hand, they are shortest and simplest in parasitic cœlomaria, which derive their fluid food already prepared from the contents of another animal's intestines. In these cases the gut may altogether atrophy; as in the _acanthocephala_ among the vermalia, the _entoconcha_ among the mollusks, and the _sacculina_ among the crustacea. The greater the extent of the body, and the more complex the organization of the higher animals, the more necessary it is to have an orderly and regular distribution of the nutritive fluid to all parts. In the cœlenteria this work is accomplished by the gastric canals (side branches from the gut, opening into its cavity) but in the cœlomaria it is done much better by means of blood-vessels (_vasa sanguifera_). These canals do not communicate directly with the gastro-canal, but are formed independently of it in the surrounding parenchyma of the mesoderm. They take up the filtered and chemically improved food-fluid, which transudes through the intestinal walls, and conduct it in the form of blood to all parts of the body. This blood generally contains millions of cells, which are of great importance in metabolism. The blood-cells of the lower cœlomaria are usually colorless (leucocytes), while those of the vertebrates are mostly red (rhodocytes). The circulation of the blood in most of the cœlomaria is effected by a heart, a contractile tube, formed by the local thickening of a skin-vessel, which contracts and beats regularly by means of its muscular bands. Originally two of these skin-vessels were developed in the abdominal wall--a dorsal vessel in the upper and ventral vessel in the lower wall (as in many of the vermalia). The heart is formed from the dorsal vessel in the mollusks and articulates, but from the ventral in the tunicates and vertebrates. The arteries are the vessels which conduct the blood from the heart; those which conduct it from the body to the heart are the veins. The finest branchlets of both kinds of vessels, which form the connecting link between them, are called capillaries; these immediately effect the interchange of matter in the tissues by osmosis. The blood-vessels co-operate very closely with the respiratory organs.