endoskeleton: the internal skeleton; bony and cartilaginous structure (especially of vertebrates)
skeletal system: the hard structure (bones and cartilages) that provides a frame for the body of an animal
elves: an acronym for emissions of light and very low frequency perturbations due to electromagnetic pulse sources; extremely bright extremely short (less than a msec) electrical flashes forming a huge ring (up to 400 km diameter) in the ionosphere