Schistosomes have a complex life cycle, in which cercariae, free-living in fresh water, can penetrate healthy human skin. The head of the cercaria transforms into an endoparasitic larva, the schistomule. The schistomules pass several days in the skin then enter the venous circulation and eventually migrate to the lungs (generally within 5 to 7 days of penetration). They then travel through the circulatory system to the heptoportal circulation (after fifteen days) where they mature into adult worms and mate. Depending on the species, the schistosomes migrate to their final infection site either on the bladder or the intestine where the females begin egg production. These eggs are attached the wall of the lumen. The eggs penetrate the wall of the lumen. They are then expelled in the feces or urine. The miracidium, liberated from the egg, seek out snail hosts where they enter a sporocyst stage that eventually results in free-living cercariae that seek out human hosts.