IV 



NEPHRIDIAL DUCTS 



265 



cloaca. But as already pointed out there is no embryological 

 support for this view. Everywhere the archinephric opening is at 

 first within the endodermal part of the alimentary canal and this 

 suggests that the communication of duct with cloaca has come 

 about in some other way. The evidence of Polypterus suggests as 

 already indicated that the opening into the cloaca represents the 

 persistent primitive communication of a mesoderm segment with the 

 enteron. It is quite conceivable that a secondary communication 

 between archinephric duct and gut may have come about in this 

 way, in correlation with the pronephric part of the archinephros 

 reaching the actively functional condition 

 at a period when the mesoderm segment 

 at the level of the anus had not yet been 

 completely separated from the endoderm. 

 Once this secondary opening was estab- 

 lished it would be a natural consequence 

 for the post-anal portion of the nephridial 

 system to atrophy and disappear. 



The hypothesis indicated in this descrip- 

 tion derives the nephridial apparatus of 

 the Vertebrata from an ancestral condition 

 resembling that characteristic of Annelids 

 — the main difference being that in the 

 Vertebrates the nephridial tubes open into 

 a longitudinal duct which at its hinder end 

 communicates with the alimentary canal. 

 It is of great interest then to find even 

 within the group of the Annelida clear 

 expressions of the tendency for the nephri- 

 dial tubes to open into such a duct. The 

 best marked case of this known up to the 

 present appears to be that of the Earthworm 

 Allolobophora antipae described by Rosa 

 (1906). Here (Fig. 139) in the posterior 



portion of the body the nephridial tubes lead into a longitudinal 

 duct which fusing posteriorly with its fellow opens into the alimentary 

 canal on its dorsal side and near its posterior end. In other words 

 in this particular case an arrangement precisely like that of the 

 vertebrate has been evolved out of an ancestral condition in which 

 segmentally placed nephridial tubes opened independently upon the 

 outer surface. 



In regard to the origin of the typical metanephric duct or ureter 

 as seen for example in a Bird there are two obvious possibilities. 

 If the metanephros represents a number of nephridial segments its 

 special duct may have originated by such steps as are represented by 

 the adult condition in male Urodeles and male Elasmobranchs i.e. by 

 the openings of the collecting- tubes into the original duct becoming 

 displaced backwards. Or on the other hand if the metanephros 



Fig. 139. — Diagram of the 

 posterior end of the body of 

 Allolobophora antipae as seen 

 from the dorsal side to show 

 the relations of the renal 

 organs (shown in black) 

 according to Eosa (1906). 



