W. K. BROOKS ON THE GENUS SALPA. 191 



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 per- 

 forations 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, according 

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

 ossophagus 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 superseded in the remote progenitors of the vertebrates by a second 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. 



Dohrn says (page 60) that these assumptions "set the relation 

 between the fishes and the ascidians in the right light," although the 

 perforation of the gut, which the hypothesis is to explain, is not only left 

 unaccounted for, but is multiplied so many times that, like the man with 

 an unclean spirit, its last state is worse than the first. 



Dohrn says that the secondary nature of the mouth of the verte- 

 brates is proved by its very late appearance in the young vertebrate after 

 its embryonic body and its great systems of organs are fully formed, and 

 by the fact that, when it does make its appearance, it does not lie at the 

 anterior end of the body, in the place which it finally occupies in the 

 great majority of vertebrates, but at a spot some distance behind this 

 place. 



