Metagenesis refers to:

Body forms: Two basic body forms are found in cnidarians which differ in structure and function from one another. These are called polyp and medusa. A polyp is a cylindrical form that remains attached to the substratum (hence sessile) whereas a medusa is an umbrella-shaped and free-swimming form. Some cnidarians exist only in polyp form, e.g., Hydra, Adamsia, while some have medusa as the dominant form in their life cycle (polyp may be reduced or absent), e.g., Aurelia (jellyfish). Some other cnidarians exist in both forms, i.e., polyp and medusa, e.g., Obelia. Those cnidarians which exist in both forms show alternation of generation in their life (metagenesis).