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theca is pentamerous (has five-part symmetry) and is homologous with the body or disc of other echinoderms. The base of the theca is formed from a cup-shaped set of ossicles (bony plates), the calyx, while the upper surface is formed by the weakly calcified tegmen, a membranous disc. The tegmen is divided into five "ambulacral areas", including a deep groove from which the tube feet

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