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A phylum that comprises the sea anemones, jellyfish, and corals, and which is known from the late Precambrian. Cnidarians are basically radially symmetrical and have tentacles. The body contains a gastrovascular cavity and the body wall is diploblastic. Cnidarians occur as two distinct types: polyps and medusae, which in many taxa constitute alternating generations. All are aquatic and most are marine. There are about 9000 extant species.

Subjects: Biological Sciences.

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