Hermit crabs rely on their shells for protection, but will leave their current shell for a new one when they get too large or if their shell is damaged. A crab will eventually choose to occupy a glass shell if it is placed in its tank and this allows them to be observed in an interesting way. The Blue Reef Aquarium in Newquay attempted this with shells that were hand-blown and based of of a design dating back to 1930. The reasoning was to be able to examine the inner workings of the hermit crab without damaging the outer shell. These pictures are from a different source, but it is quite incredible to see how the crab folds up its body to fit inside. The photo to the right shows the hermit crab displaying its interest in the shell before it climbed in.

Sources: dkimages1, dkimages2, bluereefaquarium.