The body wall consists of two epithelia, the outer epidermis and the inner gastrodermis, between which is sandwiched a thin connective tissue mesoglea. The body cavity is the gastrovascular cavity, or coelenteron, which has only one opening, the mouth. Polymorphism is common and most groups exhibit an alternation of generations. Typically a solitary asexual polypoid generation alternates with a solitary, mobile, sexual medusoid generation. Gonads may be epidermal or gastrodermal.
Cnidarians are gonochoristic.
Anthozoa Anthozoa are exclusively polypoid and there are no medusae. The class includes the stony corals, soft corals, anemones, and others. The mouth opens into a flattened, ectodermal invagination known as the pharynx, which in turn opens into the gastrovascular cavity. They may be solitary or colonial. Many have important endosymbiotic zooxanthellae.