The digestive system exists in several different forms in the biological world. In the case of some types of spider for example, digestion is begun outside of the body, with digestive enzymes pumped out of the gut and onto the prey to liquefy it for ingestion.

In most mammals, the food is orally ingested and chewed before passing into the stomach and intestine. The food is then broken down by digestive enzymes into simpler components to aid its absorption.

Some of the different forms of digestive system are described below:

• Invasive digestion – Here, an organism invades the cells of the prey to obtain its nutrients. Viruses are classic examples of this form of digestion as they invade host cells to reach the nutrients stored inside the cell vacuoles.
• Digestion by secretion – Bacteria, for example, secrete toxins to digest the food then imbibe the resulting nutrient mass.
• Digestion by channel transport – In this form of digestion, several proteins form a continuous channel that links the inner and outer membranes of the bacteria. The ABC protein, membrane fusion protein, and outer membrane protein are the three proteins involved. The bacteria secretes digestive chemicals and the transport system helps the transfer of these nutrients into the cell.
• Digestion using the molecular syringe – Some bacteria such as salmonella (that causes typhoid fever), shigella (that causes diarrhea) and Yersinia (that causes plague), can inject certain proteins into mammalian cells to help them assimilate the nutrients of the host cell.
• Digestion by conjugation – This is also typically seen in bacteria.
• Formation of outer membrane vesicles - Gram-negative bacteria can form outer membrane vesicles from pinched off portions of the outer membrane.
• Formation of a phagosome – Some bacteria form a vacuole called a phagosome by engulfing an organism or particle though phagocytosis. The particle is then digested inside the phagosome.
• Gastrovascular cavity digestion – This is a form of digestion seen in lower animals where a pouch with a single opening is the site of digestion as well as nutrient exchange. Examples of animals with this form of digestion include the flatworm and the sea anemone.
• Advanced digestive system – Mammals have a well developed gastrointestinal system that involves the mechanical breakdown of food in the mouth, followed by chemical digestion in the stomach and small intestine. The remains of the food is then passed through the large intestine and eventually forms feces that is stored in the rectum and excreted via the anus.