noun

• (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.
  "[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. …"
• (zoology, archaic) A microscopic aquatic animal, including protozoa and rotifers.
  "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."
• (obsolete) A small animal.
  "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: […]"