Recently I had a post on Subulina octona, a land snail introduced to Florida. This is another photo of the same individual caught during an act of contortion with its body turned around almost 180° relative to the opening of the shell.
Snails can do that without any trouble because their bodies are attached to their shells by only one muscle, the columellar, that twists around the axis of coiling, the columella. The lower portion of a snail's body, including its foot, is not attached to its shell. If that weren't the case, I don't think a snail would be able to go in and out of its shell so easily and quickly.