Jellyfish
. Jellyfish, common name for any of the invertebrate animals making up two classes of the cnidarian phylum. About 2700 hydrozoan and 200 scyphozoan species are known. The term jellyfish applies more specifically to the free-swimming, gelatinous organism called the medusa, the form usually taken during the sexual stage of these animals, this generation alternating with a bottom-dwelling polyp stage in which reproduction is asexual. In one class the medusae tend to be small and the polyps well developed, whereas in the other class the medusae predominate. Both classes are marine, except for a few hydrozoans, such as Hydra, that live in fresh water. Jellyfish have little nutritional value. Their stings can be painful, and a few tropical forms can be deadly to humans. As in other cnidarians, the jellyfish has only two major developmental layers (ectoderm and endoderm), no head, a gut but no anus, and a nervous system without a brain. The body exhibits radial symmetry, or symmetry about an