184 W. K BROOKS. 



such logical minuteness from a fundamental assumption which is 

 purely imaginary. 



The story, as told by Dohrn in the " Ursprung," is so consistent 

 and logical that I see no reason why animals like the tunicates 

 might not have been evolved in the way which he pictures so 

 vividly, although I believe that the actual tunicates have been pro- 

 duced in a very different way. 



I shall therefore examine the account of the origin of the gill- 

 slits which Dohrn gives in the " Ursprung," and the view of the 

 ciliated and glandular structures of the pharynx which is developed 

 in his " Studien," especially in Parts VII, YIII and IX, in order 

 to determine how far the origin of these structures is accounted for 

 by the annelidian hypothesis, and what superiority, if any, this has 

 over the much simpler hypothesis which is here advanced. 



Dohrn says (Ursprung, p. 10) that the branchial apparatus of 

 the tunicates and that of balanoglossus are so much more complicated 

 than that of the selachians, and their origin is so much more 

 difficult to understand, that they are of no help to us in our attempt 

 to trace the origin of gill-slits. 



I am quite at a loss f6r the meaning of this passage, for no 

 secondary perforation of the pharynx could possibly be less com- 

 plicated than the gill-slits of appendicularia, nor could it be de- 

 veloped in a simpler way than by the iavolution of a pit on the side 

 of the body. 



It is quite true that we do not know how the gill-slits of appen- 

 dicularia first came into existence, or what influence led to their 

 formation, but their usefulness as channels for the escape of the 

 water which, before they were formed, must have passed through 

 the intestine, is clear, and we can understand why they have been 

 preserved, by natural selection, on account of this advantage. 



We are forced to believe that the pharynx did, in some way, 

 acquire a secondary communication with the exterior, although we 

 are not able to say how it was acquired. 



Dohrn's view of the origin of gill-slits is based upon the need 

 for an explanation of the original formation of the perforation. He 

 says (Ursprung, p. 10) : "What is a. gill-slit? Perforations of the 

 body-wall do not take place directly, and still less do they form 

 connections with corresponding perforations of the digestive tract," 



