Source : Webster's Revised Unabridged Dictionary (1913)

Animalcule \An`i*mal"cule\, n. [As if fr. a L. animalculum, dim.
   of animal.]
   1. A small animal, as a fly, spider, etc. [Obs.] --Ray.

   2. (Zo["o]l.) An animal, invisible, or nearly so, to the
      naked eye. See {Infusoria}.

   Note: Many of the so-called animalcules have been shown to be
         plants, having locomotive powers something like those
         of animals. Among these are {Volvox}, the
         {Desmidiac[ae]}, and the siliceous {Diatomace[ae]}.

   {Spermatic animalcules}. See {Spermatozoa}.