Flatworms (Platyhelminthes) occupy a variety of environments, they live in the sea, in freshwater and some even on land. Many flatworms are parasites. This comprehensive book deals with the free-living taxa, parasites are treated with in a future volume. Many free-living flatworms are microscopically small and live in marine and freshwater sediments. They are almost unknown to amateurs, but diverse in morphology and in their reproductive biology. Better known are larger flatworms, either the freshwater planarians, the tropical land-planarians or the often colorful marine polyclads. Phylogenetically the free-living flatworms form the paraphyletic stock that leads to the derived parasitic Neodermata. This volume provides an in-depth insight into the free-living flatworms written by experts. It includes morphology, phylogeny and reproductive biology and contains more than 200 illustrations.