In a piece on octopus farming, Katherine Harmon mentions a fascinating fact — octopuses don't have an adaptive immune system, the handy-dandy network of different immune-response cells that allow us vertebrates to more easily fight off infections our bodies have encountered before.
That's a problem if you're trying to raise a bunch of invertebrates in close quarters (as per a farm) because you can't immunize them against pathogens that could easily spread from one octopus to another. As a random biological tidbit, though, it's just damned fascinating. Check out this doctoral thesis for more information on how the octopus immune system does work. You should also read this story that looks at the evolution of the adaptive immune system and asks a key question — does having immune "memory" really make us that much better off than the animals that don't have it?
Image: Octopus, a Creative Commons Attribution Share-Alike (2.0) image from alicecai's photostream