Dawson, Michael N. Section of Natural Sciences, University of California, Merced, California.
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An order of large true jellyfishes in the class Scyphozoa. The order Semaeostomeae includes the four earliest-evolving lineages within the Discomedusae (groups together Semaeostomeae plus Rhizostomeae): the families Pelagiidae, Cyaneidae, Drymonematidae, and Ulmaridae. The semaeostomes illustrate the evolution of scyphozoan morphology from an ancestral body plan of bell-plus-tentacles, which characterizes the scyphozoan order Coronatae and the closely related class Cubozoa, to a body plan in which a large oral structure is appended to the bell and replaces the tentacles as the principal location of prey capture, as characterizes the rhizostomes (see illustration). See also: Cnidaria; Rhizostomeae; Scyphozoa
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