The number of contractile vacuoles per cell varies, depending on the species. "Amoeba" have one, "Dictyostelium discoideum", "Paramecium aurelia" and "Chlamydomonas reinhardtii" have two, and giant amoeba, such as "Chaos carolinensis", have many. The number of contractile vacuoles in each species is mostly constant and is therefore used for species characterization in systematics. The contractile vacuole has several structures attached to it in most cells, such as membrane folds, tubules, water tracts and small vesicles. These structures have been termed the spongiome; the contractile vacuole together with the spongiome is sometimes called the "contractile vacuole complex" (CVC). The spongiome serves several functions in water transport into the contractile vacuole and in localization and docking of the contractile vacuole within the cell.