Cnidaria

Cnidaria

Kingdom:	Animalia
Phylum:	Cnidaria

Classeses

Anthozoa - corals and sea anemones
Cubozoa - sea wasps or box jellyfish
Hydrozoa - hydroids, hydra-like animals
Scyphozoa - jellyfish

The cnidarians are a phylum of some 10,000 species of relatively simple animals, found exclusively in aquatic environments (most species are marine). The corals, which are important reef-builders, belong here, as do the familiar sea anemones, jellyfish, sea pens, sea pansies and sea wasps. The name Coelenterata is sometimes applied to the group, but as it is taken to include the similar Ctenophores (comb jellies), it has been abandoned. Cnidarians are well-known in the fossil record and date back to at least the Cambrian.

The basic body shape of a cnidarian consists of a sac with a digestive cavity, with a single opening that functions as both mouth and anus. It has radial symmetry and is composed of two layers of tissues, called the ectoderm and endoderm, separated by a gelatinous mesoglea containing only scattered cells. This organization is often called diploblastic, though the mesoglea may be homologous with the mesoderm in other animals.

Cnidarians lack organs, but have various differentiated tissues. Their movement is coordinated by a decentralized nerve net and simple receptors. Respiration takes place by diffusion of oxygen directly through their tissues, without specialized structures like gills, tracheae or lungs, made possible by their small or flattened bodies. Tentacles surrounding the mouth contain cnidocysts, specialized stinging cells. The ability to sting is what gives cnidarians their name (Greek knide, nettle).

There are four main classes of Cnidaria:

Class Anthozoa (anemones, corals, etc.)
Class Hydrozoa (Portuguese man-of-war, Obelia, etc.)
Class Scyphozoa (jellyfish)
Class Cubozoa (box jellies)

Traditionally the hydrozoans were considered to be the most primitive, but evidence now suggests the anthozoans were actually the earliest to diverge. In these the organism is benthic or sessile, with its mouth directed upwards. This form is called a polyp. Sea anemones, sea fans and corals are in this class. Hydrozoa have life-cycles that alternate between asexual polyps and sexual, free-swimming forms called medusaee. These are absent from the Anthozoa. Among the Scyphozoa and Cubozoa, the medusae are the dominant form in the life-cycle, while the polyps are reduced or absent. Medusae may be up to 2 m in diameter.

The Siphonophora deserve special mention. These hydrozoans form colonies which show varying degrees of specialization, so that in extreme cases individuals function essentially as organs of the whole.

A small group of microscopic parasites, the Myxozoa, have been considered to be extremely reduced cnidarians. These attach themselves to their hosts by polar filaments similar to the stinging threads of cnidocysts. Their exact placement within the phylum is uncertain, however, and new studies suggest they may have developed from some other group of animals.

Finally, the extinct Conulariida may or may not be members of the Cnidaria.

External Links, Reference

A Cnidaria homepage maintained by University of California, Irvine
campbell, reece, mitchell. Biology. 1999.
Solomon, E.P., Berg, L.R., Martin, D.W. 2002. Biology Sixth Edition.

Brooks/Cole Thomson Learning, Australia, Canada, Mexico, Singapore, Spain, United Kingdom, and United States.

[MSN Encarta page]