SALPA IN RELATION TO EVOLUTION OF LIFE. 185 



and be therefore undertakes to study the origin and primitive 

 function of gill-slits by the aid of the law of the change of function, 

 and to find in a more primitive function an explanation of their 

 present function as channels for water. 



As his point of departure is the need for an explanation of the 

 origin of the perforation, we feel a natural hope that we are to be 

 led to this explanation, but this hope ends in disappointment. 



He regards the gill-slits as modified segmental organs, but he 

 tells us explicitly, on page 10, that " we are not able to assign any 

 reason why segmental organs should unite with the gut," and his 

 explanation of the origin of the perforations is no explanation at 

 all, since it simply assumes, but does not account for, the very 

 phenomenon which it is supposed to make clear. 



His inability to understand the direct origin of the secondary 

 perforations of the gut has one most remarkable result, for the 

 view that the gill-slits are segmental organs involves the view 

 (Ursprung, p. 57) that the anus of the tunicates is not a primary 

 anus nor a secondary one, but a tertiary one, and that the ancestors 

 of the tunicates have not only acquired two new secondary anal 

 apertures, but that they have lost one mouth and acquired a second, 

 and that they have lost this and acquired a third. As these mouths 

 are supposed to be modified segmental organs, we are, according to 

 the acknowledgment on page 10, " unable to assign any reason why 

 they should have united with the gut." 



The original mouth of the ancestors of the chordata was, accord- 

 ing to Dohrn (page 3), on what is now the dorsal surface, and the 

 primitive oesophagus passed through what is now known as the 

 fossa rhomboidea of the brain. 



This ancestral mouth degenerated and disappeared as it was 

 gradually superseded in the remote progenitors of the vertebrates 

 by a second mouth (page 5), which is the mouth of the vertebrates 

 of the present day, and of the ancestors of the tunicates (page 57) 

 as well, although it was gradually converted first into a sucker, and 

 finally into an organ for fastening the tunicata to foreign bodies, 

 while these animals gradually acquired a tertiary mouth (page 58) 

 by the formation of a secondary communication between the nasal 

 chamber and the gut. 



