Test animals used in the medical laboratories of the U.S.S. Enterprise in 2269. Because these small, rodent-like extraterrestrial creatures are nearly transparent and are extremely sensitive to changes in their environment, they prove useful as experimental subjects. When every living being aboard the Enterprise begins shrinking after being exposed to radiation by the Terratins on Stardate 5577.3 (2269), the gossamer mice become small enough to slip through the bars of their cages.