Animalcule ('little animal', from Latin animal + the diminutive suffix -culum) is an old term for microscopic organisms that included bacteria, protozoans, and very small animals. The word was invented by 17th-century Dutch scientist Antonie van Leeuwenhoek to refer to the microorganisms he observed in rainwater. Some better-known types of animalcule include: 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: A 1795 illustration of van Leeuwenhoek's animalcules by an unknown artist.