ECHINODERMATA, e-ki-no-der-ma'ta, or ECHINODERMA, one of the most dis tinct phyla of the animal kingdom, embracing the crinoids, sea cucumbers, sea urchins and starfishes. On account of a predominating radical symtnetry Cuvier united them with the Crlenterata in his type Radiates Leuckart was the first to clearly perceive that only a super ficial resemblance exists between the members of these two great groups and to him is due the elevation of the Echinodermata to the impor tance of a distinct phylum. The skin is always more or less indurated by the deposition of cal cium carbonate in the form of variously shaped i spiculcs, spines, discs or plates, either isolated or united to form a reticulum or a continuous test. There is a principal axis around which most of the organs of the body are repeated five times in a radial manner. While this pentam erism is generally predominant, certain organs (madreporic plate, stone canal, 'heart") occur in but one radial axis which, therefore, lies in a median plane to which all other organs are bilaterally related. Of the repeated organs the digestive glands, radial canals, nerves, various vessels, the eye-spots and some others, are ra dial, the genital organs (single in Holothuroi dea) and polian vesicles interradial in position. The partial suppression of one ray or two rays, and among the starfishes a multiplication of rays, may take place; furthermore, in some Echinoidea and Holothuroidea certain of the organs or rays may shift so extensively with regard to the principal axis and the median plane that the original pantamerism becomes obscured. There is a well-developed cmlom or body cavity and a closely connected "blood" or pseudoharnal system which consists of an axial organ or "heart," a circumcesophageal ring, radial vessels and a more or less extensive sys tem of lacuna. In addition to the blood vascu lar system there is a very characteristic water vascular system which, when typically devel oped, consists of a median madreporic or sieve plate and stone canal, a circumcesophageal canal bearing polian vesicles and other diverticula, and giving rise to radial canals which in turn bear rows of ampulla and tube feet. The latter are the chief organs of locomotion, and the most important function of the water-vascular system is to provide for their distension by means of the water which is strained through the madreporic plate and enters the stone canal.
In some cases the tube feet assume an exclu sively respiratory function. With few excep tions the sexes are separated and, following fer tilization in the water external to the body, a free swimming larval form develops, except in a few cases, from the egg. This is always bilaterally symmetrical, has a curved alimentary canal with mouth and anus, and a more or less complicated- ciliated tract or cilia or separate circles of cilia and a pair of ccelomic pouches. Older larva often develop irregular processes and a larval skeleton. The larva of the several classes differ considerably in structural details and have been named auricularia, bipinnaria, gluteus, etc. The Echinodermata are exclusively marine. The following classes. may be recog nized: Crinoidea, usually fixed to the sea bottom by the aboral surface; the mouth uppermost and the viscera all contained in the central disc or theca. When arms are present they are solid and usually branched.
Stone or crinoids. Here belong also the ex tinct blastids and cystids which are often re garded as forming separate classes.
Stelleroidea, never fixed to the sea bottom; the mouth or oral surface always in contact with the surface over which they are moving; tube feet confined to the oral surface; form stellate_ There are two quite distinct sub-classes, the Ophiuroidea, or brittle stars, serpent stars and basket stars, and the Asteroidea or true star fish.
Echinoidea, never fixed to the sea bottom; ambulacral areas with the tube feet extending more or less over the aboral surface; form compact, more or less discoid or spheroidal. Sub-classes, Regularia, or regular sea-urchins; and Irregularia, or irregular sea-urchins. r Holothuroiclea, never permanently fixed to the sea bottom; water-vascular system with a circle of oral tentacles; more or less elongated along the principal axis to a cucumber or worm like form; skeleton in the form of separate plates or sometimes totally wanting. See CUCUMBERS.