You can see the internal organs of glass frogs through their translucent stomachs
These crazy-looking “glass frogs” are native to southern areas of Central America and portions of the north of South America.
While the general background coloration of most glass frogs is primarily lime green, the abdominal skin of some members of this family is translucent. The internal viscera, including the heart, liver, and gastrointestinal tract are visible through this translucent skin, hence the common name.
(via Archie McPhee)