Keeping your anus up near your head means keeping your gut short. Your bum is actually located on the top right of the ‘head end’. But keeping your gut short means that you have a lot of body behind your shell to use as a muscle.

The ‘foot end’ gets longer and bigger relative to the rest of you, until it is about 2/3rds of your body.
In fact there isn’r really much to you, apart from gut and foot, which is what gives you your name: “gastro-pod” or “stomach-foot”

You never develop much of a brain, but you continue to evolve into the present day, branching out into hundreds of species of slug and snail, some in the sea, like whelks, and some on land, like common garden snails.

Slugs aren’t a single separate group, which is why they haven’t been split out here: slugs appear to have evolved from snails on many different occasions, presumably because the shell, while useful, is a big heavy thing to make. All species of slugs are just snails which happen to have stopped growing a shell. At least for now!