Well they are different species for a start, but the shell is not necessarily a difference between them !
A given slug may have a similar range of differences commared to a given snail as a Human has compared to a Gorilla.
Most slugs have a "vestigial shell" internally and some have a shell which is visible outside the soft body.
Here is a link to a picture of a "Green Shell Slug:
-- Best wishes - Majikthise.
Snails take up to 2 years to mature, and live up to twelve years, while slugs mature in one year and usually die after two years.
nothing, which is why it's weird.
lol thats what i thought.