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At first, Haeckel included a few multicellular organisms in this kingdom, but in later work, he restricted the Protista to single-celled organisms, or simple colonies whose individual cells are not differentiated into different kinds of tissues. [21] Frederick Chapman's The foraminifera: an introduction to the study of the protozoa (1902)
[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]
A unicellular organism, also known as a single-celled organism, is an organism that consists of a single cell, unlike a multicellular organism that consists of multiple cells. Organisms fall into two general categories: prokaryotic organisms and eukaryotic organisms. Most prokaryotes are unicellular and are classified into bacteria and archaea.
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). [1]
Prokaryotes (Archaea and Bacteria) reproduce asexually through binary fission, in which the parent organism divides in two to produce two genetically identical daughter organisms. Eukaryotes (such as protists and unicellular fungi) may reproduce in a functionally similar manner by mitosis; most of these are also capable of sexual reproduction.
A protist (/ ˈ p r oʊ t ɪ s t /) is any eukaryotic organism (one with cells containing a nucleus) that is not an animal, plant, or fungus.The protists do not form a natural group, or clade, since they exclude certain eukaryotes with whom they share a common ancestor; [a] but, like algae or invertebrates, the grouping is used for convenience.
Of eukaryotic groups, the protists are most commonly unicellular and microscopic. This is a highly diverse group of organisms that are not easy to classify. [62] [63] Several algae species are multicellular protists, and slime molds have unique life cycles that involve switching between unicellular, colonial, and multicellular forms. [64]