In traditional classifications a phylum of acoelomate invertebrates comprising the flatworms, characterized by a flattened unsegmented body. The simple nervous system shows some concentration of cells at the head end. The mouth leads to a simple branched gut without an anus. Flatworms are hermaphrodite but self-fertilization is unusual. Many species are parasitic. The phylum contains the classes Turbellaria (planarians), Trematoda (flukes), and Cestoda (tapeworms). Molecular evidence now suggests that the turbellarians and trematodes are paraphyletic groups, and that the majority of flatworms are secondarily acoelomate, belonging to a clade with the proposed name Platyzoa. This distinguishes them from those platyhelminths traditionally placed in the order Acoelomorpha, which are primitively acoelomate and descended from an ancient animal lineage that is neither protostome nor deuterostome.

http://www.ucmp.berkeley.edu/platyhelminthes/platyhelminthes.html Brief introduction to flatworms from the website of the University of California Museum of Paleontology