How do slugs feel?

The snail and slug clade (Gastropoda) is a very vast class of invertebrates. They do not have a true "brain" per-se, but rather a group of ganglia. While it has not been conclusively proven that sea snails such as conches and abalone feel pain, there is evidence that Heterobranchia snails (such as the common garden snail) have nociceptors (pain receptors). In addition, fear conditioning and avoidance learning have been documented in snails of this clade.

That being said, "only the animal knows" if they feel pain, as we are unable to experience what they are experiencing at any given moment. It could be that pain is felt in a different way we are as of yet unaware of.