Platyhelminthes phylum contains about 20,00 living species. These species are normally made up of soft bodied, bilaterally symmetrical, and invertebrate animals. This phylum has four classes: free-living (primary aquatic) turbellaria, trematoda, cestoda, and monogenea. The structure of most of these animals marks a major step in animal evolution. these species are most commonly known as flatworms. All flatworms have layers of longitudinal and circular muscle fibers below the epidermis, which are used for locomotion.