We should be quite unable to explain these if comparative anatomy and ontogeny did not come to our assistance. The vascular system in man and all the Craniotes is an elaborate apparatus of cavities filled with juices or cell-containing fluids. With the latter are connected the large cavities of the body, especially the body-cavity, or coeloma.

In the Dipneusts and Amphibia, in which the allantoic sac first makes its appearance, it remains within the body-cavity, and functions entirely as bladder. But in all the Amniotes it grows far outside of the body-cavity of the embryo, and forms the large embryonic "primitive bladder," from which the placenta develops in the higher mammals. This is lost at birth.

With the former they share the lack of body-cavity, anus, and vascular system; with the latter they have in common the bilateral type, the possession of a pair of nephridia or renal canals, and the formation of a vertical brain or cerebral ganglion.

The primitive mouth is surrounded by a dark ring that bears very strong and long vibratory lashes, and effects the swimming movements. Pemmatodiscus and Kunstleria may be included in the family of the Gastremaria. The Dicyemida live parasitically in the body-cavity, especially the renal cavities, of the cuttle-fishes.

It is only in the Mammals that the diaphragm forms a transverse partition of the body-cavity, completely separating the pectoral from the abdominal cavity. N nostrils, NA nasal bone, Mx upper jaw, Prf prefrontal, Fr frontal bone, A eye-pits, S temple-pits.

3A. above, on the dorsal side, the episomites, the double row of primitive or muscular segments; and 3B. below, on each side of the gut, the hyposomites, the two lateral plates that give rise to the sex-glands, and the cavities of which partly unite to form the body-cavity.

Within these nodules further division and development takes place until finally the nodule is burst open and many thousand minute rod-like organisms, sporozoites, are turned loose into the body-cavity of the mosquito.

This consists of a series of chambers each communicating with the one in front of it by an opening which is guarded by a valve. When one of these chambers contracts it forces the blood that is in it forward into the next chamber which, in its turn, sends it on. As the walls relax the valves at the sides are opened and the blood that is in the body-cavity rushes in to fill the empty chamber.

The letters have the same meaning throughout: N medullary tube, Ch chorda, M lateral muscles, Lh body-cavity, G part of the body-cavity in which the sexual organs are subsequently formed. But the body afterwards undergoes various modifications, especially in the fore-part.

In some there is another gland besides the liver, and this is taken to represent the kidneys. The body-cavity proper, or coeloma, which is filled with blood and encloses the hepatic gut, is very narrow in the Ascidia, as in the Amphioxus, and is here also usually confounded with the wide atrium, or peribranchial cavity, full of water.