The important discovery that certain aquatic mollusks act as intermediary hosts to the common cattle fluke was made almost simultaneously, but quite independently, by A. P. Thomas, in England, and by Leuckart, in Germany. It was some years later that a similar alternation of generations, involving a molluscan host, was demonstrated for several human parasites. Information on this topic has been accumulating at a rapid pace during the last decade. Malacology, which for a long time was of but little practical importance, now has become a prominent branch of Medical Zoölogy. It may be useful to bring together what is positively known of the subject, since these data are scattered through many publications and as yet inadequately treated in textbooks. A recent study of the African fresh-water mollusks, published by the author in collaboration with Dr. H. A. Pilsbry (1927), includes an account of the medical importance of these animals, which was written from the point of view of the malacologist.