What are Slugs? The simplest description is that slugs are snails without shells. Some of these mollusks do, in fact, have a shell, but those that do have internalized the shell and use it to store minerals.

Do slugs have internal shells?

Slugs evolved from snails by reducing the size of the shell and internalizing it (yes, most slugs have an internal shell), and there are likely to be consequences of reducing the shell. A snail with an external shell large enough for the body to pull back into.

Why did slugs lose their shells?

Land snails therefore have very thin shells by comparison with their marine relatives. The total shell loss that is seen in terrestrial slugs may be an adaptation to cope with a lack of calcium, and there is evidence that the original distribution of slugs was confined to low-calcium environments.