VERTEBRATA. 



143 



the sides of the pharynx, and then these break through to 

 the exterior, giving rise to gill-slits or clefts, through which 

 water taken in at the mouth can pass out. On the sides of 

 these clefts the gills proper are developed. These are thin- 

 walled leaves or filaments with a rich blood-supply, and 

 through these thin walls there is an exchange of dissolved 

 gases (oxygen and carbon dioxide) between the water and 

 the blood. 



In the septa between the gill-slits are the gill-bars or car- 

 tilages (p. 131) ; and from the septa there grow out, in the 



FIG. 67. Relations of gills, gill-openings, etc., in a shark (left) and a 



teleost (right). 



larval -batrachia, fleshy fringes, the external gills. In most 

 batrachia these external gills are later absorbed and replaced 

 by internal gills, which in turn may disappear upon the 

 assumption of an aerial respiration. 



The number of these clefts varies between four and eight, 

 but in all the anterior cleft has largely lost its respiratory 

 function. In the sharks it becomes modified into the 



