Cnidaria

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{{Taxobox | color = pink | name = Cnidaria | image = Sea nettles.jpg | image_width = 250px | image_caption = Sea nettles, Chrysaora quinquecirrha | regnum = Animalia | phylum = Cnidaria | phylum_authority = Hatschek, 1888 | subdivision_ranks = Classes | subdivision = Anthozoa - Corals and sea anemones
Cubozoa - Sea wasps or box jellyfish
Hydrozoa - Hydroids, hydra-like animals
Scyphozoa - Jellyfish }}

Cnidaria (silent c - pronounced /Template:IPA/ from New Latin cnida, fr. Gk κνιδη "nettle") is a phylum containing some 10,000 species of relatively simple animals found exclusively in aquatic, mostly marine, environments. Cnidarians get their name from cnidocytes, which are specialized cells that carry stinging organelles. The corals, which are important reef-builders, belong here, as do the familiar sea anemones, jellyfish, sea pens, sea pansies and sea wasps. The names Coelenterata and Coelentera were formerly applied to the group, but as those names included the Ctenophores (comb jellies), they have been abandoned. Cnidarians are highly evident in the fossil records, having first appeared in the Precambrian era.

The basic body shape of a cnidarian consists of a sac with a gastrovascular cavity, with a single opening that functions as both mouth and anus. It has radial symmetry, meaning that whichever way it is cut along its central axis, the resulting halves would always be mirror images of each other. The cnidarian is composed of two layers of tissue, known as the ectoderm and endoderm (or gastroderm), with a gelatinous mesoglea in between them containing only scattered cells. Thus the organisms are considered to be diploblastic, though the mesoglea may be homologous with the mesoderm in other animals.

Although Cnidarians are classified to be organized on the tissue level, they actually contain gonads as their only organs. Their movement is coordinated by a decentralized nerve net and simple receptors. Several free-swimming Cubozoa and Scyphozoa possess rhopalia, complex sensory structures that can include image-forming eyes with lenses and retinas [1], and a gravity-sensing statolith comparable in function to the otolith of the vertebrate inner ear. Respiration takes place by diffusion of oxygen directly through their tissues without specialized structures like gills, tracheae or lungs. This is made possible by their small or flattened bodies. Tentacles surrounding the mouth contain nematocysts, specialized stinging cells. The ability to sting is what gives cnidarians their name.

Cnidarians employ these stinging cnidocysts to immobilize, kill, or entangle their prey. The nematocysts are the cnidarians' main form of offence or defense and function by a chemical or physical trigger that causes the specialized cell to eject a barbed and poisoned hook that can stick into, ensnare, or entangle prey. Dead or paralyzed prey are pushed into the cnidarian's oral opening by the tentacles. Digestion occurs in the gastrovascular cavity, and all undigested food, waste material, or other secretions must exit the cnidarian through the oral opening.

There are four main classes of Cnidaria:

Class Anthozoa (anemones, corals, etc.)
Class Hydrozoa (Portuguese Man o' 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. Sea anemones, sea fans and corals are in this class. The non-anthozoan classes may be grouped into the subphylum Medusozoa.

Theoretically, members of Cnidaria have life-cycles that alternate between asexual polyps and sexual, free-swimming forms called medusae. In reality however there is a vast variation within the life-cycles of cnidarians. Within group Anthozoa, the medusal stage is virtually non-existent; the larva, once fusing with the substratum and developing into the polyp stage, grows benthic or sessile, meaning it no longer metamorphosises into the medusal stage. Among the Scyphozoa and Cubozoa, the medusae are the dominant form in the life-cycle, while the polyps are in turn reduced or absent. Medusae are extremely varied and range in size from a few millimeters to over 30 metres (with tentacles). The Hydrozoa are intermediate, with significant medusoid and polyp forms.

The Siphonophora deserve special mention. These hydrozoans form colonies that 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. Usually they are placed in their own phylum.

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

Obsolete names for groups of cnidarians include Acalephae (Hydrozoa plus Scyphozoa, based on the shared character of stinging cells; however this character is no longer thought to be primitive)

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