As to the aquatic genera but little is known at present with regard to their range; they have not been widely collected in extra-European countries. What little is known points to the conclusion that while many parts of the world have their peculiar genera (such as _Hesperodrilus_ in South America, _Phreodrilus_ and _Pelodrilus_ in New Zealand), some of the common European species are widely distributed. I have, for example, received _Henlea ventriculosa_ from Kirghiz Tartary, and from New Zealand; and a New Zealand _Tubifex_ appeared to me to be indistinguishable from the common _T. rivulorum_ of our rivers and ponds. It is possible that these and similar instances may, at least in some cases, be due to accidental importation at the hands of man, a matter into which we shall enter later. But the aquatic genera have, many of them, facilities for extending their range in a natural fashion, which are greater than those possessed by earthworms. {370}It has been pointed out that the chaetae of the aquatic Oligochaeta 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[426] to the singular habit of forming a temporary cyst which characterises one species of the genus _Aeolosoma_; this would perhaps tend to facilitate its transference in the way indicated from one spot to another.