Vegetarians and Mollusks

Why don't vegetarians eat mollusks? I thought of this question at dinner with Lisa several weeks ago. Lisa, who is a vegetarian, couldn't think of a good answer. I mean, there isn't much of a difference between a mollusk (clam, oyster, scallop) and a plant is there? Neither have brains, therefore they do not experience pain. Why don't more vegetarians eat mollusks?