In organisms that exhibit both forms, such as members of the cosmopolitan genus Obelia, the polyp is the asexual stage and the medusa the sexual stage. In such organisms the polyp, by budding, gives rise to medusae, which either detach themselves and swim away or remain permanently attached to the polyp. The medusae then produce new polyps by sexual reproduction. A medusa produces eggs or sperm, which are usually shed into the water; when an egg is fertilized, it develops into a swimming larva, which eventually settles and grows into a polyp. In addition to this elaborate means of reproduction, the polyp can form new polyps by budding. In some groups of coelenterates either the polyp or the medusa has become highly developed, with the reduction or complete loss of the other form. Where only the medusa occurs, as in many jellyfish, the larva never settles, and grows directly into a medusa. Where only the polyp exists, as in the hydra and the sea anemone, the organism has the ability to produce new polyps sexually, as well as by budding.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.