A class of the phylum Cnidaria traditionally containing four major groups—Coronatae, Rhizostomeae, Semaeostomeae, and Stauromedusae—all marine or estuarine. Most occur in two forms, the polyp, or scyphistoma, and the medusa, or scyphomedusa. However, some (the Stauromedusae) are polyplike and sessile throughout their lives, while others (such as the semaeostomes Pelagia and Stellamedusa and the coronate Periphylla periphylla) are always pelagic and lack the sessile scyphistoma stage. Among the Cnidaria, the Scyphozoa are characterized by having well-developed medusae of large size and fairly well organized scyphistomae of small size. The Scyphozoa are most closely related to the other medusae-containing classes—Cubozoa and Hydrozoa—within the Cnidaria, all of which evolved from a polyp-only ancestor currently represented by members of Class Anthozoa. Recent analyses of DNA and morphology indicate that the Stauromedusae group may represent an intermediate evolutionary stage between Class Anthozoa and Medusozoa (which would encompass Cubozoa, Hydrozoa, Coronatae, Rhizostomeae, and Semaeostomeae) and may, in fact, represent a distinct class of cnidarian (Class Staurozoa).