CritterTrail Fun-nels Straight Tubes are the perfect way to start expanding your CritterTrail home. If you can imagine it, you can build it with CritterTrail Fun-nels! The tubes are available in a variety of colors, shapes and sizes so you can create a huge playground for your pet. With the Fun-nels Straight Tubes, you can start turning your CritterTrail habitat into a fantastic fun-nel city! For hamsters, mice, gerbils or other small animals.