Which animals feel pain?

In humans, nocioceptors -- neurons specialized for sensing noxious stimuli -- are connected to a central nervous system, and the resulting signals are processed in the brain. ...the human nocioceptive system also includes endogenous opiods, or endorphins, which provide the brain with its natural "pain-killing" ability. ... If we want to know whether it is reasonable to believe that a particular kind of animal is capable of feeling pain, we may ask: Are there nocioceptors present? Are they connected to a central nervous system? What happens in that nervous system to the signals from the nocioceptors? And are there endogenous opiods? In our present state of understanding, this sort of information, together with the obvious behavioral signs of distress, is the best evidence we can have that an animal is capable of feeling pain.

-- James Rachels, "Drawing Lines", in Cass R. Sunstein and Martha C. Nussbaum (eds.), Animal Rights: Current Debates and New Directions