Anatomy A membranous structure in a hollow organ or passage, as in an artery or vein, that folds or closes to prevent the return flow of the body fluid passing through it. Any of various devices that regulate the flow of gases, liquids, or loose materials through piping or through apertures by opening, closing, or obstructing ports or passageways. The movable control element of such a device. Music A device in a brass wind instrument that permits change in pitch by a rapid varying of the air column in a tube. Biology One of the paired, hinged shells of certain mollusks and of brachiopods. Biology One of the two silicified halves of the cell wall of a diatom. Biology The entire, one-piece shell of a snail and certain other mollusks. Botany One of the sections into which the wall of a seedpod or other dehiscent fruit splits. Botany A lidlike covering of an anther. Chiefly British An electron tube or a vacuum tube. Archaic Either half of a double or folding door. To provide with a valve. To control by means of a valve.