Are Snails Slugs With a Mortgage?

[faustus]

Yesterday as I walked to the pub in the rain, I noted how many slugs there were on the pavement (after brief thoughts about dogs and their, er, droppings). Today as I walked home from the pub in the rain, I noted how many snails there were on the pavement.

Maybe the shells allow snails to survive in the damp for longer before having to evacuate. Trapped air might allow them to float. Or something. Or the gastropods are leaving the planet, species by species.