In introduction there is a brief review of the literature and an outline of the work embodied in the thesis. The thesis is divided into four main parts. Part 1. Gametogenesis and early development in Gigantocotyle bathycotyle (Fischoeder 1901), Nasmark 1937. This includes a description of the genitalia, with particular reference to the female organs and their associated ducts, an account of gametogenesis, egg shell formation and the early cleavage divisions. Part 2. The species of the genus, Paramphistomum , Fischoeder 1901, which occur in the British isles, with notes on some material from the Netherlands and France. Two new species of Paramphistomum are described and they are compared with Paramphistomum cervi (Zeder 1790 ) Fischoeder 1901 hitherto believed to be the only mammalian paramphistome occurring In this country Gametogenesis and early development are described briefly and compared with the processes in Gigantocotyle bathycotyle. The species obtained from the Netherlands and France are noted. Part 3. The development of the miracidium of Paramphistomum hiberniae from the time of deposition of the egg, until hatching. Observations are based almost, entirely on living material as only comparatively small numbers of eggs were available. The description of the miracidium includes notes on the staining reactions of various vital dyes and on the use of poly-vinyl alcohol as a means of keeping the miracidium still, without distortion. The process of hatching is described in some detail. Part 4. The attempts made to infect snails experimentally with paramphistome miracidia. An account is given of the collection and culturing of the eggs. Many hundreds of snails of various species were exposed to paramphistome miracidia. Results so far have been negative.