The presence of a species (_P. lemani_) of the characteristically marine genus _Plagiostoma_, in the Lake of Geneva, and in one or two other Swiss lakes, at depths varying from 1 to 150 fathoms, is very interesting, and is perhaps the only well-established case of the survival of a once marine Rhabdocoelid under changed conditions. _Plagiostoma lemani_ is by far the biggest of the group to which it belongs, being over half an inch in length. It is usually found in fine mud, sometimes among _Chara hispida_, and has the general appearance of an inactive white slug. We are indebted to Forel and Duplessis for the discovery of this species, and also of _Otomesostoma morgiense_, a _Mesostoma_ with an otolith, dredged in 10 to 50 fathoms in the Lake of Geneva, the Lake of Zürich, and found recently also by Zacharias in the Riesengebirge. The genus _Bothrioplana_, first found by Braun in the water-pipes of Dorpat, has been carefully investigated by Vejdovsky,[66] who places it in a special family, Bothrioplanidae, among the Alloeocoela. One species has recently been found near Manchester. A comprehensive survey of the Rhabdocoelida shows that, with the chief exception of the Proboscidae, the more lowly organised forms, the Acoela and Alloeocoela, are marine, whereas the fresh-water forms are in most cases the most highly organised {47}genera (_Mesostoma_, _Vortex_). But _Macrorhynchus helgolandicus_, though minute (1.5-2 mm. long), has a more complex structure[67] than any other species of the specialised marine genus to which it belongs, and is a remarkable instance of great complexity being associated with small size.