Dictionary of Ichthyology  

spiracle = a vestigial gill cleft between the eye and the gill slits in Elasmobranchii, Acipenseridae, Polyodontidae, and Polypteridae. Water for respiration is inhaled through the spiracles in Rajiformes. Connects with the gill cavity; often retains branchial lamellae which are called the spiracular pseudobranch.

Shark skull, a = auditory capsule, b = basibranchial, c = keratobranchial, e = epibranchial, g = gill cleft, h = hyoid, hm = hyomandibula, l = labial cartilages, m = mandible (Meckel's cartilage), o = olfactory capsule, p = pharyngobranchial, pq = pterygoquadrate, r = rostrum, s = spiracle, 2, 5 = exits of second and fifth nerves, I-V = branchial arches., from Kingsley's "Textbook of Vertebrate Zoology, 1906. Photograph by Brian W. Coad.

Canadian Museum of Nature