animalcule

1. (physiology, historical) A sperm cell or spermatozoon; also, the embryo that was formerly thought to be contained inside a spermatozoon in a fully developed state.Examples: "[Antonie van] Leeuwenhoek's most mysterious finding was yet to come, however. Inside the animalcules in the thickest part of the semen he saw / all manner of great and small vessels, so various and so numerous that I do not doubt that they be nerves, arteries and veins. …"historical

2. (zoology, archaic) A microscopic aquatic animal, including protozoa and rotifers.Examples: "If we are part of nature, then we are synonymous with it at the metaphysical level, every bit as much as the first all-but-inorganic animalcules that ever formed a chain of themselves in the blow hole of a primordial sea vent."archaic

3. (obsolete) A small animal.Examples: "Why, there is not a Man, or a Thing, now alive but has tools. The basest of created animalcules, the Spider itself, has a spinning-jenny, and warping-mill, and power-loom within its head: […]"; "Is it not evident that if a parasitic animalcule desired to call its attention it would sink a hole in its shell and so stimulate its sensory apparatus?"obsolete