Macrostomid flatworm. Differential interference contrast (DIC) light micrograph of a section through a Microstomum lineare flatworm (family Macrostomida). Macrostomids a small aquatic flatworms that inhabit most freshwater, brackish and marine environments. They measure from around . 5 to 5mm in length and are generally transparent (as here). Magnification: x57, when printed at 10 centimetres wide.