a fold of mucous membrane in the large intestine of lampreys and certain fishes that is arranged spirally with as many as 40 turns. The spiral valve increases the absorbing surface of the intestine and slows the movement of food through the intestine, thus causing a short straight intestine to resemble a long convoluted one functionally. A spiral valve characterizes sharks, rays, chimaeras, lungfishes, cartilaginous and bony ganoid fishes, and polypteroids.