EVALUATION OF CAUSES IN EISE OF AMPHIBIANS 425 



Substitution of lungs for gills, following on this general method of 

 accessory respiration, would not necessitate a reorganization of the heart 

 and arterial system. A single ventricle could still drive wholly impure 

 blood to the oxygenating organs ; thence the efferent arteries would carry 

 only pure blood to the head, and a dorsal aorta would carry only pure 

 blood to the body. Collected into the venous system, only impure blood 

 would be sent to the heart. The heart would remain two-chambered and 

 efficient during and after the substitution of air for water as the medium 

 of respiration. It would be necessary, however, for the forward part of 

 the gill chamber to be utilized in order that pure blood should be sent to 

 the head. 



A distinctly different method of supplemental oxygenation is that of 

 swallowing air and using the alimentary canal for the purpose of absorb- 

 ing the oxygen. In this method, if the accessory mode of respiration 

 should become the principal mode, a reorganization of the circulatory 

 system would become necessary. This is because blood is taken to the 

 stomach and intestines of fishes by the single cceliac artery, given off 

 from the dorsal aorta behind the pair of subclavial arteries. Conse- 

 quently, from the standpoint of the circulation system, the oesophagus 

 and intestines are parts of the body, which come to function as the 

 respiratory organ, as distinct from the pharyngeal or original respiratory 

 region. The respiratory mechanism is no longer situated between the 

 two-chambered heart and the body. As a result pure blood from the 

 lungs would be mingled with impure blood from the body before entering 

 the heart. This mixed stream would thence be sent back again both to 

 body and lungs. 



Which of these general methods of development of the accessory organs 

 of respiration was pursued by the ancestors of Devonian ganoids ? A prob- 

 able answer may be derived by the study, on the one hand, of the circula- 

 tion of those fishes which never made large use of air, and, on the other 

 hand, by the study of the surviving dipnoans and crossopterygians and 

 the descendants of the latter — the amphibians. 



In most of higher fishes, Teleostomi, the air-bladder is supplied with 

 blood by branches of the cceliac artery, with the addition of small branches 

 arising directly from the dorsal aorta. In the dipnoan Protopterus a 

 more anterior origin exists, in that the pulmonary arteries arise from the 

 two dorsal aortas between the place where these trunk arteries of the neck 

 receive the fourth efferent arteries and the place where they unite into 

 the single dorsal aorta. In Polypterus and in Neoceratodus, however, the 

 arteries for the air-bladder are derived, as shown in figure 1, B, from a 

 branch of the last or fourth pair of efferent branchial vessels. The pul- 



