DEVELOPMENT OF NEPHRIDIA OF PHERETIMA 89 



evolutionary stages between the condition in Perionyx 

 (exonephric) and that in Pheretima (enteronephric). 

 Although we can derive the diffuse condition of Pleinogaster 

 from Perionyx, through such forms as M e g a s c o 1 e x show- 

 ing an intermediate condition, we cannot ignore the fact that 

 the gap between the ' exonephric ' (p. 86) and ' enteronephric ' 

 conditions is very deep indeed. 



A few facts in the embryology of the nephridial system, 

 however, seem to throw some light on the possible evolution 

 of the enteronephric system. I have already shown (p. 67) 

 that the primary integumentary nephridia have a common 

 source of origin in the nephridial masses lying opposite the 

 intersegmental septa. We have also seen that, while the 

 integumentary nephridium has no ' funnel-cell ', it is repre- 

 sented in the rudiment of each septal nephridium, and con- 

 sequently the former lacks and the latter possesses a ' funnel ' 

 in the adult condition. It is possible to suppose that the 

 first great step in the process of evolution of the enteronephric 

 system was the severance of the connexion between the ' funnel ' 

 and the ' body ' of the nephridium. That this severance has 

 probably taken place in Pheretima is supported by very 

 strong evidence from the embryology of the nephridia of 

 Octochaetus (3), Megascolides (17), and Mahbenus 

 (8). In all these forms there is a paired meganephric condition 

 in the embryo, and each nephridium is provided with a well- 

 developed funnel. In the transformation of this embryonic 

 into the adult condition the part to degenerate first is always 

 the duct following the funnel. In Octochaetus (3), Beddard 

 found that the change took place by the disappearance of the 

 lumen in the portion nearest the funnel. Vejdovsky (16) 

 has found in Megascolides that the paired embryonic 

 nephridia have a funnel from which leads a straight duct 

 without lumen, and that this duct joins the nephridial loDps. 

 During development the connecting part of the original tube 

 (i. e. the straight solid duct) first degenerates into a mere strand 

 of connective tissue and finally breaks up entirely. But the 

 funnel remains and forms part of the large nephridium in the 



