What is an x-ray fish?

The X-ray fish, Pristella maxillaris, is also known as a goldern pristella tetra or X-ray tetra and can be found wild in South America. A popular aquarium species, it is so called because a large part of its body is transparent.

There's a picture of one at http://upload.wikimedia.org/wikipedia/commons/c/c4/Pristella_maxillaris.jpg