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.

[1] Originally a model organism for research on developmental biology[2] and the evolution of the bilaterian body plan,[3] it has since expanded to other important fields of research such as sexual selection and sexual conflicts,[4] ageing[5] and the evolution of the bilaterian body plan,[6] ecotoxicology,[7] and, more recently, genomics.

[8] The genus name "Macrostomum", meaning "big-mouthed", derives from the Greek μάκρος makros, "large", and στόμα, stoma, mouth.

Macrostomum lignano, like all other flatworms, is an unsegmented, soft-bodied bilaterian without body cavity, and no specialized circulatory or respiratory organs.

[10] Macrostomum lignano lives interstitially in sandy habitats, at the intertidal or upper-intertidal zone, usually in the upper 5–10 mm.