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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. There is a single opening to the digestive chamber that acts as both a mouth and an anus, usually surrounded by stinging tentacles. 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.
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. Hydrozoa have life-cycles that alternate between asexual polyps and sexual medusae, free-swimming forms where the mouth is on the bottom. The Cubozoa and Scyphozoa spend their whole lives as medusae.
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. The Portuguese man-o'-war is probably the best known.
A small group of single-celled parasites, the Myxozoa[?], are now generally believed to be extremely reduced cnidarians. These are characterized by a stinging thread akin to that of the cnidocysts, which is used to attach themselves to their host, either a fish or some other cold-blooded vertebrate. Their exact placement within the phylum is unknown. Also, the extinct Conulariida[?] may or may not be members of this phylum.