Interesting question.  This is posed as a philosophical question about which
we may speculate, and it is just as well, as we cannot know this for certain.
 Perhaps one day, someone will figure out a way to graph the electrical
activity in a neural pathways within the primitive molluscan nervous system
and compare it to the pain response in higher animals and then, one could
postulate a pain response.
         Since we cannot know for sure, those who find this to be a
tantalizing puzzle may freely philosophize.  As for me, I think that Mollusks
probably do sense something as they do respond to predatory threats and
irritations, but, it is possible that as their capacity for thought (as far
as we know!) is limited, perhaps their experience of pain is not as intense
as with other higher animals with more complicated nervous systems.
        One thing we may agree on: Life Loves to Live....and will do
everything possible to fulfill its innate sense of this essential Instinct.