Star-fish. (Astracanthion.)
| Fig. 155. Egg of Star-fish. | Fig. 156. Egg of Star-fish in which the yolk has been divided into two segments. | Fig. 157. Egg in which there are eight segments of the yolk. |
The egg of the Star-fish, when first formed, is a transparent, spherical body, enclosing the germinative vesicle and dot. ([See Fig. 155.]) As soon as these disappear, the segmentation of the yolk begins; it divides first into two portions ([see Fig. 156]), then into four, then into eight, and so on; but when there are no more than eight bodies of segmentation ([see Fig. 157]), they already show a disposition to arrange themselves in a hollow sphere, enclosing a space within, and by the time the segmentation is completed, they form a continuous spherical shell. At this time the egg, or, as we will henceforth call it, the embryo, escapes and swims freely about. ([See Fig. 158.]) The wall next begins to thin out on one side, while on the opposite side, which by comparison becomes somewhat bulging, a depression is formed (m a, [Fig. 159]), gradually elongating into a loop hanging down within the little animal, and forming a digestive cavity. (d, [Fig. 160].) At this stage it much resembles a young Actinia. The loop spreads somewhat at its upper extremity, and at its lower end is an opening, which at this period of the animal's life serves a double purpose, that of mouth and anus also, for at this opening it both takes in and rejects its food. We shall see that before long a true mouth is formed, after which this first aperture takes its place opposite the mouth, retaining only the function of the anus. Presently from the upper bulging extremity of the digestive cavity, two lappets, or little pouches, project (w/ w' [Fig. 161]); they shortly become completely separated from it, and form two distinct hollow cavities (w w', [Fig. 162]). Here begins the true history of the young Star-fish, for these two cavities will develop into two water-tubes, on one of which the back of the Star-fish, that is, its upper surface, covered with spines, will be developed, while on the other, the lower surface, with the suckers and tentacles, will arise. At a very early stage one of these water-tubes (w', [Fig. 163]) connects with a smaller tube opening outwards, which is hereafter to be the madreporic body (b, [Fig. 163]). Almost until the end of its growth, these two surfaces, as we shall see, remain separate, and form an open angle with one another; it is only toward the end of the development that they unite, enclosing between them the internal organs, which have been built up in the mean while.