The Radiata are the radially symmetric animals of the Eumetazoa
Eumetazoa is a clade comprising all major animal groups except sponges, placozoa and several other little known animals. Characteristics of eumetazoans include true tissues organized into germ layers, and an embryo that goes through a gastrula stage...

 subkingdom. The term Radiata has had various meanings in the history of classification. It has been applied to the echinoderm
Echinoderms are a phylum of marine animals. Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone....

s, although the echinoderms are members of the Bilateria
The bilateria are all animals having a bilateral symmetry, i.e. they have a front and a back end, as well as an upside and downside. Radially symmetrical animals like jellyfish have a topside and downside, but no front and back...

, because they exhibit bilateral symmetry in their developing stages.

Thomas Cavalier-Smith
Thomas Cavalier-Smith
Professor Thomas Cavalier-Smith , FRS, FRSC, NERC Professorial Fellow, is a Professor of Evolutionary Biology in the Department of Zoology, at the University of Oxford...

 in 1983 defined a subkingdom called Radiata consisting of the phyla
In biology, a phylum The term was coined by Georges Cuvier from Greek φῦλον phylon, "race, stock," related to φυλή phyle, "tribe, clan." is a taxonomic rank below kingdom and above class. "Phylum" is equivalent to the botanical term division....

Sea sponge
Sponges are animals of the phylum Porifera . Their bodies consist of jelly-like mesohyl sandwiched between two thin layers of cells. While all animals have unspecialized cells that can transform into specialized cells, sponges are unique in having some specialized cells, but can also have...

, Myxozoa
The Myxozoa are a group of parasitic animals of aquatic environments. Over 1300 species have been described and many have a two-host lifecycle, involving a fish and an annelid worm or bryozoan. The average size of a Myxosporea spore usually ranges from 10 μm to 20 μm and Malacosporea up...

, Placozoa
The Placozoa are a basal form of invertebrate. They are the simplest in structure of all non-parasitic multicellular animals . They are generally classified as a single species, Trichoplax adhaerens, although there is enough genetic diversity that it is likely that there are multiple,...

, Cnidaria
Cnidaria is a phylum containing over 9,000 species of animals found exclusively in aquatic and mostly marine environments. Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey. Their bodies consist of mesoglea, a non-living jelly-like substance,...

 and Ctenophora in Radiata, that is, all the animals that are not in Bilateria
The bilateria are all animals having a bilateral symmetry, i.e. they have a front and a back end, as well as an upside and downside. Radially symmetrical animals like jellyfish have a topside and downside, but no front and back...


The Five Kingdom classification of Lynn Margulis
Lynn Margulis
Lynn Margulis was an American biologist and University Professor in the Department of Geosciences at the University of Massachusetts Amherst. She is best known for her theory on the origin of eukaryotic organelles, and her contributions to the endosymbiotic theory, which is now generally accepted...

 and K. V. Schwartz keeps only Cnidaria and Ctenophora in Radiata. Cladistic
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...

 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
Anthozoa is a class within the phylum Cnidaria that contains the sea anemones and corals. Unlike other cnidarians, anthozoans do not have a medusa stage in their development. Instead, they release sperm and eggs that form a planula, which attaches to some substrate on which the cnidarian grows...

, which are now considered as the most basal and oldest group of cnidarians, are actually bilateral symmetric. Nematostella vectensis
Starlet sea anemone
The starlet sea anemone is a species of sea anemone native to the east coast of the United States, with introduced populations along the coast of southeast England and west coast of the United States....

 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 http://pharyngula.org/index/weblog/comments/bilateral_symmetry_in_a_sea_anemone/. Also the free-swimming planula
A planula is the free-swimming, flattened, ciliated, bilaterally symmetric larval form of various cnidarian species. The planula forms from the fertilized egg of a medusa, as the case in scyphozoans and some hydrozoans, or from a polyp, as in the case of anthozoans...

larvae of cnidarians exhibit bilateral symmetry. Ctenophores show biradial symmetry.
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