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.