The digestive system is that system which exists within all organisms to allow nutritive particles to be taken in, broken down, and either used by the body or removed from the body as waste. Digestive systems range from the very simple, primitive systems of the once-celled organisms, to the complex, multi-organ systems used by vertebrates (organisms with backbones).

Single-celled organisms engulf food particles with their outer membrane. The food particles are brought inside the cell; hence this type of digestion is referred to as intracellular. Food particles are ultimately engulfed by bags of potent chemicals within the cell (called digestive vacuoles) which break down the particles into usable components. Waste products may be re-packaged into little bags, and passed back out through the membrane(interestingly enough, this type of digestion is used by the immune system's white blood cells in mammals; white blood cells protect the...