There is nothing more striking in these early populations of the earth than the richness of the types. It would seem as if, before the world was prepared for the manifold existences that find their home here now, when organic life was limited by the absence of many of the present physical conditions, the whole wealth of the Creative Thought lavished itself upon the forms already introduced upon the globe. After thirty years' study of the fossil Crinoids, I am every day astonished by some new evidence of the ingenuity, the invention, the skill, if I may so speak, shown in varying this single pattern of animal life. When one has become, by long study of Nature, in some sense intimate with the animal creation, it is impossible not to recognize in it the immediate action of thought, and even to specialize the intellectual faculties it reveals. It speaks of an infinite power of combination and analysis, of reminiscence and prophecy, of that which has been in eternal harmony with that which is to be; and while we stand in reverence before the grandeur of the Creative Conception as a whole, there breaks from it such lightness of fancy, such richness of invention, such variety and vividness of color, nay, even the ripple of mirthfulness,—for Nature has its humorous side also,—that we lose our grasp of its completeness in wonder at its details, and our sense of its unity is clouded by its marvellous fertility. There may seem to be an irreverence in thus characterizing the Creative Thought by epithets which we derive from the exercise of our own mental faculties; but it is nevertheless true, that, the nearer we come to Nature, the more does it seem to us that all our intellectual endowments are merely the echo of the Almighty Mind, and that the eternal archetypes of all manifestations of thought in man are found in the Creation of which he is the crowning work.
In no group of the Animal Kingdom is the fertility of invention more striking than in the Crinoids. They seem like the productions of one who handles his work with an infinite ease and delight, taking pleasure in presenting the same thought under a thousand different aspects. Some new cut of the plates, some slight change in their relative position is constantly varying their outlines, from a close cup to an open crown, from the long pear-shaped oval of the calyx in some to its circular or square or pentagonal form in others. An angle that is simple in one projects by a fold of the surface and becomes a fluted column in another; a plate that was smooth but now has here a symmetrical figure upon it drawn in beaded lines; the stem which is perfectly unbroken in one, except by the transverse divisions common to them all, in the next puts out feathery plumes at every such transverse break. In some the plates of the stem are all rigid and firmly soldered together; in others they are articulated upon each other in such a manner as to give it the greatest flexibility, and allow the seeming flower to wave and bend upon its stalk. It would require an endless number of illustrations to give even a faint idea of the variety of these fossil Crinoids. There is no change that the fancy can suggest within the limits of the same structure that does not find expression among them. Since I have become intimate with their wonderful complications, I have sometimes amused myself with anticipating some new variation of the theme, by the introduction of some undescribed structural complication, and then seeking for it among the specimens at my command, and I have never failed to find it in one or other of these ever-changing forms.
The modern Crinoid without stem, or the Comatula, though agreeing with the ancient in all the essential elements of structure, differs from it in some specific features. It drops its stem when full-grown, though the ab-oral region still remains the predominant part of the body and retains its cup-like or calyx-like form. The Comatulae are not abundant, and though represented by a number of Species, yet the type as it exists at present is meagre in comparison to its richness in former times. Indeed, this group of Echinoderms, which in the earliest periods was the exponent of all its kind, has dwindled gradually, in proportion as other representatives of the Class have come in, and there exists only one species now, the Pentacrinus of the West Indies, which retains its stem in its adult condition. It is a singular fact, to which I have before alluded, and which would seem to have especial reference to the maintenance of the same numeric proportions in all times, that, while a Class is represented by few types, those types are wonderfully rich and varied, but in proportion as other expressions of the same structure are introduced, the first dwindle, and, if they do not entirely disappear, become at least much less prominent than before.
[Illustration: Ophiuran; showing one ray from the oral side.]
There remain only two other Orders to be considered, the Ophiurans and the Holothurians. The Ophiurans approach the Crinoids more nearly than any other group of Echinoderms, and in our classifications are placed next above them. In them the ab-oral region, which has such a remarkable predominance in the Crinoid, has become depressed; it no longer extends into a stem, nor does it even rise into the calyx-like or cup-like projection so characteristic of the Crinoids,—though, when the animal is living, the ab-oral side of the disk is still quite convex. The disk in the Ophiurans is small in comparison to the length of the arms, and perfectly circular; it does not merge gradually in the arms as in the Star-Fish, but the arms start abruptly from its periphery. In these, as in the Crinoids, the interambulacral plates are absent, and the interambulacral spaces are filled by an encroachment of the ab-oral region upon them. There is an infinite variety and beauty both of form and color in these Sea-Stars. The arms frequently measure many times the diameter of the whole disk, and are so different in size and ornamentation in the different Species that at first sight one might take them for animals entirely distinct from each other. In some the arms are comparatively short and quite simple,—in others they are very long, and may be either stretched to their full length or partly contracted to form a variety of graceful curves; in some they are fringed all along the edges,—in others they are so ramified that every arm seems like a little bush, as it were, and, intertwining with each other, they make a thick network all around the animal. In the geological succession, these Ophiurans follow the Crinoids, being introduced at about the Carboniferous period, and perhaps earlier. They have had their representatives in all succeeding times, and are still very numerous in the present epoch.
