Among the aquatic Oligochaeta and many earthworms (the families Lumbricidae, Geoscolicidae and a few other genera) the spermathecae are simple structures, as has been described.
In many Geoscolicidae, however, and certain Lumbricidae and Perichaetidae, there are several, even a large number, of pairs of very small spermathecae to each of the segments which contain them.
It is a remarkable fact that these genera, comprizing a separate family Lumbricidae, when introduced into tropical and other countries, thrive abundantly and oust the indigenous forms. In gatherings of earthworms from various extra-European countries it is always found that if the collections have been made in cultivated ground and near the coast the worms are of European species; farther inland the native forms are met with.
Inasmuch as in every case the Lumbricidae from non-European countries are identical with European species, since it has been shown that these animals are very readily introduced accidentally with plants, &c., and in view of the fact that they are impatient of sea water, it seems clear that the presence of these Lumbricidae in other continents is due to accidental transportation.