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Animalcule ('little animal', from Latin animal + the diminutive suffix -culum) is an old term for a microscopic Some better-known types of animalcule include: animal or protozoan. The word was invented by Antonie van Leeuwenhoek to refer to the microorganisms he observed in rainwater. The concept seems to have been proposed at least as early as about 30 BC, as evidenced by this translation from Marcus Varro's Rerum Rusticarum Libri Tres:
The term was also used during the 17th century by Henry Oldenburg, the first Secretary of the Royal Society and founding editor of Philosophical Transactions, to translate the Dutch words used by Anton van Leeuwenhoek to describe microorganisms that he discovered.
In Gilbert and Sullivan's The Mikado, 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...' 
A 1795 illustration of van Leeuwenhoek's animalcules by an unknown artist.