Source : Webster's Revised Unabridged Dictionary (1913)

Animalculum \An`i*mal"cu*lum\, n.; pl. {Animalcula}. [NL. See
   {Animalcule}.]
   An animalcule.

   Note: Animalcul[ae], as if from a Latin singular animalcula,
         is a barbarism.

Source : WordNet®

animalculum
     n : microscopic organism such as an amoeba or paramecium [syn: {animalcule}]
     [also: {animalcula} (pl)]