The polyp form resembles a flower. The stalk attaches at the bottom to a substrate and at the top of the stalk is the mouth, also called oral cavity, surrounded by tentacles. Familiar animals occurring in the polyp form include sea anemones, hydras, and corals. The medusa is a free-swimming form that resembles a bell. The oral cavity, located underneath the bell, is surrounded by tentacles.
Familiar animals occurring in the medusa body form include jellyfish.
Animals in both the Hydrozoan and Scyphozoan classes alternate the medusa and the polyp forms throughout their life cycle. The Anthozoan class exists only in the polyp form. Alternation of generations is when an organism alternates sexual and asexual reproduction during its life cycle. This process of reproduction is performed by jellyfish. Adult jellyfish, in the medusa body form, reproduce sexually.