Representative protozoans. The zooflagellate Trypanosoma brucei is the causative agent of …—© Merriam-Webster Inc.

Any of a group of small (usually microscopic) single-celled protists. They are found in most soils, fresh water, and oceans. While most are solitary individuals, various colonial forms exist. The taxonomic relationships of protozoans to one another and to other protists continue to be revised. The smallest known protozoans are tiny blood parasites less than 2 microns long; the largest may be 16 mm long and visible to the naked eye. Protozoan shapes vary, but all share such eukaryotic features as lipid-protein membranes and membrane-enclosed vacuoles and organelles (see eukaryote). They show wide variation in modes of movement, nutrition, and reproduction. Various classification systems exist to group the protozoans. The major phyla include Sarcomastigophora (flagellated forms and forms possessing cytoplasmic extensions called pseudopodia), Ciliophora (ciliated forms), and Apicomplexa, Microspora, and Myxozoa (spore-producing forms). Apicomplexa and Microspora are sometimes included in the single phylum Sporozoa. Commonly known protozoans include dinoflagellates, amoebas, and paramecia (see paramecium).

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