EXCRETORY ORGANS. 607 



opposed to a phylogenetic development ; and the substitution of a direct 

 fora 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 possi- 

 ble, 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 Chord ata 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 be- 

 tw r een these organs on a satisfactory basis, except in closely related groups. 



The excretory organs of the Platyelminthes are in many respects simi- 

 lar 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 ivitk 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 Verte- 

 brate pronephros 1 . 



Hatschek has compared the provisional trochosphere excretory 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. 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 Chsetopoda, 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. 



Confining our attention to the Invertebrata it appears to me fairly clear 

 that Hatschek is justified in holding the provisional trochosphere excretory 

 organs of Polygordius, Echiurus and the Mollusca to be homologous. The 

 atrophy of all these larval organs may perhaps be due to the presence of a 

 well-developed trunk region in the adult (absent in the larva), in which 

 excretory organs, probably serially homologous with those present in the 



1 This suggestion has I believe been made by Fiirbringer. 



