Merriam-Webster Collegiate Dictionary - gill slit

 noun  Date: 1854  1. any of the openings or clefts between the gill arches in vertebrates that breathe by gills through which water taken in at the mouth passes to the exterior and so bathes the gills  2. any of the rudimentary grooves in the neck region of the embryos of air-breathing vertebrates that correspond to the ~s  3. the external opening to the cavity containing the gills when a protective covering of the gills is present