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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). The corals, which are important reef-builders, belong here, as do
the familiar sea anemones and jellyfish. 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 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:
* Class Anthozoa (anemones, corals, etc)
* Class Hydrozoa
* 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
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 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.