Structure of Cnidarians
Cnidarians have two different body plans, both with radial symmetry. A polyp is shaped something like a vase, with the mouth opening at the top. Most polyps do not move around; they are adapted for a life attached to an underwater surface. The bowl-shaped medusa, such as a jellyfish, is adapted for a free-swimming life. Medusas, unlike polyps, have mouths that open downward. Some cnidarians go through both a polyp stage and a medusa stage during their lives. Others are polyps or medusas for their whole lives.