238 



BIOLOGY. 



[Book ii. 



we can even thus make them travel considerable distances without 

 killing them. Branchial surfaces and pulmonary surfaces absorb 

 oxygen, exhale carbonic acid, and transform more or less perfectly 

 arterial blood into venous blood. This double current can be 

 seen, so to speak, when we esamine, by means of the microscope, 

 the branchiae plunged in water. There is established, from their 

 contact, in the liquid medium a sort of circular movement ; the 

 corpuscules floating in the water seem to be attracted on one side 

 and refilled on the other.^ 



r?\ 





KiG. 33. 



Diverse forms of natatory bladders :—A, FolypUns Mc^ir ; B, JoJmim loiatus ; 0, Corvina 



trUpinosa .■ a, annexes of the bladder; 6, its orifice. 



We have not here to describe in detail the pulmonary ap 

 paratus of the superior vertebrates. Let us merely say that the 

 more the animal is perfect the more the pulmonary pouch is 

 divided and subdivided into lobes, lobules, terminal culs-d&sac 

 or cells, in ever increasing number, whence in the superior 

 manmiifers an enormous respiratory surface (Fig. 34). At the 

 same time that the pulmonary sac grows more perfect it separates 

 1 Dnges, loc. cit., t. II., p. 522. 



