animalcule

The term "animalcule" refers to a minute, usually microscopic organism. This term was first used in 1662.1

The development of the microscope in the 17th century led to the discovery of these previously unseen life forms. Antonie van Leeuwenhoek, a Dutch microscopist, is credited with observing various protozoans in stagnant water in 1675, which he termed animalcules. He also identified different shapes of bacteria.2

A specific type of animalcule is the "sun animalcule," which is a protozoan belonging to the order Heliozoa, notably Actinophrys sol, a freshwater organism with radiating axopodia. The discovery of animalcules challenged the long-held theory of spontaneous generation, suggesting that life could originate from nonliving matter.32