60 THE ANATOMY OF INVERTEBRATED ANIMALS. 



terior to the bindermost of the visceral arches. They receive 

 their blood from the hindermost aortic arch. It therefore 

 flows from the heart. The gradual improvement of these 

 lungs as respiratory machines is effected, first, by the increase 

 of the surface over which the venous blood brought to the 

 lungs is distributed; secondly, by changes in the walls of 

 the cavity in which the lungs are contained, by which that 

 cavity gradually becomes shut off from the peritoneal cham- 

 ber, and divided from it by a muscular partition. Concur- 

 rently with these modifications, a series of alterations takes 

 place in the accessory apparatus of respiration, whereby the 

 machinery of inspiration, which, in the lower Vertebrata, is a 

 buccal force-pump, which drives air into the lungs, in the same 

 way as water is driven through the branchiae, is replaced by 

 a thoracic suction-pump, which draws air into the lungs by 

 dilatation of the walls of the closed cavity in which they are 

 contained. Along with these changes, modifications of the 

 heart take place, in virtue of which one-half of its total 

 mechanical power becomes more and more exclusively ap- 

 propriated to the task of driving the blood through the lungs. 

 The term "double circulation" applied to the course of the 

 blood in the highest Vertebrata is, however, a misnomer. In 

 the highest, as in the lowest, of these animals, the blood com- 

 pletes but one circle, and the respiratory organ is in the 

 course of the outward current. 



Many animals are truly amphibious, combining aquatic 

 and aerial respiratory organs. 



Thus, among Mollusks, Ampullar ia and Onchidum com- 

 bine branchiae with pulmonary organs ; many Teleostean fishes 

 have the lining membrane of the enlarged branchial chamber 

 vascular and competent to subserve aerial respiration. And 

 in the Ganoids and Teleostei the presence of an air-bladder, 

 which is both functionally and morphologically of the same 

 nature as a lung, is very common. But, in the majority of 

 the Teleostei, the air-bladder is turned aside from its pulmo- 

 nary function to subserve mechanical purposes, in affecting 

 the specific gravity of the body. On the other hand, in the 

 Ganoids and Dipnoi, the whole series of modifications by 

 which the air-bladder passes into the lung are patent. In 

 such lower Amphibia as Proteus and Menobranclius, bran- 

 chial respiration is predominant, and the lungs are subsidi- 

 ary ; but, in the higher, the lungs acquire greater importance, 

 while the branchiae diminish, and eventually disappear. 



