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Protists are distributed across all major groups of eukaryotes, including those that contain multicellular algae, green plants, animals, and fungi. If photosynthetic and fungal protists are distinguished from protozoa, they appear as shown in the phylogenetic tree of eukaryotic groups.
[3] [4] Although most protists are unicellular, there is a considerable range of multicellularity amongst them; some form colonies or multicellular structures visible to the naked eye. The term 'protist' is defined as a paraphyletic group of all eukaryotes that are not animals , plants or fungi . [ 5 ]
Many eukaryotes are unicellular; the informal grouping called protists includes many of these, with some multicellular forms like the giant kelp up to 200 feet (61 m) long. [9] The multicellular eukaryotes include the animals, plants, and fungi, but again, these groups too contain many unicellular species. [10]
Its field of study therefore overlaps with the more traditional disciplines of phycology, mycology, and protozoology, just as protists embrace mostly unicellular organisms described as algae, some organisms regarded previously as primitive fungi, and protozoa ("animal" motile protists lacking chloroplasts).
A multicellular organism is an organism that consists of more than one cell, unlike unicellular organisms. [1] All species of animals , land plants and most fungi are multicellular, as are many algae , whereas a few organisms are partially uni- and partially multicellular, like slime molds and social amoebae such as the genus Dictyostelium .
Some modern authors prefer to exclude multicellular organisms from the traditional definition of a protist, restricting protists to unicellular organisms. [ 9 ] [ 10 ] This more constrained definition excludes all brown , the multicellular red and green algae , and, sometimes, slime molds (slime molds excluded when multicellularity is defined ...
Protists have been described as a taxonomic grab bag of misfits where anything that doesn't fit into one of the main biological kingdoms can be placed. [98] Some modern authors prefer to exclude multicellular organisms from the traditional definition of a protist, restricting protists to unicellular organisms.
Individual cells within multicellular organisms also show thermotaxis. For example, mammalian sperm migrate through the oviduct to the fertilization site guided by a rise in temperature. [75] [76] The investigation of how unicellular organisms migrate toward preferred temperatures began more than 100 years ago. [74]