Of this family one entire sub-family, the Criodrilinae, is very nearly purely aquatic in habit. Two genera, viz. _Callidrilus_ and _Glyphidrilus_, out of another sub-family, Microchaetinae, which contains perhaps five other genera, are also aquatic in their mode of life. In examining into the characters of the various aquatic species with a view to searching for common characters which might be put down to modifications induced by the aquatic life, there are two or three which arrest attention. In the first place the Criodrilinae never possess a well-developed gizzard, having at most a rudimentary gizzard, or even two. However this character is not of universal applicability, for both _Callidrilus_ and _Glyphidrilus_ have got a gizzard and a strong one. These later genera however have no calciferous glands or oesophageal pouches of any description, structures which are also absent among the Criodrilinae. It will be remembered that the purely aquatic families, Tubificidae, Lumbriculidae, etc., rarely show signs of a gizzard and rarely possess oesophageal pouches of any kind. In view of the fact that in the case of a life in fresh water no calcareous salts are necessary to resist the acids of the soil, and that the mud passed through the alimentary canal is already finely divided, it is not surprising to find gizzard and calciferous glands absent or rudimentary. Another not unusual feature among these aquatic Geoscolecidae is the quadrangular form of the posterior end of the body. This is shown--as its specific name denotes--by _Glyphidrilus quadrangulus_, by species of _Alma_ and in all the species of the genus _Criodrilus_. It is to be noted in this connection that a species of the partly aquatic _Eiseniella_ has been named _tetraedrus_ on account of precisely the same phenomenon. In these cases it is the posterior part of the body which is thus quadrangular; the anterior segments down to the ninth in _Criodrilus_ being rounded in the usual Oligochaetous fashion. As the paired setae are apt to lie in the four projections of the quadrangular body, one is tempted to see in this arrangement of structures a faint approach to the dorsal and ventral parapodia of the marine worms, and in any case it seems possible that by this means the worms can cling more effectively and continuously to the stems and leaves of aquatic plants among which they so largely live.