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Radiata

(Subkingdom)

Overview

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The Radiata are the radially symmetric animals of the Eumetazoa subkingdom. 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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Taxonomy

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The Subkingdom Radiata is a member of the Kingdom Animalia. Here is the complete "parentage" of Radiata:

The Subkingdom Radiata is further organized into finer groupings including:

Phyla

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Cnidaria

Cnidaria ( with a silent c) is a phylum containing over 10,000 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, sandwiched between two layers of epithelium that are mostly one cell thick. They have two basic body forms: swimming medusae and sessile polyps, both of which are radially symmetrical with mouths surrounded by tentacles that bear cnidocytes. Both forms have a single orifice and body cavity that are used for digestion and respiration. Many cnidarian species produce colonies that are single organisms composed of medusa-like or polyp-like zooids, or both. Cnidarians' activities are coordinated by a decentralized nerve net and simple receptors. Several free-swimming Cubozoa and Scyphozoa possess balance-sensing statocysts, and some have simple eyes. Not all cnidarians reproduce sexually. Many have complex lifecycles with asexual polyp stages and sexual medusae, but some omit either the polyp or the medusa stage. [more]

Ctenophora

The Ctenophora (; singular ctenophore, /'t?n?f??r/ or /'ti?n?f??r/; from the Greek ?te?? kteis 'comb' and f??? phero 'carry'; commonly known as comb jellies) are a phylum of animals that live in marine waters worldwide. Their most distinctive feature is the "combs", groups of cilia they use for swimming, and they are the largest animals that swim by means of cilia ? adults of various species range from a few millimeters to 1.5 meters (59 in) in size. Like cnidarians, their bodies consist of a mass of jelly, with one layer of cells on the outside and another lining the internal cavity. In ctenophores, these layers are two cells deep, while those in cnidarians are only one cell deep. Ctenophores also resemble cnidarians in having a decentralized nerve net rather than a brain. Some authors combined ctenophores and cnidarians in one phylum, Coelenterata, as both groups rely on water flow through the body cavity for both digestion and respiration. Increasing awareness of the differences persuaded more recent authors to classify them in separate phyla. [more]

Myxozoa

The Myxozoa (etymology: Greek: ??? myx- "slime" or "mucus" + ??a zoa "animals") 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 myxosporean spore usually ranges from 10 ?m to 20 ?m whereas that of a spore can be up to 2 mm. Infection occurs through valved spores. These contain one or two sporoblast cells and one or more polar capsules that contain filaments which anchor the spore to its host. The sporoblasts are then released as a motile form, called an amoebula, which penetrates the host tissues and develops into one or more multinucleate plasmodia. Certain nuclei later pair up, one engulfing another, to form new spores. [more]

Placozoa

The Placozoa are a basal form of invertebrate. They are the simplest in structure of all non-parasitic multicellular animals (Metazoa). They are generally classified as a single species, Trichoplax adhaerens, although there is enough genetic diversity that it is likely that there are multiple, morphologically similar species. Although they were first discovered in 1883, a common name does not yet exist for the taxon; the scientific name literally means "flat animals". [more]

Porifera

Sponges are animals of the phylum Porifera (; meaning "pore bearer").They are multicellular organisms which have bodies full of pores and channels allowing water to circulate through them, 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 specialized cells that can transform into other types, often migrating between the main cell layers and the mesohyl in the process. Sponges do not have nervous, digestive or circulatory systems. Instead, most rely on maintaining a constant water flow through their bodies to obtain food, oxygen and remove wastes. The shapes of their bodies are adapted for maximal efficiency of water flow. Water enters through the central cavity, deposits nutrients, and leaves through a hole called the osculum. All sponges are sessile aquatic animals. Although there are freshwater species, the great majority are marine (salt water) species, ranging from tidal zones to depths exceeding 8,800 metres (5.5 mi). [more]

Vendobionta

[more]

At least 5 species and subspecies belong to the Phylum Vendobionta.

More info about the Phylum Vendobionta may be found here.

References

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Sources

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Last Revised: August 24, 2012
2012/08/24 12:57:42