Whether or not you want to eat molluscs (clams, oysters, scallops etc.) is up to you. However, as molluscs are animals, eating them would mean you are not technically a vegetarian anymore.
When I became a vegetarian (and later vegan) my decision was based upon not wanting to cause animals unnecessary pain and death.
My question was not: do they have a face? but do they suffer?
Evolution has given animals the ability to feel pain in order to avoid dangerous, life threatening, situations. Molluscs have a simple nervous system, and are quick to close their shells/move away from the sensation of touch. Therefore, I err on the side of caution and believe that molluscs are most likely able to feel pain.
Vegan zoology honours student