Cnidarians take on two basic forms, a medusa and a polyp. The medusa form is a free-swimming structure which consists of an umbrella-shaped body (called a bell), a fringe of tentacles that hang from the edge of the bell, a mouth opening located on the underside of the bell, and a gastrovascular cavity. The mesoglea layer of the medusa body wall is thick and jelly-like. Some cnidarians only exhibit the medussa form throughout their life while others first pass through other phases before maturing into the medussa form.
The medusa form is most commonly associated with adult jellyfish. Although jellyfish pass through planula and polyp stages in their life cycle, it is the medusa form that is most recognized with this group of animals.
