DEVELOPMENT OF NEPHRIDIA OF PHERETIMA 55 



connective tissue. The last step is the ahsohite 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, secondly, that this becomes broken up 

 into a large number of nephridia, of which one only — the large 

 paired nephridium — retains the funnel ' (4). 



The development of nephridia in Mahbenus imperatrix 

 described by Bourne (8) is remarkably similar to that of the 

 nephridia in Megascolides described by Vejdovsky (17). 

 The only difference is that while in the former the funnel is 

 at no stage well developed, is probably never functional, and 

 afterwards entirely degenerates, in the latter the funnel is 

 retained by one pair of nephridia. In fact, the resemblance 

 in the development in the two forms is so great that there 

 is a remarkable similarity between Vejdovsky's diagram 

 (PI. 32, fig. 5) showing the development of nephridia in 

 Megascolides and Bourne's diagram (PI. 5, fig. 39) showing 

 the same in Mahbenus. 



From the foregoing account of the history of our knowledge 

 of the development of nephridia in earthworms we arrive at 

 three more or less definite broad conclusions. The first is with 

 regard to the fundamental problem of the ultimate origin 

 (ecto- or mesodermal) of the Oligochaete nephridium. As we 

 have seen, there is an overwhelming amount of evidence to 

 show that the nephridia in Oligochaetes are certainly ectodermal. 



Secondly, in all forms with the so-called ' plectonephric ' 

 system, studied so far, this adult condition is preceded in the 

 embryo by a condition of paired pronephridia in each segment. 

 In the third place, the adult condition of diffuse micronephridia 

 is derived by the breaking up into separate loops of the em- 

 bryonic pair of pronephridia, the original funnel either being 

 retained by one of the nephridia in each segment or degenerating 

 altogether. 



The present work on the development of nephridia was 

 undertaken to find an answer to the following questions : 



1. Are all the three types of nephridia in Pheretima 

 ectodermal in origin '? 



