[Illustration: Fig. 163. Profile view of larva; _b_ madreporic opening, _w'_ earlet, _a_ _d_ digestive system, _m_ mouth, _v_ _v'_ vibratile chord.] [Illustration: Fig. 164. Larva showing mode of formation of mouth _m_, by bending of digestive cavity _o_.] At about the same time with the development of these two pouches, so important in the animal's future history, the digestive cavity becomes slightly curved, bending its upper end sideways till it meets the outer wall, and forms a junction with it (_m_, Fig. 164). At this point, when the juncture takes place, an aperture is presently formed, which is the true mouth. The digestive sac, which has thus far served as the only internal cavity, now contracts at certain distances, and forms three distinct, though connected cavities, as in Fig. 163; viz. the oesophagus leading directly from the mouth (_m_) to the second cavity or stomach (_d_), which opens in its turn into the third cavity, the alimentary canal. Meanwhile the water-tubes have been elongating till they now surround the digestive cavity, extending on the other side of it beyond the mouth, where they unite, thus forming a Y-shaped tube, narrowing at one extremity, and dividing into two branches toward the other end. (Fig. 165.) [Illustration: Fig. 165. Larva in which arms are developing, lettering as before; _e'_ _e''_ _e'''_ _e^4_ _e^5_ _e^6_ arms, _o_ oesophagus.] On the surface where the mouth is formed, and very near it on either side, two small arcs arise, as _v_ in Fig. 162; these are cords consisting entirely of vibratile cilia. They are the locomotive organs of the young embryo, and they gradually extend until they respectively enclose nearly the whole of the upper and lower half of the body, forming two large shields or plastrons. (Figs. 165, 166.) The corners of these shields project, slightly at first (Fig. 165), but elongating more and more until a number of arms are formed, stretching in various directions (Figs. 166, 167), and, by their constant upward and downward play, moving the embryo about in the water. [Illustration: Fig. 166. Adult larva, so-called Brachiolaria, lettering as before; _r_ back of young Star-fish, _t_ tentacles of young Star-fish, _f_ _f'_ brachiolar appendages.] [Illustration: Fig. 167. Fig. 166 seen in profile, lettering as before.] [Illustration: Fig. 168. Star-fish which has just resorbed the larva, seen from the back; _b_ madreporic opening.] [Illustration: Fig. 169. Fig. 168, seen from the mouth side; _m_ mouth, _t_ tentacles.] [Illustration: Fig. 170. Young Star-fish which has become symmetrical, seen from the back; _t'_ odd tentacle.]