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]