Any vegans in the house?
I'm inclined to say 'yes' ...
Why would evolution develop the capacity for 'pain' in an animal that can't do anything about it?
It seems logical that physiological signals should be matched to the type of response they can trigger. Why would a mollusk need a continuous 'fight or flight' signal for a sloooow defence mechanism like pearl-making?
On the other hand, it is quite clear that whatever we call 'pain' is totally dependent on the nervous system: shut down that and there's no pain (anesthetics), turn on the signal only and no real stimulus is needed anymore (nerve damage, memory of pain in missing limbs or organs... that sort of horrid thing).
I'd bet that even mollusks must be getting some information from the outside world to activate their defense mechanisms. Still, it is hard to imagine what kind of signal is that, since mollusks aren't built to do either fight or flight so no such reflex is necessary and not much use for a signal similar to pain. It sounds like pain would be as necessary to them while making pearls to defend themselves, as it is to humans on a dentist chair - i.e. not useful at all. We developed solutions for useless pain in a hundred years or so of scientific phar macy, mollusks had hundreds of millions of years of evolution to develop their response to the environment. What works, works...
Besides, we've also got a bunch of slow, long-term biological defense mechanisms, but they are not associated with continuous pain (scarring on some internal organs and tissues), thank goodness. Like pearl-making, there is too much of a good thing with those too... And this is as far as my imagination goes; it seems to me that this sort of comparison is interesting only in the context we've got here (my colleague shrugged only).