protozoan , Any of a group of small (usually microscopic) single-celled protists that are nonfilamentous and heterotrophic (using organic carbon as a source of energy). 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 micrometres 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. Commonly known protozoans include dinoflagellates, amoebas, and paramecia (see paramecium).
