forms an effective weapon whether against the creature's enemies or its prey. [Illustration: FIG. 43.--Nephridium of a Turbellarian.] About this stage of Evolution, the exact point being somewhat difficult to fix, there appears the body cavity. This, which is altogether distinct from the digestive cavity, is a familiar feature of the anatomy of the higher animals. In it are suspended the heart and lungs and the whole of the digestive organs and glands. The question of the origin of the body cavity and the blood system is a very difficult one, and a thorough theoretical discussion would take us too far. Before proceeding to the question of the origin of the vertebrates, we may pause briefly to consider the other groups to which the worms appear to have given rise. First of these we may take the Echinoderms, which include the well-known star-fishes and sea-urchins, and the very beautiful feather stars. As already indicated, it is believed that the radial symmetry, which is so characteristic of this group, is not a primitive feature, but that, in fact, the Echinoderms are descended from bilaterally symmetrical ancestors. One reason for this view is that the larval or immature form is always markedly bilaterally symmetrical. In an ordinary star-fish, which we may take as typical of the group, the mouth is in the middle of the lower aspect, and the excretory opening of the digestive cavity in the upper side just opposite. There is no blood system, or excretory organs, and no concentration of nerve cells into any form of brain. Eyes, however, are present, and sensitiveness to light may be easily demonstrated. The most remarkable feature of the group is the water-vascular system, consisting of a series of radial canals, one in each ray, which join a circular one situated in the central portion of the body. The system of canals communicates with the exterior by means of a sieve-like plate on the upper surface, and it is kept full of water by the continual pumping action of cilia on the walls of the tube which leads down from the sieve plate. [Illustration: FIG. 44.--A Nemertine--Tetrastemma. Actual length about 1-1/2 inch.] [Illustration: FIG. 45.--Diagram of Nemertine--Nemertopsis. _cg_ and _sg_, Sense organ; _a_, eyes; _gh_, brain; _bl_, blood vessels; _n_, nerve cord; _d_, alimentary canal; _ex_, nephridia; _r_, snout, withdrawn.] The ordinary star-fish is carnivorous, and lives largely on ordinary mussels, which it bridges over with its arms, and opens by a steady and long-continued pulling, the soft parts being then sucked up by the partially protruded stomach. A few types of Echinoderms are shown in Figs. 46, 47, 48. [Illustration: FIG. 46.--Star-fishes.] [Illustration: FIG. 47.--Feather Star. Photo: Harold Bastin.]