GILL-CLEFTS 151 



contrary rests with those who deny it. Up to 

 the present there has been no brilliant demonstra- 

 tion of the diphyletic origin of gill-clefts. The 

 assumed homology between the gill-clefts of the 

 Enteropneusta and those of Amphioxus has been 

 doubted because their histological composition is 

 not identical ; but we know now that histological 

 identity is no safer guide to morphology than is 

 anatomical identity. This is where the matter 

 rests at present with regard to trematic respira- 

 tion. The differences between the gill-clefts of 

 the Enteropneusta where the tongue-bar is the 

 principal component, and the gill-clefts of Amphi- 

 oxus where the tongue-bar is a secondary com- 

 ponent, are such as one would be prepared to 

 find in allied groups which present unequal grades 

 of organisation. 



We have already mentioned several examples 

 of convergent pulmonate respiration, and it only 

 remains to be added, for the sake of complete- 

 ness, that the lungs of the higher vertebrates 

 are acknowledged, on grounds of comparative 

 anatomy, to be homologous with the air-bladder 

 of Teleostean fishes in spite of differences in the 

 vascularisation and topography of these organs. 



According to Owen (Preface to Hunter's 

 Animal Oeconomy, 1837), Harvey was the first 

 to compare the abdominal air-sacs of the bird 

 with those of reptiles and fishes ; and in this he 

 was followed by John Hunter, who discovered the 



