So much, then, for the structure of the common Star-fish. I must next say a few words on the remarkable modifications which this structure undergoes in different members of the Star-fish group.
In some species the size of the central disc is increased so as to fill up the interspaces between the rays, the whole animal being thus converted into the form of a pentagon. In other species, again, the reverse process has taken place, the rays having become relatively longer, and being at the same time very active; they look like five little snakes joined together by a circular disc (Fig. 37). Again, in another species the rays have begun to branch, these branches again to branch, and so on till the whole animal looks like a mat. But the most extreme modifications are attained in the sea-cucumbers and lily-stars (Fig. 38). Without, however, waiting to consider these, I shall go a little more particularly into the modification of Star-fish structure which is presented by the sea-urchin, or Echinus (Fig. 39).
Fig. 37.—A Brittle-star. (From Cassell's "Nat. Hist.")
Fig. 38.—A Lily-star. (From Cassell's "Nat. Hist.")]
Externally, the animal presents the form of an orange, and is completely covered with a large number of hard calcareous spines, on which account it derives its scientific name of Echinus, or hedgehog (the spines have been removed from the larger portion of the specimen represented in Fig. 39). In the living animal these spines are fully movable in all directions, each being mounted on a ball-and-socket joint, and provided with muscles at its base. On the external surface, besides the spines, we meet with pedicellariæ (Fig. 33 magnified), and also with the madreporic tubercle (Fig. 39, m). The pedicellariæ in their main features resemble those which occur in the Star-fish, though considerably larger, and the ambulacral system is constructed upon the same plan. If we shave off the spines and pedicellariæ (Fig. 39), we find that we come to a hard shell, which, if we break, we find to be hollow and filled with fluid (Fig. 40). The fluid closely resembles sea-water, but is, nevertheless, richly corpusculated; it coagulates when exposed to the air, and otherwise shows that it is something more than mere sea-water. If we look closely into the shell which has been deprived of its spines, we find that it is composed of a great number of small hexagonal plates (Fig. 41), the edges of which fit so closely together that the whole shell is converted into a box, which, when the animal is alive, is water-tight, as we have proved by submitting the contained fluid to hydrostatic pressure, under which circumstances there is no leakage until the pressure is sufficient to burst the shell. Nevertheless, if we look closely at the dried shell of an Echinus, we shall see that it is not an absolutely closed box; for we shall see that the hexagonal plates are so arranged as to give rise to five double rows of holes or pores (Fig. 41), which extend symmetrically from pole to pole of the animal (Fig. 39). It is through these holes that the tube-feet are protruded; so that if we imagine a pentagonal species of Star-fish to be curved into the shape of a hollow spheroid, and then converted into a calcareous box with holes left for its feet to come through, we should have a mental picture of an Echinus. It would only be necessary to add the curious apparatus of teeth (Figs. 40 and 42), which occurs in the Echinus, to increase the size of the spines and pedicellariæ, and to make a few other such minor alterations; but in all its main features an Echinus is merely a Star-fish with its five rays calcified and soldered together so as to constitute a rigid box.
Fig. 39.—An Echinus, partly denuded of its spines. (From Cassell's "Nat. Hist.")