I photographed this Deroceras reticulatum last Friday night at 7:30 in my backyard under a rock. An hour and a half later, the slug was still there even though the temperature had gone down to -2.6 °C. How do they do it? I don't know. But they must be able to supercool their body fluids to avoid freezing.
The little yellow balls visible at the top are the slug eggs mentioned in this post (the millipede next to them was dead). Next weekend I may bring the eggs inside to see if the embryos have survived the sub-freezing temperatures we have had since I first saw them.