As to how to visualize what’s going on internally one only needs to think about the exoskeleton in terms of support rather than armor, as it is natural to do when looking at an arthropod. Support comes first, protection is only secondary.

I’d been vaguely aware that surface features reflected internal musculature, from my interest in trilobites, but the way that is put gives it a concretely visual sense I hadn’t had before. Are there any pics around that show the external and internal aspects of the same piece of exoskeleton?