What is the only invertebrate with intelligence, and consciousness?

Cephalopods are an island of mental complexity in the sea of invertebrate animals. Because our most recent common ancestor was so simple and lies so far back, cephalopods are an independent experiment in the evolution of large brains and complex behavior. If we can make contact with cephalopods as sentient beings, it is not because of a shared history, not because of kinship, but because evolution built minds twice over. This is probably the closest we will come to meeting an intelligent alien.

Vertebrate brains all have a common architecture. When vertebrate brains are compared to octopus brains, all bets - or rather, all mappings - are off. There is no part-by-part correspondence between the parts of their brains and ours. Indeed, octopuses have not even collected the majority of their neurons inside their brains; most of the neurons are found in their arms.

One central idea is that our body itself, rather than our brain, is responsible for some of the "smartness" with which we handle the world. Our body's own structure encodes some information about the environment and how we must deal with it, so not all this information needs to be stored in the brain. The joints and angles of our limbs, for example, make motions such as walking naturally arise. Knowing how to walk is partly a matterof having the right body. As Hillel Chiel and Randall Beer put it, an animal's body structure creates both constraints and opportunities, which guide its action.

Some octopus researchers have been influenced by this way of thinking, especially Benny Hochner. Hochner believes these ideas can help us grasp the octopus/human differences. Octopuses have a different embodiment, which has consequences for their different kind of psychology.

I agree with that last point. But the doctrines of the embodied cognition movement do not really fit well with the strangeness of the octopus's way of being. Defenders of embodied cognition often say that the body's shape and organization encodes information. But that requires that there be a shape to the body, and an octopus has less of a fixed shape than other animals. The same animal can stand tall on its arms, squeeze through a hole little bigger than its eye, become a streamlined missile, or fold itself to fit into a jar. When advocates of embodied cognition such as Chiel and Beer give examples of how bodies provide resources for intelligent action, they mention the distances between parts of a body (which aid perception) and the locations and angles of joints. The octopus body has none of those things - no fixed distances between parts, no joints, no natural angles. Further, the relevant contrast in the octopus case is not "body rather than brain" - the contrast usually emphasized in discussions of embodied cognition. In an octopus, the nervous system as a whole is a more relevant object than the brain: it's not clear where the brain itself begins and ends, and the nervous system runs all through the body. The octopus is suffused with nervousness; the body is not a separate thing that is controlled by the brain or nervous system.

The octopus, indeed, has a "different embodiment," but one so unusual that it does not fit any of the standard views in this area. The usual debate is between those who see the brain as an all-powerful CEO and those who emphasize the intelligence stored in the body itself. Both views rely on a distinction between brain-based and body-based knowledge. The octopus lives outside both the usual pictures. Its embodiment prevents it from doing the sorts of things that are usually emphasized in the embodied cognition theories. The octopus, in a sense, is disembodied. That word makes it sound immaterial, which is not, of course, what I have in mind. It has a body, and is a material object. But the body itself is protean, all possibility; it has none of the costs and gains of a constraining and action-guiding body. The octopus lives outside the usual body/brain divide.

Octopuses and other cephalopods have exceptionally good eyes, and these are eyes built on the same general design as ours. Two experiments in the evolution of large nervous systems landed on similar ways of seeing.
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The most dramatic similarity is the eyes. Our common ancestor may have had a pair of eyespots, but it did not have eyes anything like ours. Vertebrates and cephalopods separately evolved "camera" eyes, with a lens that focuses an image on a retina.