Some members of the amphibian family called Centrolenidae have little pigmentation in their abdominal skin, especially when young. This means that some of their internal organs can be seen from without.
Hypsiboas pellucens is an example.

Hmm, I came out to check the computer, and SWMBO followed.

First thing I clicked on was this.

“What a great photo” was SWMBO’s immediate reaction.

I must concur.

With the black background of this site it stands out in a stunning relief.