To show the correspondence of the Holothurians with the typical formula of the whole class of Echinoderms, I will return to the Sea-Urchins, since they are more nearly allied with that Order than with any of the other groups. We have seen that the Sea-Urchins approach most nearly to the sphere, and that in them the oral region and the sides predominate so greatly over the ab-oral region that the latter is reduced to a small area on the summit of the sphere. In order to transform the Sea-Urchin into a Holothurian, we have only to stretch it out from end to end till it becomes a cylinder, with the oral region or mouth at one extremity, and the ab-oral region, which in the Holothurian is reduced to its minimum, at the other. The zones of the Sea-Urchin now extend as parallel rows on the Holothurian, running from one end to the other of the long cylindrical body. On account of their form, some of them have been taken for Worms, and so classified by naturalists; but as soon as their true structure was understood, which agrees in every respect with that of the other Echinoderms, and has no affinity whatever with the articulated structure of the Worms, they found their true place in our classifications.
[Illustration: Holothurian.]
The natural attitude of these animals is different from that of the other Echinoderms: they lie on one side, and move with the oral opening forward, and this has been one cause of the mistakes as to their true nature. But when we would compare animals, we should place them, not in the attitude which is natural to them in their native element, but in what I would call their normal position,—that is, such a position as brings the corresponding parts in all into the same relation. For instance, the natural attitude of the Crinoid is with the ab-oral region downward, attached to a stem, and the oral region or mouth upward; the Ophiuran turns its oral region, along which all the suckers or ambulacra are arranged, toward the surface along which it moves; the Star-Fish does the same; the Sea-Urchin also has its oral opening downward; but the Holothurian moves on one side, mouth foremost, as represented in the adjoining wood-cut, dragging itself onward, like all the rest, by means of its rows of suckers. If, now, we compare these animals in the various attitudes natural to them, we may fail to recognize the identity of parts, or, at least, it will not strike us at once. But if we place them all—Holothurian, Sea-Urchin, Star-Fish, Ophiuran, and Crinoid—with the oral or mouth side downward, for instance, we shall see immediately that the small area at the opposite end of the Holothurian corresponds to the area on the top of the Sea-Urchin; that the upper side of the Star-Fish is the same region enlarged; that, in the Ophiuran, that region makes one side of the small circular disk; while in the Crinoid it is enlarged and extended to make the calyx-like projection and stem. In the same way, if we place them in the same attitude, we shall see that the long, straight rows of suckers along the length of the Holothurian, and the arching zones of suckers on the spherical body of the Sea-Urchin, and the furrows with the suckers protruding from them along the arms of the Star-Fish and Ophiuran, and the radiating series of pores from the oral opening in the Crinoid are one and the same thing in all, only altered somewhat in their relative proportion and extent. Around the oral opening of the Holothurian there are appendages capable of the most extraordinary changes, which seem at first to be peculiar to these animals, and to have no affinity with any corresponding feature in the same Class. But a closer investigation has shown them to be only modifications of the locomotive suckers of the Star-Fish and Sea-Urchin, but ramifying to such an extent as to assume the form of branching feelers. The little tufts projecting from the oral side in the Sea-Urchins, described as gills, are another form of the same kind of appendage.
The Holothurians have not the hard, brittle surface of the other Echinoderms; on the contrary, their envelope is tough and leathery, capable of great contraction and dilatation. No idea can be formed of the beauty of these animals either from dried specimens or from those preserved in alcohol. Of course, in either case, they lose their color, become shrunken, and the movable appendages about the mouth shrivel up. One who had seen the Holothurian only as preserved in museums would be amazed at the spectacle of the living animal, especially if his first introduction should be to one of the deep, rich crimson-colored species, such as are found in quantities in the Bay of Fundy. I have seen such an animal, when first thrown into a tank of sea-water, remain for a while closely contracted, looking like a soft crimson ball. Slowly, almost imperceptibly, as it becomes accustomed to its new position, it begins to elongate; the fringes creep softly out, spreading gradually all their ramifications, till one end of the animal seems crowned with feathery, crimson sea-weeds of the most delicate tracery. It is much to be regretted that these lower marine animals are not better known. The plumage of the tropical birds, the down on the most brilliant butterfly's wing, are not more beautiful in coloring than the hues of many Radiates, and there is no grace of motion surpassing the movements of some of them in their native element. The habit of keeping marine animals in tanks is happily growing constantly more popular, and before long the beauty of these inhabitants of the ocean will be as familiar to us as that of Birds and Insects. Many of the most beautiful among them are, however, difficult to obtain, and not easily kept alive in confinement, so that they are not often seen in aquariums.
Having thus endeavored to sketch each different kind of Echinoderm, let us try to forget them all in their individuality, and think only of the structural formula that applies equally to each. In all, the body has three distinct regions, the oral, the ab-oral, and the sides; but by giving a predominance to one or other of these regions, a variety of outlines characteristic of the different groups is produced. In all, the parts radiate from the oral opening, and join in the ab-oral region. In all, this radiation is accompanied by rows of suckers following the line of the diverging rays. It is always the same structure, but, endowed with the freedom of life, it is never monotonous, notwithstanding its absolute permanence. In short, drop off the stem of the Crinoid, and depress its calyx to form a flat disk, and we have an Ophiuran; expand that disk, and let it merge gradually in the arms, and we have a Star-Fish; draw up the rays of the Star-Fish, and unite them at the tips so as to form a spherical outline, and we have a Sea-Urchin; stretch out the Sea-Urchin to form a cylinder, and we have a Holothurian.