Experiments based on contenta
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The phylum Coelenterata is composed of three classes: Hydrozoa (e.g., Hydra, hydroids), Scyphozoa (the true jellyfish), and Anthozoa (e.g., sea anemones, sea fans, and corals). Although some, such as the corals and sea whips, are truly sessile, most coelenterates are capable of some form of movement, ranging from creeping on a pedal disc and burrowing to freely swimming. Coelenterates include both marine and freshwater species. They are found from the deepest reaches of the ocean to the intertidal zone, and in some habitats they are one of the dominant animals. Representatives of the phylum have been identified in Precambrian fossils and there are an estimated 11,000 extant species (Russell-Hunter, 1969). The phylum Coelenterata must therefore be counted among the oldest and more successful of the animal groups.