Cnidaria

Anthozoa - Corals and sea anemones

Related Topics:
Anthozoa - Coral - Sea anemone

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Cubozoa - Sea wasps or box jellyfish

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Hydrozoa - Hydroids, hydra-like animals

Related Topics:
Hydrozoa - Hydroid - Hydra

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Scyphozoa - Jellyfish

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Cnidaria (from New Latin cnida nematocyst, fr. Gk knide "nettle") is a phylum containing 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 names Coelenterata and Coelentera were formerly applied to the group, but as those names included the Ctenophores (comb jellies), they have been abandoned. Cnidarians are highly evident in the fossil record, having first appeared in the Precambrian era.

Related Topics:
New Latin - Nematocyst - Gk - Phylum - Species - Animals - Cnidocyte - Organelle - Corals - Reef - Sea anemones - Jellyfish - Sea pen - Sea wasp - Ctenophores - Fossil - Precambrian

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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 whichever way it is cut along its central axis, the resulting halves would always be mirror images of each other. The cnidarian is composed of two layers of tissue, known as the ectoderm and endoderm (or gastroderm), which are held together 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.

Related Topics:
Digestive - Mouth - Anus - Radial symmetry - Diploblastic - Mesoderm

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Although Cnidarians are classified to be organized on the tissue level, they actually contain gonads as their only organs. 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. This is made possible by their small or flattened bodies. Tentacles surrounding the mouth contain nematocysts, specialized stinging cells. The ability to sting is what gives cnidarians their name (Greek cnidos, "nettle").

Related Topics:
Gonads - Organs - Nerve - Respiration - Oxygen - Gills - Tracheae - Lungs - Sting

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Cnidarians employ these stinging cnidocysts to immobilize, kill, or entangle their prey. The nematocysts are the cnidarians' main form of offence or defense and function by a chemical or physical trigger that causes the specialized cell to eject a barbed and poisoned hook that can stick into, ensnare, or entangle prey. Dead or paralyzed prey are pushed into the cnidarian's oral opening by the tentacles. Digestion occurs in the gastrovascular cavity, and all undigested food, waste material, or other secretions must exit the cnidarian through the oral opening.

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