Page:First course in biology (IA firstcourseinbio00bailrich).pdf/245

 the deer family, and several other families. These are all placed together in the next higher systematic unit called an order, in this case, the order of ruminants.

The ruminants, because they are covered with hair and nourish the young with milk, are in every essential respect related to the one-toed horses, the beasts of prey, the apes, etc. Hence they are all placed in a more inclusive division of animals, the class called mammals.

All mammals have the skeleton, or support of the body, on the inside, the axis of which is called the vertebral column. This feature also belongs to the classes of reptiles, amphibians, and fishes. It is therefore consistent to unite these classes by a general idea or conception into a great branch of animals called the vertebrates.

Returning from the general to the particular by successive steps, state the branch, class, order, family, genus, and species to which the cow belongs.

The Eight Branches or Sub-kingdoms.—The simplest classification divides the whole animal kingdom into eight branches, named and characterized as follows, beginning with the lowest: I. One-celled. II. Many openings. III. Circular; cup-like; having only one opening which is both mouth and vent. IV. Circular; rough-skinned; two openings. V. No skeleton; usually with external shell. VI. Elongate body, no jointed legs. VII. External jointed skeleton; jointed legs. VIII. Internal jointed skeleton with axis or backbone.