2 EARTHWORMS AND THEIR ALLIES [ch. 



habits, and range of the earthworms and their im- 

 mediate allies, the aquatic Oligochaeta. These three 

 aspects of the animals dovetail into each other more 

 thoroughly than is the case with some other groups. 

 This is due to the fact that they have of late years 

 been very thoroughly studied from the anatomical 

 and distributional side. So lately as 1889, M. Vaillant 

 in a very comprehensive treatise was only able to 

 enumerate 369 species, of which a large number 

 were but incompletely differentiated, and some are 

 no longer admitted. There are at the moment of 

 writing perhaps 1500 species, the vast majority of 

 which are well known owing to careful investigation. 

 Furthermore there are but few parts of the world, 

 and these are not of large area, from which earth- 

 worms at any rate have not been gathered. Though 

 there can be no doubt that a very considerable number 

 of species await discovery, it would seem that we are 

 in possession of information which is not likely to be 

 seriously affected by future researches. 



The Anatomy of Earthworms. 



Although it is not contemplated to make the 

 present volume a guide to the structure of this 

 group of worms, it is necessary to give some little 

 anatomical sketch of the group in order first of all 

 to illustrate their diversity of structure, secondly to 



