The oral aperture is occupied by five plates, which may be called jaws, remembering always that here again this word signifies the function, and not the structure usually associated with the presence of jaws in the higher animals; and each of these jaws or plates terminates in a tooth. Even the mode of eating in these animals is controlled by their radiate structure; for these jaws, evenly distributed about the circular oral aperture, open to receive the prey and then are brought together to crush it, the points meeting in the centre, thus working concentrically, instead of moving up and down or from right to left, as in other animals. From the oral opening the ten zones diverge, spreading over the whole surface, like the ribs on a melon, and converging in the opposite direction till they meet in the small space which we have called the ab-oral region opposite the starting-point.
Here the broad zones terminate in five large plates differing somewhat from those that form the zones in other parts of the body, and called ovarian plates, because the eggs pass out through certain openings in them; while the five narrow zones terminate in five small plates on each of which is an eye, making thus five eyes alternating with five ovarian plates. The centre of this area containing the ovarian plates and the visual plates is filled up with small movable plates closing the space between them. I should add that one of the five ovarian plates is larger than the other four, and has a peculiar structure, long a puzzle to naturalists. It is perforated with minute holes, forming an exceedingly delicate sieve, and this is actually the purpose it serves. It is, as it were, a filter, and opens into a canal which conducts water through the interior of the body; closed by this sieve on the outside, all the water that passes into it is purified from all foreign substances that might be injurious to the animal, and is thus fitted to pass into the water-system, from which arise the main branches leading to the minute suckers which project through the holes in the narrow zones of plates.
[Illustration: Star-Fish from the ab-oral side.]
Now in order to transform theoretically our Sea-Urchin into a Star-Fish, what have we to do? Let the reader imagine for a moment that the small ab-oral area closing the space between the ovarian plates and the eye-plates is elastic and may be stretched out indefinitely; then split the five broad zones along the centre and draw them down to the same level with the mouth, carrying the ovarian plates between them. We have then a star, just as, dividing, for instance, the peel of an orange into five compartments, leaving them, of course, united at the base, then stripping it off and spreading it out flat, we should have a five-rayed star.
[Illustration: One arm of Star-Fish from the oral side.]
But in thus dividing the broad zones of the Sea-Urchins, we leave the narrow zones in their original relation to them, except that every narrow zone, instead of being placed between two broad zones, has now one-half of each of the zones with which it alternated in the Sea-Urchin on either side of it and lies between them. The adjoining wood-cut represents a single ray of a Star-Fish, drawn from what we call its lower side or the oral side. Along the centre of every such ray, diverging from the central opening or the mouth, we have a furrow, corresponding exactly to the narrower zones of the Sea-Urchin. It is composed of comparatively small perforated plates through which pass the suckers or locomotive appendages. On either side of the furrows are other plates corresponding to the plates of the broad zones in the Sea-Urchin. Where shall we look for the five eyes? Of course, at the tip of every ray; exactly where they were when the rays were drawn up to form the summit of a sphere, so that the eyes, which are now at their extremities, were clustered together at their point of meeting. Where shall we look for the ovarian plates? At each angle of the five rays, because, when the broad zones of which they formed the summit were divided, they followed the split, and now occupy the place which, though it seems so different on the surface of the Star-Fish, is nevertheless, relatively to the rest of the body, the same as they occupied in the Sea-Urchin. Assuming, as we premised, that the central area of the ab-oral region, forming the space between the plates at the summit of the zones in the Sea-Urchin, is elastic, it has stretched with the spreading out of the zones, following the indentation between the rays, and now forms the whole upper surface of the body. All the internal organs of the animal lie between the oral and ab-oral regions, just as they did in the Sea-Urchin, only that in the Star- Fish these regions are coequal in extent, while in the Sea-Urchin the ab-oral region is very contracted, and the oral region with the parts belonging to it occupies the greater part of its surface.
Such being the identity of parts between a Star-Fish and a Sea-Urchin, let us see now how the Star-Fish may be transformed into the Pedunculated Crinoid, the earliest representative of its Class, or into a Comatula, one of the free animals that represent the Crinoids in our day.
[Illustration: Crinoid with branching crown; oral side turned upward.]
We have seen that in the Sea-Urchins the ab-oral region is very contracted, the oral region and the parts radiating from it and forming the sides being the predominant features in the structure; and we shall find, as we proceed in our comparison, that the different proportion of these three parts, the oral and ab-oral regions and the sides, determines the different outlines of the various Orders in this Class. In the Sea-Urchin the oral region and the sides are predominant, while the ab-oral region is very small. In the Star-Fish, the oral and ab-oral regions are brought into equal relations, neither preponderating over the other, and the sides are compressed, so that, seen in profile, the outline of the Star-Fish is that of a slightly convex disk, instead of a sphere, as in the Sea-Urchin. But when we come to the Crinoids, we find that the great preponderance of the ab-oral region determines all that peculiarity of form that distinguishes them from the other Echinoderms, while the oral region is comparatively insignificant. The ab-oral region in the Crinoid rises to form a sort of cup-like or calyx-like projection. The plates forming it, which in the Star-Fish or the Sea-Urchin are movable, are soldered together so as to be perfectly immovable in the Crinoid. Let this seeming calyx be now prolonged into a stem, and we see at once how striking is the resemblance to a flower; turn it downwards, an attitude which is natural to these Crinoids, and the likeness to a drooping lily is still more remarkable The oral region, with the radiating ambulacra, is now limited to the small flat area opposite the juncture of the stem with the calyx; and whether it stretches out to form long arms, or is more compact, so as to close the calyx like a cup, it seems in either case to form a flower-like crown. In these groups of Echinoderms the interambulacral plates are absent; there are no rows of plates of a different kind alternating with the ambulacral ones, as in the Sea-Urchins and the Star-Fishes, but the ab-oral region closes immediately upon the ambulacra.
It seems a contradiction to say, that, though these Crinoids were the only representatives of their Class in the early geological ages, while it includes five Orders at the present time, Echinoderms were as numerous and various then as now. But, paradoxical as it may seem, this is nevertheless true, not only for this Class, but for many others in the Animal Kingdom. The same numerical proportions, the same richness and vividness of conception were manifested in the early creation as now; and though many of the groups were wanting that are most prominent in modern geological periods, those that existed were expressed in such endless variety that the Animal Kingdom seems to have been as full then as it is to-day. The Class of the Echinoderms is one of the most remarkable instances of this. In the Silurian period, the Crinoids stood alone; there were neither Ophiurans, Asteroids, Echinoids, nor Holothurians; and yet in one single locality, Lockport, in the State of New York, over an area of not more than a few square miles, where the Silurian deposits have been carefully examined, there have been found more different Species of Echinoderms than are living now along our whole Atlantic coast from Maine to Florida.