The jellyfish are included in two of these three phyla, the Hydrozoa and the Schyphozoa. Both the Hydrozoa and Scyphozoa phylum have dimorphic life cycles that exhibit alternation of generations . An organism with a dimorphic life cycle is one that has two or more different body shapes throughout its life. 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.