The bearing of embryology upon classification is becoming every day more important, rendering the processes of development among animals one of the most interesting and instructive studies to which the naturalist can devote himself, in the present state of his science. The accuracy of this test, not only as explaining the relations between animals now living, but as giving the clew to their connection with those of past times, cannot but astonish any one who makes it the basis of his investigations. The comparison of embryo forms with fossil types is of course difficult, and must in many instances be incomplete, for while, in the one case, death and decay have often half destroyed the specimen, in the other, life has scarcely stamped itself in legible characters on the new being. Yet, whenever such comparisons have been successfully carried out, the result is always the same; the present representatives of the fossil types recall in their embryonic condition the ancient forms, and often explain their true position in the animal kingdom. One of the most remarkable examples of this in the type we are now considering, is that of the Comatula already mentioned. Its condition in the earlier stages of growth, when it is provided with a stem, at once shows its relation to the old stemmed Crinoids, the earliest representatives of the class of Echinoderms. These coincidences are still more striking among living animals, where they can be more readily and fully traced, and often give us a key to their relative standing, which our knowledge of their anatomical structure fails to furnish. This is perhaps nowhere more distinctly seen than in the type of Radiates, where the Acalephs in their first stages of growth, that is, in their Hydroid condition, remind us of the adult forms among Polyps, showing the structural rank of the Acalephs to be the highest, since they pass beyond a stage which is permanent with the Polyps; while the adult forms of the Acalephs have in their turn a certain resemblance to the embryonic phases of the class next above them, the Echinoderms. Within the limits of the classes, the same correspondence exists as between the different orders; the embryonic forms of the higher Polyps recall the adult forms of the lower ones, and the same is true of the Acalephs as far as these phenomena have been followed and compared among them. In the class of Echinoderms the comparison has been carried out to a considerable extent, their classification has hitherto been based chiefly upon the ambulacral system, so characteristic of the class, but so unequally developed in the different orders. This places the Holothurians, in which the ambulacral system has its greatest development, at the head of the class; next to them come the Sea-urchins or Echinoids; then the Star-fishes; then the Ophiurans and Crinoids, in which the ambulacral system is reduced to a minimum. Another basis for classification in this type, which gives the same