skeleton of Echinoderms differs from the "shell" of a crab or lobster, which is formed by a hardening of the skin itself. [Illustration: FIG. 39.--_A_, Head of a Stone Lily or Encrinite, _Encrinus liliformis_, a fossil from the Muschelkalk of Brunswick, natural size. _B_, Rock with stalks of encrinites. _C_, Section of a stalk.] The Crinoidea, Encrinites or Stone-Lilies, form another group of the Echinodermata. Though still represented by living forms, they attained their maximum development in past ages. The English "Mountain Limestone" of the Carboniferous period is full of their fossilized remains, which form a marble often used for ornamental purposes. The so-called "Stone Lily" consists of a "head" comparable with the body of a Star-fish or other Echinoderm, which is borne at the end of a long fixed stalk. The marble above named owes its ornamental appearance to the presence of these stalks, often very long, and cut through at every possible angle. The Crinoids have their living representative in English Seas, _Antedon_, the Feather-Star (Fig. 40). On the side opposite the mouth, where, in the Encrinite, the stalk would be, there are a group of elongated processes called cirrhi, by means of which the animal can attach itself to stones or seaweeds. When not thus fixed, it swims about, by moving its fringed arms, each of which is forked. It will be seen that when the animal is fixed by its cirrhi, it stands mouth upwards, so that its position compared with that of the Starfish or Sea-Urchin is upside down. The young of the Feather-Stars have stalks by which they are fixed, like the Encrinites; but afterwards the stalk is lost. [Illustration: FIG. 40.--A Feather-Star, _Antedon bifida_, British Seas, three-quarters of the natural size. The short threads in the middle are the cirrhi.] 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.