One night last week, Jerry accidently stepped on a snail on our front walkway, and killed it. The next day, I noticed a slug on top of what was left of it, eating it.
Actually, it looked as if it had just finished eating the shell. The slug seemed to be cleaning up where the snail's innards connected to the last bit of shell. Once the shell was gone, the slug stopped eating.
I had never seen a slug eat a snail shell before. I tried taking a stray bit of shell and offered it to another slug that was passing by, but he wasn't interested.