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Animalcule ("little animal", from Latin animal + the diminutive suffix -culum) is an older term for a microscopic animal or protozoan. The word was coined by Antonie van Leeuwenhoek to refer to the microorganisms he observed in rainwater. The concept appears to have been proposed at least as early as around 30 BC, as evidenced by this translation from Marcus Varro's Rerum Rusticarum Libri Tres:
"Note also if there be any swampy ground, both for the reasons given above, and because certain minute animals, invisible to the eye, breed there, and, borne by the air, reach the inside of the body by way of the mouth and nose, and cause diseases which are difficult to be rid of."
Some better-known animalcules include:
The term was also used in the 17th century by Henry Oldenburg, the first Secretary of the Royal Society and founding or of Philosophical Transactions, to translate the Dutch words used by Anton van Leeuwenhoek to describe microorganisms that he discovered.