Question

Explain why soft-bodied invertebrates, like slugs, die when you pour salt on them.

Answer 1

Because the salt drys them up 

Answer 2

The process of osmosis is triggered and large amounts of fluid from inside the slug rush to the surface to restore equilibrium and dilute the concentration of salt. This results in the thick goo you see. The unfortunate slug rapidly loses more fluid than it can tolerate, and quickly dies from dehydration.