Amoeba, a single celled organism, a member of the group Protista, formerly called protozoans, which sends out cytoplasmic arms called pseudopodia to surround the food to form a food vacuole.

There are many forms of protist, some are covered by cilia and are therefore referred to as the Ciliata. One of these called Paramecium is able to generate a current which carries small particles into a groove at the base of which small food vacuoles are formed. These vacuoles travel around a defined route in the cytoplasm. Acid and then enzymes are added to digest the bacteria trapped in the vacuoles. The undigested remains are expelled through a weak point in the external surface. This cytoplasmic pathway appears to be analogous to the gut of metazoans (multicellular organisms) and the fixed expulsion point parallels the anus.

In the Coelenterata (jellyfish, sea anemones) the gut is a hollow internal space with a single opening, the stomadeum, which is surrounded by tentacles which possess stinging cells to catch the prey. The food is taken into this simple gut space and extracellular digestion occurs by the secretion of enzymes into the space. Particles of food are then absorbed into food vacuoles by the cells lining the gut space and digestion continues intracellularly. In the phylum Platyhelminthes the single gut opening is still present but it takes the form of a muscular pharynx which allows food to be sucked in. Digestion in this group is primarily extracellular.

In the more advanced invertebrates the gut forms a continuous tube with an opening at each end. This is a big advance since it allows food to be taken in at one end and then passed through a series of specialised gut areas where different digestive processes can be carried out. The soluble products can then be absorbed into the organism and the waste material passed out of the other end.