Truncatella is a genus of coastal-terrestrial snails that have been the subjects of several posts on this blog (for example, this one and this one).
In the discussion of Truncatella subcylindrica in British Prosobranch Molluscs by Fretter & Graham (1994), there is this statement: "...on occasions, a bubble of air may be seen in the [mantle] cavity."
I have noticed an air bubble in the mantle cavities of other species of Truncatella. Here is a photo of Truncatella pulchella crawling in sea water. Notice the large air bubble in the mantle cavity under the shell.
What is the function of the air bubble? Is it an oxygen reserve for the snail?