The Collaborative International Dictionary of English v.0.48:

Volvox \Vol"vox\, n. (Bot.)
   A genus of minute, pale-green, globular, organisms, about one
   fiftieth of an inch in diameter, found rolling through water,
   the motion being produced by minute colorless cilia. It has
   been considered as belonging to the flagellate Infusoria, but
   is now referred to the vegetable kingdom, and each globule is
   considered a colony of many individuals. The commonest
   species is Volvox globator, often called globe
   animalcule.
   [1913 Webster]

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):

Volvox
    n 1: type genus of the Volvocaceae; minute pale green
         flagellates occurring in tiny spherical colonies; minute
         flagella rotate the colony about an axis [syn: Volvox,
         genus Volvox]