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. [Illustration: Fig. 176. Adult larva of Toxopneustes, _f_ brachiolar appendages.] [Illustration: Fig. 177. Fig. 176 seen endways.] [Illustration: Fig. 178. The Sea-urchin resorbing the arms of the larva.] [Illustration: Fig. 179. Half a young Sea-urchin immediately after resorption of the larva; _s''_ _s''_ spines, _t'_ _t'_ ambulacral tentacles.] [Illustration: Fig. 180. Young Sea-urchin older than Fig. 179; _t_ _t'_ tentacles, _s''_ _s'''_ spines.] [Illustration: 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. [Illustration: Fig. 182. Ophiuran which has resorbed the whole larva except the two long arms, _y_ _y'_ limestone rods of young Ophiuran, _r_ middle of back, lettering as in Fig. 183.] [Illustration: Fig. 183. Larva of Ophiuran; _e'-e^iv_ arms, _r'_ _r^iv_ solid rods, _v_ _v'_ vibratile chord, _w_ _w'_ water system, _b_ madreporic body, _a_ _d_ digestive system.] [Illustration: Fig. 184 Young Ophiuran which has resorbed the whole larva; _r_ middle plate of back.] [Illustration: Fig. 185 Cluster of eggs of Star-fishes placed over the mouth of the parent.]