A phylum of the invertebrates, commonly called the flatworms. They are bilaterally symmetrical, nonsegmented worms characterized by lack of coelom, anus, circulatory and respiratory systems, and exo- and endoskeletons. Many species are dorsoventrally flattened. They possess a protonephridial (osmoregulatory-excretory) system, a complicated hermaphroditic reproductive system, and a solid mesenchyme which fills the interior of the body. Some parasitic species, that is, some trematodes, have secondarily acquired a lymphatic system resembling a true circulatory system. Some species of trematodes, the schistosomes, have separate sexes.