This slide shows a pair of adult blood flukes in copulation.  Blood flukes differ from most other flukes by being dioecious (i.e., having separate sexes).  Males are larger and have a large, ventral groove called a gynecophoric canal posterior to the ventral sucker that holds the smaller  (more darkly stained) female during copulation, which is continuous. Schistosoma mansoni is one of the three species of blood flukes responsible for the disease in humans called schistosomiasis.  Humans get infected when the tailed cercaria larvae (which escape from freshwater snails that serve as their intermediate hosts) burrow into the exposed skin  of individuals bathing, swimming or working in such habitats.