 Smith &amp; Boyd (1991) assess the evidence for the pain-sensing capabilities of animals in the categories of whether nociceptors are connected to the central nervous system, whether endogenous opioids are present, whether analgesics affect responses, and whether the ensuing behavioral responses are analogous to those of humans (see table 2.3 in Varner 1998, p. 53, which updates the one presented by Smith &amp; Boyd).  On the basis of these criteria, Varner follows Smith &amp; Boyd in concluding tentatively that the most obvious place to draw a line between pain-conscious organisms and those not capable of feeling pain consciously is between vertebrates and invertebrates. However, Elwood &amp; Appel (2009) conducted an experiment on hermit crabs which they interpret as providing evidence that pain is experienced and remembered by these crustaceans. Varner also expressed some hesitation about the evidence for conscious pain in “lower” vertebrates: fish, reptiles and amphibians. Allen (2004) argues, however, that subsequent research indicates that the direction of discovery seems uniformly towards identifying more similarities among diverse species belonging to different taxonomic classes, especially in the domains of anatomy and physiology of the nociceptive and pain systems.