Why does a slug shrivel up when you put salt on it?

Sprinkle ordinary salt on a slug and something startling happens: within seconds it starts to wrinkle, foam and shrink, as if the life is being squeezed out of it. But salt isn't acid and it isn't poison in the burning sense; it's the same stuff you put on chips. So how can a few harmless grains of salt make a whole animal shrivel before your eyes?