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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). 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 name Coelenterata is sometimes applied to the group, but as it is taken to include the similar CtenophoresTentaculata Nuda Ctenophores are jellyfish-like animals commonly called comb jellies , sea gooseberries , sea walnuts , or Venus' girdles . Comb jellies are voracious marine predators on plankton. The word ctenophore (pronounced teen-oh-for) comes from Gr (comb jellies), it has been abandoned. Cnidarians are well-known in the fossil record and date back to at least the CambrianThe Cambrian is a major division of the geologic timescale that begins about 542 million years before the present (BP) at the end of the Proterozoic eon and ended about 490 million years BP with the beginning of the Ordovician period. It is the first peri.

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 symmetryIn biology, the term radial symmetry is used for cylindrical symmetry of multicellular organisms, a characteristic that is used to help classify them. It refers to the fact that it's possible to make any cut through the center of the organism, with the pl, meaning that no matter how you were to cut it the resulting halves would always be mirror images of each other, and is composed of two layers of tissues, called the ectoderm and endoderm, separated 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.

Cnidarians lack organs, but have various differentiated tissues. 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 gillsGill In aquatic organisms In aquatic organisms, gills are a respiratory organ for the extraction of oxygen from water and for the excretion of carbon dioxide. Many small aquatic animals absorb oxygen through the surface of their bodies in general, but mor, tracheaeThe trachea ( IPA tr'eik-i-a), or windpipe is a tube extending from the larynx to the bronchi in mammals, and from the pharynx to the syrinx in birds, carrying air to the lungs. It is lined with ciliated cells which push particles out and reinforced with or lungsThe lung is an organ belonging to the respiratory system and interfacing to the circulatory system of air-breathing vertebrates. Its function is to exchange oxygen from air with carbon dioxide from blood. The process in which this happens is called " exte, made possible by their small or flattened bodies. Tentacles surrounding the mouth contain cnidocysts, specialized stinging cells. The ability to sting is what gives cnidarians their name (Greek knide, nettle).

Cnidarians use special means in capturing their prey. They use nematocysts which are stinging cells that are used to render their prey unable to defend themselves. The nematocysts are the Cnidarians main form of defense. Cnidarians administer the stinging cells when they are able to sense chemically or physically the presence of another entity. Dead or paralyzed prey are pushed into the cnidarian's mouth by the tentacles. Digestion occurs in the gastrovascular cavity, and any undigested food exits the body via the mouth.