Pawson, David L Department of Invertebrate Zoology, Smithsonian Institution, Washington, DC.
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An order of extant echinoderms known as the brittle stars. Living brittle stars comprise two orders: Ophiurida or brittle stars (see illustration), and Euryalida or basket stars. In Ophiurida, the arms are unbranched and they lack rings of hooks. The central disk may be covered by a soft, thick skin (family Ophiomyxidae), or more usually by scales, spines, tubercles, or granules (approximately 13 other families). One madreporite (a delicately perforated sieve plate) is present. In contrast, members of the Euryalida have very flexible arms, which may bear rings of microscopic hooks; the arms can coil, often branching several times to form a feeding fan or basket, and multiple madreporites may be present. Of the approximately 2000 species of ophiuroids, more than 1800 are referred to the order Ophiurida. In Ophiurida, the five (rarely six) arms possess vertebrae that articulate by means of ball-and-socket joints; the arms move from side to side, and they do not coil in the vertical plane, as they do in Euryalida. The superficial arm skeleton consists of four rows of plates: one dorsal, two lateral, and one ventral. See also: Echinodermata; Ophiuroidea
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