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In 21st-century systems of biological classification, the Protozoa are defined as a diverse group of unicellular organisms.
(2009). 9780071623261, McGraw Hill Professional. .
Historically, protozoa were defined as single-celled animals or organisms with -like behaviors, such as and . The group was regarded as the zoological counterpart to the "", which were considered to be plant-like, as they are capable of .

The terms protozoa and protozoans are now mostly used informally to designate single-celled, non- , such as the , and .

The term Protozoa was introduced in 1818 for a taxonomic class, but in later classification schemes the group was elevated to higher ranks, including phylum, subkingdom and kingdom. In several classification systems proposed by Thomas Cavalier-Smith and his collaborators since 1981, Protozoa is ranked as a kingdom. The seven-kingdom scheme proposed by Ruggiero et al. in 2015, places eight phyla under Protozoa: , , , , , , and . This kingdom does not form a , but an evolutionary grade or group, from which the fungi and animals are specifically excluded.

The use of Protozoa as a formal has been discouraged by some recent researchers, mainly because the term, which is formed from the Greek protos "first" + zoia, plural of zoion, "animal", implies kinship with animals (metazoa) and promotes a separation of "animal-like" from "plant-like" organisms, neither of which they encourage. Modern ultrastructural, biochemical, and genetic techniques have shown that protozoa, as traditionally defined, belong to widely divergent lineages, and can no longer be regarded as "primitive animals." For this reason, the terms "protists," "Protista" or "Protoctista" are sometimes preferred for the high-level classification of microbes. In 2005, members of the Society of voted to change the name of that organization to the International Society of .

History and terminology
The word "protozoa" (singular protozoon or protozoan) was coined in 1818 by zoologist Georg August Goldfuss, as the Greek equivalent of the German Urthiere, meaning "primitive, or original animals" ( ‘proto-’ + ‘animal’). Goldfuss erected Protozoa as a class containing what he believed to be the simplest animals. Originally, the group included not only , but also some "lower" multicellular animals, such as , , , , and .

In 1848, in light of advancements in pioneered by and Matthias Schleiden, the anatomist and zoologist C.T. von Siebold proposed that the bodies of microbial organisms such as and were made up of single cells, similar to those from which the multicellular tissues of plants and animals were constructed. Von Siebold redefined Protozoa to include only such unicellular forms, to the exclusion of all metazoa. At the same time, he raised the group to the level of a containing two broad classes of microbes: (mostly ciliates and flagellated ), and Rhizopoda (). The definition of Protozoa as a phylum or sub-kingdom made up of "unicellular animals" was adopted by the zoologist Otto Bütschli—celebrated at his centenary as the "architect of protozoology"—and the term came into wide use.

As a phylum under Animalia, the Protozoa were firmly rooted in the old "two-kingdom" classification of life, according to which all living beings were classified as either animals or plants. As long as this scheme remained dominant, the protozoa were understood to be animals and studied in departments of Zoology, while photosynthetic microbes and microscopic fungi—the so-called Protophyta—were assigned to the Plants, and studied in departments of Botany.

Criticism of this system began in the latter half of the 19th century, with the realization that many organisms met the criteria for inclusion among both plants and animals. For example, the algae and have for , but can also feed on organic matter and are . In 1860, John Hogg argued against the use of "protozoa", on the grounds that "naturalists are divided in opinion—and probably some will ever continue so—whether many of these organisms, or living beings, are animals or plants." As an alternative, he proposed a new kingdom called Primigenum, consisting of both the protozoa and unicellular algae (protophyta), which he combined together under the name "Protoctista". In Hoggs's conception, the animal and plant kingdoms were likened to two great "pyramids" blending at their bases in the Kingdom Primigenum.

Six years later, also proposed a third kingdom of life, which he named . 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.

Despite these proposals, Protozoa emerged as the preferred taxonomic placement for heterotrophic microbes such as amoebae and ciliates, and remained so for more than a century. In the course of the 20th century, however, the old "two kingdom" system began to weaken, with the growing awareness that fungi did not belong among the plants, and that most of the unicellular protozoa were no more closely related to the animals than they were to the plants. By mid-century, some biologists, such as , Robert H. Whittaker and , advocated the revival of Haeckel's Protista or Hogg's Protoctista as a kingdom-level eukaryotic group, alongside Plants, Animals and Fungi. A variety of multi-kingdom systems were proposed, and Kingdoms Protista and Protoctista became well established in biology texts and curricula.

