Cnidaria
| Cnidaria | ||||
|---|---|---|---|---|
| Scientific classification | ||||
| ||||
| 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). 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 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, meaning that no matter how you were to cut it the resulting halves would always be mirror images of each other, and is composed of two layers of tissues, called the ectoderm and endoderm, separated by a gelatinous mesoglea containing only scattered cells. Thus the organisms are considered to be 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).
Cnidarians use special means in capturing their prey. They use nematocysts which are stinging cells that are used to render their prey unable to defend themselves. The nematocysts are the Cnidarians main form of defense. Cnidarians administer the stinging cells when they are able to sense chemically or physically the presence of another entity. Dead or paralyzed prey are pushed into the cnidarian's mouth by the tentacles. Digestion occurs in the gastrovascular cavity, and any undigested food exits the body via the mouth.
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)
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.
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