A close cousin to the octopus with a very high brain-to-body-size ratio, the squid also swims onto this list. We're used to eating this cephalopod as calamari – fried, grilled or maybe folded into a seafood stew. As with an octopus (but not a human), the squid's brain is quite decentralized. In fact, three-fifths of it resides in the arms and tentacles, making individual tentacles able to act on their own [source: Williams]. However, the squid's brain is made of the same types of brain cells we have – neurons. Thanks to this shared cellular anatomy, we're actually able to learn a lot about ourselves by studying squid.
Squid are more difficult than octopuses to keep in the laboratory, but scientists have found that they too are good problem solvers, especially when it comes to escaping predators. And like humans, newborn squid use their brains to learn through a process of trial and error [source: Schwartz]. Perhaps the smartest part of a squid that scientists know the most about? Their skin. With thousands of color-changing cells called chromatophores just below the surface of their skin, squid can change color to blend into their backgrounds in the blink of an eye.