What does "animalcule" mean?

Animalcule: (Zoöl.) 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 Desmidiacæ, and the siliceous Diatomaceæ. Spermatic animalcules. See Spermatozoa.