During the eighteenth century the works upon the Crinoïdæ were very numerous, though not very correct. They sometimes reported these organic remains to be vegetable; sometimes they were beings allied to the star-fishes; at others they were the vertebral column of fishes. Towards the year 1761, however, Guettard, one of the most learned naturalists of his time, understood the real nature of these productions. He had occasion to examine a recent Encrinus sent from Martinique under the name of Sea-Palm, which was in reality Pentacrinus caput Medusæ. The comparison of the living individual with the fossil fragment described by his predecessors, and of which he had specimens in his collections, enabled him to ascertain the real origin of the fossil Encrinoidæ. The beautiful fragment which still exists in the Museum of Natural History at Paris was long considered unique, but it is now known that ten others exist in different museums. Since that date the Crinoïdæ have been examined and described by observers such as Miller, Forbes, D'Orbigny, and Pictet, and very elaborately by Major Austin.
Fig. 108. Pentacrinus Europæus (Thompson).
"The species of fixed Crinoïdæ actually living are Pentacrinus caput Medusæ (Fig. 107), and Pentacrinus Europæus (Fig. 108). These curious zoophytes resemble a flower borne upon a stem, which terminates in an organ called the calyx, but which is, properly speaking, the head of the animal. Arms, more or less branching, spring from this calyx, their ramifications, so formed, consisting of many pieces articulated to each other. The calyx is supported by a stem, varying in height, formed of pieces secreted by the living tissues which surround them. The articulations of this stem are usually very numerous, cylindrical, and present a series of rays striated upon their articulated faces. In Pentacrinus they are prismatic and pentagonal; that is, they present five projecting angles, and on their articulated face a star with five branches, or, better still, a rose with five petals. At the base of the stem of this animal-plant, in many of the Crinoïdæ, we find a sort of spreading root, which is implanted in the rocks, and is capable of growing by itself, of nourishing the stem, and of producing new ones.
The root and stem of the fixed encrinites seem to indicate that the animal can only live with the head erect. Their normal condition is thus quite different from that of any other of the Echinoderms, almost all of which keep their mouths invariably directed downwards.
The Medusæ heads are chiefly found on rocky beds, or in the midst of banks of corals, at great depths. There, firmly fixed by their roots, their long stems raise themselves vertically; then, with expanded calyx and long-spreading arms, they wait for the prey which passes within their reach in order to seize it.
The Pentacrinus caput Medusæ have, as we have said, been fished up from great depths in the Antilles. Its very small calyx is borne upon a stem of from eighteen to twenty inches in height, terminating in long movable arms, the internal surface of which bears its tentacles in a groove. In the middle of the arms is a mouth, and at the side the orifice for the expulsion of the digested residuum.
In the Medusæ head and European Pentacrine (P. Europæus, Fig. 108), the presence of a digestive apparatus has been distinctly traced. It is a sort of irregular sac, with a central mouth on the upper surface, and another orifice situated at a little distance from the mouth, and evidently intended as an outlet for the products of digestion. The arms of these creatures, which are spreading or folded up according to their wants, are provided with fleshy tentacula, which, serving at once as organs of absorption and as vibratile cilia, are at the same time organs of respiration. Such are these curious beings: they occupy a sort of middle or transition state between animals permanently fixed to some spot and those capable of motion, representing in our own times the last remains of extinct generations. Every type of the Crinoïdæ furnished with arms presents incontestable evidence of their mode of reproduction or redintegration—that is, of the power of restoring those parts of the body broken or destroyed by accident; but as we have already drawn the attention of the reader to this strange faculty of renewing organs which many of the zoophytes possess, we will not here enlarge further upon the subject.
The Crinoïdæ are not all like the two species which have been described. There is an entire family of animals belonging to this class, namely, the Comatula, which are fixed in their early days, but separate themselves from the rooted stem in their adult age, and, throwing off the bonds imposed on their youth, live side by side with the asterias, with whose company they seem much pleased. The encrinites and the star-fishes thus live in company, and that at prodigious depths, and under a body of water which no light can reach. Imagine the existence of animals which pass their lives in such eternal funereal darkness. The family of Comatula are found in the seas of both hemispheres. Their bodies are flat—a large calcareous plate formed like a cuirass upon their backs—presenting, besides, cirri composed of numerous curling articulations, the last of which terminates in a hook. The ventral surface presents two orifices: the one in the centre corresponding to a mouth, the other evidently intended for the discharge of the products of digestion. This animal is provided with five arms, which diverge directly from the centre plate or cuirass. The branches of these arms have ambulacral grooves, comprehending a double row of fleshy tentacles, in the centre of which is the ambulacral groove, properly so called, clothed with vibratile cilia over their whole surface. These cilia or hairs guide the current which drives the various substances on which it feeds, such as the organic corpuscles of sea-weeds, and microscopic animalcules floating in the sea, towards its mouth. They are also powerful aids to respiration.
The movements of these curious creatures are very slow, their only object being to catch the bodies of animals and marine plants, or, by extending or contracting their arms, to feel their way through the water to some new locality. Sometimes, also, in order to change their feeding-ground, the Comatula abandon the submarine forests, herbage, and sea-wracks, and float through the water, moving their arms with considerable rapidity in search of a new station.