A diverse phylum of aquatic animals that are characterized by the possession of (1) intrinsic nematocysts, (2) a single body space that opens to the external medium, (3) radial or biradial symmetry, (4) tissue level of organization, and (5) diploblastic structure. With few exceptions, a cnidarian also has tentacles surrounding the mouth (the opening of the body cavity to the exterior). Most definitions of the phylum include the feature metagenesis; however, although the life cycle of many cnidarians involves this alternation between an asexually reproducing polyp and a sexually reproducing medusa, this is not true of two entire classes (Anthozoa and Staurozoa) and of some members of class Hydrozoa. The name Coelenterata, which has been used for this phylum, is being supplanted because, as sometimes used, it included the sponges (Porifera) and comb jellies (Ctenophora), as well as the animals now called cnidarians.