II] MODE OF LIFE 45 



and species much evidence of particular resemblance 

 with the purely aquatic familes of Oligochaeta. It is 

 therefore particularly interesting to examine into 

 the characteristics of these water-living genera ; for 

 we may expect to be able to deduce from them some 

 hints as to what characters are really to be associated 

 with the purely aquatic life. We can in fact hope 

 to diflferentiate between adaptive and fundamental 

 characters in these animals. 



These secondarily aquatic species can be referred 

 to two categories. There are examples of particular 

 species which differ from their congeners in being 

 aquatic ; and there are whole genera, even sub-families, 

 which are exclusively, or very nearly so, aquatic in 

 habit. The former division need not detain us ; for 

 the actual occurrence of the worms in fresh water 

 instead of upon dry land may be a temporary affair 

 and not a mark of habitual sojourn. Thus I myself 

 found the British and European earthworm Eiseniella 

 (Alluri(s) tetraedrus in the River Plym in Devonshire, 

 while it has been generally met with upon land. The 

 Patagonian and Falkland Island species Notiodrl- 

 lus aquarum didcium was so called on account of its 

 having been collected in fresh water. But its near 

 ally N. georgiamis (which is perhaps even identical 

 with it) was found on the sea shore in the same 

 region of the world. While the differences which the 

 small species of Notlodrdus shows from other purely 



