tristoma

from The Century Dictionary.

• noun A genus of monogeneous trematoid worms, typical of the family Tristomidæ: so called from one large ventral sucker behind two smaller adoral ones. They are of broad and flat oval or discoid form, and infest the skin and gills of fishes.
• noun A worm of the above genus.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Zoöl.) 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.