By using the scientific method of naming invented by Linnæus, confusion is made impossible. The Latin names of this naturalist have the great advantage that they not only give a perfectly distinct name to any particular species, but also at the same time show the genus to which it belongs. Each kind of animal possesses, in fact, two names; just in the same way as every person possesses at least two names, a Christian name and a surname. The generic name comes first, and is, of course, common to all animals of the same genus. The second name is the specific one, and belongs exclusively to animals of the same species. The hare and rabbit, for example, are both included in the genus _Lepus_. The Latin name of the first is _Lepus timidus_; that of the second, _Lepus cuniculus_. Horse = _Equus caballus_; ass = _Equus asinus_. II. Review of the Structure and Vital Phenomena of Animals. I select as a point of departure the human body, and the bodies of domestic animals, because my readers are best acquainted with these. The limbs consist, beginning on the outside, of skin, flesh, and bone. The same parts can also be distinguished in the head, neck, and trunk; but in these divisions of the body they enclose a cavity, the _body-cavity_, which, again, contains various parts (“organs”), which are not everywhere attached to the body-wall. Fig. 1 represents a longitudinal section through the body. The skin is represented by a line, flesh and internal lining are shaded, while the bones are black. These parts form together the _body-wall_. In front the body-wall encloses a cavity, the body-cavity (_Kh._), which in Mammals is divided into two sections (thoracic cavity, _Bz.h_, and abdominal cavity, _B.h._) by the _midriff_ (diaphragm). In the thoracic cavity are found the lungs and heart (_H_), also most of the gullet or upper part of the gut; the abdominal cavity contains the remainder of the often much-coiled gut, which in one place widens into the stomach (_M_), also the kidneys, spleen, and parts connected with the gut (_e.g._ the liver). The cavities are bounded behind by the backbone (vertebral column), which is made up of many flattened vertebræ. The uppermost vertebra supports the skull, which encloses a _cranial cavity_ (_Sch.h._) continuous with a _vertebral canal_ bounded by the vertebræ. Cranial cavity and vertebral canal form together a second body-space, in which are contained the brain and spinal cord. [Illustration: FIG. 1.—Schematic Longitudinal Section of the Human Body. ]