308 



SYSTEMATIC ZOOLOGY. 



in the septa between the gill-slits are the gill-arches or 

 cartilages (p. 294); and from the septa there grow out, 

 in the larval Amphibia, fleshy fringes, the external gills. 



FIG. 115. Relations of gills, gill-openings, etc., in a shark (left) and a teleost 



(right). 



In most Amphibia these external gills are later absorbed 

 and replaced by internal gills, which in turn may disap- 

 pear upon the assumption of an aerial respiration. 



The number of these clefts varies between four and 

 eight (more in some cyclostomes), but in all gnathostomes 

 the anterior cleft has largely lost its respiratory function. 

 In the sharks it becomes modified into the spiracular 

 cleft; in the higher vertebrates it enters into the struc- 

 ture of the ear, giving rise to the cavity of the drum and 

 to the Eustachian tube (p. 303). 



In the sharks (fig. 115) each cleft opens separately to 

 the exterior; but in ganoids and teleosts the hyoid sep- 

 tum gives rise to a fold (operculum) or gill-cover, which 

 grows back over the external openings, so that there is 



