A molecular hypothesis of shell coiling in Gastropoda. (A) In a snail with a coiled shell, dpp (red) is expressed asymmetrically in the mantle, and Dpp diffusion causes an asymmetric concentration gradient in the mantle. (B) Asymmetric mantle expansion is induced by asymmetric Dpp localization, because Dpp controls cell proliferation in the mantle . (C, D) As a result of the asymmetric mantle expansion, non-uniform shell growth occurs, and produces a coiled shell. By contrast, in the limpets, a non-coiled shell is formed because the lack of expression of dpp in the mantle results in symmetric mantle expansion and shell growth. L, left; R, right.