56 EARTHWORMS AND THEIR ALLIES [oh. 



the question of a character which thus merely shows 

 affinity is not yet eliminated. It is therefore of 

 particularly great importance that Dr Cognetti de 

 Martiis should have met with the South American 

 genus Periscolex which, undoubtedly a member of 

 a totally distinct family, the Geoscolecidae, yet shows 

 the same complete circle of setae. The reason for 

 dwelling upon this particular anatomical character 

 in the present connection is because it would seem to 

 be a character specially suited to an underground life 

 where there is an equal pressure all round the body 

 and where progression would seem therefore to be 

 best attained by a continual leverage round the 

 circular body. And this view is strengthened by the 

 sporadic occurrence of this modification in different 

 families. We thus come to the conclusion that the 

 opposite state of affairs is a remnant of an aquatic life, 

 a conclusion which it is the object of the present section 

 to discuss. More than this, it would seem that an equal 

 disposition of the two bundles of setae on each side 

 of the body was a less modified state of affairs than 

 the restriction of the two bundles or pairs of setae to 

 the ventral surface, such as occurs for example in the 

 genus Dichogaster and which is very obvious in some 

 of the larger-sized members of this extensive genus. 

 For the restriction of the setae to the ventral surface 

 obviously favours progression upon a surface and not 

 through a medium. And it is only among the 



