Animals eat to replace expended energy and to provide the raw materials necessary for the structural components of life. Heterotrophic organisms must engulf biochemicals they cannot synthesize. Many have become highly specialized feeders.
Different animals have different ways of obtaining and digesting foods.
More complex organisms have more complex digestive systems. The gastrointestinal tract extends from the mouth to the anus and includes the accessory structures. In the mouth and esophagus both mechanical and chemical digestion occur. The stomach functions in storage, in digestion, and in pushing food into the small intestine. The small intestine completes digestion and absorbs the resulting nutrients.