Most of our planet’s biodiversity is still unknown, particularly in the sea. Although around the island of Sylt in the North Sea, the small zoobenthos (meiofauna) has been studied intensively since the 1950s, repeating previous surveys revealed an unexpected wealth of new species in addition to the 330 species of free-living microturbellarians (non-parasitic Platyhelminthes) already known from this area. Extrapolation from well-known to less-known habitat types suggests that a total of some 520 Platyhelminth species should be expected around this island, about 670 in the North Sea ecoregion, and 830 in the ‘Northern European Sea’ ecoprovince. Assuming that the other biogeographic provinces of the planet harbour a similar diversity, a total of some 20,000 marine microturbellarian species is estimated for the global shelf zones. Less than 10% of these are known by now. As a contribution to fill that gap, ten new taxa are described: Coelogynopora minuta n. sp., Coelogynopora sopottehlersae n. sp., Cirrifera paraculeata n. sp., Boreocelis fragilis n. sp., Postbursoplana noldti n. sp., Promesostoma wehrenbergi n. sp., Ptyalorhynchus oculatus n. sp., Acrorhynchides canaliculatus n. sp., Dactyloplana n. gen., and Dactyloplana tridigitata n. sp.