From The Collaborative International Dictionary of English v.0.48 [gcide]: Platyelminthes \Plat`y*el*min"thes\, n. pl. [NL. See Platy-, and Helminthes.] (Zool.) A class of helminthes including the cestodes, or tapeworms, the trematodes, and the turbellarians. Called also flatworms. [1913 Webster]