PHYLOGENETIC ARRANGEMENT 



173 



combine the characters of those two groups, which are substantially the same as 

 the Microdrili and the Megadrili of the scheme advocated here. It is also a form 

 which, as regards habitat, is on the border-line between the two divisions ; it lives 

 both in the water and on the land. I look upon Phreoryctes as representing, more 

 nearly than any other existing form, the common type whence the Megadrili and the 

 Microdrili have been derived. The recognition of this worm, as occupying the 

 most primitive position in the two groups, seems to be at vaiiance with the assump- 

 tion supported above of the Perichaetidae being a primitive type on account of the 

 continuous circles of setae. The question is whether it is conceivable that the 

 perichaetous condition could have twice been reduced to the condition of four groups 

 of setae ; this may perhaps be a little difficult of conception ; but the necessary 

 alternative is that from the eight setae per segment the perichaetous condition has 

 been at least twice independently arrived at. The one hypothesis is to my mind 

 at least as difficult as the other. In conclusion, the following scheme embodies the 

 result of the foregoing remarks. 



Fig. 34- 



Acanthodrilidae. Cryptodrilidae. 



Eudrilidae. 



Naidomorpha. Tubificidae. 



Lumbriculidae. 



Moniligastridae. 



- Perichaetidae. 



Lumbiioidae. 



Phreoryctidae. 



Aeolosoma. 



Archichaetopod. 



