To have interests, in a strict, nonmetaphorical sense, a being must be capable of suffering or experiencing pleasure. At this point, he goes into an interesting discussion of whether fish can suffer and what constitutes this suffering. Since it is so easy to avoid eating them, I now think it better to do so. Another source I found that addressed the same concern is in a chapter called Animal Awareness in Tom Regan's The Case For Animal Rights. Where one draws the line regarding the presence of consciousness is no easy matter, but our honest uncertainty about this should not paralyze our judgement in all cases.
Endorphins are chemicals that have strong painkilling effects in humans.
This report led animal rights advocate Henry Rollin to conclude that the evidence suggests that invertebrates do feel pain. His entire book reads as a literature review and dispute of prominent animal liberationists. Leahy is representative of those who think that the animal rights activists are taking animals too seriously. In Against Liberation, he basically says that the liberationists' arguments are not convincing enough for him and that their paths converge in the wrong direction. Leahy also tries to define pain and admits that is difficult to know where to start in relating animals and pain.