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