They correspond to the gill-slits of fishes and tadpoles.
"The Outline of Science, Vol. 1 (of 4)" by J. Arthur Thomson
The first gill-slit, or, as Rathke here prefers to call it, pharyngeal slit, closes completely in snakes and in Urodeles.
"Form and Function" by E. S. (Edward Stuart) Russell
Behind the head, and in front of the fore fin (pectoral fin), are five gill slits (g.s.
"Text Book of Biology, Part 1: Vertebrata" by H. G. Wells
Gill-slits, 146, 147, 150-153.
"Darwin, and After Darwin (Vol. 1 and 3, of 3)" by George John Romanes
Jocelyn watched her out of sight, then turning the pickerel over, he slit the firm, white belly from vent to gill.
"A Young Man in a Hurry" by Robert W. Chambers
There are, therefore, as many gill-pouches as there are gill-slits and as many gill-pores as pouches.
"Encyclopaedia Britannica, 11th Edition, Volume 3, Part 1, Slice 2" by Various
The most significant of these is that in which the embryo is closely assimilated to the fish, by the possession of gill slits.
"Man And His Ancestor" by Charles Morris
The majority of them had two fins just behind the gill slits, typical fish tails and blunt, sloping heads.
"Astounding Stories of Super-Science September 1930" by Various
I could see even the hard, bony cover that protected these slits, and I realized that these were gills!
"Astounding Stories of Super-Science, May, 1930" by Various
Their gill-openings are reduced to a short slit.
"Encyclopaedia Britannica, 11th Edition, Volume 5, Slice 5" by Various
How does the number of gills and gill slits change in the series?
"A Guide for the Study of Animals" by Worrallo Whitney
The mouth cavity represents two gill-slits united in the front line.
"Degeneracy" by Eugene S. Talbot
They are situated on each side of the pharynx, and they correspond to the gill slits in lower vertebrates.
"Embryology" by Gerald R. Leighton
Some day I'll slit you open, tail to gills, see if I don't!
"Hopalong Cassidy" by Clarence E. Mulford
The typical functional gill cleft forms a vertical slit, having on each side a gill septum which separates it from its neighbours in the series.
"Encyclopaedia Britannica, 11th Edition, Volume 14, Slice 3" by Various
Note the seven gill slits and the lappetlike paired fins.
"The Elements of Geology" by William Harmon Norton
The external gills project from all the gill-slits including the spiracle.
"The Works of Francis Maitland Balfour, Volume 1" by Francis Maitland Balfour