i] STRUCTURAL AND SYSTEMATIC 15 



characteristically from Notiodrilus by the fact that 

 the paired nephridia alternate in position in successive 

 segments, being now in front of the dorsal, and in 

 other segments in front of the ventral, pairs of setae. 

 While these genera are somewhat degenerate, the 

 New Zealand Plagiochaeta has undergone specialisa- 

 tion in an upward direction. For the setae of each 

 segment are increased to a large number much ex- 

 ceeding eight. 



It is not a long step to the sub-families Diplocar- 

 diinae and Trigastrinae. In the first of these, an 

 American race confined to the northern and central 

 parts of that continent, the male pore shows a 

 tendency to move backwards, being situated on any 

 of segments xviii-xxi. The two spermiducal glands 

 follow it, but are always placed one pair in front 

 and one behind, as in Notiodrilus. In this group we 

 get a new feature of specialisation in the duplication 

 or triplication of the gizzard. 



So too with the Trigastrinae where there are 

 either two or three gizzards ; but in this sub-family 

 another modification has become apparent. The 

 paired nephridia have disappeared and their place 

 is taken by several, often quite numerous, pairs of 

 much smaller nephridia called on that account 'mi- 

 cronephridia ' instead of * meganephridia.' To this 

 sub-family belong the especially African but also 

 American and Malayan Dichogaster, whose name is 



