Here is what was written about the periwinkles
The Zig Zag Periwinkles will consume algae, detritus and diatoms on your aquarium's glass and rocks. They are small, growing to a size of only about a 3/5 of an inch at most. All snails have the ability to leave your aquarium with their gravity defying ability to climb, however, of all the snails we sell this species has a greater tendency to evacuate than others. They should only be kept in a tank with a lid.
We have found them to be extremely hardy, the hardiest aquarium species that we know of, and an excellent addition to the aquarium that focuses on diversity
though I didnt see the part about them crawling out of the tank. I guess I will have to keep an eye out. I ordered 20 to try them out.
T