From Wikipedia, the free encyclopedia. The flatworms, or Platyhelminthes, Plathelminthes, or platyhelminths (from the Greek πλατύ, platy, meaning flat and ἕλμινς (root: ἑλμινθ-), helminth- , meaning worm) are a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrates.hese are often called flukes, as most have flat rhomboid shapes like that of a flounder (Old English floc). Flóc they have 11,000 11000, species more than all other platyhelminthes, combined and second only to roundworms among parasites on. metazoans