FIGURE 29.8
Polyp and medusa forms of cnidarians. The body wall of a polyp (top) or medusa (bottom) has two layers of cells, an outer layer of epidermis specialized for protection and an inner layer of gastrodermis for digestion. After the animal ingests food, the gastrodermis secretes digestive enzymes into the gastrovascular cavity. The gastrodermal cells engulf small pieces of the partially digested food by phagocytosis, and digestion is completed within the cells in food vascuoles. Flagella on the gastrodermal cells keep the contents of the gastrovascular cavity agitated and help distribute nutrients. Sandwiched between the epidermis and gastrodermis is a gelatinous layer of mesoglea. In many medusas, the mesoglea is thick and jellylike-thus their name, jellyfish.