Crinoidea
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Crinoidea
Crinoidea
a class of benthic invertebrates of the phylum Echinodermata. They have a cup-shaped body, or calyx, in the middle of which is a mouth; the body is crowned by feathery, generally branching, upwardly directed rays. There are five rays at the base of the calyx and ten or more (up to 200) further up. The loose crown forms a net for catching small planktonic organisms and detritus, upon which the crinoids feed.
Stalked crinoids, or sea lilies, have a segmented stalk, which is up to 1 m long. Fixed to the base of the calyx, the stalk attaches itself to the sea bottom. Stalkless crinoids, or comatulids, have numerous movable processes (cirri) and can crawl and swim. Sea lilies, of which there are approximately 80 species, are found at depths to 9,700 m. Stalkless forms, of which there are about 550 species, are most diverse in the shallow waters of tropical seas and are often brightly colored. Adult invertebrates develop from free-swimming larvae and a sessile stalked stage.
More than 5,000 species of fossil crinoids have been traced to the Lower Ordovician. They reached the height of their development in the Middle Paleozoic (several subclasses). By the end of the Paleozoic, most species had become extinct. The subclass Articulata, to which all extant crinoids belong, has been traced back to the Triassic.
REFERENCES
Zhizn’ zhivotnykh, vol. 2. Moscow, 1968.Hyman, L. H. The Invertebrates, vol. 4. New York-London, 1955.