I] STRUCTURAL AND SYSTEMATIC 3 



give reasons for the classification of them, 

 and thirdly to enable the reader to realise 

 certain structural details which it is abso- 

 lutely necessary to give some account of in 

 order to explain other matters. 



It is for example impossible to attempt 

 any account of the fitness of some of these 

 animals for their terrestrial life and of 

 others for an aquatic life without treating 

 of anatomy to some extent. 



I shall take one particular species as a 

 type and indicate later the principal diver- 

 gencies shown by other forms. According 

 to the general opinion among those who have 

 studied the Oligochaeta I take as a repre- 

 sentative form a Megascolecid (this and the 

 other families are dealt with seriatim on 

 p. 14 et seq.), as this group is presumed to 

 be the oldest, and within that group a 

 representative of the genus Notiodrilus 

 which is with some reason held to be the 

 most primitive genus in the group. Finally 

 I have no particular reason for selecting the 

 species Notiodrilus tamajusi except that 

 there happens to be a longer and fuller 

 description of it than of many. 



Fig. 1. Notiodrilus tamajusi. The worm shown from the ventral 

 surface. About natural size. (After Eisen.) 



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