\In`fu*so"ri*a\, n. pl. [NL.; -- so called because
found in infusions which are left exposed to the air for a
time. See {Infuse}.] (Zo["o]l.)
One of the classes of Protozoa, including a large number of
species, all of minute size.
Note: 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.