Cercaria

from WordNet (r) 3.0 (2006)

cercaria
    n 1: tadpole-shaped parasitic larva of a trematode worm; tail
         disappears in adult stage

from 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]