on it. The yolk is, in great measure, received into the abdomen of the chicken a little before the time of its being hatched, and serves for its support, like the milk of a quadruped, and like the cotyledons of young plants, until the system is become sufficiently strong for extracting its own food out of the ordinary nutriment of the species.