This extraordinary process of development which we have analyzed thus at length in the history of the Star-fish, but which is equally true of all Echinoderms, has been hitherto described (so far as it was known) under the name of the plutean stages of growth. In these early stages the young, or the so-called larvæ of Echinoderms, have received the name of Pluteus on account of their ever-changing forms. Let us look for a moment at the plutean stages of the Sea-urchin, as they differ in some points from those of the Star-fish. In the Pluteus of our common Sea-urchins ([see Fig. 176]), the arms are supported by a framework of solid limestone rods, which do not exist in that of the Star-fish, and which give to the larva of the Sea-urchin a remarkable rigidity. They are formed very early, as may be seen in [Fig. 173], representing the little Sea-urchin before any arms are discernible, though the limestone rods are quite distinct. [Figs. 173],[ 174],[ 175], may be compared with [Figs. 160], [ 162],[ 165], of the young Star-fish, where it will be seen that the general outline is very similar, though, on account of the limestone rods, the Pluteus of the Sea-urchin seems somewhat more complicated. In [Fig. 176] the young Sea-urchin has so far encroached upon the Pluteus that it forms the essential part of the body, the arms and rods appearing as mere appendages. [Fig. 177] shows the same animal when we looked down upon it in its natural attitude; the Sea-urchin is carried downward, and the arms stretch in every direction around it. In [Fig. 178] the Plutens is already in process of absorption; in [Fig. 179] it has wholly disappeared; in [Figs. 180] [and 181] we have different stages of the little Sea-urchin, with its spines and suckers of a large size and in full activity. The appearance of the Sea-urchin, as soon as this larva or Pluteus is completely absorbed, is much more like that of the adult than is the Star-fish at the same stages, in which, as we have seen, there is a transition period of considerable duration.

[fig 176]


Fig. 176. Adult larva of Toxopneustes, f brachiolar appendages.
Click on image to view larger size.
[fig 177]
Fig. 177. Fig. 176 seen endways.
[fig 178]
Fig. 178. The Sea-urchin resorbing the arms of the larva.
[fig 179]
Fig. 179. Half a young Sea-urchin immediately after resorption of the larva;
s'' s'' spines, t' t' ambulacral tentacles.

[fig 180]
Fig. 180. Young Sea-urchin older than [Fig. 179]; t t' tentacles, s'' s''' spines.
[fig 181]
Fig. 181. Still older Sea-urchin; t t tentacles, a anus, p pedicellariæ;
shell one sixteenth of an inch in diameter.

Ophiurans.

[Fig. 183] represents an Ophiuran undergoing the same process of growth, at a period when the larva is most fully developed, and before it begins to fail. By the limestone rods which support the arms, the Pluteus of the Ophiuran, here represented, resembles that of the Sea-urchin more than that of the Star-Fish, while by the character of the water-tubes and by its internal organization it is more closely allied to the latter. It differs from both, however, in the immense length of two of the arms; these arms being the last signs of its plutean condition to disappear; when the young Ophiuran has absorbed almost the whole Pluteus, it still goes wandering about with these two immense appendages, which finally share the fate of all the rest. [Fig. 182] represents an Ophiuran at the moment when the process of resorption is nearly completed, though the arms of the Pluteus, greatly diminished, are still to be seen protruding from the surface of the animal.

[fig 182]