128 EARTHWORMS AND THEIR ALLIES [ch. 



porterage. Moreover they are apt to be deposited 

 rather deep down and among the roots of grasses, and 

 in situations where they are not so likely to become 

 entangled in the feet of drinking birds. Assuming, 

 however, that these difficulties can be got over there 

 remains another difficulty. A single cocoon among 

 the terrestrial Oligochaeta does not contain a large 

 number of embryos, as has been pointed out on a 

 previous page. It is true that Allolobophora foetida 

 has six within one cocoon, but most of our indigenous 

 forms have but from one to three embryos in a single 

 cocoon. Thus, if successfully imported, it is hardly 

 likely that the developed embryos scattered after their 

 emergence would come together for breeding purposes; 

 and in cocoons with but one embryo the accidental 

 importation in this way would have to be very frequent 

 to produce any result. 



The case here is exactly the reverse of that afforded 

 by the aquatic families (or many of them). In these 

 Annelids the attachment of the cocoon to water plants, 

 which are liable to be entangled in the feet of shore- 

 frequenting birds, would tend to favour migration. 

 And in addition to this the cocoons are naturally 

 smaller and often contain a considerable number of 

 embryos. We are to note that the aquatic forms are 

 on the whole distinctly wider in their range than are 

 the earthworms. 



