Echinozoa

(invertebrate zoology) A subphylum of free-living echinoderms having the body essentially globoid with meridional symmetry and lacking appendages.

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, originating in the Middle Ordovician; (3) Ophiocistioidea of the Lower Ordovician through Middle Devonian; and (4) Holothuroidea, which apparently first appeared in the Devonian. Two other extinct echinoderm classes can be placed here for convenience: (5) Lower Cambrian Camptostromatoidea and (6) Lower Cambrian Helicoplacoidea. The latter class may be the sister group of both echinozoans and crinozoans.

The oldest definite echinozoans are stromatocystitids of the Lower and Middle Cambrian. This group, which may have camptostromatids as its sister group, may have been ancestral to other edrioasteroids and, perhaps, other echinozoans. The asterozoans (asteroids and ophiuroids) may have been derived from the echinozoans, making the subphylum paraphyletic. See also Camptostromatoidea; Echinodermata; Echinoidea; Helicoplacoidea; Holothuroidea.