Group of simply built organisms like polyps, medusae, sea anemones, coral polyps.

A mobile form (medusa) and a sessile form (polyp) are often alternating during the development of members of this group. Typically, the animals have various forms of stinging cells, used for paralysing or killing their prey, which they subsequently digest within the central cavity.

Particularly interesting animals are the siphonophores, "super-organisms", formed by the combination of many, specialized individuals, and the coral polyps, responsible for the formation of coral reefs, due to their limestone skeletons.