Protist

In all biological taxonomy schemes, protists (/ˈproʊtɨst/) were a large group of diverse eukaryotic microorganisms, mainly unicellular animals and plants, that do not form tissues. Formerly, these were assigned to the now-obsolete kingdom Protista. However in modern taxonomy the Protista are understood to be paraphyletic (not a clade), so the term remains in use only for convenience, similar to ""invertebrate"". An equivalent term Protoctista is used for these organisms by various organisations and institutions. Molecular analyses in modern taxonomy have been used to redistribute former members of this group into diverse and sometimes distantly related phyla. When used, the term “protists” is now considered to mean similar-appearing but diverse phyla that are not related through an exclusive common ancestor, and which have different life cycles, trophic levels, modes of locomotion, and cellular structures. Besides their relatively simple levels of organization, the protists do not have much in common.The term protista was first used by Ernst Haeckel in 1866. Protists were traditionally subdivided into several groups based on similarities to the ""higher"" kingdoms: the unicellular ""animal-like"" protozoa, the ""plant-like"" protophyta (mostly unicellular algae), and the ""fungus-like"" slime molds and water molds. These traditional subdivisions, largely based on superficial commonalities, have been replaced by classifications based on phylogenetics (evolutionary relatedness among organisms). However, the older terms are still used as informal names to describe the morphology and ecology of various protists.Protists live in almost any environment that contains liquid water. Many protists, such as algae, are photosynthetic and are vital primary producers in ecosystems, particularly in the ocean as part of the plankton. Other protists include pathogenic species such as the kinetoplastid Trypanosoma brucei, which causes sleeping sickness and species of the apicomplexan Plasmodium which cause malaria.