||Pro`to*zo"a (?), n. pl. [NL., fr. Gr. &?; first + &?; an animal.] (Zoöl.) The lowest of the grand divisions of the animal kingdom.

&fist; 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 Vorticellæ. 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, Gregarinæ, and Infusoria. See also Foraminifera, Heliozoa, Protoplasta, Radiolaria, Flagellata, Ciliata.

• (zoology) Animal-like (i.e., motile and heterotrophic) eukaryotic unicellular organisms.