Answer: Cephalopoda

When you think about particularly intelligent animals, you likely think of our primate relatives, like monkeys and apes. Or, perhaps, you thought about dolphins, known for their animal intellect, or even perhaps something more domestic like the dog.

While these creatures are all quite diverse from each other in terms of habitats, diets, and so forth, there’s one thing linking them all together: all of them, like humans, are vertebrates with backbones, spinal cords, and brains that are more alike than they are different—in fact, simply being a vertebrate, even if you’re a lowly one like a rat, is generally a good indicator that you’re far more intelligent than any invertebrate, like a worm or a jellyfish.

The most pronounced exception to that rule is found in the invertebrate class, Cephalopoda, that includes octopuses, squid, and cuttlefish (the Coleoidea subclass). The Octopoda order, especially, is home to some of the most intelligent invertebrates on the planet.

Their intelligence has been shown through maze and problem solving abilities, evidence of strong short and long term memory storage, tool usage, and their near legendary ability to escape their enclosures (despite safeguards put in place to prevent them from doing so). This is interesting in and of itself, but it has proven to be an enticing area of research as their entire nervous system is so fundamentally different from that of vertebrates.

Unsurprisingly, cuttlefish and octopuses have the highest brain-to-body mass ratio of all invertebrates.