(594 



EEPTILIA. 



permanent gills as well as lungs ; and thirdly, the metamorphoses that 

 take place in the construction of the breathing organs during the de- 

 velopment of the lungs, and the obliteration of the branchiae in those 

 forms in which the branchiae are not persistent. 



(1944.) The lungs of Reptiles are two capacious membranous sacs 

 occupying a considerable portion of the visceral cavity, which, as there 

 is no diaphragm as yet developed, cannot properly be divided into tho- 

 rax and abdomen, as it is in Mammalia. From the internal surface of 

 the walls of each lung membranous septa project inwards, so as par- 

 tially to divide the interior of the organ into numerous polygonal cells, 

 which are themselves subdivided into smaller compartments in a similar 

 manner. This structure is well seen in the lung of the Tortoise 

 (fig. 341). 



Fig. 341. 



Lung of the Tortoise. 



(1945.) The pulmonary cells are most numerous and complete towards 

 the anterior extremity of the lung, and it is here that the pulmonary 



