194 



longitudinal duct of Lanice, Polymnia, Polygordius and others, 

 to the segmental canal, or primitive ureter of Vertebrates. 



Professor Wilson in his remarkable paper on the germ-bands of 

 Lumbricus goes even so far as to consider this homology as 

 evident. 



On this point I venture to recall attention to Professor Haddon's 

 hypothesis as to the phylogenetic origin, and epiblastic nature of the 

 segmental duct of Vertebrates ^ ). 



The existence of such an evidently adaptive structure as the epi- 

 dermic duct of w e n i a seems to give a remarkable confirmation to 

 his suggestion as to how a continuous groove into which the nephridia 

 opened, may have been converted into a canal. 



It is not evident at all that the segmental duct really is an un- 

 segmented part of a cell-row homologous to that of Clepsine^) or 

 Lumbricus^). It may have appeared at a much later period of 

 the phylogenetic evolution, and have been at a given moment a new 

 structure corresponding to new wants, just as the epidermic duct of 

 Oweuia corresponds to a peculiar disposition of the protective tube 

 of the worm. The coexistence of a segmental duct analogous to the 

 epithelial duct of Ovvenia, and of a structure homologous to the longi- 

 tudinal canal of Lanice is even possible. 



I do not affirm that the epidermic duct of w e n i a really repre- 

 sents the segmental duct at an early stage of its phylogenetic develop- 

 ment. I rather think that we have here a case of homoplasy, not of 

 homogeny. I believe however that the homology of the primitive 

 ureter is not settled as the American professor would have it, but 

 remains a question open to further investigation. 



1) Haddon, Suggestion respecting the epiblastic origin of the seg- 

 mental duct. Proc. Royal Soc. Dublin, Vol. V, 1886—87. 



2) "Whitman, A contribution to the history of the germ-layers in 

 Clepsine. Journal of Morphology, 1887, Vol. I, 



3) Wilson, The germ-bands of Lumbricus. Ibid. 



