Protozoa

A single-cell organism that can only divide within a host organism. Malaria is caused by a protozoa: Plasmodium. Other protozoan parasites Giardia and Toxoplasma.

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Formerly considered a phylum, now regarded as a subkingdom of the animal kingdom, including all of the so-called acellular or unicellular forms. They consist of a single functional cell unit or aggregation of nondifferentiated cells, loosely held together and not forming tissues, as distinguishes the Animalia or Metazoa, which include all other animals. P. were formerly divided into four classes: Sarcodina, Mastigophora, Sporozoa, and Ciliata; new classifications employ higher taxa (phyla, subphyla, and superclasses) and a number of major subdivisions. [proto- + G. zoon, animal]

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Pro·to·zoa .prōt-ə-'zō-ə n pl a phylum or subkingdom of chiefly motile unicellular protists (as amebas, trypanosomes, sporozoans, and paramecia) that consist of a protoplasmic body either naked or enclosed in an outer covering, that have holophytic, saprophytic, or holozoic modes of nourishment, that reproduce asexually by nuclear division usu. with a more or less modified mitosis associated with cytoplasmic binary fission or with multiple fission or budding or often sexually by various means, that have the life cycle simple (as in an ameba) or extremely complex (as in many sporozoans), that are represented in almost every kind of habitat, and that include pathogenic parasites of humans and domestic animals compare METAZOA (1)

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pl. n.

a group of microscopic single-celled organisms. Most protozoa are free-living but some are important disease-causing parasites of humans; for example, Plasmodium, Leishmania, and Trypanosoma cause malaria, kala-azar, and sleeping sickness respectively. See also amoeba.

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Pro·to·zoa (pro″to-zoґə) [proto- + Gr. zoon animal] a subkingdom (formerly a phylum) comprising the simplest organisms of the animal kingdom, consisting of unicellular organisms that range in size from submicroscopic to macroscopic; most are free living, but some lead commensalistic, mutualistic, or parasitic existences. According to newer classifications, Protozoa is divided into seven phyla: Sarcomastigophora, Labyrinthomorpha, Apicomplexa, Microspora, Acetospora, Myxozoa, and Ciliophora. Cf. Metazoa.