Species belonging to the family schistosomatidae are digenean flukes with some characteristics that are not typical of their many relatives. Schistosomes are elongate worms, uncharacteristically having separate sexes and living in the blood vessels of their definitive hosts. Their development omits the redial and metacercarial stages typical of many digenean species (Dawes, 1968). Like nearly all digenean flukes, however, schistosomes develop in snails. Twelve species of schistosome have been found to infect humans (List, Chapter 1), but in some cases infection does not progress beyond penetration of the skin followed by cercarial dermatitis (see Coombs and Crompton, 1991). Five species have public health significance.