Octopuses: three hearts, nine brains, blue blood
The closest thing to an alien living on Earth.
1An octopus has three hearts: two pump blood through the gills, one serves the rest of the body. Stranger still, the main heart pauses when the octopus swims, which is exhausting — so octopuses prefer to crawl and save the effort.
2It also has nine 'brains': one central brain, plus an independent cluster of neurons in each of its eight arms, letting every arm act — and react — on its own. The blood is blue, because it carries oxygen with a copper-based molecule (haemocyanin) rather than our iron-based haemoglobin.
3Three hearts, nine brains, colour-changing skin and an intelligence that can unscrew a jar lid: it is no wonder the octopus fascinates scientists more than any other invertebrate.
Two hearts for the gills, one for the body; every arm has its own nerve centre.