Fell, Howard B. Formerly, Museum of Comparative Zoology, Harvard University, Cambridge, Massachusetts.
Sepkoski, J. John, Jr. Department of Geophysical Sciences, University of Chicago, Chicago, Illinois.
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A subphylum of free-living echinoderms in which the body is essentially globoid with meriodional symmetry. They lack arms, brachioles, or other appendages, and do not at any time exhibit pinnate structure. The Echinozoa range from the Early Cambrian to the present day. There are four classes that can definitely be placed here: (1) Edrioasteroidea, Lower Cambrian to lower Carboniferous echinozoans in which the mouth and anus were both directed upward and ambulacra (three to five in number) served as food-collecting areas; (2) Echinoidea, the existing and fossil sea urchins, originating in the Middle Ordovician; (3) Ophiocistioidea of the Lower Ordovician through Middle Devonian, with a domed aboral surface cover with large polygonal plates and a flat adoral side with a mouth and five radiating ambulacra; and (4) Holothuroidea, the existing and fossil sea cucumbers, which apparently first appeared in the Devonian. Two other extinct echinoderm classes can be placed here for convenience: (5) Lower Cambrian Camptostromatoidea, conical or domal animals with plates of varying size that overlapped on the lower theca; and (6) Lower Cambrian Helicoplacoidea, cylindrical animals with a spirally plated test and three ambulacra on the surface. The latter class may be the sister group of both echinozoans and crinozoans. See also: Camptostromatoidea; Echinoidea; Helicoplacoidea; Holothuroidea
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