da quello che ho letto su applenail.net sembra il collasso del mantello :
The mantle is a sac-like structure that envelops the snail's body and that contains very important organs like the lung and the gills. In normal conditions, the mantle of the snail is aligned with the inside of the shell. Only a thin layer of fluid and slime fills the space between the snail's body tissue and the shell. When the mantle, however, becomes separated from the shell, it collapses. With the collapse of the mantle cavity, the lung collapses as well. Not only the lung function is severely affected, the gills are also suffering a decrease in function, as the water isn't able to flow around the gills. While the snail might survive this condition for weeks, it often turns out to be fethal disease. Unluckily it's not clear what causes the mantle to loosen from the shell and no remedy is known (to the author). If your snail is in this condition, you might choose to kill the snails of the situation does not improve within days
