The problems of Gastropodan structure and phylogeny can only be solved with the help of an adequate knowledge of the embryology of the more primitive members of this group. It is unfortunate that the only Gastropod whose development has been followed in detail from the egg to the adult is Paludina , the relatively specialized form which was the subject of Erlanger’s classic investigations (1891). Among the more primitive forms, Acmea and Haliotis have been examined, though only superficially, by Boutan (1899). Another type almost as primitive is represented by Patella , in which the originally paired kidneys are both retained. The development of Patella has an added interest inasmuch as the egg possesses comparatively little yolk and the larva is free-swimming from the beginning. Under these conditions it is reasonable to expect a much greater retention of primitive characters than otherwise.