The Collaborative International Dictionary of English v.0.48:
Cercaria \Cer*ca"ri*a\, n.; pl. Cercarle [NL., fr. Gr. ke`rkos tail.] (Zool.) The larval form of a trematode worm having the shape of a tadpole, with its body terminated by a tail-like appendage. [1913 Webster]WordNet (r) 3.0 (2006):
cercaria n 1: tadpole-shaped parasitic larva of a trematode worm; tail disappears in adult stage