Protist
From Freepedia
| Protists | ||||
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| Image:Paramecium.jpg Paramecium aurelia, a ciliate | ||||
| Scientific classification | ||||
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| Typical phyla | ||||
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Protists are a heterogeneous group of living things, comprising those eukaryotes that are neither animals, plants, nor fungi. They are usually treated as a kingdom Protista or Protoctista, first introduced by Haeckel. The protists are a paraphyletic grade, rather than a natural group, and do not have much in common besides a relatively simple organization (unicellular, or multicellular without highly specialized tissues).
Protists were traditionally subdivided based on similarities to the higher kingdoms. The animal-like protozoa are mostly single-celled, motile, and feed by phagocytosis, though there are numerous exceptions. They are usually only 0.01-0.5 mm in size, generally too small to be seen without a microscope. They are ubiquitous throughout aqueous environments and the soil, commonly surviving dry periods as cysts or spores, and include several important parasites. Based on locomotion, protozoa are grouped into:
| Flagellates | with long flagella | e.g., Euglena |
| Amoeboids | with transient pseudopodia | e.g., Amoeba |
| Ciliates | with multiple, short cilia | e.g., Paramecium |
| Sporozoa | non-motile parasites; form spores | e.g., Plasmodium |
The plant-like algae use light energy through photosynthesis. These include many single-celled creatures that are also considered protozoa, such as Euglena and Paramecium bursaria, which have acquired chloroplasts through endosymbiosis. Others are non-motile, and some are truly multicellular. Of these the green and red algae appear to be close relatives of other plants, and so some authors treat them as Plantae despite their simple organization. However, the brown algae developed separately.
There are also fungal protists: the slime moulds, spore-forming amoeboids which spend part of their life cycle aggregated into multi-cellular fruiting bodies, and the water moulds and labyrinthulids. The latter two are related to brown algae, golden algae, diatoms and the like, forming a group called the heterokonts. They may be considered a separate kingdom, the Chromista, in which case the remaining protists may be treated as a paraphyletic kingdom Protozoa.
The classification of protists has undergone periodic changes. Except for ciliates and water moulds, the traditional groups are polyphyletic and frequently overlap. Newer classification criteria attempt to present monophyletic groups based on ultrastructure, biochemistry, and genetics. However, an understanding of the evolutionary relationships between protists has only recently begun to emerge, and there are still many groups whose placement is uncertain. A partial picture is given by the list of phyla at the right; the traditional protist groups are given by the links in the table above.
