Animalcule ("little animal", from Latin animal + the diminutive suffix -culum) is an older term for a microscopic animal or protozoan. Some better-known animalcules include:
The term was also used by Anton van Leeuwenhoek, the 17th-century preformationist and discoverer of microorganisms, to describe spermatozoa.
The word appears in the Major-General's Song, in which Major-General Stanley sings, "I know the scientific names of beings animalculous..."