In cnidarians, a medusa is a body form that is often free living. They can vary from being bell-shaped to the shape of a thin disk. The upper surface is called the exumbrella and the lower surface the subumbrella. Their mouth is on the lower surface, which is sometimes partially closed by a membrane extending inward from the margin.
During the complete life cycle, this form may alternate with a form called a polyp, which is often fixed in one place.
Associated Terms: Medusae