ORGANIC EVOLUTION. 67 



the organs of respiration, or branchiae. But 

 in all the other vertebrata {i.e. except fish and 

 amphibia) the gill-slits do not develop branchiae, 

 become closed (with the frequent exception of 

 the first), and so never subserve the function of 

 respiration. Or, as Mr. Darwin states it, " At 

 this period the arteries run in arch-like branches, 

 as if to carry the blood to branchiae which are 

 not present in the higher vertebrata, though 

 the slits on the sides of the neck still remain, 

 marking their former position." 



The heart is "at first a simple pulsating vessel, 

 like the heart of the lowest fishes, and the 

 excreta are voided through a common cloacal 

 passage — an anatomical feature so characteristic 

 of the lower vertebrata, that it occurs in no fully 

 formed member of the mammalian group, with 

 the exception of the bird-like order of mono- 

 tremata, which takes its name from presenting 

 so striking a peculiarity. 



F 2 



