[Illustration: Fig. 152. Fossil Pentacrinus.]
The Crinoids are very scantily represented in the present creation. They had their day in the earlier geological epochs, when for some time they remained the sole representatives of their class, and were then so numerous that the class of Echinoderms, with only one order, seemed as full and various as it now does with five. The different forms they assumed in the successive geological periods are particularly instructive; these older Crinoids combined characters which foreshadowed the advent of the Ophiurans, the true Star-fishes, and the Sea-urchins; and so prominently were their prophetic characters developed, that many of them are readily mistaken for Star-fishes or Sea-urchins.
In later times the group of Crinoids has been gradually dwindling in number and variety. Its present representatives are the Pentacrini of Porto Rico and the coast of Portugal, the lovely little Rhizocrinus of the Atlantic, dredged first by the younger Sars on the coast of Norway, attached throughout life to a stem, and the Comatula, which has a stem only in the early stages of its growth, but is free when adult. The Pentacrinus bears the closer relation to the more ancient Crinoids (Fig. 152), which were always supported on a stem, while it is only in more recent periods that we find the free Crinoids, corresponding to the Comatula.
One large species of Comatula (_Alecto Eschrichtii_ M. & T.) is known on our coast, off the shores of Greenland, where it has been dredged at a depth of about one hundred and fifty fathoms, and young specimens of the same species have been found as far south as Eastport, Maine. The species selected for representation here, however, (Fig. 153,) is one quite abundant along the shores of South Carolina. It is introduced instead of the northern one, because the latter is so rare that it is not likely to fall into the hands of our readers. The annexed drawing (Fig. 154, magnified from Fig. 153) represents a group of the young of the Charleston Comatula, still attached to the parent body by their stems, and in various stages of development. At first sight, the Comatula, or, as it is sometimes called, the feather-star, resembles an Ophiuran; but on a closer examination we find that the arms are made up of short joints; and along the sides of the arms, attached to each joint, are appendages resembling somewhat the beards of a feather, and giving to each ray the appearance of a plume; hence the name of feather-star. On one side the arms are covered with a tough skin, through which project the ambulacræ, and on the same side of the disk are situated the mouth and the anus; the latter projects in a trumpet-shaped proboscis. On the opposite side of the disk the Comatula is covered with plates, arranged regularly around a central plate, which is itself covered with long cirri.