Nudibranch is a suborder of toxic sea snails. There are around 3.000 known species, and they have pretty colours:

Why is it that these snails dont have a shell on their back?

The bottom one looks like he's on fire! love em

Not all snails have a shell. There are also some shell-less land snails. (Quite common in the Netherlands, so surely you must know about their existence.)

These snails don't need a shell, because they protect themselves by being poisonous, and making a kind of slime that tastes bad, so predators won't want to eat them.

venoumus snales,
venom,
Can we feed JJ to 'em zarra?

But seriously, cool snales!