{ With vertebræ { Hot Blood { Viviparous I. MAMMALIA. { { { Oviparous II. BIRDS. Body { { Cold red Blood { With lungs III. AMPHIBIA. { { With gills IV. FISHES. { Without vertebræ Cold white Blood { Having antennæ V. INSECTS. { Having tentacula VI. WORMS. ORDERS OF MAMMALIA. The first class, or Mammalia, consists of such animals as produce living offspring, and nourish their young ones with milk supplied from their own bodies; and it comprises both the quadrupeds and the cetacea. This class was divided by Linnæus into seven Orders: viz. _primates_, _bruta_, _feræ_, _glires_, _pecora_, _belluæ_, and _cetacea_ (this order was called Cete by Linnæus) or whales. The characteristics of these were founded, for the most part, on the number and arrangement of the teeth; and on the form and construction of the feet, or of those parts in the seals, manati, and cetacea, which supply the place of feet: I. PRIMATES.--Having the upper front teeth, generally four in number, wedge-shaped, and parallel; and two teats situated on the breast, as the apes and monkeys. II. BRUTA.--Having no front teeth in either jaw; and the feet armed with strong hoof-like nails, as the elephant. III. FERÆ.--Having in general six front teeth in each jaw; a single canine tooth on each side in both jaws; and the grinders with conic projections, as the dogs and cats. IV. GLIRES.--Having in each jaw two long projecting front teeth, which stand close together; and no canine teeth in either jaw, as the rats and mice. V. PECORA.--Having no front teeth in the upper jaw; six or eight in the lower jaw, situated at a considerable distance from the grinders; and the feet with hoofs, as cattle and sheep. VI. BELLUÆ.--Having blunt wedge-shaped front teeth in both jaws; and the feet with hoofs, as horses. VII. CETACEA.--Having spiracles or breathing-holes on the head; fins instead of fore feet; and a tail flattened horizontally, instead of hind feet. This order consists of the narvals, whales, cachalots, and dolphins. ORDERS OF BIRDS. 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. 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. That part of Zoology which treats of Birds is called Ornithology.