Cercariae are larval stages of flukes. They emerge from mollusks (usually snails) after the mollusk has been infected by a miracidium. Within the mollusk the miracidium undergoes asexual reproduction to produce many cercariae. A cercaria may emerge to infect another intermediate host (as a metacercaria) or, in some taxa, a cercaria infects the primary host.

Schistosoma
Cercaria of the shistosoma parasite