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Protozoa

Protozoa

Protozoa are a very diverse group of single-celled organisms, with more than 50,000 different types represented. The vast majority are microscopic, many measuring less than 1/200 mm, but some, such as the freshwater Spirostomun, may reach 0.17 in (3 mm) in length, large enough to enable it to be seen with the naked eye.

Scientists have discovered fossilized specimen of protozoa that measured 0.78 in (20 mm) in diameter. Whatever the size, however, protozoans are well-known for their diversity and the fact that they have evolved under so many different conditions.

One of the basic requirements of all protozoans is the presence of water, but within this limitation, they may live in the sea, in rivers, lakes, stagnant ponds of freshwater, soil, and in some decaying matters. Many are solitary organisms, but some live in colonies; some are free-living, others are sessile; and some species are even parasites of plants and animals (including humans). Many protozoans form complex, exquisite shapes and their beauty is often greatly overlooked on account of their diminutive size.

The protozoan cell body is often bounded by a thin pliable membrane, although some sessile forms may have a toughened outer layer formed of cellulose, or even distinct shells formed from a mixture of materials. All the processes of life take place within this cell wall. The inside of the membrane is filled with a fluid-like material called cytoplasm , in which a number of tiny organs float. The most important of these is the nucleus , which is essential for growth and reproduction. Also present are one or more contractile vacuoles, which resemble air bubbles, whose job it is to maintain the correct water balance of the cytoplasm and also to assist with food assimilation.

Protozoans living in salt water do not require contractile vacuoles as the concentration of salts in the cytoplasm is similar to that of seawater and there is therefore no net loss or gain of fluids. Food vacuoles develop whenever food is ingested and shrink as digestion progresses. If too much water enters the cell, these vacuoles swell, move towards the edge of the cell wall and release the water through a tiny pore in the membrane.

Some protozoans contain the green pigment chlorophyll more commonly associated with higher plants, and are able to manufacture their own foodstuffs in a similar manner to plants. Others feed by engulfing small particles of plant or animal matter. To assist with capturing prey, many protozoans have developed an ability to move. Some, such as Euglena and Trypanosoma are equipped with a single whip like flagella which, when quickly moved back and forth, pushes the body through the surrounding water body. Other protozoans (e.g., Paramecium) have developed large numbers of tiny cilia around the membrane; the rhythmic beat of these hairlike structures propel the cell along and also carry food, such as bacteria , towards the gullet. Still others are capable of changing the shape of their cell wall. The Amoeba, for example, is capable of detecting chemicals given off by potential food particles such as diatoms , algae, bacteria or other protozoa. As the cell wall has no definite shape, the cytoplasm can extrude to form pseudopodia (Greek pseudes, "false"; pous, "foot") in various sizes and at any point of the cell surface. As the Amoeba approaches its prey, two pseudopodia extend out from the main cell and encircle and engulf the food, which is then slowly digested.

Various forms of reproduction have evolved in this group, one of the simplest involves a splitting of the cell in a process known as binary fission. In species like amoeba, this process takes place over a period of about one hour: the nucleus divides and the two sections drift apart to opposite ends of the cell. The cytoplasm also begins to divide and the cell changes shape to a dumb-bell appearance. Eventually the cell splits giving rise to two identical "daughter" cells that then resume moving and feeding. They, in turn, can divide further in this process known as asexual reproduction, where only one individual is involved.

Some species that normally reproduce asexually, may occasionally reproduce through sexual means, which involves the joining, or fusion, of the nuclei from two different cells. In the case of paramecium , each individual has two nuclei: a larger macronucleus that is responsible for growth, and a much smaller micronucleus that controls reproduction. When paramecium reproduce by sexual means, two individuals join in the region of the oral groovea shallow groove in the cell membrane that opens to the outside. When this has taken place, the macronuclei of each begins to disintegrate, while the micronucleus divides in four. Three of these then degenerate and the remaining nucleus divides once again to produce two micronuclei that are genetically identical. The two cells then exchange one of these nuclei that, upon reaching the other individual's micronucleus, fuse to form what is known as a zygote nucleus. Shortly afterwards, the two cells separate but within each cell a number of other cellular and cytoplasmic divisions will continue to take place, eventually resulting in the production of four daughter cells from each individual.

