Slug Fact

Snails and slugs are both gastropod mollusks. The big difference of course is the shell. Snails live inside their shells. Most slugs you’re likely to run into live around their shell. It’s internal. (There are three families of slugs that don’t have shells at all.)

This decomposing slug shows off the calcareous, plate-like form, eroded out, as it were. These internal shells largely function as calcium storage for the animal.