190 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



believe that the actual tunicates have been produced 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, VIII 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 for the meaning of this passage, for no secondary 

 perforation of the pharynx could possibly be less complicated than the 

 gill-slits of appendicularia, nor could it be developed in a simpler way 

 than by the involution of a pit on the side of the body. 



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

 laria 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," and he 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 