(1974). 9781461569466, Springer US. .

While many taxonomists have abandoned Protozoa as a high-level group, Thomas Cavalier-Smith has retained it as a kingdom in the various classifications he has proposed. As of 2015, Cavalier-Smith's Protozoa excludes several major groups of organisms traditionally placed among the protozoa, including the ciliates, and (all members of the ). In its current form, his kingdom Protozoa is a group which includes a common ancestor and most of its descendents, but excludes two important clades that branch within it: the animals and fungi.

Protozoa, as traditionally defined, are mainly microscopic organisms, ranging in size from 10 to 52 . Some, however, are significantly larger. Among the largest are the deep-sea–dwelling , single-celled foraminifera whose shells can reach 20 cm in diameter. Free-living forms are restricted to moist environments, such as soils, mosses and aquatic habitats, although many form which enable them to survive drying. Many protozoan species are , some are , and some are predators of bacteria, and other protists.

Motility and feeding
Organisms traditionally classified as protozoa are abundant in environments and , occupying a range of . The group includes (which move with the help of whip-like structures called ), (which move by using hair-like structures called ) and (which move by the use of foot-like structures called ). Some protozoa are sessile, and do not move at all.

Protozoa may take in food by , absorbing nutrients through their ; or they may feed by , either by engulfing particles of food with pseudopodia (as amoebae do), or taking in food through a mouth-like aperture called a . All protozoa digest their food in stomach-like compartments called .

The pellicle is a thin layer supporting the in various protozoa, such as ciliates, protecting them and allowing them to retain their shape, especially during locomotion, allowing the organism to be more . The pellicle varies from flexible and elastic to rigid. Although somewhat stiff, the pellicle is also flexible and allows the to fit into tighter spaces. In and , it is formed from closely packed vesicles called alveoli. In , it is formed from strips arranged spirally along the length of the body. Familiar examples of protists with a pellicle are the and the ciliate . In some protozoa, the pellicle hosts bacteria that adhere to the surface by their fimbriae (attachment pili). Protozoa in biological research

Life cycle
Some protozoa have life phases alternating between proliferative stages (e.g., ) and dormant . As cysts, protozoa can survive harsh conditions, such as exposure to extreme temperatures or harmful chemicals, or long periods without access to nutrients, water, or oxygen for periods of time. Being a cyst enables parasitic species to survive outside of a host, and allows their transmission from one host to another. When protozoa are in the form of (Greek tropho = to nourish), they actively feed. The conversion of a trophozoite to cyst form is known as encystation, while the process of transforming back into a trophozoite is known as excystation. Protozoa reproduce asexually by or multiple fission. Many protozoan species exchange genetic material by sexual means (typically, through ); however, sexuality is generally decoupled from the process of reproduction, and does not immediately result in increased population.

Although meiotic sex is widespread among present day , it has, until recently, been unclear whether or not eukaryotes were sexual early in their evolution. Due to recent advances in detection and other techniques, evidence has been found for some form of meiotic sex in an increasing number of protozoans of ancient lineage that diverged early in eukaryotic evolution.Bernstein H, Bernstein C (2013). Bernstein C, Bernstein H eds. Evolutionary Origin and Adaptive Function of Meiosis'. Meiosis. InTech. ISBN 978-953-51-1197-9 (See eukaryote reproduction.) Thus, such findings suggest that meiotic sex arose early in eukaryotic evolution. Examples of protozoan meiotic sexuality are described in the articles , , , Plasmodium falciparum biology , , Toxoplasma gondii , Trichomonas vaginalis and Trypanosoma brucei''.

The classification of protozoa has been and remains a problematic area of taxonomy. Where they are available, DNA sequences are used as the basis for classification; however, for the majority of described protozoa, such material is not available. Protozoa have been and still are organized mostly on the basis of their morphology, means of locomotion, and for the parasitic species their hosts. As a phylum the Protozoa were, historically, divided into four subphyla reflecting the means of locomotion:

These systems are no longer considered to be valid.

