THE RESPIRATORY SYSTEM 329 



Dipnoi, have narial passages. But it is not generally believed that the 

 Dipnoi are in the direct line of amphibian ancestry. 



The story of gills is one of great multiplication in number in forms like 

 the protochordates which use the pharynx both for obtaining food and for 

 gaseous exchange. In the fishes and amphibia, however, the gills are 

 considerably modified, are reduced in number and finally in higher verte- 

 brates disappear. Startling changes of function occur. Supporting 

 skeletal elements are converted into a sound-conducting apparatus. 

 Gill-slits degenerate into blind pharyngeal pouches, which in turn become 

 endocrinal glands. 



The transformation of a ventral air bladder into lungs is sufficiently 

 well attested to be plausible. The chief evolutionary change which lungs 

 undergo is an enormous increase of respiratory surface so that, even within 

 the hmits of the mammalian chest, they expose many square yards of 

 moist surface for gaseous exchange. 



To meet respiratory needs, two sorts of organs have emerged in 

 animals, branchial organs or gills found in aquatic animals and pulmonary 

 organs characteristic of land forms. 



A. The Branchial System. The fact that lungs are wanting in all 

 classes of protochordates as well as in the more primitive groups of verte- 

 brates, proves that the primary respiratory system is that series of paired 

 pharyngeal gills which form the branchial system of chordates. Rem- 

 nants of this system persist in all higher vertebrates. The transition 

 between gilled and lunged forms occurs in the amphibia most of which, at 

 least at some time in their individual development, have both gills and 

 lungs and which thus bridge the gap between aquatic and terrestrial life. 



Gills, like lungs, function as respiratory organs by bringing a network 

 of blood capillaries in close contact with moistened membranes through 

 which gaseous exchange takes place. Their efficiency is increased either 

 by the activity of cilia which cover the surface of the gills or by the con- 

 traction of muscles which pump a stream of water through the pharynx, 

 or by waving the gills to and fro, as in Necturus. 



It may be doubted whether we have any adequate explanation of the 

 substitution of lungs for gills as respiratory organs. The fact that lungs 

 are much better adapted to the needs of land animals than gills which tend 

 to dry in air does not explain their origin. It is to be noted however 

 that in this change of life animals have played safe. Even before they 

 abandoned the water for a land Hfe, they had acquired an organ, the 

 air bladder, which would serve as a substitute for gills. 



Gills are not the pharyngeal openings through which water passes in 

 respiration; these are gill sUts or gill clefts. Two sorts may be distin- 

 guished, internal gills within the body wall and external gills. Those of 

 most animals are internal; a few fishes and amphibians have external. 



