gill slit

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun One of several narrow external openings connecting with the pharynx, characteristic of sharks and related fishes, through which water passes to the exterior, thereby bathing the gills.
• noun Embryology One of several rudimentary invaginations in the surface of the embryo, present during development of all air-breathing vertebrates and corresponding to the functional gill slits of aquatic species.

from The Century Dictionary.

• noun A visceral cleft between any two visceral arches of the neck; a passage-way through gill-arches from the mouth or pharynx to the exterior; a branchial cleft.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun anatomy, fish An individual opening to gills, which lacks an operculum; characteristic of Cartilaginous fish such as sharks.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun one of a series of slit openings in the pharynxes of fishes and aquatic amphibians through which water passes