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 cnidarian is consisted of a sac with gastrovascular cvaity. This gastrovascular cavity has a function as mouth and anus, and it is a central digestive compartment. The ability to sting is in fact what gives the group its name (Greek cnidos, nettle) and is caused by unique cells called cnidocysts which are capable of ejecting barbed hooks tipped with poison. Cnidarians are carnivores their digestion begins at gastrovascular. The cnidarian body is radially symmetric and diploblastic - that is, composed of two layers of tissues, which are not differentiated into organs. These are called the ectoderm and endoderm, and are separated by a gelatinous layer called the mesoglea which contains only a few scattered cells. Cnidarians' respiration is done by diffusion(Berg, Solomon, Martin. 2002). Their typical small body size allows oxygen to diffuse from water through their thick membrane, so no specialized respiratory structures like gills, trachea, or lungs are needed (Berg, Solomon, Martin. 2002).

The body shape has two variations: sessile poly and floating medusa. Adult jellyfish is a good example of medusa. They float in water and have umbrella-like shape. Poly are cynlindrical forms that adhere to the substratum by the aboral end of the body.

Their movement is coordinated by a nerve net. They do not have centralized nerve net nor brain. Instead Cnidarians have simple receptors.

There are four main classes of Cnidaria:

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

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. Hydrozoa have life-cycles that alternate between asexual polyps and sexual medusae, free-swimming forms, mouth down version of polyp.

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.

The extinct Conulariida may or may not be members of the Cnidaria.

Class scyphozoa are all marine. The medusal generally prevails in the life cycle. For that reason, most of them are free-swimming. Jellies, sea wasp, and sea nattles are part of this class, and the poly stage reduced. Medusas are big as up to 2m in dimater.

Class Anthozoa occur as polyps only. Sea anemones, sea fans and corals are in this class, and medusa stage is completely absent. Thus, they are sessile.

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