This is a small order of invertebrates, consisting of the medusas and the hydras. There are three families, Clavidae, Hydridae and Petasidae. Within the Clavidae, there is a single species, Cordylophora caspia, recognized by Ohio EPA. Within the Hydridae, there are the various species of Hydra, which are not taken further than genus. Within the Petasidae (Olindiadidae) there is one species, recognized by Ohio EPA, Craspedacusta sowerbyi.