I was looking for reference photos for a stuffed octopus I was making, when I stumbled across some interesting information on cephalopod eyes.
I already knew a bit about how cephalopod eyes are different from vertebrate eyes--the eye of the octopus is often used as an example of convergent evolution (that is, where similar structures evolve from different beginnings due to similar uses)--and I knew that in some ways, the octopus eye works better than the human eye (it doesn't have the same blind spot from the optic nerve, for example), but there are some really cool things I didn't know about cephalopod eyes.
The most obvious difference between cephalopod eyes and human eyes is that cephalopods have horizontal pupils. Not only that, but because the eyes can rotate in a way that most vertebrate eyes cannot, and because cephalopods have a balance organ called a statocyst, they can always keep their pupils horizontal, no matter what position their body is in. This means their brains can always interpret visual information the same way, and not have to account for the position of the eye.
Cephalopod eyes can also see polarized light, allowing them to communicate by creating changing patterns on their skin that we humans can't see except with the help of special cameras.
The difference between cephalopod and vertebrate eyes partly stems from their very beginnings. While vertebrate eyes develop as an extension of the brain, cephalopod eyes started out as light-sensitive skin cells that folded inwards to form the structure they have now.
Both types of eyes developed retinas, corneas, irises and lenses, but the way those structures are arranged and used is different. The light-receptive cells in cephalopod eyes point directly outward into the light, while those of vertebrates point inward, instead catching light reflected off the back of the eye.
Vertebrate eye lenses are flexible and the can can be focused by special muscles that change the shape of the lens. Cephalopod eye lenses are inflexible and have their focus fixed on a relatively nearby point, but can be focused with muscles that move the entire lens closer to or father from the retina.
So there you have another reason why cephalopods are cool: they've got really interesting eyes that are both very similar to ours, and very different.
Image credit: "Difference between human and octopus eyes" by Jerry Crimson Mann, 2005, used under the Creative Commons Attribution ShareAlike 3.0 License and found on Wikimedia Commons.