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The Collaborative International Dictionary

Protozoa \Pro`to*zo"a\, n. pl. [NL., fr. Gr. ? first + zo^,on an animal.] (Zo["o]l.) The lowest of the grand divisions of the animal kingdom.

Note: The entire animal consists of a single cell which is variously modified; but in many species a number of these simple zooids are united together so as to form a compound body or organism, as in the Foraminifera and Vorticell[ae]. The reproduction takes place by fission, or by the breaking up of the contents of the body after encystment, each portion becoming a distinct animal, or in other ways, but never by true eggs. The principal divisions are Rhizopoda, Gregarin[ae], and Infusoria. See also Foraminifera, Heliozoa, Protoplasta, Radiolaria, Flagellata, Ciliata.


Protozoon \Pro`to*zo"["o]n\ (-[o^]n), n.; pl. Protozoa. [NL.] (Zo["o]l.)

  1. One of the Protozoa.

  2. A single zooid of a compound protozoan.

Douglas Harper's Etymology Dictionary

1828, from Modern Latin Protozoa, coined 1818 by German zoologist Georg August Goldfuss (1782-1848) from Greek protos "first" (see proto-) + zoia, plural of zoion "animal" (see zoo). Originally including sponges and corals; current sense is from 1845. Related: Protozoon (aingular); Protozoan.


n. 1 (plural of protozoan English) 2 (plural of protozoon English)

  1. adj. of or relating to the Protozoa [syn: protozoal, protozoic]

  2. n. any of diverse minute acellular or unicellular organisms usually nonphotosynthetic [syn: protozoon]

  3. [also: protozoa (pl)]


See protozoan


In some systems of biological classification, the Protozoa are a diverse group of unicellular eukaryotic organisms. Historically, protozoa were defined as single-celled organisms with animal-like behaviors, such as motility and predation. The group was regarded as the zoological counterpart to the " protophyta", which were considered to be plant-like, as they are capable of photosynthesis. The terms protozoa and protozoans are also used informally to designate single-celled, non- photosynthetic protists, such as ciliates, amoebae and flagellates.

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: Euglenozoa, Amoebozoa, Metamonada, Choanozoa, Loukozoa, Percolozoa, Microsporidia and Sulcozoa. This kingdom does not form a clade, but an evolutionary grade or paraphyletic group, from which the fungi and animals are specifically excluded.

The use of Protozoa as a formal taxon has been discouraged by some researchers, mainly because the term, which is formed from the Greek protos "first" + zoia, plural of zoion, "animal", misleadingly implies kinship with animals (metazoa) and promotes an arbitrary separation of "animal-like" from "plant-like" organisms. 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 eukaryotic microbes. In 2005, members of the Society of Protozoologists voted to change the name of that organization to the International Society of Protistologists.

Usage examples of "protozoa".

Fabulously early date of evolution, preceding even simplest Archaean protozoa hitherto known, baffles all conjecture as to origin.

In their guts they harbour colonies of microscopic, one-celled protozoa, which break down the cellulose for them and make it digestible.

The largest taxonomic divisions distinguish between plants and animals, or between those organisms with poorly developed nuclei in their cells (such as bacteria and blue-green algae) and those with very clearly demarcated and elaborately architectured nuclei (such as protozoa or people).

Maybe those ciliated protozoa that had a variant genetic code were descended from some cilia who had been in symbiosis with other cells in the past, developing genetic-code variations for the same safety-net reasons mitochondria had but, unlike the cilia we still retained, had subsequently broken off the symbiosis and returned to stand-alone life.

Into this category I would place many of the protozoa in our alimentary canals.

Because they are born without these protozoa in their intestines, they must acquire them by eating each other's faecal matter.