Flatworms are in the phylum Platyhelminthes. There are more than 20,000 species of flatworms. They are bilaterally symmetrical and most of them are hermaphroditic (having both types of sexual organs). Most of them (80%) are parasitic and live on or inside other creatures. There are four classes in the Platyhelminthes phylum: flukes (trematoda), tapeworms (Cestoda), planarians (Turbellaria), and Monogenea.