LUNGS 



277 



pleural cavity, separated from the rest of the coelom by the dia- 

 phragm (p. 140). 



DIPNOI. — In Ceratodus (fig. 290, D) there is a single lung sac; Protopterus 

 and Lepodosiren have paired lungs, the two being united in front at the entrance 

 of the air duct. In all three the inner surface is divided more or less regularly 

 into groups of alveoli, separated by more prominent partitions. The pulmonary 

 arteries arise from the last efferent branchial artery of either side, and hence the 

 blood supply, under normal conditions, is arterial and the lungs cannot act as 

 respiratory organs. In times of drought {Protopterus) or of foul water 

 {Ceratodus) the gills no longer function and the pulmonary arteries bring 

 venous blood to the lungs. 



AMPHIBIA. — In the lower urodeles the two lungs are elongate 

 (the left the longer) and are united at 

 their bases, true bronchi being absent. 

 Internally they may be entirely 

 smooth as in Necturus, or there may 

 be alveoli in the basal portion (fig. 

 3cx>), the whole representing a termi- 

 nal vesicle either connected directly 

 with the trachea {A) or by the in- 

 tervention of an alveolar duct {B). 

 In the ca?cilians the left lung is very 

 short, the other elongate, with alveoli 

 developed throughout. In the frogs 

 (fig. 300, C) the two lungs are distinct, 

 and their walls are divided into a series 

 of sacs or infundibula lined with 

 alveoli. The infundibula open into 



a central chaniber, which, since it is ciliated and has numerous glands 

 in its walls, may be compared to a bronchiole. In the toads and 

 aglossa the alveoli are more extensively developed in correlation 

 with the more terrestrial habits 



It has recently been shown that a number of terrestrial urodeles are lungless 

 in all stages of development, and that no traces of larynx or trachea occur, even 

 after the gills are absorbed. In these species there is a great development of 

 capillaries in the skin and in the walls of the mouth and pharynx, the respiratory 

 functions being transferred to these parts. In the frogs the skin is also respira- 

 tory and it is largely supplied by the cutaneous arteries which arise from the 

 same arch as the pulmonary arteries. 



In the amphibia the air ducts enter the anterior end of the lungs, 

 but in the amniotes the lungs extend anteriorly to the entrance of the 



Fig. 300. — DiflFerent t)T)es of am- 

 phibian lungs. A, Necturus, without 

 alveoli; B, alveoli in the proximal por- 

 tion; C, frog, alveoli throughout. 



