protozoa
Protozoa are a diverse group of unicellular eukaryotic organisms,many of which are motile. Historically, protozoa were defined as unicellular protists with animal-like behaviour, such as movement or motility. Protozoa were regarded as the partner group of protists to protophyta, which have plant-like behavior, e.g. photosyntheis. The term protozoan has become highly problematic due to the introduction of modern ultrastructural, biochemical, and genetic techniques. Today, protozoan are usually single-celled and heterotrophic eukaryotes 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 faeces bacteria and algae. There are an estimated 30,000 protozoan species.