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Any of various invertebrate animals of the phylum Cnidaria (or Coelenterata), having a body with radial symmetry and tentacles that bear microscopic stinging capsules called nematocysts. The tentacles surround a mouth that opens into a saclike internal cavity and that is used both for ingesting food and for eliminating wastes. Cnidarians evolved in the Precambrian Era, but it is not known from what type of organism. Cnidarians include the jellyfishes, hydras, sea anemones, and corals. Also called coelenterate.