EXCRETORY ORGANS. 737 



a phylogenetic development has most probably been rendered possible by 

 the fact that the anterior part of the mesonephros continued all the while 

 to be unaffected and to remain as the main excretory organ during foetal 

 life. 



The most serious difficulty urged by Fiirbringer against the homology is 

 the fact that the ureter of the metanephros develops on a type of its own, 

 which is quite distinct from the mode of development of the ureters of the 

 metanephros of the Ichthyopsidan forms. It is however quite possible, though 

 far from certain, that the ureter of Amniota may be a special formation 

 confined to that group, and this fact would in no wise militate against the 

 homology I have been attempting to establish. 



Comparison of the Excretory organs of the Chordata and 

 Invertebrata. 



The structural characters and development of the various forms of 

 excretory organs described in the preceding pages do not appear to me to 

 be sufficiently distinctive to render it possible to establish homologies 

 between these organs on a satisfactory basis, except in closely related 

 groups. 



The excretory organs of the Platyelminthes are in many respects similar 

 to the provisional excretory organ of the trochosphere of Polygordius 

 and the Gephyrea on the one hand, and to the Vertebrate pronephros 

 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 1 . 



Hatschek has compared the provisional trochosphere excretory organ of 

 Polygordius to the Vertebrate pronephros, and the posterior Chastopod 

 segmental tubes to the mesonephric tubes ; the latter homology having 

 been already suggested independently by both Semper and myself. With 

 reference to the comparison 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 an- 

 teriorly, and directly to the exterior. 



(2) The pronephros is situated within the segmented region of the 

 trunk, and has a more or less distinct metameric arrangement of its parts ; 

 while the provisional trochosphere organ is placed in front of the segmented 

 region of the trunk, and is in no way segmented. 



The comparison of the mesonephric tubules with the segmental excre- 

 tory organs of the Chaetopoda, though not impossible, cannot be satisfac- 

 torily admitted till some light has been thrown upon the loss of the supposed 

 external openings of the tubes, and the origin of their secondary connection 

 with the segmental duct. 



1 This suggestion has I believe been made by Fiirbringer. 

 B. III. 47 



