Protista

This is a kingdom comprised of simple, unicellular 'animals' that do not fall easily into the plant and animal kingdoms.

Protists exhibit an enormous range of body form, even though they are largely microscopic. They may occur singly or in colonies. They may swim freely, travel along a surface, or be fixed to a surface. They may be housed in a shell, clothed in scales or other adhering matter, or be naked. They may or may not be coloured. They may be parasites; or they may live, apparently harmlessly, attached to or within plants or animals.

Protists were traditionally (for the last 150 years) subdivided into several groups based on similarities to the higher kingdoms: the animal-like protozoa and the plant-like algae and the fungi-like slime moulds. These traditional groups often overlap. The traditional classification is fundamentally flawed and new systems are proposed and likely to be adopted. In some newer schemes, most algae are classified in the kingdoms Plantae and Chromista, and in such cases the remaining forms may be classified as a kingdom Protozoa. The name is misleading, since they are not truly 'animals'.

Protozoa (in Greek proto = first and zoa = animal) are single-celled eukaryotes (organisms whose cells have nuclei) that show some characteristics usually associated with animals, most notably mobility and heterotrophy. Protozoa have traditionally been divided on the basis of locomotion, as below, although this is no longer believed to represent genuine relationships:

	Flagellates
	Amoeboids
	Sporozoans: Apicomplexa, Myxozoa, Microsporidia
	Ciliates

Most protozoans are too small to be seen with the naked eye - most are around 0.01-0.05 mm, although forms up to 0.5 mm are still fairly common - but can easily be found under a microscope. Protozoa are ubiquitous throughout aqueous environments and the soil, and play an important role in their ecology. Protozoa occupy a range of trophic levels. As predators upon unicellular or filamentous algae, bacteria, and microfungi, protozoa play a role both as herbivores and as consumers in the decomposer link of the food chain. Protozoa also play a vital role in controlling bacteria population and biomass. As components of the micro- and meiofauna, protozoa are an important food source for microinvertebrates. Thus, the ecological role of protozoa in the transfer of bacterial and algal production to successive trophic levels is important. Protozoa are also important as parasites and symbionts of multicellular animals.

Much of whats above is taken from Wikipedia, the free encyclopedia - see http://en.wikipedia.org.

Links

The website of the British Section of the (international) Society of Protozoology (BSSP) is at http://www.bssp.org.