PROTOZOA in which the organism is provided with cilia, flagella, or vibratile membranes as organs of locomotion and ingestion of food, or for one or the other purpose/ Contour of the body stable, sometimes amoeboid, and then for the most part only in the last stage of the life-history. The cilia, flagella, and vibratile membranes are stable structures but retractile under certain conditions.

There are three classes, the Acinetaria, Infusoria s. Ciliata, and the Mastigophora.