Dictionary of Ichthyology  

spiral valve = a spiral or helical fold in the midgut of Petromyzontiformes, Elasmobranchii, Holocephali and some primitive Teleostomi. Functions to increase the surface area for processing food without lengthening the intestine.

Guts of Protopterus from Kingsley's "Textbook of Vertebrate Zoology, 1906. C = caecum, G = gall bladder, HV = hepatic vein, L = liver, O = oesophagus, OD = oviduct, PV = pyloric valve, R = rectum, RD = renal (mesonephric) duct, S = stomach, SP = spleen, SV = spiral valve, V = vent. Photograph by Brian W. Coad.

Canadian Museum of Nature