Is there "pain" in Invertebrates?

In contrast to nociception, the perception of pain, or pain experience, remains a subjective notion applicable to humans, but untestable with animals. Yet, when defined operationally as a physiological response induced in an animal by stimuli painful to humans, and resulting in a protective stimulus avoidance response, pain is amenable to testing with non-human subjects. This paper considers a series of examples showing responses to stimuli that are both painful (nociceptive) and responsible for eliciting natural self-preserving behavior in Invertebrates. Consideration is also given to the evolution and possible mechanism underlying the "pain-system" in Invertebrates.