At the outset I would desire to express my deep obligations to Professor Rolleston in the matter of this paper. I was first informed of the existence of Land-Planarians in Ceylon by Professor Rolleston, and of the importance of investigating the correctness or incorrectness of Schmarda's description of a ganglionated nerve-cord in Sphyrocephalus , to which Professor Rolleston has referred in his ‘Forms of Animal Life,’ as have also many other authors. Professor Rolleston at first agreed that the paper should be a joint one, and himself prepared a large number of sections of Rhynchodemus , one of which is figured, but subsequently decided that my name only should appear in the matter. I have to thank him for suggestions and assistance rendered during the whole of the investigation, which took more than two months’ constant work, and also for help in the getting up of the bibliography of the subject. The work was done in the Anatomical Department of the Oxford Museum. The Land-Planarians the anatomy and histology of which are described in the present memoir were obtained in Ceylon during the months of January and February last year (1872) by the author, in the Royal Botanic Gardens, Peradeniya, Ceylon, through the kind assistance of G. H. K. Thwaites, Esq., F. R. S., the distinguished Curator of those Gardens.