stant than when young. The anal portion is kept in advance when moving, and the larva rotates about, but not as frequently as when young; moving more generally with either the ventral or dorsal side uppermost, and more rarely in such a way that the profile can be seen. When at rest, they invariably assume one and the same position; that is, turn slightly obliquely below the anal portion, with the dorsal surface downwards. In this way they often remain for a long period, simply carried about by the currents; the only movements being the expansion and contraction of the oesophagus, and the slow bending and twisting of the arms in every direction. Up to the stage represented in Fig. 9, all the larvæ were raised by artificial fecundation from eggs of Asteracanthion berylinus Ag. At the time when I discovered these larvæ I immediately examined the ovaries of our star-fishes, and found that in one species, the berylinus, the eggs were not yet sufficiently advanced to be fecundated, while the eggs of the second species, which is so common on our rocks, the Ast. pallidus, had all escaped. I had, however, been fortunate enough to find quite young larvæ of this second species in which the water-tubes were still exceedingly small, and had made a complete series of drawings of general outlines from their youngest stage up to the time when the star-fish is formed, so that I am certain that all the young I have represented as belonging together are those of one species, as the interval between the time when these two species spawn is more than three weeks. The time of spawning is very short; three or four days after the Ast. berylinus began to spawn it was quite difficult to find females which had not lost their eggs, and a week after that period I found none. Owing to this great difference in the time of spawning, and its short duration, the dates at which I caught these star-fish larvæ floating about leave no doubt to which of the species the larvæ belonged. A careful comparison of the youngest specimens also shows very striking differences, which will always enable an observer to distinguish readily the larvæ of these two species, even in their earlier stages. The males can easily be distinguished from the females by their difference of color; the females being always slightly bluish, while the males have a decidedly reddish tint. The same difference in color is noticed in our Sea-urchin, Toxopneustes drobachiensis. The females are of a light green at the time of spawning, while the males are of a dull vermilion color. In eggs which have been fecundated artificially (Fig. 1), the spermatic particles surround like a halo the whole of the outer envelope, beating about its surface with the greatest violence. The yolk soon begins to contract after the germinative vesicle and dot have disappeared, and then divides into two spheres. (Fig. 2.) The segmentation takes place very rapidly, and as soon as there are eight spheres (Fig. 3), they arrange themselves in such a manner as to enclose the remaining space, which is more and more separated as the spheres become more numerous; finally, there is a complete envelope formed before the young makes its escape from the egg. The young when it escapes is spherical. The walls of the envelope are of the same thickness. One side becomes thicker (Fig. 4); the embryo flattens on the thick side. This wall is then bent in so as to form a slight cavity, in which fluid circulates. (Fig. 5.) This cavity extends half the length of the larva (Fig. 6), then swells at the extremities. The walls become thinner; the pouch formed at the end of the cavity develops laterally to form two smaller pouches (Fig. 7), which soon become small hollow bodies entirely separated from the main cavity. (See Fig. 10.) The main cavity bends slightly to one side (Fig. 8), and eventually forms a junction with a depression opposite to it, and there the mouth is formed. The other opening, which was the first to be developed, thus becomes the anus. This bent tube divides into three distinct regions, forming the œsophagus, the digestive cavity, and the alimentary canal. (Fig. 9.) The small hollow bodies, the water-tubes, which are not connected with one another in the young embryos (Fig. 10), differ one from the other. One, the left (when seen from above), connects with the surrounding medium by means of an opening, the water-pore. (Fig. 9.) In older specimens these two tubes extend to the extremity of the digestive cavity, and towards one another, beyond the mouth, where they unite, forming a Y-shaped tube. (Fig. 11.) Arms are developed which are edged with rows of vibratile cilia. Some of these arms are of a different character, having different appendages (see Figs. 12, 13). On these water-tubes is developed the star-fish; one of the water-tubes (the one with the water-pore) developing the actinal side and the tentacles (Fig. 13), the other developing the spines and the abactinal area (Fig. 12). These opposite parts of the star-fish are open (Fig. 13) pentagonal spiral surfaces, not in the same plane, but making nearly a right-angle with one another. The water-pore becomes the madreporic body. |