The presence of these two chambers with the solid partition between them is a primary fact in the anatomy of the body; it shows that man is a vertebrate animal, that is to say, is a back-boned animal, and belongs to the same great group as fishes, reptiles, birds, and beasts* : sea anemones, clams, and insects are invertebrate animals, and built on quite different plans; sections made through any of them from the head to the opposite end, would show nothing like those two main cavities with a backbone between them which exist in our own bodies.

What lies between the haemal and neural cavities? Of what is the spine composed?

Fig. 1. Diagrammatic longitudinal section of the body, a, the neural tube, with its upper enlargement in the skull cavity at a'; N, the spinal cord; N', the brain; ee. vertebræ forming the solid partition between the dorsal and ventral cavities; b, the pleural, and, c, the abdominal division of the ventral cavity, separated from one another by the diaphragm, d; i, the nasal, and o, the mouth chamber, opening behind into the pharynx, from which one tube leads to the lungs, l, and another to the stomach, f; h, the heart; a kidney the sympathetic nervous chain. From the stomach, f. the intestinal tube leads through the abdominal cavity to the posterior opening of the alimentary canal.