Among fossil Echinoderms there are two groups of stalked forms which have no living representatives. These are the Cystoidea and the Blastoidea. In both of these the stalk bears, as in Encrinites, a calyx or head, which is comparable, with the body of the free Echinoderms.
The Sea-Urchins possess a swimming larval stage, which goes through remarkable changes after passing out of the two-layered (Gastrula) form. It becomes provided with cilia, which are arranged in bands, and outgrowths of peculiar form are established in the case of the Sea-Urchins, while the larvæ of the other groups also present characteristic shapes. Within the larva the adult form develops, the outside of the larva being finally thrown off.
In the young Feather-Star, a subsequent stage of the young animal has a stalk, by which, like the Encrinite, it is fixed. This animal therefore is at first free-swimming, afterwards fixed, and again free in its final stage—a remarkable series of changes.
These queer-shaped things, the Sea-Urchins and their allies, are perhaps the last creatures amongst which we should think of looking for relations of the Worms. Yet the earliest stages of the larva are considered to present a certain amount of resemblance to the Wheel-ball larva, which has been referred to elsewhere (pp. [42] and [72]). Still more startling fact, these larvæ have been compared to that of Balanoglossus, the lowest member of the Chordata, and a relation of the Vertebrates themselves (see [p. 143]).
TABLE SHOWING THE CLASSIFICATION OF THE ECHINODERMATA
| ECHINODERMATA. | ECHINOIDEA, or
Sea-Urchins. ASTEROIDEA, or Star-Fishes. OPHIUROIDEA, or Brittle-Stars. CRINOIDEA, or Feather-Stars and Stone-Lilies. HOLOTHUROIDEA, or Sea-Cucumbers. |