Protozoans have evolved to live under a great range of environmental conditions. When these conditions are unfavorable, such as when food is scarce, most species are able to enter an inactive phase, where cells become non-motile and secrete a surrounding cyst that prevents desiccation and protects the cell from extreme temperatures. The cysts may also serve as a useful means of dispersal, with cells being borne on the wind or on the feet of animals. Once the cyst reaches a more favorable situation, the outer wall breaks down and the cell resumes normal activity.

Many species are of considerable interest to scientists, not least because of the medical problems that many cause. The tiny Plasmodium protozoan, the cause of malaria in humans, is responsible for hundreds of millions of cases of illness each year, with many deaths occurring in poor countries. This parasite is transferred from a malarial patient to a healthy person by the bite of female mosquitoes of the genus Anopheles. As the mosquito feeds on a victim's blood the parasites pass from its salivary glands into the open wound. From there, they make their way to the liver where they multiply and later enter directly into red blood cells. Here they multiply even further, eventually causing the blood cell to burst and release from 6-36 infectious bodies into the blood plasma. A mosquito feeding on such a patient's blood may absorb some of these organisms, allowing the parasite to complete its life cycle and begin the process all over again. The shock of the release of so many parasites into the human blood stream results in a series of chills and feverstypical symptoms of malaria. Acute cases of malaria may continue for some days or even weeks, and may subside if the body is able to develop immunity to the disease. Relapses, however, are common and malaria is still a major cause of death in the tropics. Although certain drugs have been developed to protect people from Plasmodium many forms of malaria have now developed, some of which are even immune to the strongest medicines.

While malaria is one of the best known diseases known to be caused by protozoans, a wide range of other equally devastating ailments are also caused by protozoan infections. Amoebic dysentery , for example, is caused by Entamoeba histolytica.; African sleeping sickness , which is spread by the bite of the tsetse fly, is caused by the flagellate protozoan Trypanosoma; a related species T. cruzi causes Chagas' disease in South and Central America; Eimeria causes coccidiosis in rabbits and poultry; and Babesia, spread by ticks, causes red water fever in cattle.

Not all protozoans are parasites however, although this is by far a more specialized life style than that adopted by free-living forms. Several protozoans form a unique, nondestructive, relationship with other species, such as those found in the intestine of wood-eating termites. Living in the termites' intestines the protozoans are provided with free board and lodgings as they ingest the wood fibers for their own nutrition. In the process of doing so, they also release proteins which can be absorbed by the termite's digestive system, which is otherwise unable to break down the tough cellulose walls of the wood fibers. Through this mutualistic relationship, the termites benefit from a nutritional source that they could otherwise not digest, while the protozoans receive a safe home and steady supply of food.

See also Amoebic dysentery; Entamoeba histolytica; Epidemiology, tracking diseases with technology; Waste water treatment; Water quality

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Protozoa

Protozoa

Protozoa (meaning "first animals") are heterotrophic, single-celled or colonial eukaryotes. Individuals are microscopic and range in size from a few to hundreds of micrometers, depending on the species. Most protozoa are animal-like (heterotrophic) because their carbon and energy must be obtained by eating or absorbing organic compounds originating from other living organisms. As eukaryotes they have several organelles , including at least one nucleus that contains most of the cell's deoxyribonucleic acid (DNA).

Beyond this broad description, it is difficult to define protozoa because they are so diverse and only distantly related to each other. While the term "protozoa" is commonly used, it has little basis in evolutionary history, or phylogeny, of these organisms. Taxonomic systems try to assign organisms to a monophyletic group, that is, one that includes an ancestor and all of its descendants. Plants, animals, and fungi are monophyletic groups; protozoans are not. (The understanding of evolutionary relationships of uni-cellular eukaryotes is in a state of flux.) Further complicating a precise definition of protozoa is the close relationship between some protozoa and unicellular algae. Modern taxonomic treatments recognize these similarities and group protozoa, photosynthetic unicellular algae, and slime molds together as protists or protoctists. Whichever term one prefers, the classification is not monophyletic. Despite the fact that protozoa is not a proper taxonomic name, it is a useful, functional term. Ecologists differentiate between autotrophic and heterotrophic components of an ecosystem, and it is natural to separate the animal-like protozoa from the photosynthetic algae based on their nutritional mode. (However, Euglena, which can be induced to lose their chloroplast, illustrate why unicellular algae are included with protozoa.)

