Beginning of Essay
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 p ....

Middle of Essay
.... is partly digested in the coelenteron, a gut-like cavity, and the residue is completely digested in cells of the body wall. Scientists believe that intracellular digestion in organisms such as Hydra has evolved from such protozoans as amoebae and paramecia. In most higher animals digestion is completed not in the cell in the cavity of a digestive tract (the stomach and intestine). Animals with this type of digestion include crustaceans, insects, cephalopods, tunicates, and all vertebrates. A few animals with digestive tracts also partially digest their food before eating it. Some spiders, for example, pierce their prey with fangs that pump diges ....