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Cnidaria (silent c - pronounced // from New Latin cnida, fr. Gk κνιδη "nettle") is a phylum containing some 11000 species of relatively simple animals found exclusively in aquatic, mostly marine, environments. 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 records, having first appeared in the Precambrian era.

The basic body shape of a cnidarian consists of a sac with a gastrovascular 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. Their movement is coordinated by a decentralized nerve net and simple receptors. Several free-swimming Cubozoa and Scyphozoa possess rhopalia, complex sensory structures that can include image-forming eyes with lenses and retinas *, and a gravity-sensing statolith comparable in function to the otolith of the vertebrate inner ear. Tentacles surrounding the mouth contain nematocysts, specialized stinging cells, which they use to catch prey and defend themselves from predators. The ability to sting is what gives cnidarians their name.

There are four main classes of Cnidaria:

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Coral Reefs :: Marine Life
Corals :: Invertebrates

 
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Fri, 05 Sep 2008 00:00:00 -0400
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McMurdo Sound, Antarctica - Spectacular photographs of Antarctica's sea anemones, jellyfish, hydroids and soft coral.

Cnidaria - Information and resources for researchers studying all aspects of cnidarian biology.

Cnidaria (Coelenterata) - An attractive and informative site from the Tree of Life Project, by Daphne G. Fautin and Sandra L. Romano.

Coral Health and Monitoring Program - Services to help improve and sustain coral reef health throughout the world. Part of an international network of coral reef researchers.
Meta Description: [ NOAA's Coral Health and Monitoring Program ListServer. ]

Introduction to the Cnidaria - Sea anemones, medusae, jellyfish, corals.

Phylum Cnidaria - General information about the phylum that includes jellyfish, coral, hydra.

Reef Corals of the Indo-Malayan Seas - Identification key to the corals of the waters of Indonesia and adjacent territories, including members of classes Hydrozoa (hydrocorals), and Anthozoa (black corals, stony corals, soft corals and sea pens). Includes about 3500 colour pictures and interactive distribution maps.
Meta Description: [ Identification key to 180 (sub)genera of coral reefs of the waters of Indonesia and adjacent territories. ]

404 ReefBase - World Conservation Monitoring Center - A global database on coral reef systems and their resources.

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