Echinoderm at a (ekinodermata)

pl. [NL., neut. pl. of echinoderm at us: see echino derma to us.] A phylum or subkingdom of metazoic animals; the echinoderms. They represent one of the most distinct types of the animal kingdom, agreeing with cælenterates in having a radiate or act i nom eric arrangement of parts, usually pentamerous or by fives or tens, a digestive canal, a water-vascular or ambulacral apparatus, a true blood-vascular system, and the integument indurated by calcareous deposits, as either granules, spicules, or hard plates forming a shell. The alimentary canal is distinct from the general body-cavity; there is a deuterostomatous oral orifice or mouth, and usually an anus. The sexes are mostly distinct. The species undergo metamorphosis; the free-swimming ciliated embryo is known as a pluteus, in some cases as an echinopædium (see cut under echinopædium ); the adult form is usually a umed by a complicated kind of secondary development from the larval form, which is mostly bilateral. The Echinoderm at a were so named by Klein in 1734, and in Cuvier's system were the first cla of his Radiata; they are still sometimes reduced to a cla with the Cœlen terata. As a subkingdom they are divisible into four cla es: Crinoidea, Ech i no idea, Asteroidea, and Holothu rioidea, or the crinoids, sea-urchins, starfishes, and sea cucumbers. As a cla they are sometimes divided directly into seven orders: Ech i no idea (sea-urchins), Asteroidea (starfishes), Ophiuroidea (sand-stars and brittle-stars), Crinoidea (feather-stars), Cystoidea (extinct), Blastoidea (extinct), and Holothurioidea (sea-cucumbers). All are marine. Also Echinoderm a. The organization of the Echinoderm at a does in fact appear so different from that of the cœlenterates, and seems belong so much higher grade of development, the combination of the two groups as Radiata is inadmis sible, and so much the more so since the radial arrangement of the structure exhibits some transitions towards a bilateral symmetry. The Echinoderm at a are separated from the Cœlen terata by the po e ion of a separate alimentary canal and vascular system, and also by a number of peculiar features both of organization and of develop ment. Claus , Zoology (trans.), I. 267.