Ecological role
As components of the micro- and meiofauna, protozoa are an important food source for microinvertebrates. Thus, the ecological role of protozoa in the transfer of bacterial and algal production to successive is important. As predators, they prey upon or filamentous algae, , and . Protozoan species include both and in the decomposer link of the . They also control bacteria populations and to some extent. On average, protozoa eat ~ 100 to 1,000 bacteria per hour. Protozoa can stimulate decomposition of organic matter, digest in the of cows and termite guts, and can play a role in nutrient mobilization.


In humans

A number of protozoan are , causing diseases such as (by ), , , , cryptosporidiosis, , , , African trypanosomiasis (sleeping sickness), amoebic dysentery, acanthamoeba keratitis, and primary amoebic meningoencephalitis (naegleriasis).

In other animals
The protozoan Ophryocystis elektroscirrha is a parasite of butterfly , passed from female to caterpillar. Severely infected individuals are weak, unable to expand their wings, or unable to , and have shortened lifespans, but parasite levels vary in populations. Infection creates a effect, whereby infected migrating animals are less likely to complete the migration. This results in populations with lower parasite loads at the end of the migration. This is not the case in laboratory or commercial rearing, where after a few generations, all individuals can be infected.

  • Dogiel, V. A., revised by J.I. Poljanskij and E. M. Chejsin. General Protozoology, 2nd ed., Oxford University Press, 1965.
  • Hausmann, K., N. Hulsmann. Protozoology. Thieme Verlag; New York, 1996.
  • Kudo, R.R. Protozoology. Springfield, Illinois: C.C. Thomas, 1954; 4th ed.
  • Manwell, R.D. Introduction to Protozoology, second revised edition, Dover Publications Inc., New York, 1968.
  • Roger Anderson, O. Comparative protozoology: ecology, physiology, life history. Berlin etc.: Springer-Verlag, 1988.
  • Sleigh, M. The Biology of Protozoa. E. Arnold: London, 1981.

  • Jahn,T.L.- Bovee, E.C. & Jahn, F.F. How to Know the Protozoa. Wm. C. Brown Publishers, Div. of McGraw Hill, Dubuque, Iowa, 1979; 2nd ed.
  • Lee, J.J., Leedale, G.F. & Bradbury, P. An Illustrated Guide to the Protozoa. Lawrence, Kansas, U.S.A: Society of Protozoologists, 2000; 2nd ed.
  • Patterson, D.J. Free-Living Freshwater Protozoa. A Colour Guide. Manson Publishing; London, 1996.
  • Patterson, D.J., M.A. Burford. A Guide to the Protozoa of Marine Aquaculture Ponds. CSIRO Publishing, 2001.

  • Harrison, F.W., Corliss, J.O. (ed.). 1991. Microscopic Anatomy of Invertebrates, vol. 1, Protozoa. New York: Wiley-Liss, 512 pp.
  • Pitelka, D. R. 1963. Electron-Microscopic Structure of Protozoa. Pergamon Press, Oxford.

Physiology and biochemistry
  • Nisbet, B. 1984. Nutrition and feeding strategies in Protozoa. Croom Helm Publ., London, 280 pp.
  • Coombs, G.H. & North, M. 1991. Biochemical protozoology. Taylor & Francis, London, Washington.
  • Laybourn-Parry J. 1984. A Functional Biology of Free-Living Protozoa. Berkeley, California: University of California Press.
  • Levandowski, M., S.H. Hutner (eds). 1979. Biochemistry and physiology of protozoa. Volumes 1, 2, and 3. Academic Press: New York, NY; 2nd ed.
  • Sukhareva-Buell, N.N. 2003. Biologically active substances of protozoa. Dordrecht: Kluwer.

  • Capriulo, G.M. (ed.). 1990. Ecology of Marine Protozoa. Oxford Univ. Press, New York.
  • Darbyshire, J.F. (ed.). 1994. Soil Protozoa. CAB International: Wallingford, U.K. 2009 pp.
  • Laybourn-Parry, J. 1992. Protozoan plankton ecology. Chapman & Hall, New York. 213 pp.
  • Fenchel, T. 1987. Ecology of protozoan: The biology of free-living phagotrophic protists. Springer-Verlag, Berlin. 197 pp.

  • Kreier, J.P. (ed.). 1991-1995. Parasitic Protozoa, 2nd ed. 10 vols (1-3 coedited by Baker, J.R.). Academic Press, San Diego, California, [4].

Lee, J. J., & Soldo, A. T. (1992). Protocols in protozoology. Kansas, USA: Society of Protozoologists, Lawrence, [5].

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