Shell fish have no nervous system, and they cant feel pain.
I don't know where you came up with that idea, but it's definitely not true. While I'm not intimately familiar with the anatomy of clams, mussels, scallops, etc., they do respond to touch. As for shrimp, crab, & lobster - they DEFINITELY have excellent well-documented nervous systems.
If you understandably enjoy eating shellfish, so be it. But please don't do so & call yourself "vegetarian" because you erroneously believe they're creatures without feeling.