The Collaborative International Dictionary of English v.0.48:

Animalculum \An`i*mal"cu*lum\, n.; pl. Animalcula. [NL. See
   Animalcule.]
   An animalcule.
   [1913 Webster]

   Note: Animalcul[ae], as if from a Latin singular animalcula,
         is a barbarism.
         [1913 Webster]

WordNet (r) 3.0 (2006):

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