44 EARTHWORMS AND THEIR ALLIES [oh. 



kind of need that we can as yet appreciate. Com- 

 paring the Terricolous Oligochaeta with other large 

 groups of the animal kingdom, all or nearly all the 

 members of which lead a closely similar life, such as 

 the frogs and toads, or the land mollusca, or snakes, 

 we get a much wider range of structural change in 

 the Oligochaeta than in any of these. 



We shall now consider their mode of life and their 

 relations to the environment. 



The mode of life of earthworms seems at first sight 

 to need no special chapter or section. They simply 

 live in and beneath the soil, leaving it at times to 

 wander over the surface especially at night and 

 during wet weather. But there are a number of 

 species which habitually lead an aquatic life and 

 whose characteristics therefore demand consideration. 



Aquatic Earthworms. 



Although it is perhaps somewhat of a contradic- 

 tion to speak of aquatic earthworms the collocation 

 of words will serve to emphasise the fact that there 

 are species of Oligochaeta belonging to the tribe 

 Megadrili or terrestrial group, which are as purely 

 aquatic in their habits as is a Tubifex or Limno- 

 drilus. In such cases we may fairly assume rather a 

 return to an aquatic life than the persistence of such 

 a habit. For we do not find among these genera 



