STRUCTURE OF THE VERTEBRATES 213 



from the pharynx is the larynx; the undivided upper portion of 

 the tube is the trachea; and the two tubes into which the 

 trachea divides are the bronchi. Urodele lungs are simple ex- 

 pansions of each bronchus, and are less lobulated than those of 

 the dipnoan fish. During the larval period of development all 

 amphibia have the gill and lung combination for respiration, and 

 this double system is retained by many urodeles. 



The homology of the swim-bladder is shown by (1) the com- 

 parative anatomy and function of the two structures; (2) by the 

 mode of embryological development, which is practically iden- 

 tical in the crossopterygian and dipnoan fish and the land verte- 

 brates; and (3) the vascular supply. In all cases the supply is 

 from the sixth aortic artery. It is only in the mammals that 

 there is a complete separation of the pulmonary arteries, which 

 arise as branches of the sixth branchial, and the ventral aorta. 



2. Comparative Anatomy of the Lungs. Beginning with the 

 unlobulated, elongated lungs of the primitive urodeles, the lungs 

 evolved toward more complex divisions. Salamanders have the 

 anterior part of the lungs divided into shallow pockets or alveoli, 

 giving the lungs a honeycomb appearance. Each alveolus opens 

 wide into the central cavity, but the low septa add to the ab- 

 sorptive area of the epithelium. 



Frogs show a definite advance over the salamanders. The lungs 

 are more rounded in correlation with their body shape, and the 

 alveoli are deeper and cover the entire inner surface of the 

 lungs. The toads, which live in drier areas, have lungs of the 

 frog pattern; but the dividing septa close partly over the alveoli, 

 giving them an almost spherical shape with a circular opening 

 into the central lung cavity. 



Sphenodon (a New Zealand primitive reptile) and some turtles 

 have lungs which are no better developed than those of a frog. 

 A definite advance in lung structure appears in the lizards. Dur- 

 ing development the alveoli move further from the central 

 cavity, and are connected with it by small ducts, homologous 

 with the bronchioles of the birds and mammals. 



Alligators and crocodiles have the best developed lungs among 

 the reptiles. As the lungs develop each bronchus divides into 

 several branches, thus cutting the lung into several lobes, each 

 with its own cavity. From the air cavity of each lobe numerous 



