These secondarily aquatic species can be referred to two categories. There are examples of particular species which differ from their congeners in being aquatic; and there are whole genera, even sub-families, which are exclusively, or very nearly so, aquatic in habit. The former division need not detain us; for the actual occurrence of the worms in fresh water instead of upon dry land may be a temporary affair and not a mark of habitual sojourn. Thus I myself found the British and European earthworm _Eiseniella_ (_Allurus_) _tetraedrus_ in the River Plym in Devonshire, while it has been generally met with upon land. The Patagonian and Falkland Island species _Notiodrilus aquarum dulcium_ was so called on account of its having been collected in fresh water. But its near ally _N. georgianus_ (which is perhaps even identical with it) was found on the sea shore in the same region of the world. While the differences which the small species of _Notiodrilus_ shows from other purely terrestrial members of the same genus are trifling, further information may prove that this case is on all fours with that of _Eiseniella_ referred to above. There are plenty of similar instances which I shall not enumerate. We may now therefore pass on to the second category. These examples are obviously much more important because they are of worms which appear to be wholly aquatic, or very nearly so, and which belong to definite genera easily distinguishable as such from their allies. The examples are not however very numerous. And they belong practically exclusively to the family Geoscolecidae, a family which, it will be seen later, is confined to South America, South Africa, Madagascar, certain parts of India and Burmah and of Europe. It is not a family which has reached the greater part of the East or which has been carried to the Antarctic parts of the globe. It is furthermore very important to bear in mind that there are reasons for regarding this family Geoscolecidae as one of the more modern branches of the Oligochaeta; this latter statement tends to prove that the aquatic life is, as already suggested, a secondary matter in these worms, and is not due to their belonging to an ancient race which has never left the waters of the land.