The Scyphozoa, referred to as the true jellyfish, are related to corals and anemones (anthozoans), however they differ from these in having a more complex life cycle.
Most species of Scyphozoa have a life cycle which involves an alternation between sessile (fixed in one place) polyp phase and a free-swimming medusa stage, though the medusa stage, usually predominates. The medusa stage typically consists of an umbrella-shaped body with stinging tentacles around the edge. All Scyphozoa are called jellyfish in their free-swimming medusa phase.
As medusae, they eat a variety of crustaceans and fish, which they capture using stinging cells on their nematocysts. The nematocysts are located throughout the tentacles that radiate downward from the edge of the umbrella dome, and also cover the four or eight oral arms that hang down from the central mouth. Some species, however, are instead filter feeders, using their tentacles to strain plankton from the water.