Radiata
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| Radiata Fossil range: Ediacaran - Recent |
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 A jellyfish, Chrysaora
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The Radiata are the radially symmetric animals of the Eumetazoa subregnum. The term Radiata has had various meanings in the history of classification. It has been applied to the echinoderms, although the echinoderms are members of the Bilateria, because they exhibit bilateral symmetry in their developing stages.
Thomas Cavalier-Smith in 1983 defined a subkingdom called Radiata consisting of the phyla Porifera, Myxozoa, Placozoa, Cnidaria and Ctenophora in Radiata, that is, all the animals that are not in Bilateria.
The Five Kingdom classification of Lynn Margulis and K. V. Schwartz keeps only Cnidaria and Ctenophora in Radiata. Cladistic classifications do not recognize Radiata as a clade. The radiata, in this sense, are diploblastic, meaning they have 2 primary germ layers: endoderm and ectoderm. (Cavalier-Smith's use of the term Radiata includes animals with a single germ layer such as sponges.)
Although radial symmetry is usually given as a defining characteristic of radiates, a few members of the class Anthozoa, which are now considered as the most basal and oldest group of cnidarians, are actually bilateral symmetric. Nematostella vectensis is one such example. Newer research strongly indicates that bilateral symmetry evolved before the split between Cnidaria and Bilateria, and that the radially symmetrical cnidarians have secondarily evolved radial symmetry, meaning the bilaterism in species like N. vectensis have a primary origin [1]. Also the free-swimming planula larvae of cnidarians exhibit bilateral symmetry. Ctenophores show biradial symmetry.
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[edit] References
- This article incorporates text from the Encyclopædia Britannica Eleventh Edition, a publication now in the public domain.
- Zoological Philosophy of J. B. Lamarck
- Taxon: Subkingdom Radiata
- The development of radial and biradial symmetry: The evolution of bilaterality - retrieved February 2, 2006
- Origins of Bilateral Symmetry: Hox and Dpp Expression in a Sea Anemone - retrieved February 2, 2006
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