The position of the pedicellariæ is quite different from that which they occupy in the Sea-urchin, where they are scattered singly between the spines and tentacles, though more regularly and closely grouped along the lines upon which the refuse food is moved off. In the Star-fish, on the contrary, these singular organs seem to be grouped for some special purpose around the spines, on the upper surface of the body. Every such spine swells near its point of attachment, thus forming a spreading base (Fig. 143), around which the pedicellariæ are arranged in a close wreath, in the centre of which the summit of the spine projects; they differ also from those of the Sea-urchin in having two prongs instead of three. Other pedicellariæ are scattered independently over the surface of the animal, but they are smaller than those forming the clusters and connected with the spines. The function of these organs in the Star-fish remains unexplained; the opening on the upper surface, through which the refuse food is thrown out, is in such a position that they evidently do not serve here the same purpose which renders them so useful to the Sea-urchin. Occasionally they may be seen to catch small prey with these forks, little Crustacea, for instance; but this is probably not their only office. The Star-fish has a fourth set of external appendages in the shape of little water-tubes. (Seen in Fig. 143.) The upper surface of the back consists of a strong limestone network (Fig. 144), and certain openings in this network are covered with a thin membrane through which these water-tubes project. It is supposed that water may be introduced into the body through these tubes; but while there can be no doubt that they are constantly filled with water, and are therefore directly connected with the circulation through the madreporic body (Fig. 145), no external opening has as yet been detected in them. The fact, however, that when these animals are taken out of their native element, the water pours out of them all over the surface of the back, so that they at once collapse and lose entirely their fulness of outline, seems to show that water does issue from those tubes. The ends of the arms are always slightly turned up, and at the summit of each is a red eye-speck. The tentacles about the eye become very delicate and are destitute of suckers. [Illustration: Fig. 143. Single spine of Star-fish, with surrounding appendages; magnified.] [Illustration: Fig. 144. Limestone network of back of Star-fish.] [Illustration: Fig. 145. Madreporic body of Star-fish; magnified.]