respond with those of the female, and the common
urogenital duct (= vestibule), the urethra, is prolonged into an erectile
penis (P.) surrounded by a fold of skin, the prepuce. A prostate gland
(pr.), contributes to the male sexual fluid. The character of the
essential male element, the spermatozoon, the general nature of the
reproductive process, will be conveniently deferred until the chapters
upon development are reached.

9. _Classificatory Points_

Section 141. The following facts of classificatory importance may now
be considered, but their full force will be better appreciated after the
study of other vertebrate types. They are such as come prominently
forward in the comparison of the rabbit with other organisms.

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
ch