[Illustration: Fig. 158. Larva just hatched from egg; a thickened pole.]
[Illustration: Fig. 159. Larva somewhat older than Fig. 158; _m_ _a_ depression at thickened pole.]
[Illustration: Fig. 160. Larva where the depression has become a digestive cavity _d_, opening at _a_.]
[Illustration: Fig. 161. Earlets, _w_ _w'_ (water-tubes), developed at the extremity of the digestive cavity _d_; _m_ mouth.]
[Illustration: Fig. 162. More advanced larva; _a_ _d_ _c_ digestive system, _v_ vibratile chord, _m_ mouth.]
[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.