370 OLIGOCHAETA 



It has been pointed out that the ehaetae of the aquatic Oligo- 

 chaeta are generally hooked at the extremity and bifid, which 

 would give them a greater chance of holding on to the feet or 

 feathers of aquatic birds' ; I am not myself disposed to lay much 

 stress on the possibilities of migration by these means, since the 

 tender bodies of the small worms would be liable to be soon dried 

 up by wind while in the act of migration. More likely in 

 every way is a migration when enclosed in the cocoon. The 

 cocoons being small, and often deposited at the edges of ponds 

 frequented by aquatic birds, there would be many chances of 

 their being carried away with tolerable frequency ; moreover, as 

 Dr. Michaelsen has pointed out, the cocoons of some species, 

 particularly among the Enchytraeidae, contain a large number of 

 embryos ; so that when such a cocoon reached a foreign shore 

 there would be a better chance of the species establishing itself 

 there. I have referred elsewhere 1 to the singular habit of 

 forming a temporary cyst which characterises one species of the 

 genus Aeolosoma ; this would perhaps tend to facilitate its trans- 

 ference in the way indicated from one spot to another. 



Earthworms, on the other hand, have not such easy means of 

 travelling from country to country; the assistance which the 

 cocoons in all probability give to the smaller aquatic Oligo- 

 chaeta cannot be held to be of much importance in facilitating 

 the migrations of the earthworms. In the first place, the animals 

 themselves are of greater bulk, and their cocoons are naturally 

 larger, and thus less easy of transportation. Secondly, they are 

 deposited as a rule upon dry land, where the chances of their 

 sticking to the feet of birds would be less ; and thirdly, they 

 are often deposited deep in the ground, which is a further bar to 

 their being taken up. Another possible method by which earth- 

 worms could cross the sea is by the help of floating tree-trunks ; 

 it is, however, the case with many species that they are fatally 

 injured by the contact of salt water. There are, it is true, a few 

 species, such as Pontodrilus of the Mediterranean coast, which 

 habitually live within reach of the waves ; but with the majority 

 any such passage across the sea seems to be impossible. 2 On the 

 other hand, -rivers and lakes are not a barrier to the dispersal of 

 the group. There are a few species, such as Allurus tetragonurus, 



1 Ann. Mag. Nat. Hist. (6) ix. 1892, p. 12. 

 2 Darwin, Vegetable Mould and Earthworms, p. 121. 



