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