320 BULLETIN OF THE 



other hand, claimed that the transverse tubules of the mesonephros are 

 homologous with the segmental tubes of Annelids, an opinion which is 

 shared by Beard, Haddon, Kollmann ('82 a ) and others. 



Riickert ('88, p. 2G2) has recently denied this homology, and asserted 

 that the Annelidan nephridia are represented in the pronephros alone, 

 since the latter is the only portion of the system of metameric tubes 

 which comes in contact with the ectoderm. With Riickert, I would 

 admit that the evidences of an Annelidan origin of the excretory system 

 are to be sought mainly in the pronephros; but, in view of the intimate 

 relations which exist between pronephros and mesonephros, it seems to 

 me more probable that both have had the same phylogenetic origin. 

 OstroumofF ('88, pp. 80 et seq.) also has asserted that the development of 

 the pronephros indicated that this system had been inherited from 

 Annelidan ancestors. 



On the other hand, a number of authors maintain that, although the 

 mesonephros may be derived from the segmental organs of Annelids, 

 the pronephric system is represented by other Invertebrate nephridia. 

 Thus, Semper ('7G, pp. 387, 388) suggests the possibility that the duct 

 (pronephric system so far as present in Selachians J ) may represent the 

 unsegmented excretory tubules developed in the larva of Xephelis, or 

 may even represent an inheritance from the Plathelminthan water-vas- 

 cular system ; and Balfour ('81, p. GOT) was led to accept this view. 



In addition to his hypothesis of an origin of the excretory system of 

 Vertebrates from that of Plathelminthes, Furbringer suggests the possi- 

 bility that the entire system may have arisen from Gephyreans, in which 

 unsegmented and segmented excretory organs are stated to coexist. This 

 view was adopted by Kollmann ('82 a ), and is offered as a suggestion in 

 Wiedersheim's ('86, p. 731) text-book. 



Xone of the views which claim a double origin for the excretory system 

 seem to me to be tenable, in consequence of the fact that the pronephros, 

 an undoubtedly segmented element, develops in strict continuity with 

 the duct, the so-called unsegmented element. The view according to 

 which the excretory system is derived from that of Gephyreans, more- 

 over, is liable to special criticism. This claim that Gephyreans present 

 an excretory system of a double nature, segmented and unsegmented, 

 doubtless refers to the coexistence of two or three pairs of nephridia, 

 together with the so-called Analschlauche of Echiurids. I am imable to 

 see the slightest evidence that the Vertebrate excretory system is made 



1 By this suggestion, Semper allows some justification to W. Mailer's conception 

 of a pronephros, although he had earlier contested it. 



