Whilst engaged in arranging the materials for their Monograph on the Nudibranchiate Mollusks now in course of publication by the Ray Society, it became evident to Messrs. Alder and Hancock, that not only the external characters, but also the internal strucutre, of this interesting group of animals, should be fully investigated. Since then the writers of the following paper have taken up the anatomical part of the subject, and have already been enabled to publish in the 'Annals of Natural History' an account of the anatomy of one genus, Eolis ; another genus, Doris , forms the subject of the present communication. On entering on the present investigation, we were naturally anxious to avail ourselves of the stock of knowledge already collected by our predecessors in this walk of science.