The Kingdom Protista or Protoctista is one of the commonly recognized biological kingdomss, including all the eukaryotes except for the plants, fungi, animals, and sometimes other groups which are treated in separate kingdoms. A few forms are multicellular, for instance the brown and red algae. The vast majority, though, comprise the single-celled organisms, and are typically only 0.01-0.5 mm in size, usually too small to be seen without a microscope. Protists are ubiquitous throughout aqueous environments and the soil, commonly surviving dry periods in the form of cysts; a few are important parasites.
Traditionally protists have been divided into:
- Plant-like forms that have chloroplasts, the algae;
- Fungus-like forms, the slime molds and water molds;
- Animal-like forms, the protozoa, generally divided on the basis of morphology and locomotion into:
More recently attempts have been made to divide protists into more genuine groups on the basis of ultrastructural, chemical, and genetic features. In newer classification systems these are often treated as separate kingdoms. However, there are still many different lines of protists whose relationships are not understood. Many scientists now consider the various protist clades as direct subgroups of the eukaryotes, with the admission that we do not yet know enough about them to properly arrange them into a hierarchy. These various clades are listed on our evolutionary tree and under the linked pages above.