Often, protozoa is just an informal term for unicellular, heterotrophic protists.
Both protozoa and protozoans are popular as the plural form.
Protozoa also play a vital role in controlling bacteria populations and biomass.
An organism belonging to this group may be known as a protozoan (plural protozoans) or protozoon (plural protozoa).
Many free-living protozoans have eye-spots that can detect changes in the quantity and quality of light, and some sense touch and chemical changes in their environment (Towle 1989).
Towle (1989) reports that over 56,000 species of protozoa have been identified, the great majority (over 60 percent) of which move by pseudopodia, false feet or temporary projections of the cell.
Among well-known protozoans are amoebas, ciliates, paramecia, and dinoflagellates.
The phyla into which protozoa fall vary with the taxonomic scheme.
Most protozoans are too small to be seen with the naked eye—most are around 0.01–0.05 mm—but can easily be found under a microscope.
Some protozoa are important as parasites and symbionts of multicellular animals.
The name protozoa ("first animals") is misleading, since they are not animals (with the possible exception of the Myxozoa).
Protozoa may live freely or as parasites, and may live as single cells or in simple colonies without any differentiation into tissues.
Protozoa span several phyla that are generally placed not as part of the kingdom Animalia, but rather as part of the kingdom Protista, sometimes known as the "other" kingdom or the "left-overs" kingdom.
Ubiquitous in aquatic environments and the soil, protozoans prey upon algae, bacteria, and other organisms and are themselves consumed by animals such as microinvertebrates.