Protists (or protoctista) are an extremely diverse group of predominantly microscopic organisms. All protists are eukaryotic, and while many are unicellular and have only one nucleus, they may be multicellular, have multiple nuclei, or form colonies of identical, unspecialized cells. Although most are microscopic, some are much larger, reaching lengths of over an inch (just over 2 millimeters). In early, traditional classifications, they were listed on the basis of being neither plant nor animal nor fungus; further evidence suggested that protists exhibit characteristics of the plant, animal, and fungi king-doms—but their classification is still highly debated.