The present paper will include the data we have accumulated during our studies at the University of Michigan Biological Station on the effect of infections with larval trematodes in lymnaeid snails on their second intermediate host relations to the strigeid trematode, Cotylurus flabelliformis. Three of the papers of this series have already dealt with certain aspects of this phase of the subject (Winfield, 1932; Nolf and Cort, 1933; Cort, Olivier and Brackett, 1941). In the four papers of this series already published an account has been given of the life cycle of this trematode, the development of its metacercariae, and the methods used in our studies, which need not be repeated here. For the purpose of comparing the extent of development, the metacercariae counted in our experiments were frequently classified into three arbitrary groups according to the stage of development, viz., (1) (all stages up to the largest and those beginning to reorganize), (2) pre-cysts (those with a distinct hind-body but without a cyst wall, and (3) cysts (those that had completed their development and were surrounded by a cyst wall). For the convenience of the reader, the names of the trematode species mentioned in this paper and the pertinent literature on their larval stages are listed in Table 1. It has been shown in a recently published paper (Cort, Brackett and Olivier, 1944) that the metacercariae of C. flabelliformis can complete their development in several different species of lymnaeid snails. When such snails do not harbor the germinal sacs of trematodes, the metacercariae are localized in the hermaphroditic gland which lies imbedded in the digestive gland. In this location they go through their profound metamorphosis from the cercarial body to the encysted metacercarial stage (tetracotyle) in close contact with the host's tissues and feed directly on its substance. When, however, sporocysts or rediae of trematode parasites are present in the digestive gland of the snails, the metacercariae of C. flabelliformis develop as hyperparasites inside them. In this location they are cut off completely from the tissues of the snail and must feed on the material absorbed by the germinal sacs and elaborated by them for the nourishment of their developing progeny. Whenever the cercariae of C. flabelliformis are able to get into sporocysts or rediae, the metacer-