Why do slugs shrivel up when you put salt on them?

As with most non-ocean living animal and plant cells, salt applied externally creates a situation where the concentration of salt outside the cells is greater than that inside. Ions move into the cells and water then moves out to try and balance the difference to create an equalibrium. This loss of water within the cells causes them to shrivel up and die, furthermore causing the slug to die as well due to the lack of water.