On Immunity Reactions of Snails to the Penetration of the Cercariae of the Strigeid Trematode, Cotylurus flabelliformis (Faust)

Abstract

The fact that in the life cycle of the strigeid of the duck, Cotylurus flabelliformis, the same species of snails serve both as intermediate and second intermediate hosts has made possible the study of the relation of previous infestation of these snails with larval trematodes to the penetration of the cercariae and the development of the metacercariae of this species. Since C. flabelliformis is a common paratype of the duck in the region where the University of Michigan Biological Station is located it was possible to obtain material for this study in the summer of I932. The life cycle of C. flabelliformis has been worked out in its various stages by a group of workers in the last sixteen years (Faust, 1917; Cort, 1917; Cort and Brooks, 1928; Hughes, 1929; Van Haitsma, 1931). The sporocysts of this species develop in Lymnaea stagnalis appressa Say, L. stagnalis perampla Walker and Stagnicola (Lymnaea) emarginata angulata (Sowerby). Cercariae begin to escape from these snails about six weeks after the penetration of the miracidia. These cercariae then penetrate into these same species of snails and also into a smaller form Fossaria obrussa Say and develop in a period of about six weeks going through a very great metamorphosis into mature encysted metacercariae of the tetracotyle type. When snails containing these fully developed tetracotylids are eaten by ducks the adults of C. flabelliformis are able to develop to sexual maturity in about four to six days. Winfield (1932) has recently shown that individuals of L. stagnalis appressa and L. stagnalis perampla which were infested with the sporocysts of C. flabelliformis were highly resistant to the penetration of its cercariae, although these two snails varieties serve very effectively both naturally and experimentally as the second intermediate host of this species. He suggested that this immunity plays an important part in the survival of snails harboring the sporocyst stages of this trematode since it prevents them from being overwhelmed by the repenetration of the large numbers of cercariae which escape. The results reported in this paper confirm Winfield's work and extend the studies to include