314 W. BLAXLAND BENHAM. 



nephric condition of Acanthodrilus as related to that of 

 Perichseta and other worms; and it would have no syste- 

 matic meaning at all, but might have been developed in any 

 genus, and at any period in its history. This assumption 

 would, no doubt, explain many difficulties, such as the pre- 

 sence of a network in Perichseta, and of large nephridia in 

 Perionyx, or the differences in different species of Acan- 

 thodrilus. In fact, the character of the nephridium would 

 be no guide as to the affinities of worms. 



(2) The assumption that the meganephric condition is ar- 

 chaic would, I think, plunge us into a sea of difficulties ; for 

 if a large nephridium is the more primitive, then we have to 

 explain how it comes about that in some genera and species 

 the paired nephridia are in relation to the inner couple of 

 setse, and that in others they are in relation to the outer 

 couple; how is it that in some worms, e.g. Perionyx 

 saltans and Ac. novae-zealandise, the position of the ne- 

 phridiopores alternates from segment to segment? and finally 

 how can we explain the presence of two pairs of nephridia per 

 segment in Brachydrilus ? For if a large nephridium is 

 ancestral, was there but a single pair, or two pairs, or four 

 pairs ? The last assumption would of course explain all the 

 above queries. 



If the plectonephric condition is derived from the branching 

 of one pair of large nephridia per segment, then the double 

 pair in Brachydrilus would have to be regarded as a secon- 

 dary condensation of tubules after the appearance of a plec- 

 tonephric condition ; and the possibility would then arise that 

 a large nephridium might be either primitive or secondary. 



Ontogeny has not yet revealed to us any sufficient evidence 

 on this point; the only plectonephric form studied is Acan- 

 thodrilus. According to Beddard (* Proc. Roy. Soc.,' 1890) 

 there is at first a single pair of large nephridia in each seg- 

 ment, and these by branching give rise to the characteristic 

 network. This looks like strong evidence in favour of Horst's 

 view of an ancestral paired meganephric condition. But, as 

 just stated, we should then have to meet the above difficulties 