As is appropriate for heterotrophic organisms that capture food, most protozoa are motile (able to move). The way they move is one of the important characteristics historically used to divide them into major groups: amoebae, flagellates, and ciliates. Apicomplexa, formerly called Sporazoa, is a fourth group of generally obligate parasitic protozoa. Amoebae crawl along surfaces by extending a cytoplasm -filled pseudopod (false-foot) that bulges outward from any edge of the cell. Flagellates and ciliates use specialized organelles, flagella and cilia, that differ primarily in length and number, to propel the cells through water. Flagella are whiplike structures that usually occur one to a few per cell and have an undulating motion. Cilia are shorter and move in concert, like oars, with alternating power and recovery strokes. Sporozoa are either nonmotile or very slow.

Other organelles that are widely distributed among protozoa include food vacuoles, in which ingested particles are digested, and lysosomes that fuse with food vacuoles and supply digestive enzymes. Contractile vacuoles, common in freshwater protozoa, eliminate water that moves into the cells by osmosis . Extrusomes are associated with the membrane of many protozoa and contain material that can be ejected from the cell. Some extrusomes secrete an amorphous material that is involved in formation of a capsule or cyst, and others discharge a pointed projectile that may serve for protection or predation. The thousands of "trichocysts" distributed over the surface of the ciliate Paramecium are extrusomes that discharge rapidly in response to physical stimulation and are probably effective deterrents to some predators. Ciliates are unique among protozoa in having two kinds of nuclei: the micronucleus, which is involved only in sexual reproduction; and the macronucleus, which is involved only in the production of messenger ribonucleic acid (RNA) for cell function.

Most protozoa reproduce most of the time by equal binary fission, in which a cell divides into two daughter cells after the chromosomes have been duplicated and distributed between them. This asexual mode of reproduction leads to rapid population growth of a clone of genetically identical cells. However, sex is widespread in protozoa and complicated life histories do exist. Sexuality is associated with environmental change and interrupts asexual reproduction; sex in protozoa usually marks the end of the existence of a genetically unique individual, when it becomes the gamete (reproductive cell) or gametes.

Protozoa are ubiquitous (found everywhere); they are present in all aquatic or moist environments, and their cysts can be found in even the most inhospitable parts of the biosphere. Most are free-living and eat bacteria, algae, or other protozoa. Protozoa are important components of aquatic and soil ecosystems, where they eat bacteria that are too small to be efficiently captured by most animals and are in turn eaten by other organisms. Bacterivorous protozoa also are abundant in activated sludge sewage treatment plants and, in fact, are necessary for their proper functioning. There are several protozoa of medical and economic importance. Examples include the flagellate Trypanosoma, which causes African sleeping sickness; the amoeba Entamoeba histolytica, which can attack the intestinal wall and cause amoebic dysentery, and the sporozoans of the Plasmodium species, which cause malaria.

Protozoa have many features linking them to the other kingdoms of life. Scientists widely believe that animals evolved from protozoan ancestors, probably colonial choanoflagellates. New tools and methods from molecular biology are leading to a better understanding of the evolutionary relationships to multicellular organisms and among protozoa.

see also Algae; Cell Motility; Cytoskeleton; Lysosomes; Osmoregulation; Plankton; Protista; Protozoan Diseases

Robert W. Sanders

Bibliography

Anderson, O. Roger, and Marvin Druger, eds. Explore the World Using Protozoa. Arlington, VA: National Science Teachers Association and the Society of Protozoologists, 1997.

Lee, John J., Seymour H. Hutner, and Eugene C. Bovee, eds. An Illustrated Guide to the Protozoa, 2nd ed. Lawrence, KS: Society of Protozoologists, 2001.

Patterson, David J. Free-Living Freshwater Protozoa: A Color Guide. New York: John Wiley & Sons, 1996.


EUGLENA

Species in the genus Euglena are photosynthetic members of a diverse group of pigmented and colorless flagellates in the order Euglenida. Both protozoologists and botanists traditionally have studied them. The cylindrical shape of the body is maintained by a flexible pellicle composed of the cell membrane and a layer of protein strips. Euglena have large, bright green chloroplasts and two flagella that arise from within a pocket on the anterior end. Usually only one flagellum is long enough to emerge from the reservoir. Beside the reservoir is a small, red-pigmented spot, the stigma, which is associated with a light-sensing region. Many euglenids have visible rods or rings made of paramylum, which, like starch, is a glucose storage molecule.

