The body of Cnidaria shows a radial symmetry and is composed of two adjacent epithelia connected by an extracellular matrix. Three of the four major groups show a life cycle including an alteration of generations from a sessile or sedentary polyp to a free-swimming medusa. Their exclusive feature is their cnidocytes, a highly specialized cell type that plays an important role in prey acquisition. A high regenerative capacity combined with unique adaptations of reproductive modes and only a few natural enemies have made cnidarians a widespread metazoan group. Apart from some freshwater species, most cnidarians live in marine habitats.