Platyhelminthes. are commonly called flatworms. There are both. parasitic. (dependent on a host organism for survival) and. free-living. forms of this group. Flatworms, either parasitic or free-living forms, have both sexes on the same individual, a condition referred to as. hermaphroditic. .ost Nematodes are free-living, but some are important parasites of plants and animals. As for the flatworms, nematode development includes all three embryonic tissue layers. Roundworms have a fluid-filled body cavity, that is lined on the outside by tissue derived from mesoderm.