1979 Edition


  • D. Nichols
Reference work entry

The main structural characteristics separating the phylum Echinodermata (Gr: Echinos , spiny; derma, skin) from all other animal phyla are: (1) the possession of hydraulic tube feet, connected by a tubular water-vascular system; (2) the possession of a basic five-rayed symmetry; and (3) the reticular nature of thecalcite of which the skeletal elements are composed. Of these, only the first is diagnostic: all known modern echinoderms have tube feet, and it seems highly probable that all the extinct ones had them too. The nearest thing to tube feet in any other phylum is probably the lophophoral filaments of brachiopods, which also have a coelomic lumen; but their structural and functional range is far less extensive. As for the other criteria, pentamerous symmetry may well have arisen as a response to developmental requirements coupled with demands for optimal all-round command in a radial animal (Stephenson, 1974). Though there are exceptions among both extinct and modern echinoderms,...

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© Dowden, Hutchinson & Ross, Inc. 1979

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  • D. Nichols

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