210 W. B. BENHAM. 



The nephridium here has all the appearance of that in the 

 common earthworm. 



For this reason I have separated it from its former allies ; 

 and although, at present, we know but few points of apparent 

 difference between Perionyx and Perichseta^ yet it may 

 be possible to separate some of the species of the latter genus 

 and place them in the former genus. 



Assuming with Beddard and Baldwin Spencer that the 

 excretory network is more primitive than paired nephridia, 

 and regarding the pericheetous condition as secondary, it 

 is not impossible to conceive this condition making its 

 appearance both in worms which still retain the network, 

 and in those which have acquired the large nephridia; 

 or perhaps Perionyx is a descendant from forms which 

 had become perichsetous whilst still retaining the network, 

 but which have lost this latter character and retained the 

 former. 



The histological structure of the nephridia still remains to 

 be worked out, although, thanks to Beddard and Spencer, we 

 have a fair knowledge of the details in the case of the small 

 tubules of the network in Acanthodrilus and Megasco- 

 lides. These, like the larger nephridia, are made up of a 

 series of '^ drain-pipe ^^ cells — no doubt ciliated in some part of 

 the tubule — ^which form the mass of the tubules; but this 

 intracellular lumen becomes converted into an intercellular 

 lumen near its external opening, and the wall is here frequently 

 provided with muscle-fibres. 



Of the exact arrangement of the convolutions of the more 

 or less elongated tube of the nephridia of other forms we 

 have little or no information. Gegeubaur's well-known figure 

 of the nephridium of Lumbricus still remains the only 

 accurate drawing of such a nephridium. The recent drawing 

 given by Goehlich (' Schneider's Beitrtige,' Bd. ii) is not quite 

 accurate; and he is mistaken in thinking that the cilia are 

 continuous from the funnel to the muscular duct. This is 

 not the case ; certain regions of the duct are provided with 

 cilia, and others are deprived of them. 



