Jellyfish

General name referring to various marine and fresh-water, free-swimming members of the phylum Cnidaria, belonging to various classes based on different morphology; families include Cubozoa (about 20 species), Scyphozoa (about 200 species), Staurozoa (about 50 species), and Hydrozoa (about 1000-1500 species, all of which are not jellyfish). Most jellyfish have in common a rounded, nearly transparent gelatinous body with a mouth on the underside surrounded by stinging tentacles; their bodies alternately expand and contract, allowing them to trap prey in their tentacles as they jet upward and parachute down again.