Cnidaria
Anthozoa - Corals and sea anemones
Cubozoa - Sea wasps or box jellyfish
Hydrozoa - Hydroids, hydra-like animals
Scyphozoa - Jellyfish
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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.
Classes
There are four main classes of Cnidaria:
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 medusae. 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 two meters 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
