Metagenesis (or alternation of generation) Coelenterates exhibit two body forms: polyp and medusa.
A. Polyp = Sessile and cylindrical like Hydra, Adamsia.
B. Medusa = Free swimming, umbrella shaped like Aurelia (jelly fish). Those cnidarians in which both polyp and medusa alternates with each other, are said to exhibit metagenesis. During metagenesis, polyp (asexual stage) reproduce asexually to form medusa, whereas the medusa (sexual stage) sexually reproduce to form polyp, e.g., Obelia.