The Collaborative International Dictionary of English v.0.48:

Tristoma \Tris"to*ma\ (tr[i^]s"t[-o]*m[.a]), n. [NL., from Gr.
   tri- (see Tri-) + sto`ma mouth.] (Zool.)
   Any one of numerous species of trematode worms belonging to
   Tristoma and allied genera having a large posterior sucker
   and two small anterior ones. They usually have broad, thin,
   and disklike bodies, and are parasite on the gills and skin
   of fishes.
   [1913 Webster]