Which structure do slugs use to them move

Foot

Explanation:

The bottom side of a slug, which is flat, is called the 'foot'. Like almost all gastropods, a slug moves by rhythmic waves of muscular contraction on the underside of its foot. It simultaneously secretes a layer of mucus that it travels on, which helps prevent damage to the foot tissues.[3] Around the edge of the foot in some slugs is a structure called the 'foot fringe'.

the answer is:

carbon dioxide