This is the infective stage for the mammalian host. The cercariae invade the second intermediate host, a crustacean such as a crab or crayfish, where they encyst and become metacercariae. Miracidia go through several developmental stages inside the snail : sporocysts, rediae, with the latter giving rise to many cercariae, which emerge from the snail. In the external environment, the eggs become embryonated, and miracidia hatch and seek the first intermediate host, a snail, and penetrate its soft tissues. The eggs are excreted unembryonated in the sputum, or alternately they are swallowed and passed with stool.