Clearly, I Can See You’ve Got Guts
Meet the glassfrog, a strange South American amphibian with a nearly transparent underbelly. The evolutionary significance of the clear abdomen is unknown, as light can be harmful to organ tissues (although the frogs get around that with a cool adaptation).
Robert Gonzalez has an interview with a glassfrog researcher at io9 highlighting their odd biological adaptations. I suppose the clear belly could be a survival technique to avoid being cut open by high school biology students?