The life cycles of Clytia linearis and Clytia noliformis, two common campanulariids previously known solely from their polyp and young medusa stages, were investigated. Adult medusae of C. linearis have a bell diameter of 2·5–3·6 mm, and up to 29 tentacles. The bell is almost flat when relaxed and C-type microbasic mastigophore nematocysts form a cluster at the tip of the tentacles. Mature medusae of C. noliformis have a more hemispherical, saucer-shaped bell (3·5–4·5 mm in diameter), up to 16 tentacles, and no diagnostic nematocysts. Polyps of both species also differ in morphology, colour, and cnidome, and a possible resting structure is described for C. linearis. The relevance of characters from the perisarc and hydranths for taxonomy within the Campanulariidae is also evaluated, and whether the size of nematocysts is variable or not, and therefore appropriate for species identification.