Euglena swim with a gyrating motion using their emergent flagellum, which pull the organism forward like a propeller. When not swimming, Euglena often alternately contract and elongate the pellicle, causing a bulge to move from one end of the cell to the other in a characteristic "euglenoid motion." Euglena are most common in organically rich freshwater environments.



AMOEBA

Most amoebae live on surfaces in moist soil or aquatic sediment. They are easily over-looked because they are small, predominantly transparent, and slow moving. Amoebae are characterized by the flow of granular cytoplasm into lobes of cell membrane (pseudopodia) that serve the dual functions of motility and food capture. These protozoa typically lack a fixed external anatomy and are flexible but can be categorized by the shape and number of the pseudopodia. Pseudopodia can occur as multiple rounded or needlelike projections or as a single advancing front. When a food particle is encountered, pseudopodia surround it with a membrane-enclosed sac that pinches off internally to form a food vacuole. In addition to food vacuoles, a contractile vacuole and a single large nucleus or many small nuclei can be distinguished. Some amoebae excrete tests, or shells. The marine foraminiferans harden their shells with calcium carbonate, and fossilized foram shells make up a large proportion of some marine sediments and terrestrial deposits (like the White Cliffs of Dover, England).


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Protozoa

Protozoa

Protozoa are a varied group of single-celled animal-like organisms belonging to the kingdom Protista. More than 50,000 different types of protozoa have been described. Their name comes from two Greek words, protos, or "first," and zoön, or "animal." The vast majority of protozoa are microscopic, many measuring less than 1/200 millimeter. The largest, however, may reach 3 millimeters (0.1 inch) in length, large enough to be seen with the naked eye. Scientists have even discovered some fossil specimens that measured 20 millimeters (0.8 inch) in diameter.

Whatever their size, protozoa are well known for their diversity and the fact that they have evolved under so many different conditions. One of the basic requirements of all protozoa is the presence of water. Within this limitation, they may live in the sea; in rivers, lakes, or stagnant ponds of freshwater; in the soil; and even in decaying matter. Many are solitary organisms, but some live in groups. Some are free-living, while others are attached to other organisms. Some species are parasites of plants and animals, ranging from other protozoa to humans. Many protozoa form complex, exquisite shapes, although their beauty may be overlooked because of their very small size.

Life patterns

The vast majority of protozoa are heterotrophic. That is, they cannot manufacture their own food, but must obtain it by eating other organisms. A few protozoa contain the green pigment chlorophyll, which allows them to make their own food.

All protozoa reproduce asexually, by dividing into two parts at regular intervals. Some species, however, have evolved the ability to reproduce sexually also.

Words to Know

Chlorophyll: A green pigment that makes possible the conversion of carbon dioxide and water to complex carbohydrates.

Cilia: Tiny hairlike projections on the surface of a cell.

Cyst: A stage in a protozoan's life when it is covered by a tough outer shell and has become dormant.

Cytoplasm: The semifluid substance of a cell containing organelles and enclosed by the cell membrane.

Dormant phase: A period in which an organism is inactive.

Flagella: Whiplike structures used by some organisms for movement.

Heterotrophs: Organisms that cannot make their own food and that must, therefore, obtain their food from other organisms.

Parasitism: A situation in which one organism lives in connection with and at the expense of a second organism.

Protists: Members of the Kingdom Protista; primarily single-celled organisms that are not plants or animals.

Pseudopods: Extensions of an organisms cytoplasm used for movement and capturing food.

Symbiosis: A pattern in which two or more organism's live in close connection with each other, often to the benefit of both or all organisms.

Protozoa have evolved mechanisms that allow them to live under a great range of environmental conditions. When these conditions are unfavorable, most species are able to enter an inactive, or dormant, phase. They secrete a thick protective outer wall that prevents them from losing water and protects the cell from extreme temperatures. This tough little package, called a cyst, may also serve as a means of dispersal (to spread widely). Cysts are carried away on the wind or on the feet of animals. Once a cyst reaches a more favorable environment, its outer wall breaks down and the cell resumes normal activity.

