So that’s what a hermit crab looks like inside its shell!
Update I: In June I managed to find where this photo came from! It was taken by wildlife photographer Frank Greenaway for the publisher Dorling Kindersley, probably about ten years ago. For more information, see this post.
Update II: In August, my sister told me about an interview with marine biologists in New Zealand who are also using glass shells to help study hermit crabs. These crabs voluntarily moved into the hand-blown glass shells with 7-10 days of the glass shells being added to the tanks. (The scientists would not have forced the crabs in, this would not be deemed acceptable by the institution’s ethics committee that oversees all experimental design involving animals and humans)