diatom (n.)

"microscopic unicellular algae," 1845, from Modern Latin genus name Diatoma, coined from Greek diatomos "cut in two," from diatemnein "to cut through," from dia "through" (see dia-) + temnein "to cut" (from PIE root *tem- "to cut"). So called because they typically appear to have been cut in half. Related: Diatomic; diatomaceous.