Plural form of animalcule; tiny animals or microorganisms visible only under a microscope, especially single-celled organisms like protozoans.
From Latin 'animula' (diminutive of 'anima,' little animal or soul) + diminutive suffix '-cula', literally meaning 'tiny little animals' that were invisible until microscopes were invented.
In the 1600s-1700s, when scientists first looked through microscopes, they were astonished by 'animalcula'—tiny beings swimming in pond water—and it sparked the birth of microbiology!
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