Gossamer mice
The gossamer mouse was a type of sensitive lab animal kept in the medical lab on board the USS Enterprise. This species of mammal, as well as its organs, were naturally translucent in appearance.
In 2269, James T. Kirk called sickbay to see if the mysterious spiroid epsilon waves the ship was just probed with had any effect on the sensitive lab animals. Dr. McCoy reported that the gossamer mice showed no sign of shock.
Once the crew and all organic material aboard ship began to shrink, it was observed by Christine Chapel that the gossamer mice had all escaped from their cages, as they were now small enough to slip through the bars. (TAS: "The Terratin Incident")