The Ferplast Modular Hamster Gym Add-on Unit is a great way to add extra space to your existing hamster cage or to use this as an exploration turnpike to where you can connect multiple tunnels. The unit includes an exercise wheel, three connecting ports, and multiple ventilation points. The clear plastic design makes it easy to view into the hamster gym, and only Ferplast play tubes and tunnels are compatible with the connecting ports.