210 STRUCTURE OF THE VERTEBRATES 



is a simple opening through the body wall, connecting the 

 pharynx with the exterior. In the Chondrostei (page 40) a bony 

 covering, the operculum, shields the gill slits. In this group there 

 is a degeneration of the distal part of the septum, so that the 

 body covering of skin is no longer present in the region. The 

 reduction of the septa proceeds further in the Holostei, and in 

 the Teleostei the septa are reduced to a small fleshy membrane 

 along the gill bar. This modification obscures the primitive and 

 embryological origin of the gill slits. It gives, however, greater 

 function to the gills, for they are protected by the operculum 

 and the gill filaments are in freer contact with the water. 



3. External Gills. External gills make their first appearance 

 in the Crossopterygian and Dipnoan fish. In the larvae of these 

 animals the gills develop on the outer side of the branchial septa. 

 Their position is shown in the drawing on page 46. As the in- 

 ternal gills develop both types of gill are present for a time, 

 and then the external disappear. Almost the same condition is 

 found in the amphibia, the external gills being wholly ectodermal 

 and appearing before the gill pouches break through. They are 

 very prominent structures in the early larvae. In Urodeles with 

 persistent gills both sets remain; but in the salamanders and 

 anurans the external gills disappear when the internal gills and 

 operculum develop. The operculum is a fleshy membrane not 

 homologous with that of the fish. At metamorphosis the lungs 

 replace the gills. Exceptions to this rule, due to accessory respira- 

 tory structures, will be discussed later. 



The vascular supply of the external gills comes from the 

 branchial arches by branches which pass out into the gills. With 

 the disappearance of the gills the blood follows the more primi- 

 tive course, circling through the gill support from the ventral 

 to the dorsal aorta. The branchial arteries vary considerably in 

 the different groups of amphibia, in correlation with the reduc- 

 tion in number of gills and the branchial arches. The second 

 and fifth branchial pouches are rarely functional in amphibia. 



B. Lungs 



Lungs are the functional respiratory organs of land animals. 

 Their internal position keeps the epithelium moist. Like the gill 



