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(brittle stars), a class of benthic marine animals of the phylum Echinodermata. The body consists of a flat disk, which usually measures about 2 cm across (sometimes up to 10 cm). Five or, less frequently, ten flexible arms extend from the disk. The length of each arm is several times (sometimes 20 to 30 times) greater than that of the disk. In contrast to the arms of starfishes, the arms of brittle stars are sharply marked off from the disk and are jointed, consisting of numerous vertebrae. Most brittle stars have simple, unbranched arms. The disk and the arms are covered with thin calcareous plates.
In most species the sexes are separate. Development is usually by metamorphosis; the free-swimming larva is called an ophiopluteus. Some species are viviparous; others are capable of reproducing by division.
Brittle stars crawl by flexing their arms, or they bury themselves in the bottom. They feed on small animals or detritus. Many tropical species that inhabit shallows are brightly colored. Some species are capable of luminescence. The regeneration of arms is well developed.
Brittle stars are found throughout the world on ocean and sea bottoms (to depths of 8 km). They often form large colonies and serve as food for fishes. Some species live commensally with algae, sponges, corals, and sea urchins. Of the approximately 2,000 species, about 120 are encountered in the seas of the USSR. Fossils of extinct species have been traced to the Ordovician.
G. M. BELIAEV