“…As a matter of convenience, therefore, it seems appropriate to leave the animal where it has long been placed, as the most primitive type of living opisthobranch. This work on Acteon, and recent work on Siphonaria (Hubendick, 1947), on the ellobiid pulmonates (Morton, 1954),on Onchidella (Fretter, 1946),on Omalogyraand Rissoella (Fretter, 1948),on the pyramidellids (Fretter & Graham, 1949)and on other tectibranchs (Lloyd, 1952), demonstrate, however, with great emphasis, how close the relationship of all the three major groups of gastropods is at some central point such as this. It seems likely that further research will show that it is from some archaeogastropod nucleus, represented to-day by such molluscs as the trochids, that all these groups have arisen.…”