Cnidaria

Corals, sea anemones and jellyfish are the most familiar of the cnidarians. Two lesser-known groups are tiny moss-like creatures called hydroids and the ocean-going siphonophores. Siphonophores are colonies of highly specialised individuals
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Cnidarians have two basic body forms, medusa and polyp. The medusa form is a free-swimming structure which consists of an umbrella-shaped body (called a bell), a fringe of tentacles that hang from the edge of the bell, a mouth opening located on the underside of the bell, and a gastrovascular cavity.

The polyp is a sessile form which attaches to the sea floor and often forms large colonies. The polyp structure consists of a basal disc that attaches to a substrate, a cylindrical body stalk, inside of which is the gastrovascular cavity, a mouth opening located on the top of the polyp, and numerous tentacles which radiate out from around the edge of the mouth opening.
Cnidaria is a phylum containing over 10,000 species of animals found exclusively in aquatic.

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Most cnidarians are carnivorous and feed on small crustacans
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Bamboo corals are named for the striking appearance of their jointed skeletons. When alive they are covered with polyps. Bamboo corals can grow several metres high and form dense thickets. They live in deep water.
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Some Cnidarians are the longest animals in the world, one of them that can stretch as long as 40m
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There are four main classes of cnidarians:
- Anthrozoa (corals, sea anemones)
- Scyphozoa (jellyfish, siphonophores)
- Cubozoa (box jellyfish)
- Hydrozoa (various spectrums)

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What does a Cnidaria look like? Most cnidarians have tentacles with stinging cells surrounding their mouth. The mouth is the only body opening. All cnidarians have radial symmetry. Cnidarians are said to be the simplest organisms at the tissue grade of organization. Their cells are organized into true tissues. Cnidarians are essentially bags made of two cell layers. The outer layer, or epidermis, contains the cnidocysts, the stinging cells that are characteristic of the phylum. The inner layer, or gastrodermis, lines the gut. In between epidermis and gastrodermis is the mesoglea, a layer of jelly-like substance, which contains scattered cells and collagen fibers. The mouth is often, but not always, surrounded by a ring of tentacles.]]

What does a Cnidaria look like?
Most cnidarians have tentacles with stinging cells surrounding their mouth. The mouth is the only body opening. All cnidarians have radial symmetry. Cnidarians are said to be the simplest organisms at the tissue grade of organization. Their cells are organized into true tissues. Cnidarians are essentially bags made of two cell layers. The outer layer, or epidermis, contains the cnidocysts, the stinging cells that are characteristic of the phylum. The inner layer, or gastrodermis, lines the gut. In between epidermis and gastrodermis is the mesoglea, a layer of jelly-like substance, which contains scattered cells and collagen fibers. The mouth is often, but not always, surrounded by a ring of tentacles.

What does a Cnidarian eat?
Many eat small creatures or trap detritus, plankton and other microscopic titbits. But many also capture and eat large prey. Many cnidarians, however, more supplement their meals with food provided by photosynthetic zooxanthellae (symbiotic single-celled algae) that they harbour in their tentacles and bodies. These algae produce food from sunlight.

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