ECOLOGY. About 3000 living echinoderms are known. They are all marine, and the species are scattered through all seas of the globe and are found at all depths, but they are most abundant in the warm seas of the tropics. The only fixed members of the branch are those included in the sub-hraneh Pelmatozoa, comprising the erinoids, cystoids, and hlastoids. Many echinoderm,s, espe cially the crinoids, are gregarious, and they are found associated together in vast numbers. Echinoderms arc largely bcnthonic organisms, crawling slowly on the bottom of the sea by means of the tube feet or arms, as in the star fish and sea-nrehins, or by the oral tentacles, as in holothurians. The fixed pelmatozoans are either benthonie, when fixed to the bottom, or pseudo-planktonic, when attached to floating wood. and the free-swimming crinoids almost fall among the pelagic animal; or plankton.
xrION. The Exhith)dermata are di Vided the following seven classes: llolo thuroidea, Echinoidea, Asteroidea. ophinroidea, rinoidea, Cystoidea, and Illastoidea. The last three groups, comprising mostly attached forms, are often as subclasses under a class We prefer to associate the first four in a subbranch Eleutherozoa, and the last three in a second sub-branch. Pelmatozoa. The character, of the various classes are as follows: I. tholothurians, or sea-eu cumbers I. The body is cylindrical or worm-like, and has a soft. leathery skin, in which are im bedded minute calcareous spicules or plates. A certain degree of bilateral symmetry is apparent, but there is little trace of radial pentameral symmetry. The mouth is at the anterior end of the body and is surrounded by tentacles: the anus is at the posterior end. The only portions of holothurians that are apt to be preserved as fossils are the calcareous bodies of the skin. These are known in rocks of Carboniferous to recent time, but they are never common. See
llotortmaAN.
11. I:cm:Noun:A (sea-urchins). In this group the body is inclosed in a more or less rigid box of polygonal stony plates, arranged in rows that form five amlnilacral and five interambulaeral areas. In the more regular, radial forms both the mouth and the anus are central on opposite surfaces of the body. lit other irregular forms, exhibiting less radial and greater bilateral sym metry. the mouth may be placed toward the an terior end and the anus is variously situated, either on the dorsal. lateral, or ventral surface, but always in the middle line. The majority of. echinoids have complicated jaws. About 300 living and '2500 fossil species are known. The earliest appear in Ordovician rocks. See Ecm OIDEA; Ill. ASTEROMEA (starfish). The body of the animals of this class are flattened dorso-ven traily, and the ambnlaera are produced into five (or sonic multiple of live) radiating arms. The arms are not at all demarcated front the cethral portion of the body, and they contain prolonga tions of the Nelome and of most of the organs. Radial symmetry is quite marked. The tough skin contains calcareous plates of irregular form that are not united to each other, so that the body-wall is to sonic extent flexible. The andm laeral plates occupy the larger portion of the under sides of the arms, and the tube feet are well developed. The large mouth-opening is in the centre of the ventral surface of the disk. and the small anal opening is near the centre of the dorsal surface, except in a few genera, where it is wanting. and the inte-tine is then closed. The earliest starfish appear in rocks of Ordovician age. and they have continued, though never be coming abundant. through all subsequent periods down to the present time. See STARFISH.