The Collaborative International Dictionary of English v.0.48:

Infusoria \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.] (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]

   Note: (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]

The Collaborative International Dictionary of English v.0.48:

Molluscoidea, including Brachiopoda and Bryozoa. Mollusca,
including Cephalopoda, Gastropoda, Pteropoda, Scaphopoda,
Lamellibranchiata or Acephala. Echinodermata, including
Holothurioidea, Echinoidea, Asterioidea, Ophiuroidea, and
Crinoidea. C[oe]lenterata, including Anthozoa or Polyps,
Ctenophora, and Hydrozoa or Acalephs. Spongiozoa or
Porifera, including the sponges.
Protozoa, including Infusoria and Rhizopoda. For
definitions, see these names in the Vocabulary.
[1913 Webster] Animalcular

WordNet (r) 3.0 (2006):

Infusoria
    n 1: in some recent classifications, coextensive with the
         Ciliata: minute organisms found in decomposing infusions of
         organic matter [syn: Infusoria, subclass Infusoria]