(Science: zoology) One of the classes of protozoa, including a large number of species, all of minute size.
They are found in all seas, lakes, ponds, and streams, as well as in infusions of organic matter exposed to the air. They are distinguished by having vibrating lashes or cilia, with which they obtain their food and swim about.They are devided into the orders flagellata, Ciliata, and Tentaculifera. See these words in the vocabulary. Formely the term infusoria was applied to all microscopic organisms found in water, including many minute plants, belonging to the diatoms, as well as minute animals belonging to various classes, as the rotifera, which are worms; and the rhizopoda, which constitute a distinct class of protozoa. Fossil infusoria are mostly the siliceous shells of diatoms; sometimes they are siliceous skeletons of Radiolaria, or the calcareous shells of Foraminifera.
Origin: NL.; so called because found in infusions which are left exposed to the air for a time. See infuse.