Digestion is the breakdown of food by a living organism into chemically simpler forms that can be readily assimilated and absorbed by the body. It requires the action of digestive enzymes. It most animals, digestion is an extracellular process. In mammals, in particular, it involves the gastrointestinal tract, which, together with accessory structures, such as the teeth, tongue, liver, pancreas, and gall bladder, makes up the digestive system. In simpler organisms, such as protozoans and cnidarians, digestion is primarily intracellular, for example involving phagocytic cells.