II] MODE OF LIFE 55 



and in the bottom waters of deep lakes must produce 

 a very vast diiference in physiological conditions. 



We have already dealt with the characteristics 

 of the aquatic genera of earthworms, not only in 

 detailing the general characters of the families which 

 are found in this situation but also in studying the 

 features which earthworms show in those cases where 

 they have reverted to an aquatic mode of life. It 

 remains in the present section to attempt to descry 

 in the purely terrestrial forms the remnants of 

 adaptations to an aquatic life which are no longer of 

 service to them. 



It is a noteworthy fact, that the continuous circle 

 of setae which is met with in certain earthworms 

 is by no means a character of such classificatory 

 importance as it was at one time, perhaps, thought 

 to be. It is true that we meet with this character in 

 the genera Megascolex and Pheretima which are not 

 very far from each other in the ystem and are at 

 any rate members of the same sub-family, the Mega- 

 scolecinae. But we also find the continuous circle of 

 setae well developed in Plagiochaeta which is not so 

 near to Pheretima, and an approach towards it in 

 Dinodrilus and Dinodriloides which are equally 

 remote perhaps from both Pheretima and Megascolex 

 on the one hand and Plagiochaeta on the other. 

 Still it may be urged that all of these genera are at 

 least members of the family Megascolecidae and that 



