Section 142. In the first place, the rabbit is a metazoon, one of the metazoa, i.e., a multicellular organism, as compared with the amoeba, which belongs to the protozoa or one-cell animals (Section 55). In the next place, it is externally bilaterally symmetrical, its parts balance, and where, in its internal anatomy, it departs from this symmetry (as in the case of the aorta, the stomach and intestines, and the kidneys), the departure has an appearance of being the results of partial reductions and distortions of an originally quite symmetrical plan. And the facts of development strengthen this idea; in the very earliest stages we have paired aortic arches, of which, the left only remains, a straight alimentary canal, and less asymmetrical kidneys. In the vast majority of animals the same bilateral symmetry is to be seen, but in the star-fish and sea-urchins, and in the jelly-fish, corals, sea anemones, and hydra, the general form of the animal is, instead, arranged round a centre, like a star and its rays, and the symmetry is called radial. Section 143. We also see in various organs of the rabbit, and especially in the case of the limbs and vertebral column, what is called metameric segmentation, that is, a repetition of parts, one behind the other, along the axis of the body. Thus the bodies and arches of the vertebrae repeat each other, and so do the spinal nerves. The renal organ of the rabbit, some time before birth, displays a metameric arrangement of its parts; but this disappears, as development proceeds, into the compact kidney of the adult. But the metameric segmentation in the rabbit's organism is not nearly so marked as that of an earthworm, for instance, which is visibly a chain of rings. If the student wants a perfect figure of metameric segmentation he should think of a train of precisely similar carriages, or a string of beads. One bead, one carriage, one vertebra, would be a metamere. Section 144. In contrast to metameric segmentation is the antimeric repetition of radial symmetry (Section 142), in which each ray of the star is called an antimere. It is possible to have bilateral symmetry without a metameric arrangement of parts, as in the mussel and the cuttle-fish; but metameric segmentation without complete or reduced bilateral symmetry does not occur. Section 145. We are now in a position to appreciate the fact that the old and more popularly know division of animals into vertebrata and invertebrata scarcely represents the facts of the case, that the primary division should be into protozoa and metazoa, and that the vertebrata are one of several groups of metazoa with a fundamental bilateral symmetry and imperfect metameric segmentation. The rabbit is one of the vertebrata, and, in common with all the other animals collected under this head, it has--