Animalcule ("little animal", from Latin animal + the diminutive suffix -culum) is an older term for a microscopic animal or protozoan. The concept appears to have been understood at least as early as c. 30 BCE, as evidenced by the following translation from Marcus Varro's Rerum Rusticarum Libri Tres.
"Like precautions must be taken against swampy places for the same reasons and particularly because as they dry, swamps breed certain animalculae which cannot be seen with the eyes and which we breathe through the nose and mouth into the body where they cause grave maladies."
Some better-known animalcules include:
The term was also used by Anton van Leeuwenhoek, the 17th-century preformationist and the discoverer of microorganisms, to describe them.
The word appears in adjectival form in the Major-General's Song, in which Major-General Stanley sings, "I know the scientific names of beings animalculous..."