Animals need nutrients in order to obtain energy and provide raw materials for the body's growth and repair. Animals get the nutrients they need by taking in food, or ingestion. Once the food is inside the body, it is processed into molecules that are small enough for the body to absorb.

This process is called digestion.

Digested food is taken into the cells and tissues by absorption, and any undigested material is then eliminated from the body as waste material.

Animals are adapted to ingest and digest food in many different ways, depending on the type of food source and the size and structure of the animal’s body. Structurally simple organisms like this amoeba don’t have a digestive system at all: they take in and break down food by intracellular digestion.

An amoeba engulfs its meal by phagocytosis and creates a food vacuole inside the cell. … The food is mixed with digestive enzymes inside the vacuole, which is separated from the main part of the cell by a membrane.

Many protists, as well as multicellular sponges, use this method of digestion. In all multicellular animals except sponges, digestion takes place outside of cells. This requires a digestive system for holding the food while it's being broken down.

In simple invertebrates, such as hydras and flatworms, the digestive system is a sac, with a single opening. A hydra, for example, has a central cavity that holds prey: the cavity is lined by a layer of specialized cells that secrete digestive enzymes to break down the prey’s tissues.

These cells then absorb the smaller particles and finally digest them, in much the same way that protists do. The digestive system of the hydra and flatworm represents a midpoint in complexity between the simple intracellular digestion of protists and the complete digestive system of most animals.

A complete digestive system consists of a digestive tract, or alimentary canal. This has two openings: the mouth, where food is initially ingested, and the anus, where undigested waste is eliminated.

The evolution of the complete digestive system provided a number of advantages, which we'll explore in this activity. First we'll take a look at vertebrate digestive systems and see how structure and function are related in this group of animals. Next, we’ll follow the path of food through the human digestive system. Finally, we’ll see how digestion is regulated by the nervous system and endocrine system.

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