Kaytee CritterTrail Fun-Nels Straight Tubes are fun and exciting add-ons for all tubular small animal systems. The CritterTrail System is a colorful network of interchangeable, connecting tubes which allow for an endless array of shapes and sizes. The unique bubble wave shape is designed for easy climbing and comfort for all hamsters and gerbils. Also compatible with Penn-Plax S.A.M. and Hagen Habit rail Systems. 2" diameter tubes.