DEVELOPMENT OF NEPHRIDIA OF PHERETIMA 87 



adult nephridia, I came to the conclusion that each nephridium 

 is a separate and discrete structure, that there is no network 

 of any kind connecting one nephridium with the other, and 

 that it is a mistake to describe the nephridial system of P h e r e - 

 tima as a ' coelomic network ' or as ' plectonephric ', implying 

 the idea of a reticular connexion between the nephridia. This 

 conclusion is now confirmed by the evidence we have from the 

 embryology of the excretory system of this worm. We now 

 know that each nephridium, integumentary or septal, originates 

 independently of the others, and, therefore, even the embryonic 

 connexion found in Megascolides and Mahbenus is 

 wanting in this worm. 



10, PhyloCtEny of the Oligochaete Nephridial 



System. 



Before any developmental facts were known with regard to 

 the excretory organs of Oligochaetes, it was commonly held 

 ' that the paired nephridia (meganephridia) of most Oligochaeta 

 were formed by reduction from a network such as now exists 

 in Perichaeta and many other genera ' (4). But after the 

 embryology of the excretory system in Octochaetus and 

 Megascolides had been elucidated, and it was found that 

 a meganephridial condition preceded the diffuse condition, 

 this view had to be given up. In speaking of the phylogeny of 

 the system, Beddard (4) says that ' it does not follow that the 

 diffuse nephridia are the outcome of a branching and specializa- 

 tion of the paired nephridia ; what the developmental facts 

 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 view was definitely formu- 

 lated by Vejdovsky (16), who says, ' Es hat daher meiner 

 Ansicht nach sowohl das " Plecto- als Meganephridium " 

 gleiche genetische Bedeutung. Beiden muss ein einfacher, 

 paarig in jedem Segmente sich anlegender Strang — das Pro- 

 nephridium — vorausgehen, aus welchem erst secundar seitliche 

 Wucherungen entstehen, die sich als zahlreichere oder spar- 

 lichere Nephridiallappen erweisen. In grosser Menge bilden 



