Mass-position distributions of the clumps A to I in bubble N131. The position axis of the labelled clumps is offset from the bubble centre. Filaments AD, BC, and the pre-existing clump AB have a similar projected length of about 10 arcmin. Filament BC; clumps E, F, G, H, I (i.e. the ring-like shell); and filament AD have a mass of 11 000, 9000, and 11 000 M_⊙, respectively. The error bar is 10% for each mass value.