Macrostomum lignano is a free-living, hermaphroditic flatworm. It is transparent and of small size (adults reaching about 1.7 mm), and is part of the intertidal sand meiofauna of the Adriatic Sea. Originally a model organism for research on developmental biology and the evolution of the bilaterian body plan,it has since expanded to other important fields of research such as sexual selection and sexual conflicts, ageing and the evolution of the bilaterian body plan, ecotoxicology, and, more recently, genomics.