Have you ever noticed that crabs have their head inside their body? That can feel a little weird if you think about it too much. It’s called a cephalothorax: a head-chest.
Here’s one with its head firmly buried inside its chest on Westray:
Being covered entirely in a hard shell, an exoskeleton, means it’s normally impossible for Shore Crabs to mate. Females wear a near-permanent chastity belt all over their bodies. Males have to wait until the female is almost ready to moult and then carry the female around for days, waiting. Here are my notes from my Nature Notebooks on the Shore Crab from over forty years ago:
Their young live as plankton, floating in the sea, for two or three years before they settle on the seabed.
Here’s an abandoned carapace on Westray’s shoreline:
I let the living crab go very quickly and it walked off, sideways, with its head held high, metaphorically.