Scyphozoa is the scientific name of the true jellyfish. They are in the Animalia kingdom, the Cnidaria phylum and the Medusozoa subphylum. Other similar organisms are classified as Hydrozoa and Cubozoa.
True jellyfish are soft-bodied invertebrates. They have no bones, shell or exoskeleton. Jellyfish life cycles include the planula, polyp, ephyra and medusa forms. The medusa form is the most familiar.
Jellyfish can range in size from one-half inch long to more than 6 feet across. The largest jellyfish have tentacles over 131 feet long. Jelly fish are carnivores that feed on zooplankton, invertebrates and small fish. When feeding, the jellyfish inject prey with venom to immobilize it.