Can invertebrates suffer? Or, how robust is argument-by-analogy?

It is a popular notion that, compared to vertebrates, invertebrates have a reduced capacity to experience suffering. This is usually based on argument s that invertebrates show only simple forms of learning, have little memory capacity, do not show behavioural responses to stimuli that would cause 'h igher' vertebrates to exhibit responses indicative of pain, and have differ ences in their physiology that would preclude the capacity for suffering. B ut, how convincing is this 'evidence' of a reduced capacity to suffer? Suff ering is a negative mental state - a private experience - and, as such, it cannot be measured directly. When assessing the capacity of an animal to ex perience suffering, we often compare the similarity of its responses with t hose of 'higher' animals, conceptualized in the principle of argument-by-an alogy. By closely examining the responses of invertebrates, it can be seen that they often behave in a strikingly analogous manner to vertebrates. In this paper, I discuss published studies that show that invertebrates such a s cockroaches, flies and slugs have short- and long-term memory; have age e ffects on memory; have complex spatial, associative and social learning; pe rform appropriately in preference tests and consumer demand studies; exhibi t behavioural and physiological responses indicative of pain, and, apparent ly, experience learned helplessness. The similarity of these responses to t hose of vertebrates may indicate a level of consciousness or suffering that is not normally attributed to invertebrates. This indicates that we should either be more cautious when using argument-by-analogy, or remain open-min ded to the possibility that invertebrates are capable of suffering in a sim ilar way to vertebrates.