138 AQUATIC EARTHWORMS. [CH. Ill 



Dispersal of Oligochseta. 



The involuntary migration of animals is mainly con- 

 fined to the invertebrates and smaller vertebrates. Some 

 of the former possess special facilities for being carried 

 about from place to place, and it is invariably the case 

 that these species or genera are the most widely dis- 

 tributed. We will commence with a few examples taken 

 from the terrestrial and fresh -water Annelids. Earth- 

 worms as already mentioned are not easily moved from 

 place to place except by their own exertions ; there is 

 here but little assisted emigration. Rivers it is true 

 could, and doubtless do, convey them for considerable 

 distances, as many if not all are capable of surviving a 

 prolonged immersion in fresh water. M. Perrier kept an 

 earthworm for some weeks in a vessel of water, and I have 

 made a similar though not so prolonged an experiment 

 myself. This however would not be of much use as a 

 distributing agent unless they were thus enabled to 

 traverse a desert otherwise impassable. It is possible 

 that in this way a peculiar genus of earthworms, Siphono- 

 gaster, distinguished by a pair of long appendages of 

 problematical use, has been able to pass from tropical 

 Africa to Egypt, the Nile serving as the path. It is 

 difficult however to see how earthworms could be conveyed 

 across the sea 1 . Floating trunks have been observed far 



1 In the case of this as in so many other groups oceanic islands require 

 more study. Earthworms do occur in oceanic islands, but it seems 

 probable that such widely distributed forms as Eudrilus eugenia 

 (St Helena) and Pontoscolex arenicola (Fernando de Noronha) have been 

 accidentally imported. 



