Okanishi, Masanori Department of Science, College of Science, Ibaraki University, Ibaraki, Japan.
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One of two extant orders of brittle stars, comprising the basket stars and snake stars, in the class Ophiuroidea (phylum Echinodermata). The order Euryalida currently includes approximately 190 valid species worldwide, which represents a little less than 10% of the total number of species in the class Ophiuroidea (phylum Echinodermata); the remaining and vast majority of brittle stars are assigned to the order Ophiurida. Sixty-one species in 23 genera, collectively termed the basket stars, have branching arms (Fig. 1). In contrast, 125 species in 26 genera have unbranched, simple arms (Fig. 2), and their arms coil in the vertical plane to cling to host substrates; these species are termed the snake or serpent stars. Many species, at least members of the families Asteronychidae and Euryalidae, have thick skins on their bodies, but members of the families Asteroschematidae and Gorgonocephalidae (for example, Asteroschema and Astrochele) may carry external ossicles (calcareous elements), for example, granules, scales, and/or tubercles. Members of the family Astrocharidae are completely plated by polygonal scales. There is usually one madreporite (a delicately perforated sieve plate), but some species have multiple madreporites (for example, Astroglymma sculptum has five). Rings of tiny hooks are present on the aboral surfaces of arms (sometimes also on the disk) in gorgonocephalids (Fig. 2). All euryalids possess lateral arm plates, but ventral arm plates are sometimes absent and dorsal arm plates are basically absent. See also: Echinodermata; Ophiurida; Ophiuroidea
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