Protozoa

In some taxonomy schemes, Protozoa are a diverse group of mostly motile unicellular eukaryotic organisms, but the term is no longer commonly used in modern taxonomy.

Taxonomy
Historically, protozoa were defined as unicellular protists with animal-like behavior, such as movement. The term protozoan has become highly problematic due to the introduction of modern ultrastructural, biochemical, and genetic techniques, which have showed that the group does not form a clade as required by modern classifications.

The term is still used informally, especially in high-school education, and today, protozoa are usually single-celled and heterotrophiceukaryotes containing non-filamentous structures that belong to any of the major lineages of protists. They are restricted to moist or aquatic habitats (i.e., they are obligate aquatic organisms). Many protozoan species are symbionts, some are parasites, and some are predators of feces bacteria and algae. There are an estimated 30,000 protozoan species.

