[Cubo-/Scyphomedusae] In some rhizostomes, in addition to the numerous minute mouthlets on the mouth-arms there is a mouth-opening in the centre of the subumbrella as in semaeostomes, termed a primary mouth opening. Whether it developmentally represents the embryonic mouth is not implied. One occurs for example in Stomolophus, and in that genus it may have been secondarily acquired so that the term "primary" may be misleading.

Alternative form for Primary mouth : Primary mouth opening.