116 EARTHWORMS AND THEIR ALLIES [ch. 



comparatively small part of the vast African continent. 

 It also obviously follows, and it is advisable to state 

 this fact however obvious, that no species are common 

 to the two sides of the continent except indeed the 

 ubiquitous Eudrilus, whose range over the world has 

 been more than once referred to in this book. 



On the other hand the genus Dichogaster offers 

 quite different facts, which are in contradiction to 

 those just enumerated. This genus as already said 

 is very characteristic of tropical Africa, and a large 

 preponderance of the known species are confined to 

 that continent. Although there is some variation in 

 structural characters among the many species which 

 compose this genus, there is but little doubt that 

 they are all rightly referred to one genus with 

 perhaps some doubtful, though not very striking, 

 exceptions. In any case the utmost divergence of 

 structure between worms usually placed together 

 in this genus is nowhere near to that which separates 

 the genera of Eudrilidae from each other. Of the 

 African members of the genus the species are pretty 

 evenly divided between the eastern and western 

 halves of the continent ; they are, like the Eudrilidae, 

 tropical in range, not occurring to the southward, 

 where their place is taken by the Acanthodrilinae 

 and Geoscolecidae. There are it is true a few species, 

 such as D. gracilis and D. bolavi, which are common to 

 the two sides of Africa ; but in these cases we clearly 



