Protists (
IPA /'pro?t?st/),
Greek protiston -a meaning the (most) first of all ones, are a diverse group of
organisms, comprising those
eukaryotes that cannot be classified in any of the other eukaryotic
kingdoms as
fungi,
animals, or
plants. They are usually treated as the
kingdom Protista or
Protoctista.
Protoctists (or protists) are a
paraphyletic grade, rather than a natural, (
monophyletic) group, and so do not have much in common besides a relatively simple organization -- either they are
unicellular, or they are
multicellular without highly specialized tissues. The term
protista was coined by
Ernst Haeckel in
1866.
Protists were traditionally subdivided into several groups based on similarities to the "higher" kingdoms the one-celled animal-like protozoa, the plant-like protophyta (mostly one-celled algae), and the fungus-like slime molds and water molds. Because these groups often overlap, they have been replaced by phylogenetic-based classifications. However, they are still useful as informal names for describing the morphology and ecology of protists.
At one time, the non-nucleated bacteria were also considered protists under the three-kingdom system of Animalia (comprising the many-celled animals or metazoans), Plantae (which then included fungi as well as green land plants), and Protista (which included everything else, except viruses). However, most current textbooks treat bacteria (and the newly-discovered archaea) as either a separate kingdom or domain.
Protists obtain nutrients and digest nutrients in a complex acquirement and assimilation system. Most protists also feed on bacteria. Protists acquire their food material through internal digestion. They extend their cell wall and cell membrane around the food material to form a food vacuole via endocytosis (usually phagocytosis; sometimes pinocytosis).
