The medusa is the dominant form in the life cycle of ‘true jellyfish’; the polyp is restricted to a larval stage. Medusae move by pulsating muscular contractions of the bell, which is supported by a thick gelatinous and fibrous mesogloea. Only weak swimmers, scyphozoans are carried great distances in the plankton by the seas’ currents.
Plankton are animals and plants that live suspended in the sea but are unable to swim against water currents due to their small size or weak powers of movement. Nekton are animals, like fish, that are powerful enough to swim against water currents.
Organisms living on the sea-floor like hydroids and sponges are members of the Benthos.
Right: Chrysaora sp. from the Pacific. These species can grow up to a metre in diameter.