He has been half in/ half out of his shell for several hours. Earlier he was like this but moving his antennae around and looking like he was deciding whether to open up or not. When I got kinda close to the tank he flinched, so I figured I should leave him alone and he'd be fine. But just now I checked on him and was surprised to see he hadn't moved yet. He's several years old- either 2 or 4 years old (I have two that look alike that I got at different times).