The digestive systems works on nutrients taken from the environment, breaking them down into simpler products, and then absorbing the products together with water and salts so they can be used in metabolism. The individuals of all species contain many different protiens or carbohydrates in the cell walls of bacteria, chitin in the external skeletons of arthropods, and so on and foreign proteins or carbohydrates are rarely incorporated unchanged. They are usually first broken down to their constituents before being built up again as the protei ....