Living World?s Exercise Ball provides a safe, mobile environment that lets small pets get the exercise they need. Simply place your pet inside the Exercise Ball for safe, interactive and healthy activity, and be sure to monitor your pet closely. Exercise is very important for small animals, as it helps keep them strong and healthy.
The Exercise Ball is made from non-toxic, non-breakable, clear plastic and is well-ventilated to ensure continuous air circulation. Cleaning is easy, just use warm water and a mild detergent and rinse thoroughly. For dwarf hamsters and mice. Small.

Other sizes available :

61725 Living World Exercise Ball with stand for hamsters, medium
61730 Living World Exercise Ball for guinea pigs & ferrets, large,