No kidding? They do keep growing even without changing out shells? I never realized that, as they didn't SEEM bigger to me, but then I didn't exactly measure them with a caliper or anything, just kind of eyeballed it, ya know?
I guess we all learn something new everyday! Anyway, the point of my post was that I think it's amazing how adaptable nature is - in the absence of natural shells, the animal found a "suitable" substitute to it's liking in the bottle.