Succession of sexually and asexually formed groups of offspring belonging to a given species of organisms; metagenesis: Examples of the alternation of generations are microspheric and megalospheric foraminifers, polyps, and medusa stages of some coelenterates (invertebrate lacking a backbone or spinal column), etc.

"Coelenterates" are radially symmetrical animals having saclike bodies with only one opening and tentacles with stinging structures; they occur in polyp and medusa forms.