Trends and PerspectivesT R E N D S AND P E E S P E C T I V B S is a series of articles which are intended to: 1. Focus attention on developing and growing topics in parasitology. 2. Identify new and old problems in need of research. 3. Explain how results from one area of biology might have some bearing on current studies in parasitology. 4. Present teachers and students with concise summaries of recent discoveries and new theories. None of the articles will take the form of a detailed historical survey of the literature. In seeking contributions for the series, the editors have urged authors to present clear and stimulating personal points of view about those topics which have excited their curiosity and imagination. The editors hope that a wide readership will gain enjoyment and benefit from this enterprise. Paraaitology (1985), 90, 205-216 2 0 5

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Life history traits expressed by organisms vary due to ecological and evolutionary constraints imposed by their current environmental conditions and genetic heritage. Trematodes often alter the life history of their host snails by inducing parasitic castration. Our understanding of the variables that influence the resulting changes in host growth, fecundity and survivorship is insufficient to confidently predict specific outcomes of novel snail-trematode combinations. In a literature review of the last 30 years, we found 41 publications examining various life history characteristics of trematode-infected snails. These publications reported 113 different field and laboratory experiments involving 30 snail species and 39 trematode species and provided a data set for assessing factors that potentially affect life history outcomes. Analysis of the diverse responses across various snail-trematode systems and experimental conditions teased out general patterns for the expression of host growth, fecundity and survival. These were used to address existing hypotheses and develop several new ones relating the response of snail-trematode interactions to environmental and genetic factors. Finally, we propose directions for future experiments that will better assess the ecological and evolutionary factors influencing snail life history responses to trematode parasitism.