THE NEPHRIDIUM OF LUMBRICUS. 315 



by supposing the branching and anastomosing of four pairs 

 of large nephridia in order to give rise to the condition found 

 in Ac. multiporus, where we have eight groups of tubules 

 per segment ; a further extension of the anastomosis leads to 

 the more complete plectonephric condition of Perichseta 

 aspergillum. 



Megascolides would be explained by one of the four 

 pairs remaining simple (in the posterior part of the body), the 

 rest branching ; still further, reduction to two pairs leads to 

 Brachydrilus, and again to Lumbricus, LJrochseta, &c. 



But it seems to me that this is a strained hypothesis. For, 

 after all, the occurrence in the ontogeny of Ac. multi- 

 porus of the paired nephridia may be merely coenogenetic, 

 and have no meaning of an ancestral nature ; it would come in 

 the same category as the formation of the heart in mammalia 

 from a double rudiment. 



The paired nephridia of Acanthodrilus^ may, therefore^ 

 be merely provisional, like the embryonic nephridia described 

 by Vejdovsky in Rhynchelmis and in Lumbricus, and 

 those of embryo Pulmonates and other animals. 



(3) It therefore remains for us to regard the branching tubes 

 of Brachydrilus and other '^ meganephric " genera, as well 

 as of the strictly " plectonephric genera," as homogenetic, 

 and the plectonephric condition as the more archaic. This 

 is the position taken up by Beddard and by Baldwin Spencer, 

 although there is some difference as to details in the two 

 theories. 



Spencer has pointed out the parallel series of stages in 

 Oligochseta and in Hirudinea ; and it is interesting to com- 

 pare the nephridium of Hirudo (see 14, p. 485) with its net- 

 work of fine intra-cellular tubes around the main intra-cellular 

 tube with that in Brachydrilus; and the less branched and 

 non-anastomosing tubule in Ciepsine (14, p. 483) with the 

 condition met with in Lumbricus and other earthworms. 



* If this be the case, then it would appear that the genital ducts in Acan- 

 thodrilus (and perhaps in other worms) arise, in part at least, by a modifi- 

 cation of the embryonic provisional nephridia. 



