RESPIRATORY ORGANS. 25 



the perennibranch urodeles these external gills persist through 

 life.^ In the other urodeles they are lost without replacement. 



Fig. 27. Head of young Amphiuma showing the external gills, partially covered 

 at the base by the backward extension of opercular fold. 



In the anura, on the other hand, the external gills are early- 

 lost, and are replaced by internal gills upon the sides of the 

 clefts, which, however, are said to be of ectodermal origin. 



Air-bladder. — From the 

 pharyngeal or oesophageal 

 region there arises also in 

 most ganoids and teleosts 

 the air- or swim-bladder. It 

 starts as a diverticulum from 

 the dorsal ^ wall of the phar- 

 ynx, the distal portion of 

 which enlarges into a thin 

 walled sac, the air-bladder or 

 pneumatocyst ; the proximal 

 portion forms the pneumatic 

 duct. This duct remains 

 open throughout life in the 

 ganoids and the lower tele- 

 osts, but in the higher tele- 

 osts it closes and is reduced 



to a fibrous cord.^ The bladder itself usually lies dorsal to 

 the aorta and urogenital system next the vertebral column. In 



^ Cope, however, claims that in Siren the embryonic gills are lost, and that the per- 

 sistent gills of the adult are new structures. 



2 The pneumatic duct empties laterally in some characinidae, ventrally in Polypierus^ 

 into the oesophagus; but until the development is known, we cannot say how far this- 

 condition is secondary. 



3 The teleostei were formerly subdivided into physostomi, with permanent pneumatic 

 duct, and physoclisti with it closed. 



Fig. 28. Relations of the air-bladder 

 to the alimentary canal, after Dean. Ay 

 in most physostomous teleosts; B, in Ery- 

 thrimis; C, in Folypterus, CalamoichthySy 

 and dipnoans. 



