246 CHORDATE ANATOMY 



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 amphibians, 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 higher verte- 

 brates 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 limits of the mammahan 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 the 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 amphibians 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. 



Gills are not the pharyngeal openings through which water passes in 

 respiration; these are gill-slits 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. 

 The gills of elasmobranchs may be taken as typical. They are modifica- 

 tions of the branchial bars or arches which alternate with the gill-slits 

 and serve to keep them open. Each branchial arch consists of an inter- 

 branchial septum of connective tissue which is covered on the surface of 

 the body by skin, and which includes near the pharyngeal lining a car- 

 tilaginous arch as a support. Within the septum are branches of the 

 dorsal and ventral aortae which supply the gills with blood. The septa 



