 Fig. 168. Star-fish which has just resorbed the larva, seen from the back; b madreporic opening. |  Fig. 170. Young Star-fish which has become symmetrical, seen from the back; t' odd tentacle. | |
 Fig. 169. Fig. 168, seen from the mouth side; m mouth, t tentacles. |
At this stage of the growth of the embryo, we have what seems quite a complicated structure, and might be taken for a complete animal; this is after all but the prelude to its true Star-fish existence. While these various appendages of the embryo have been forming, changes of another kind have taken place; on one of the two water-tubes above mentioned (w'), at the end nearest the digestive cavity, a number of lobes are formed (t, [Fig. 166]); this is the first appearance of the tentacles. In the same region of the opposite water-tube (w) a number of little limestone rods arise, which eventually unite to form a continuous network; this is the beginning of the back of the Star-fish (r, [Fig. 166]), from which the spines will presently project. When this process is complete, the whole embryo, with the exception of the part where the young Star-fish is placed, grows opaque; it fades, as it were, begins to shrink and contract, and presently drops to the bottom, where it attaches itself by means of short arms (f f', [Fig. 166]), covered with warts, which act as suckers, and are placed just above the mouth. As soon as the Star-fish has thus secured itself, it begins to resorb the whole external structure described above; the water-tubes, the plastrons, and the complicated system of arms connected with them, disappear within the little Star-fish; it swallows up, so to speak, the first stage of its own existence; it devours its own larva, which now becomes part and parcel of the new animal. Next the two surfaces, the back and lower surface, on which the arms are now marked out, while the tentacles, suckers, and spines have already assumed a certain prominence, approach each other. At this time, however, the arms are not in one plane; both the back and the lower surface are curved in a kind of spiral; they begin to flatten; the arms spread out on one level,—and now the two surfaces draw together, meeting at the circumference, and enclosing between them the internal organs, which, as we have seen, are already formed and surrounded by walls of their own, before the two walls of the body, close thus over them. [Fig. 168] represents the upper surface of the Star-fish just before this junction takes place. The complicated structure of the Brachiolaria, as the larva of the Star-fish has been called, hitherto so essential to the life of the animal, by which it has been supported, moved about in the water, and provided with food during its immature condition, has made a final contribution to its further development by the process of resorption described above, and has wholly disappeared within the Star-fish. At this stage the rays are only just marked out, as five lobes around the margin; [Fig. 169] represents the lower surface at the same moment, with the open mouth (m), around which the tentacles (t) are just beginning to appear; while [Fig. 170] shows us the animal at a more advanced stage, after the two surfaces have united. It has now somewhat the outline of a Maltese cross, the five arms being more distinctly marked out, while the tentacles have already attained a considerable length ([Fig. 171]), and the dorsal plates have become quite distinct. [Fig. 172] represents the same animal, at the same age, in profile. This period, in which we have compared the form of the Star-fish to that of a Maltese cross, is one of long duration; two or three years must elapse before the arms will elongate sufficiently to give it a star-shaped form, and before the pedicellariæ make their appearance, and it is only then that it can be at once recognized as the young of our common Star-fish. Even then, after it has assumed its ultimate outline, it lacks some features of the adult, having only two rows of tentacles, whereas the full-grown Star-fish has four.
Fig. 171. Lower side of ray of young Star-fish; m mouth, b madreporic body, e eye-speck. | Fig. 172. Young Star-fish seen in profile; t' odd tentacle at extremity of arm. |
 Fig. 173 |  Fig. 174 | |
 Fig. 175 | ||
| Fig. 173, 174, 175. Young larvæ of Toxopneustes in different stages of development; e'-eiv arms, v-v'' vibratile chord, w w' earlets (water-tubes), a o d c digestive system, r'-r''' solid rods of arms, m mouth, b madreporic opening. | ||
Sea-urchins.