THE RESPIRATORY SYSTEM 339 



including mammals, and that they play a part in the disposition 

 of the arterial arches although they cease to function as 

 respiratory organs. 



The adult Amphibia (or most of them, i.e. those which 

 have not lost the lungs) and all higher vertebrates breathe by 

 lungs. (The use of the skin as a breathing organ in Amphibia 

 is made possible by the fact that their skin is moist and 

 uncovered.) 



Lungs are also present in some fish. In Polyp terus, there 

 is a trachea leading out from the ventral side of the oesophagus, 

 and forking into two lungs. The cavity of these lungs is 

 divided into small spaces or " cells," which has the result of 

 increasing the internal surface. Such lungs are called cellular, 

 and they are supplied with blood by pulmonary arteries, i.e. 

 branches from the last (6th) pair of branchial arterial arches. 

 From them, blood returns (to near the sinus venosus) by paired 

 pulmonary veins. In the Dipnoi, there are paired lungs in 

 Protopterus and Lepidosiren, but a single one only in 

 Ceratodus. Their relations are similar to those of Polyp terus, 

 except that the lungs, together with the pulmonary arteries and 

 veins, have been displaced to a dorsal position by passing round 

 the right side of the oesophagus. In Ceratodus the pulmonary 

 veins open into the left side of the auricle. Lungs were 

 almost certainly present in the Osteolepidoti. These animals 

 lived or live in fresh water in which the oxygen- content is 

 low (owing to desiccation and accumulation of decomposing 

 organic debris), and branchial respiration is supplemented by 

 the intake of bubbles of air through the mouth. Indeed, 

 Protopterus is able to withstand periods of drought when the 

 swamps in which it lives dry up, by burying itself in the mud 

 and breathing by its lungs. The lungs of higher vertebrates 

 are easily derived from those of the fish just described. It is 

 possible that the lungs respresent a pair of gill-pouches behind 

 the remainder, and which ceased to open to the exterior. They 

 are formed from the endoderm and communicate with the 

 alimentary canal, and they preserve their blood-supply from 

 the vessel of the last branchial arch. 



In the higher bony fish, the lung is single and modified. 



