CLASS IV.—_Mammalia_ (milk givers). Though there are some very low species in this order, it reaches upward and includes the highest forms of animal organization, all of what ever degree, that have the mammary glands, and secrete milk for the nourishment of their young. There are more than two thousand species of these in North America alone. They have several distinguishing characteristics. The heart is four chambered, having two auricles and two ventricles. They are warm blooded, the blood having two kinds of corpuscles, the red predominating, and being globular. The impure blood from the body pours into the right auricle, passes to the right ventricle, and thence to the lungs. In the lungs, which are spongy and full of air cells, it is supplied with oxygen from the air freely circulating through them. Being thus changed to purer arterial blood, it passes back through the left auricle to the left ventricle, and, by a muscular action, is forced through the great aorta and its innumerable branches to every part of the body. Mammals always breathe by lungs enclosed in a membranous sack called the pleura. They are viviparous, or produce their young alive, though some when in a very immature, helpless state. They are for convenience subdivided: ORDER I. _Monotremata._[18] These animals, the lowest of mammals, have long, flattened beaks, webbed feet, and other bird-like characteristics. But they are of little importance, few in number, and not widely distributed, being confined mostly to Australia and Tasmania. Belonging to this class is the Water-Mole, with a broad, duck-like, horny bill. It lives on worms and vegetables, and burrows in the banks of streams, having the opening to its quarters under the surface of the water. ORDER II. _Marsupial_, or pouched animals. The young are brought forth in a very premature state, but are immediately placed in a marsupium or pouch under the abdomen, where, attached to the little teats, they receive their nourishment. They are thus protected and carried by the mother as long as such care is necessary. The opossum, the only native marsupial of the United States, is about twenty inches long, exclusive of the peculiar rat-like, prehensile tail, by which the animal often suspends itself from the branches of trees. When in danger, it feigns death as a way of escape—and can survive injuries that would be fatal to most animals. They were once very numerous in this country, and though destructive to fruit and poultry, are of some value. Their flesh is used by some, and the skins are in demand, the hair or coarse fur being wrought into felt. ORDER III. _Edentata_[19] (toothless). Animals without incisors, and having separate clawed toes, are included under this order. The chief representatives are the Armadillos, Sloths and Ant-eaters.