Delicate, almost transparent rodents kept as laboratory animals aboard the Enterprise, whose behaviour helps Doctor McCoy study the shrinking effect in the animated episode The Terratin Incident.
Gossamer mice are small rodents whose bodies are so pale and translucent that they are hard to see at all. Starfleet medical staff use them as laboratory animals, and Leonard McCoy keeps at least one in sickbay.
They appear in the animated episode The Terratin Incident. When a burst of spiroid-epsilon waves from the planet Terratin strikes the Enterprise in 2269, every organic thing aboard begins to contract, crew and animals alike. McCoy examines his gossamer mouse while trying to work out what is happening to the ship's organic matter.
The mouse is one of two lab species featured in the story, the other being the colour-changing halo fish.