The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   2. (Zool.) An animal, invisible, or nearly so, to the naked
      eye. See Infusoria.
      [1913 Webster]

   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].
         [1913 Webster]

   Spermatic animalcules. See Spermatozoa.
      [1913 Webster]

WordNet (r) 3.0 (2006):

animalcule
    n 1: microscopic organism such as an amoeba or paramecium [syn:
         animalcule, animalculum]