Dictionary of Ichthyology  

gill cleft = gill slit, gill opening, an external opening leading from the gill chamber. Also called gill slit or branchial opening. Elasmobranchii have 5-7 openings or slits on each side of the head while bony fishes have only one. The opening allows water to exit after passing over the gills for respiration. 

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