[an.nu.lus] n, pl -li also -lus.es [ML, fr. L anulus finger ring, fr. anus ring--more at anus] (1563) 1: ring
2: a part, structure, or marking resembling a ring: as a: a line of cells around a fern sporangium that ruptures the sporangium by contracting b: a growth ring (as on the scale of a fish) that is used in estimating age