Jellyfish, familiar name for the free-swimming stage of various invertebrate animals of the phylum Cnidaria. They often have a pulsating jellylike bell and trailing tentacles. Many Cnidarian classes display an alternation of generations, where a single species may be represented by a polyp form, usually asexual, and a medusoid, sexually reproductive stage. These medusoid forms are frequently referred to as jellyfish. The true jellyfish all belong to the class Scyphozoa, where the medusa is the dominant phase and the polyp or hydroid is reduced or absent. Jellyfish are radially symmetrical. Rings of muscle around the margin of the bell contract to expel water and propel the jellyfish forward.