An X-ray image of a Monterey skate (Raja montereyensis) reveals a spine that extends like a tail out from the pelvic fin. The skeletons of skates, rays, chimaeras, and sharks are made of cartilage, rather than bone. Scientists in the Division of Fishes at the Smithsonian's National Museum of Natural History use X-ray images, like the one shown, to study the complex bone structure and diversity of fish without having to dissect or damage the specimen.
In 2012, the National Museum of Natural History hosted "X-Ray Vision: Fish Inside Out," a temporary exhibit that showcases fish evolution and diversity through 40 black and white X-ray images prepared for research purposes. See more photos from the exhibit.