Block diagram showing the location of the most frequent habitats colonized by Galba truncatula (○) and Omphiscola glabra (&z.cirf;) in a central French grassland on acid soil. Most of these meadows have a surface drainage system to evacuate runoff. In the example shown in this figure, four swales on each side open into the main ditch. Two temporary sources (TS), located on the hillsides, discharge their water at the upstream end of the drainage ditch. HZ, hygrophilous zone; MZ, mesophilous zone.