Schistosoma flatworms, coloured environmental scanning electron micrograph (ESEM). This parasitic worm causes the disease bilharzia (schistosomiasis). It has a flat body, 10-15 millimetres long, with a longitudinal groove. Here, several females are coupling with and seated in the grooves of the male flatworms. This flatworm has two cup-like structures on its head. The oral cup is at the front end of the body and opens into the oesophagus. On the underside of its head is the ventral fastener (pink) used to attach to its host. This parasite is found in the blood vessels around the bladder.