The Astraea snail obviously didn't evolve in a fish tank. I have had to rescue countless snails that have somehow fallen off the sides of my tank. The snails always fall with the shell side down. Kind of like toast always falling with the butter side down. (Yeah, I know the Mythbusters disproved this). Once upside down the snails cannot turn themselves over. They'll try for a while. They'll stretch as far as they can to reach something solid but usually they can only grab sand. They will eventually give up and retract into their shell and pray that they will be noticed before they die. Usually my son notices them and I have to reach in and put it back on a rock. Surely this happens in the wild too. However, in the wild, the shell must protect them from death more often than it entombs them. I wonder how these snails would look if they evolved in an environment with vertical walls over sand.