[Illustration: Fig. 154. Magnified view of the group of young Comatulæ of Fig. 153.] We are indebted to Thompson for the explanation of the true relations of the young Comatula to the present Pentacrinus and the fossil Crinoids. Supposing these young to be full-grown animals, he at first described them as living representatives of the genus Pentacrinus; it was only after he had watched their development, and ascertained by actual observation that they dropped from their stem, to lead an independent life as free Comatulæ, that he fully understood their true connection with the past history of their kind, as well as with their contemporaries. In Fig. 153, a faint star-like dot (_y_) may be seen attached to the side of the disk by a slight line. In Fig. 154, we have that minute dot as it appears under the microscope, magnified many diameters; when it is seen to be a cirrus of a Comatula, with three small Pentacrinus-like animals growing upon it, in different stages of development. In the upper one, the branching arms and the disk, with its many plates, are already formed; and though in the figure the rays are folded together, they are free, and can be opened at will. In the larger of the two lower buds, the plates of the disk are less perfect, and the arms are straight and simple, without any ramifications, though they are free and movable, whereas, in the smaller one, they are folded within the closed bud. * * * * * EMBRYOLOGY OF ECHINODERMS. All Radiates have a special mode of development, as distinct for each class as is their adult condition, and in none are the stages of growth more characteristic than in the Echinoderms. In the Polyps, the division of the body into chambers, so marked a feature of their ultimate structure, takes place early; in the Acalephs, the tubes which traverse the body are hollowed out of its mass in the first stages of the embryonic growth, and we shall see that in the Echinoderms also, the distinctive feature of their structure, viz. the enclosing of the organs by separate walls, early manifests itself. This peculiarity gives to the internal structure of these animals so individual a character, that some naturalists, overlooking the law of radiation, as prevalent in them as in any members of this division, have been inclined to separate them, as a primary division of the animal kingdom, from the Polyps and Acalephs, in both of which the body-wall furnishes the walls of the different internal cavities, either by folding inwardly in such a manner as to enclose them, as in the Polyps, or by the cavities themselves being hollowed out of the general mass, as in the Acalephs. _Star-fish_. (_Astracanthion_.) [Illustration: Fig. 155. Egg of Star-fish.] [Illustration: Fig. 156. Egg of Star-fish in which the yolk has been divided into two segments.] [Illustration: Fig. 157. Egg in which there are eight segments of the yolk.]