The term "torture them for pleasure" seems rather incongruous in the context
of your querry.  Can someone "torture" an organism that cannot experience
pain?  I find it difficult to imagine someone "torturing" a plant, or a
scallop, for pleasure or for any other reason, since the purpose of torture
is to inflict pain; and one who derives pleasure from the infliction of pain
experiences that pleasure in the reactions of the victim.  Now, if a scallop
could scream . . .  Most human - genuinely human - persons abhor the idea of
intentionally and unnecessarily inflicting pain; but I think the idea of
"torturing" primitive life forms raises questions of  feasibility more than
acceptability.
Apparently such concerns have been voiced though. Recently I had to do a
demonstration of various forms of microscopy, and to make it a little more
interesting, I ordered live cultures of a few common protozoans (Amoeba,
Paramecium, etc).  Believe it or not, the shipment included an insert
explaining how to dispose of the cultures "humanely" (they suggested
microwaving them, or pouring alcohol into them).  Sheesh!
"Sentient" is a loaded term these days, and you'll need to define it
carefully before you propose any hypotheses based on it.  Does sentience
require rationality? thinking? decision making? consciousness? awareness?
reactivity?  When a diver's shadow passes over a scallop bed, and they take
off swimming madly in all directions, there is no doubt as to their
reactivity.  And, they certainly give the impression of being  "aware" of
the diver's presence, in some sense. Are they "conscious"? Hmmmmmmm. Well,
that depends on how you define "conscious".
One last observation - "Brain" is a relative term.  Of course mollusks don't
have a complex brain with all the subdivisions found in the mammalian brain.
But they do have a nervous system, and they do have specialized aggregates
of nerve cells which send out signals to the more peripheral parts of the
system, and receive incoming impulses caused by external stimuli - which is
essentially the same thing our brain, or any brain does.  The computer which
runs my windshield wipers - which I recently replaced at a cost of $300 -
could not run Ebay or AOL - but it is still a computer.