Types of protozoa

Protozoa are classified according to the ways in which they move about. One phylum, the Sarcodina, moves by pushing out portions of their cytoplasm forming pseudopods, or "false feet." They capture their food by extending their pseudopods around it, engulfing it, and digesting it. Probably the best known example of the Sarcodina is the amoeba.

Members of the phylum Ciliophora get their name from tiny hair-like projections known as cilia on the surface of the cell. These protozoa swim around by waving their cilia back and forth, like the oars on a boat. Cilia are also used to obtain food. As they beat back and forth, the cilia create a whirlpoollike effect that brings food close enough for the organism to ingest. A common example of the ciliates is the paramecium.

The phylum Mastigophora consists of one-celled organisms that move about by means of flagella. Flagella are whiplike structures somewhat similar to cilia. The major difference between the two structures is that flagella are much larger than cilia. Also, flagellates have anywhere from one to several hundred flagella, while cilia never occur individually. The majority of flagellates live inside other organisms in either a symbiotic (mutually beneficial) or parasitic relationship. A well-known example of the flagellates is the organism that causes African sleeping sickness, the trypanosoma.

Members of the phylum Sporozoa have no means of movement. Instead, they form sporelike structures and attach themselves to other organisms. They are parasitic and depend on their hosts for all their food and survival. The best known of all Sporozoa are probably members of the genus Plasmodium. These organisms are responsible for malaria, a disease that is transmitted by the Anopheles mosquito.

[See also Cell; Parasite; Plankton; Reproduction ]

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protozoa

protozoa A group of unicellular or acellular, usually microscopic, organisms now classified in various phyla of the kingdom Protoctista (see Apicomplexa; Ciliophora; Rhizopoda; Zoomastigota). They were formerly regarded either as a phylum of simple animals or as members of the kingdom Protista. They are very widely distributed in marine, freshwater, and moist terrestrial habitats; most protozoans are saprotrophs, but some are parasites, including the agents causing malaria (Plasmodium) and sleeping sickness (Trypanosoma), and a few contain chlorophyll and carry out photosynthesis, like plants. Protozoan cells may be flexible or rigid, with an outer pellicle or protective test. In some (such as Paramecium and Trypanosoma) undulipodia (cilia or flagella) are present for locomotion; others (such as Amoeba) have pseudopodia for movement and food capture. Contractile vacuoles occur in freshwater protozoans. Reproduction is usually asexual, by binary fission, but some protozoans undergo a form of sexual reproduction (see conjugation).

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Protozoa

Protozoa, formerly, the name of an animal phylum comprising a large, diverse assortment of microscopic or near-microscopic one-celled heterotrophic organisms (protozoans). The term "protozoan" (or the collective plural "protozoa" ) continues to be used informally; the organisms are now more commonly placed in any of five phyla in the kingdom Protista.

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Protozoa

Protozoa (kingdom Protoctista) A phylum (division) of eukaryotic, single-celled micro-organisms which show a wide variety of forms. The phylum includes some plant-like forms and animal species that are free-living, commensal, mutualistic, and parasitic, and some important disease-causing organisms.

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protozoa

protozoa (proh-tŏ-zoh-ă) pl. n. a group of microscopic single-celled organisms. Most protozoa are free-living but some, such as Plasmodium and Leishmania, are important disease-causing parasites of humans. See also amoeba.
protozoan adj., n.

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protozoa

protozoa (sing. protozoon; adj. protozoan) See PROTOZOA.

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Protozoa

Protozoa Former animal phylum of eukaryotic, single-celled micro-organisms. In some classifications the Protozoa are grouped with other simple eukaryotic organisms in the kingdom Protista.

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protozoa

protozoaanoa, Balboa, blower, boa, foregoer, goer, grower, hoer, jerboa, knower, Krakatoa, Lebowa, lower, moa, mower, Mururoa, Noah, o'er, proa, protozoa, rower, Samoa, sewer, Shenandoah, shower, sower, spermatozoa, Stour, thrower, tower •shadower • widower • racegoer •theatregoer (US theatergoer) •churchgoer • echoer •follower, swallower •snowblower • lawnmower • genoa •winnower • harrower • winegrower •borrower • burrower • vetoer

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