Evolution of digestive organs

In unicellular organisms (amoeba, ciliate) food is digested inside the cell in a food vacuole. Cnidarians have a digestive cavity and digestion is partly outside, partly inside cells; the gut has a single opening. Flatworms have a branched blind gut, and from roundworms on, a one-way gut with mouth and anus appeared. Annelids separated oesophagus, crop, stomach and intestine; molluscs developed a rasping tongue (radula) and a liver, arthropods jaws and salivary glands (a spider softens prey outside its body with digestive juices). In chordates the gut divides into parts: mouth, pharynx, oesophagus, stomach, small and large intestine; the liver and gallbladder exist from fish onward. Amphibians and reptiles have a longer gut opening into the cloaca; birds have no teeth, the crop and muscular gizzard grind food, and the gut is short – an adaptation for flight. Mammals have specialised teeth (incisors, canines, premolars and molars), three pairs of salivary glands and a stomach that changes with diet: in herbivorous ruminants a complex, multi-chambered stomach in which bacteria and protozoa break down cellulose.