text: why does salt kill snails?   land mollusks are not as "waterproof" as the reptiles, birds and mammals.  if you think about it, most mollusks are aquatic (snails, mussels, oysters, squids, etc.), and the species that have "conquered" life on land are not very impermeable, and must keep their body surfaces hydrated (using mucus) in order to avoid dessication. they're more vulnerable to dessicant agents such as salt (or talcum powder, or dust, for that matter) that take away their internal water.