Metagenesis is the alternation of generations between sexual and asexual reproduction, but not similar to alternation of generations as found in plants. In plants, the alternation of generations occurs between a diploid sporophyte and a haploid gametophyte, and not between sexual and asexual reproduction.

Alternation of asexual and sexual phases in the life cycle of Obelia is called metagenesis. In Obelia, both the medusa(the motile stage) and polyp(the sessile stage) are diploid. Medusa is the sexual phase and polyp is the asexual phase.