Classificatory Points

Text Book of Biology, Part 1: Vertebrata by H. G. Wells, is part of the HackerNoon Books Series. You can jump to any chapter in this book here. Classificatory Points

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.). 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.), 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., 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.

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