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--
(a) A skeletal axis (the vertebral column) between its central nervous system and its body cavity. In the adult rabbit this consists of a chain of vertebrae, but in the embryo (i.e., the young rabbit before birth) it is represented by a continuous chord, the notochord, and it remains as such in some of the lowest vertebrata throughout life. In other words, in these lower vertebrata, the vertebral axis is not metameric.
(b) A dorsal and -Tubular_ nervous axis. ([Section 131], the central canal)
(c) It has, though in the embryo only, certain slits between the throat and the exterior, like the gill slits of a fish. Such slits are-- with one or two remarkable exceptions outside the sub-kingdom-- distinctly vertebrate features, and remain, of course, in fishes throughout life.
The presence of true cartilage and bone mark a vertebrate, but vertebrata occur in which -these tissues- [bone] -are- [is] absent.
Section 146. The rabbit shares the following features with all the vertebrata, except the true fishes, which do not possess any of them--
(a) Lungs (but many fish have a swimming bladder which answers to the lungs in its anatomical relations.)
(b) Limbs which consist of a proximal joint of one bone an intermediate part of two, and a distal portion which has five digits, or is evidently a reduced form of the five-digit limb.*
(c) The absence of a median fin supported by fin rays.**
* The frog shows indications of a sixth digit.
** The frog's tadpole has a median fin, but no fin rays.
Section 147. The rabbit shares the following features with all the vertebrata above the fishes and amphibia (= frogs, toads, newts, and etc.)--
(a) Absence of gills (not gill slits, note) at any stage in development.
(b) An amnion, and
(c) An allantois in development.
The meaning of (b) and (c) we shall explain to the student in the chapters on embryology. We simply mention them here to render our table complete.
Section 148. The rabbit shares with all mammals, and differs from all other vertebrata (i.e., birds, reptiles, amphibia, and fishes), in having--