Glass frogs

the general background coloration of most glassfrogs is primarily lime green; the ventral skin of some members of this family is transparent. The internal viscera, including the heart, liver and digestive tract can be seen due to the translucency of the ventral skin.

http://en.wikipedia.org/wiki/Centrolenidae

http://biodiversitynews.blogspot.com/2006/08/glassfrogs-of-yanayacu.html

http://www.kidsbiology.com/animals-for-children.php?animal=Glass%20Frog