Introduction

There are about 400 species of freshwater snail (including the limpets) in Africa. Most are known only to specialists, but some are all too familiar to residents and visitors alike, as the source of the ‘peril in the water’—the microscopic cercariae that produce the parasitic infection in man that causes the disease schistosomiasis (bilharzia). To find out more about the species of snail which are the intermediate hosts for schistosomes became a priority about 40 years ago, when the World Health Organisation and some national health authorities began to give serious consideration to the possibility of controlling schistosomiasis. The resulting financial support for malacological studies has advanced knowledge of many species besides those of medical or veterinary importance. Collectors have searched areas where the aquatic molluscan fauna was unknown. Whole animals have been obtained for the study of many species described by earlier taxonomists from only empty shells. Comparative morphology, and the use of biochemical and cytological characters in taxonomy, as well as growing knowledge of distribution and ecology, are all contributing to a better understanding of the species as biological units, and of their histories. Studies focused on the remarkable radiation of prosobranch snails in Lake Tanganyika are providing new insights into the processes of speciation within ancient lakes.