CHAPTER I 



STRUCTURAL AND SYSTEMATIC 



The group of segmented, bristle-bearing, worms, 

 termed Oligochaeta by zoologists, comprises what are 

 popularly known as earthworms together with certain 

 forms, inhabitants of ponds, lakes and rivers, which 

 are not so well known as to have received a more 

 distinctive name than merely 'worms.' Their next 

 allies are apparently the leeches and a little more 

 remote marine bristle-bearing worms termed Poly- 

 chaeta ; the three groups, together with perhaps a 

 certain number of other forms belonging to smaller 

 groups, constitute the Annelida which are a distinct 

 and separate assemblage of invertebrate animals. 



The most interesting features about these Oligo- 

 chaetous worms are their very great anatomical varia- 

 tion and the facts of their distribution over the globe. 

 Their importance as geological agents in levelling 

 the ground was made known a long time ago by 

 Darwin, and that aspect of earthworms has remained 

 in much the same position as Darwin left it. We 

 shall concern ourselves here only with the structure, 



B, E. 1 



