Some animals, however, rely on intracellular digestion. In this case tiny food particles are phagocytized into cells and packaged in vesicles with digestive enzymes. Extracellular digestion is much more common, and is typical of mammals, including humans. Intracellular digestion is much rarer, being characteristic of highly specialized animals such as the sponges. Intracellular digestion is similar to the process by which heterotrophic protistans get their nutrients.
Many animals have simple tube digestive systems.
The nematode is an example. Without compartments, specialized processing cannot readily occur. In humans, absorption of the products of digestion occurs in the small intestine. The surface of this columnar epithelium is formed into finger-like folds called villi. A further increase in surface area for absorption is provided by having the top of each epithelial cell folded into microvilli.