uppermost rays to it, and then, letting go its other attachments, swing from the old support to the new. The activity and co-ordination manifested in these acrobatic movements, says Dr. Romanes, are surprising, and give to the animal an almost intelligent appearance.
The feet of astropecten are partly rudimentary, having lost their terminal suckers, and these star-fish assist themselves in locomotion by the muscular movements of their rays. The brittle-stars are still further removed in the same direction from the common star-fish; their tubular feet are of no use for crawling, while their rays are so long, flexible, and muscular, as to enable them to shuffle quite rapidly over horizontal surfaces. Two opposite arms are used upon the floor with the motion of swimming, the animal leaping forward about two inches at each stroke, and, as these leaps follow one another quickly, the starfish is able to travel at the rate of six feet a minute. A common star-fish can crawl only two inches a minute. Some of the Comatulæ, in which the muscularity of the rays has proceeded still further, are able actually to swim by the co-ordinated movements of their rays.
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| Fig. 6.—An Echinus partly denuded of its Spines.(From Cassell's "Natural History.") |
The sea-urchin, or echinus, is a modification of the star-fish structure, having the form of a flattened sphere, and is covered with hard spines (Fig. 6). In the living animal these spines are movable in all directions, each being mounted on a ball-and-socket joint, and provided with muscles at its base. Like the star-fish, the echinus has a madreporic tubercle, pedicellariae, and feet. If we shave off the spines and pedicellariae, we come down to a hard shell, which is hollow and filled with liquid. The liquid resembles sea-water, but is richly corpusculated, and coagulates when exposed to the air. Five double rows of holes extend symmetrically from pole to pole of the shell. It is through these holes that the feet are thrust out, so that in its main features an echinus is merely a star-fish with its five rays curved into the shape of a hollow spheroid, and then converted into a rigid box, with holes left for its feet to come through. The urchin crawls in the same way as the common star-fish, but makes use of its spines also to help push itself along. The suckers, moreover, in being protruded from all sides of a globe instead of from the under side of a flat organism, are of much more use as feelers than they are in the star-fish. If
