The photographs feature fish that have been specially treated to make the stained skeletal tissues visible through the skin and flesh. The technique, developed by Dr. Adam Summers, uses dyes, hydrogen peroxide, a digestive enzyme and glycerin to make the flesh seem to disappear. Photo: This image of the butterfly ray (Gymnura crebripunctata) helped scientists study the joints in its wings. (Photo by Adam Summers)

Yawalapiti children play during the preparations for the celebration of “quarup”, a ritual held to honor in death a person of great importance to them, in the Xingu National Park, Mato Grosso State, May 7, 2012. (Photo by Ueslei Marcelino/Reuters)

A dancer poses for a photograph as part of the “Dance as Art” photo project in Times Square in New York, on September 22, 2014. (Photo by Carlo Allegri/Reuters)

The 26th annual National Geographic Traveler Photo Contest is now accepting submissions. Official categories include: “Travel Portraits”, “Outdoor Scenes”, “Sense of Place” and “Spontaneous Moments”. The contest is accepting entries until June 30. Photo: “Forest lights”. Aurora Borealis over the forest of the Pyhae Luosto National Park, Finland. (Photo and caption by Nicholas Roemmelt/National Geographic Traveler Photo Contest)

Men at work at the Cardiff Institute, making huge baskets, unique in size and construction, for loading oil cake in South Africa. (Photo by Maeers/Getty Images). 1938

Mr. Ben, a pygmy slow loris, is administed anesthetic during one of Bristol Zoo's in-house veterinary clinic routine health check-up and teeth clean as part of the Zoo's standard animal husbandry and welfare procedure at Bristol Zoo on June 18, 2009 in Bristol, England. (Photo by Matt Cardy/Getty Images)

Marie Lou looks out of the window of gypsy folklore wagon on May 24, 2016 in Staintes Maries de la Mere near Arles, France. Gypsies from all over Europe worship “Sara the Black”, their Saint and patroness, for one week. Sara's statue is situated in the crypt of the church. She wears a multicoloured dress and will carried by the gypsies to the sea at this afternoon. (Photo by Thomas Lohnes/Getty Images)

A publican serves a glass of stout to a donkey in the East End of London, 1960s. (Photo by Steve Lewis/Getty Images)