In some biological taxonomy schemes, Protozoa are a diverse group of mostly motile unicellular eukaryotic organisms, but the term is no longer commonly used in modern taxonomy. Historically, protozoa were defined as unicellular protists with animal-like behaviour, such as movement. Protozoa were regarded as the partner group of protists to protophyta, which have plant-like behaviour, e.g. photosynthesis. 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. Modern unicellular clades within Eukaryotes which may be viewed as approximately collectively replacing the class of protozoa include: Excavata, Amoeba, Chromalveolata and Rhizaria. The term is still used informally, especially in high-school education, and today, protozoa 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.