Any eukaryotic organism that is essentially unicellular, colonial, or multicellular in form and lacks true cellular differentiation into tissues and organs. Protists include all eukaryotes except the plants, fungi, and animals. They display a diversity of forms, ranging from nonmotile or motile independent cells to leaflike or filamentous forms and sheets of cells. They can be autotrophic, saprotrophic, heterotrophic, or mixotrophic. Radical revisions to relationships between eukaryotic groups have been made in the light of molecular studies, and ‘protist’ is now purely a descriptive term. The kingdom Protista was originally proposed by Ernst Haeckel in 1866 to include the algae, bacteria, fungi, and protozoa; it was later restricted first to unicellular organisms, and then to protozoa, unicellular algae, and organisms then regarded as simple fungi.