Shells

Add water to your container to watch your hermit crab come out of his/her shell.  Hermit Crabs use the empty shells from molluscs.

Molluscs show an amazing degree of diversity, yet all
have certain features in common. All have soft bodies and have a strong muscular
foot, which is used for movement or grasping. They also have gills, a mouth and
an anus. A feature unique to molluscs is a file-like, rasping tool called a
radula. This structure allows them to scrape algae and other food off rocks and
even to drill through the shell of prey or catch fish.
 
Add water to your container to watch your hermit crab grow.  Hermit crabs are crustaceans that move into  empty shells from molluscs.