Radiata is a taxonomic rank that has been used to classify radially symmetric animals. The term Radiata has united several different groupings of animals, none of which form a monophyletic group under current views of animal phylogeny. Because of this and problems of homoplasy associated with using body symmetry as a phylogenetic character, the term is used mostly in a historical context.
In the early 19th century, Georges Cuvier united ctenophores and cnidarians in the Radiata. Thomas Cavalier-Smith, in 1983, redefined Radiata as a subkingdom consisting of Myxozoa, Placozoa, Cnidaria and Ctenophora. Lynn Margulis and K. V. Schwartz later redefined Radiata in their Five Kingdom classification, this time including only Cnidaria and Ctenophora.
Although radial symmetry
is usually given as a defining characteristic in animals that have been classified in this group, there are clear exceptions and qualifications. Echinoderms, for example, exhibit unmistaken bilateral symmetry
. Ctenophores exhibit biradial or rotational symmetry, defined by tentacular and pharyngeal axes, on which two anal canals are located in two diametrically opposed quadrants.
Some species within the cnidarian class Anthozoa
are bilaterally symmetric (For example, Nematostella vectensis
). It has been suggested that bilateral symmetry may have evolved before the split between Cnidaria and Bilateria, and that the radially symmetrical cnidarians have secondarily evolved radial symmetry, meaning the bilaterality in cnidarian species like N. vectensis
have a primary origin.
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- ^ Cuvier, Georges (1817). Le Règne Animal Distribué Selon son Organisation, pour Servir de Base à l’Histoire Naturelle des Animaux et d’Introduction à l’Anatomie Comparée. Paris: Déterville.
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