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Ophiocistioidea (ophiocistioids; phylum Echinodermata, subphylum Echinozoa) Class of extinct, free-living echinozoans, which possess a low, dome-shaped test lacking arms. The peristome is centrally placed on the ventral surface and is enclosed in a complex jaw system. The ambulacra are confined to the ventral surface and are composed of three rows of plates. The interambulacra are narrow and consist of one plate column only. The class is known from the Ordovician to Devonian.