First off, the vast majority of UV filters for cameras are Linear glass, just like autoglass. More expensive circular UV filters are sometimes made from other elements but still quarts silica based.
UV light is filtered by glass. Many bus companies use glass alternatives.

As for arsenic in glass, this hasn't been done for sometime and very few if any companies still use arsenic. It was more prevalent in brown glass than any other. Just an FYI.

Back to the UV lens mention, my personal UV lens on my SLR is a tiften filter and the only real function it serves is to preven scratches to my primary lens when I am not using a filter or any other sort.

As for the crabs being injured by the shell I doubt it. Granted I'm sure there are ways to coax a hermit crab out of its shell or some other way to not give it any other choice, but I doubt it is the case.