466 Scientific Proceedings, Royal Dublin Society. 



with the primitive pronephros. With the segmentation of the 

 trunk the latter structure itself may have acquired the more or less 

 definite metameric arrangement of its parts." 



" Another possible view is, that the segmental tubes may be 

 modified derivatives of posterior lateral branches of the prone- 

 phros, which may at first have extended for the whole length of 

 the body cavity. If there is any truth in this hypothesis, it is 

 necessary to suppose that, when the unsegmented ancestor of the 

 Chordata became segmented, the posterior branches of the primi- 

 tive excretory organ became segmentally arranged, and that, in 

 accordance with the change thus gradually introduced in them, 

 the time of their development became deferred, so as to accord to 

 a certain extent with the time of formation of the segments to 

 which they belonged. The change in the mode of development 

 which would be thereby introduced is certainly not greater than 

 that which has taken place in the case of segmental tubes, which, 

 originally developed on the Elasmobranch type, have come to de- 

 velop as they do in the posterior part of the mesonephros of 

 Salamandra, Birds, &c." 



In his " Comparison of the Excretory Organs of the Chordata 

 and Invertebrata" (/. c. p. 607), Balfour states : — " The excretory 

 organs of the Platyelminths are in many respects similar to the 

 provisional excretory organ of the trochosphere of Polygordius 

 and the Grephyrea on the one hand, and to the Vertebrate pro- 

 nephros on the other ; and the Platyelminth excretory organ, with 

 an anterior opening, might be regarded as having given origin to 

 the trochosphere organ, while that with a posterior opening may 

 have done so for the Vertebrate pronephros (this suggestion has, I 

 believe, been made by Fiirbinger). 



" Hatschek has compared the provisional trochosphere excre- 

 tory organ of Polygordius to the Vertebrate pronephros, and the 

 posterior Chsetopod segmental tubes to the mesonephric tubes, the 

 latter homology having been already suggested, independently, by 

 both Semper and myself [Balfour]. With reference to the com- 

 parison of the pronephros with the provisional excretory organ of 

 Polygordius, there are two serious difficulties : — 



" (1) The pronephric (segmental) duct opens directly into the 

 cloaca, while the duct of the provisional trochosphere excretory 

 organ opens anteriorly, and directly to the exterior. 



