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The Kingdom Protista (one of the five kingdoms in Linnaean taxonomy) includes all the Eukaryotes except for the Plantae, Fungi, and Animalia. A few forms are multicellular, i.e. the Phaeophyta (brown algae) and Rhodophyta (red algae). The vast majority, though, comprise the single-celled organisms, and are typically only 0.01-0.5 mm in size, 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:

Except for the Ciliates and Oomycota, these are all polyphyletic and frequently overlapping. More recently protists have been divided into more genuine groups on the basis of ultrastructural and chemical features. Aside from basal forms, these can be assorted into a few broad categories on the basis of the the form of the mitochondria and in particular the cristae within them.

In early classifications the Protozoa were considered a phylum of animals, and the algae and slime molds were placed among several divisions of plants. Many forms were classified under both kingdoms and researched by zoologists and botanists alike. Eventually the kingdom Protista was created to house these forms, with the classes of protozoa (corresponding roughly to the above) being promoted to phyla, but this still entails polyphyletic groups and multiple classifications. Most workers now consider the various protozoan clades as direct subgroups of the Eukaryota, with the admission that we do not know nearly enough about their relationships to construct a genuine hierarchy. The various clades are listed on our evolutionary tree and under the linked pages above.