THE ANATOMY. GEOGRAPHICAL DISTRIBUTION 149 



The means of dispersal (other than through human agency) possessed by Oligochaeta 

 are less than those which are possessed by many other animals. Salt water is 

 fatal to, at any rate, many (although Pontodrilus, Glitellio, Vermiculus, and various 

 Enchytraeids are, of course, exceptions) ; and even prolonged, though in many cases 

 it has to be very prolonged, immersion in fresh water will drown some earthworms. 

 Consequently, floating tree-trunks can hardly be safely made use of to explain the 

 presence of a similar fauna on the opposite sides of a tract of sea, even if this sea 

 were, through melting ice, comparatively fresh. 



The wide distribution of many land MoUusca, which, like the Oligochaeta, are 

 incapable of much active migration on their own account, is believed to be at least 

 partly explicable by their transference, as adults, or as eggs in the mud which 

 clings to the feet of birds. The cocoons of the Oligochaeta seem in many cases 

 suitable for carriage in this fashion^ — more especially perhaps those of the aquatic 

 Oligochaeta, which are not only deposited in the situations frequented by wading 

 birds, but are small in size. With earthworms the case is rather different, for 

 the cocoons are deposited deep in the ground. Michaelsen has properly drawn 

 attention to the fact that many Enchytraeidae have cocoons from which a large 

 number of young emerge ; in such cases the transference of even a single cocoon 

 might be sufficient to stock a new country. Among earthworms the number of 

 embryos that reach maturity in a single cocoon is very limited. 



The conditions of life of Aeolosoma favour its wide migration. The practice of 

 forming cysts by the mature worm would give it a double chance (for there is 

 presumably a cocoon formed too, though nothing is known about the matter) of 

 being carried away accidentally. 



Corresponding to the restricted capacities of earthworms for migration, we do not 

 find, with a few exceptions to be noted immediately, that species are widely 

 distributed. I may remark that I am compelled to practically leave aside the 

 aquatic forms, owing to our comparatively small knowledge of them. The species of 

 Allolohophora and Lwmhricus form the most conspicuous exception to the above 

 statement ; but, as I point out later, there is every reason for considering their 

 wide distribution to be due to human agency. The following species, not belonging 

 to those families, have also a very wide range. 



Eudrilus eugeniae, Africa, America, New Zealand, Ceylon, &c. 



Pontoscolex corethrurus, America, Australia, India, &c., and a few Perichaeta 

 (P. afflnis, P. indica, and P- houUeti). 



As to the second of these instances, it is noteworthy that the species can live 

 on the sea-shore (as indeed its name denotes) ; hence there are possibly not such 



