One of the best things about dealing with shells is how interesting they are and how much I learn from them every day. Many shells are a creation from sea snails and have adapted to their environments in the most fabulous way.
Take the catseye for example. These are actually created by sea snails to protect themselves from the environment and close the aperture of the shell when the soft parts of the animal are retracted. They grow on the snails “foot” and act as a lid or “trapdoor” for the entry of their little shells (homes). In their perfect state the circumference of the catseye (the Operculum) matches the opening of the main shell. Its basically there to close the “entrance” of the shell.