Anthomedusae are small, delicate jellyfish. Often transparent, they ghost around the ocean, camouflaged among the tiny organisms that make up plankton. Unlike plankton, they actively swim using jet propulsion. The top of an anthomedusa is an umbrella-shaped bell. Muscles around the edge of the bell cause it to pulse like an umbrella opening and closing. The pulsing propels the anthomedusa through the water. Little sensory structures along the edges of the bell react to outside stimuli, such as water currents or contact with an object. They send nerve impulses through a network of nerves on the underside of the bell. The nerve impulses affect muscles to change swimming speed or direction. Anthomedusae also have a lifestage (polyp) in which they attach to something and do not swim around. Depending on the species, they may attach to a coral reef, a rock, a snail's shell, or even the skin of a fish.

Like their close relatives, hydroids have a life cycle that includes two body types. The polyp type is stationary, anchoring on the ocean bottom or other substrate. The medusa type swims around and looks like a jellyfish. In hydroids, the polyp type tends to be more common, although many species have both types. Polyps typically form colonies containing up to thousands of small individuals. A colony usually has either all male or all female polyps. One polyp may start a colony, then enlarge it by budding off new polyps (asexual reproduction). Some mature polyps have the special ability to make mini medusae that detach and swim away. When they are grown up, female medusae release eggs into the sea, while male medusae release sperm (sexual reproduction). A fertilized egg becomes a larva that either develops into another medusa or into a polyp. A new polyp anchors, which restarts the cycle.