Octopuses change their texture using small regions in their skin known as papillae. In these structures, muscle fibers run in a spiderweb pattern, with both radial spokes and concentric circles. When these fibers contract, they draw the soft tissue in the papillae towards the center. And since that tissue doesn’t compress very well, the only direction it can go is up. By arranging the muscle fibers in different patterns, the octopus can turn flat, two-dimensional skin into all manner of three-dimensional shapes, including round bumps, sharp spikes, and even branching structures.
[ed. Aliens (but tasty).]
