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