genus of fresh-water algae, 1798, from Latin volvere "to roll," from PIE root *wel- (3) "to turn, revolve," with derivatives referring to curved, enclosing objects (cognates: Sanskrit valate "turns round," ulvam "womb, vulva;" Lithuanian valtis "twine, net," vilnis "wave," apvalus "round;" Old Church Slavonic valiti "roll, welter," vl?na "wave;" Greek eluo "wind, wrap," helix "spiral object," eilein "to turn, squeeze;" Gothic walwjan "to roll;" Old English wealwian "roll," weoloc "whelk, spiral-shelled mollusk;" Old High German walzan "to roll, waltz;" Old Irish fulumain "rolling;" Welsh olwyn "wheel"). So called from their motion.