52 OLIGOCHAETA 



degenerating into a mere strand of connective tissue. The last step is the absolute 

 severance of the connexion. Thus it appears firstly that the nephridial system of 

 this worm originates from a pair of pronephridia to each segment ; and that this 

 becomes broken up into a large number of nephridia of which one only, the large 

 nephridia described by SPENCER, retains the funnel. If there is, as has been described 

 by SPENCER, a plexus formed by the interconnexions of the small nephridia it must 

 be secondary ; but at any rate it is noteworthy that at first when the several 

 nephridia are in course of formation out of the pronephridium there is a connexion 

 at least between the numerous nephridia of the same segment. As to the continuous 

 longitudinal ducts described by SPENCER the most careful search failed to show them 

 in the embryo ; they also must therefore be secondary structures. 



4. Phylogeny of excretory system. 



The facts just detailed concerning the development of the plectonephric system 

 might appear at first sight to argue for the theory that this condition is secondary 

 and that the paired nephridia of Lumbricus, &c. are to be looked upon as the primitive 

 condition. The facts seem to negative my view, supported by SPENCER and BENHAM, 

 that the paired nephridia of the majority of Oligochaeta are formed by reduction from 

 a network such as now exists in Perichaeta and many other genera. Before any 

 developmental facts were known this course of evolution seemed to be supported 

 by many considerations. In the first place a progress from a more generalized to 

 a more specialized condition is seen in the evolution of other organs. Then there are 

 certain resemblances between the network nephridia and the excretory organs in the 

 Platyhelminths. Traces of the supposed primitive condition also existed in those 

 worms which are now provided with the paired form of excretory organ ; thus in 

 Anachaeta the nephridium has occasionally more than one funnel ; in many forms 

 there is a branching and anastomosis of the fine tubes of the nephridium, for example 

 in Microchaeta, to which reference has already been made, and in other forms also. 

 This view must evidently be now given up ; but, on the other hand, it is not by 

 any means permissible to adopt the converse view already suggested. It does not 

 follow that the diffuse nephridia are the outcome of a branching and specialization 

 of the paired nephridia ; on the contrary the developmental facts absolutely disprove 

 this. What they do prove is that both paired and diffuse nephridia are formed 

 out of similar pronephridia ; that in fact both kinds of excretory organs are equally 

 ancient. This opinion, practically arrived at by myself after the study of the 

 development of Odochaetus, was more definitely formulated by VEJDOVSKV (9). 



