Space saving Snap-on Comfort Wheel creates an external exercise area for small animals. Safe and quiet operation means very little disturbing noise. Easy to clean, removable Waste Collector Cup keeps the pet's area clean and sanitized. Bubble Wave Fun-nels design features a comfort grip that is safe for your pet's paws. Snap-on Wheel is 6.75" diameter, 4.5" wide and available in assorted colors. Universally connectable, easily attaches to all CritterTrail, Habitrail and S.A.M. systems.