THE GILL SACS. 



265 



the true respiratory appendages, the non-respiratory part is often rudimentary or 

 absent; and 3, that in the higher forms of arachnids, the respiratory part of the 

 appendage is deeply infolded to form the walls of a gill sac. 



We conclude from the above facts that approximately the same group of 



Fig. 168. — Amblystoma larv^, showing the vestigeal cephalic appendages in the form of external gills, or the 



"balancers" of the oral arches. 



Fig. 169. — Amblystoma tadpole, showing the balancers," c.a.p., at the height of their development. 



appendages, which in the arachnids have become partly or wholly infolded to 

 form the respiratory sacs, have retained that function in primitive vertebrates, and 

 have given rise to the ectodermic portion of the visceral clefts, or branchial cham- 

 bers. We also conclude that no vertebrate, however primitive, ever possessed 



