In the field of biology of Biology, it is known as a protist to that kingdom of eukaryotic microorganisms that have a miniscule size and that unlike other kingdoms such as fungi, plantae and animalia, is characterized by the fact of having cells eukaryotes and also due to the fact that they have different organs or tissues. In general, most protist organisms are unicellular, it is normal to come across multicellular protists. In addition to this, the protist kingdom is considered a paraphyletic group, that is, it is a group that does not have all the descendants of the same ancestor and therefore there are representatives both unicellular and multicellular, autotrophs and heterotrophs, phagrophs and also osmotrophs.
This term was used for the first time in the middle of the 19th century during World War II, to be more specific in the year 1866, being the German biologist Ernst Heinrich August Haeckel the first to apply it. This scientist devised this term with the aim that through it, all those unicellular organisms and certain multicellular organisms could be identified or named, which were not admitted either within the plant or animal kingdom and which according to his hypothesis were first that had populated the Earth.
For this reason, the protist kingdom includes eukaryotic-type organisms that, due to their characteristics, are not accepted in the rest of the kingdoms of this class. Although most protists are unicellular, it is important to note that multicellular protists also exist. Most of the protists, on the other hand, include the pseudopodia, cilia and flagella, which gives them the ability to have their own mobility.
It is important to note that protist organisms are not adapted to external environments with full existence of air, which is why their habitats are generally aquatic, terrestrial but humid or, failing that, they can live inside other larger organisms.
For their part, the shape and size of the protists is very varied. With regard to their shape, some protists are very similar to plants, and there are others that resemble animals. In terms of size, it varies from tens of meters to just millimeters.
