Comb jellies have radial symmetry and differentiated tissues without organ systems. They do not sting, and are usually globular. Ninety-five percent of a comb jelly is water. Some have a pair of tentacles equipped with adhesive cells, or oral lobes, or neither. Their mouth leads to their pharynx and stomach; from there canals take the digested food where it is needed. They are carniverous with a wide range of prey.

The comb plates have transverse rows of cilia fused by a thin membrane and arranged in eight lines down the axis of the jelly. The comb jelly beats these plates to move through the water using the apical organ. When it changes position gravity stimulates the nervelike tissue beneath each line of comb plates to regulate the beat. A comb jelly is hermaphroditic and shed both sperm and egg at the same time. Some hold fertilized eggs in their canals to become larvae. They are bioluminescent and their whole bodies can become outlined in light.