Cnidarians exhibit two basic body forms called polyp and medusa. The former is a sessile and cylindrical form like Hydra, Adamsia, etc. Whereas the latter is umbrella-shaped and free-swimming like Aurelia or jellyfish. The above diagram indicates the outline of the body form in Aurelia (Medusa). Those cnidarians which exist in both forms exhibit alternation of generation i.e., polyps produce medusae asexually and medusae from the polyps sexually.
