, pl. cercariae The free-swimming trematode larva that emerges from its host snail; it may penetrate the skin of a final host (as in Schistosoma of humans), encyst on vegetation (as in Fasciola), in or on fish (as in Clonorchis), or penetrate and encyst in various arthropod hosts. Body and tail are greatly varied in form, and specialized function is adapted to the particular life cycle demands of each species.sporocyst (1), redia. [G. kerkos, tail]