Radiata
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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, the free-swimming planula larvae of cnidarians exhibit bilateral symmetry, as do some adult cnidarians. Ctenophores show biradial symmetry.
[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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