Two Skeletons?
This is too fascinating to pass up!
According to this article on Scientific American, blue crabs have two skeletal systems. The first, which is familiar to us all, is their shell. But, when they molt, they use another kind of skeletal system which relies on hydrostatic pressure to get around.
This leads me to imagine an intelligent alien that has a second, invisible, “back-up” skeleton, for emergencies. Or perhaps to make them flexible in some way. Imagine a shelled creature that escapes from a jail by shedding its shell and slithering out an air vent. Or, a humanoid wrestler or warrior who’s neck can’t be “broken”. Really, the possibilities are endless.
Just think about them some, then write about them!