Abstract
The fixation of gonochorism (separate sexes) from an hermaphrodite ancestor in the Schistosomatidae may have been the result of selection for dimorphism, under the particular constraint of having to expel eggs from the vertebrate circulatory system. The hermaphroditic system, which exists in blood parasites closely related to Schistosomatidae, e.g. Spirorchidae and Sanguinicolidae, may result from a weaker constraint caused by the different pathways of egg output, and a weaker aggregation of parasites in the host population that will act against the fixation of gonochorism. The importance of these two factors relative to two others (inbreeding depression and allocation to female function in hermaphrodites) in the evolution of gonochorism in blood parasites is evaluated using an analytical model.