Invertebrate animals are usually seen as a kind of “aliens” which do not deserve any moral consideration. However, there is a growing amount of evidenceindicating that many of them do have the capacity to experience pain. The samecriteria that are usually applied in order to infer that vertebrates are sentient beings (behavioral response, learning capacity, memory, a certain specific neurophysiological structure…) lead to the idea that many invertebrates aresentient as well. Therefore, under the skeptical premise that we have no directevidence (...) of the experience of pain in vertebrates, we are forced to hold that itexists in both vertebrates and invertebrates. (shrink)