Radiata or Radiates is a historical taxonomic rank that was used to classify animals with radially symmetric body plans, and is no longer accepted. The term Radiata has united several different groupings of animals, not forming a monophyletic group under current views of animal phylogeny: similarities such as radial symmetry are taken to be the result of both incorrect evaluations by early researches as well as convergent evolution, rather than an indication of a common ancestor. Because of this, the term is used mostly in a historical context.
Temporal range: Ediacaran – Present
|Louis Agassiz in 1870, with drawings of animals then considered Radiates|
In the early 19th century, Georges Cuvier united Ctenophora and Cnidaria in the Radiata (Zoophytes). 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. This definition is similar to the historical descriptor Coelenterata, which has also been proposed as a group encompassing 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 as larvae. 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 has a primary origin.
|Wikispecies has information related to Radiata|
- Hadzi, J. (1963). The Evolution of the Metazoa. New York, NY, USA: The Macmillan Company. pp. 56–57. ISBN 0080100791.
- 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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- Margulis, Lynn (1988). Five Kingdoms: An illustrated Guide to the Phyla of Life on Earth. New York: W. H. Freeman. ISBN 0716730278.
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