I have just said that this three or four layered stage is shared by all the Metazoa, except those very lowest forms--such as sponges and jelly-fish--which do not pass on to it. But from this point the developmental histories of all the main branches of the Metazoa diverge--the Vermes, the Echinodermata, the Mollusca, the Articulata, and the Vertebrata, each taking a different road in their subsequent evolution. I will therefore confine attention to only one of these several roads or methods, namely, that which is followed by the Vertebrata--observing merely that, if space permitted, the same principles of progressive though diverging histories of evolution would equally well admit of being traced in all the other sub-kingdoms which have just been named. In order to trace these principles in the case of the Vertebrata, it is desirable first of all to obtain an idea of the anatomical features which most essentially distinguish the sub-kingdom as a whole. The following, then, is what may be termed the ideal plan of vertebrate organization, as given by Prof. Häckel. First, occupying the major axis of body we perceive the primitive vertebral column. The parts lying above this axis are those which have been developed from the ectoderm and mesoderm--viz. voluntary muscles, central nervous system, and organs of special sense. The parts lying below this axis are for the most part those which have been developed from the endoderm--namely, the digestive tract with its glandular appendages, the circulating system and the respiratory system. In transverse section, therefore, the ideal vertebrate consists of a solid axis, with a small tube occupied by the nervous system above, and a large tube, or body-cavity, below. This body-cavity contains the viscera, breathing organs, and heart, with its prolongations into the main blood-vessels of the organism. Lastly, on either side of the central axis are to be found large masses of muscle--two on the dorsal and two on the ventral. As yet, however, there are no limbs, nor even any bony skeleton, for the primitive vertebral column is hitherto unossified cartilage. This ideal animal, therefore, is to all appearance as much like a worm as a fish, and swims by means of a lateral undulation of its whole body, assisted, perhaps, by a dorsal fin formed out of skin. [Illustration: FIG. 45.--Ideal primitive vertebrate, seen from the left side. (After Häckel.) _na_, nose; _au_, eye; _g_, ear; _md_, mouth; _ks_, gill-openings; _x_, notochord; _mr_, spinal tube; _kg_, gill-vessels; _k_, gill-intestine; _hz_, heart; _ms_, muscles; _ma_, stomach; _v_, intestinal vein; _c_, body-cavity; _a_, aorta; _l_, liver; _d_, small intestine; _e_, ovary; _h_, testes; _n_, kidney canal; _af_, anus; _lh_, true or leather-skin; _oh_, outer-skin (epidermis); _f_, skin-fold, acting as a fin.] [Illustration: FIG. 46.--The same in transverse section through the ovaries; lettering as in the preceding Fig.]