314 BULLETIN OF THE 



ric type of structure. I have already sketched the manner in which 

 the mesonephros may be derived from the pronephros by supposing 

 the mctameric segmentation of the body to extend to that portion of the 

 ccelom from which the neplirostomes emerge. The account given in the 

 preceding section of this paper regarded tiie tubules as passive in such a 

 metamorphosis. It is possible, however, that the transference of the 

 tubules to a segmented portion of the coelom may have been in part 

 effected by a dorsalward shifting of the nephrostomes. In either case, 

 I am of opinion that the mode of development which I have now sug- 

 gested is applicable alike to the pronephros and the mesonephros, and I 

 may also add to the metanephros (see Sedgwick, '80). 



I have now presented, in a suggestive manner rather than as a com- 

 plete argument, certain indications of the phylogeny of the excretory 

 system which may be obtained from internal evidence. It still remains 

 for us to consider what conclusions are justified by a comparative study 

 of the excretory system, and whether the phylogenetic stages suggested 

 in the foregoing account are to be found in any group of living animals. 



The sole purpose of this discussion is to ascertain the most probable 

 phylogenetic line of development for the excretory system of Vertebrates. 

 For this reason, I shall avoid any discussion, which would necessarily be 

 lengthy, respecting the interrelationships of the diverse excretory organs 

 found in Invertebrates, simply endeavoring to seek out those classes 

 which possess nephridia similar to those of Vertebrates, and shall ignore 

 the further consequences which would follow from the assumption of a 

 homology in any single case. 



In the preceding account, I have provisionally accepted the view 

 that Amphioxus belongs to the Vertebrate phylum, and have endeav- 

 ored to interpret its kidneys in accordance with that view. With 

 Tunicates it is quite different ; not merely do they afford no assistance 

 in the solution of the problem in hand, but it has hitherto proved im- 

 possible to find any organs in this group which can be regarded as 

 homologous to Vertebrate nephridia.^ In my opinion, it cannot be 

 objected that the absence of such organs in Tunicates proves that the 

 Vertebrate nephridia arose within the Vertebrate phylum. A rigid ad- 

 herence to such a system of restriction in the case of other organs would 

 quickly lead to absurd conclusions. 



The only classes of animals in which we need seek for a homology of 



1 Hatschck ('84, p. 519) regarded the single nephridium described by him in 

 Amphioxus homologous with the neural gland of Tunicates ; but I have already 

 pointed out the probable inaccuracy of this observation. 



