The digestive systems of animals are quite diverse. Sponges, the simplest animals, do not have specialized digestive systems. Food particles filtered from the water are simply digested within individual cells (intracellular digestion). One of the first steps toward a complex digestive system in animals, and the processing of larger prey, is the evolution of a gastrovascular cavity, a digestive sac with a single opening to the external environment. The gastrovascular cavity serves as a protected space for extracellular digestion inside the animal, and at the same time allows distribution of the digested material to most cells of the body. Following extracellular digestion in the gastrovascular cavity, the digestion products from carbohydrates, proteins, and fats are taken up by cells lining the gastrovascular cavity, where digestion is completed intracellularly.

A One-Way Digestive Tract

Cnidarians and flatworms have a gastrovascular cavity. Cnidarians, such as the hydra, use their...