‖n. pl. [ NL.; -- so called because found in infusions which are left exposed to the air for a time. See Infuse. ] (Zool.) One of the classes of Protozoa, including a large number of species, all of minute size. Formerly, the term was applied to any microbe found in infusions of decaying organic material, but the term is now applied more specifically to one of the classes of the phylum Ciliophora, of ciliated protozoans. [ 1913 Webster +PJC ]

☞ (From 1913 dictionary): 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. [ 1913 Webster ]

a. (Zool.) Belonging to the Infusoria; composed of, or containing, Infusoria; as, infusorial earth. [ 1913 Webster ]

Infusorial earth (Geol.), a deposit of fine, usually white, siliceous material, composed mainly of the shells of the microscopic plants called diatoms; also called diatomaceous earth, kieselguhr, and diatomite. It is used in polishing powder, and in the manufacture of dynamite. [ 1913 Webster +PJC ]

n. (Zool.) One of the Infusoria. [ 1913 Webster ]