This is the semi-transparent eyelid of the red-eyed tree frog, Agalychnis Callidryas. Frogs aren't the only creatures with 'third eyelids' that keep their eyes moist without blocking vision completely. Sharks, cats, crocodiles, polar bears and camels have them too.

This is what an eye looks like after keratoprosthesis: a surgical procedure where a diseased cornea is replaced with an artificial cornea.

As soon as we're born, we're destined to age. Some of us age gracefully - we enjoy years of youthfulness - while others experience hair loss, wrinkles, and greying earlier than expected.

Found in the core of each hair, a pigment known as melanin gives rise to a person's hair color. This universal pigment is also found in our skin and eyes, giving use our unique physical traits. Dark and light versions of melanin - eumelanin and phaeomelanin, respectively - combine in various combinations to create all natural hair colors.

Melanin itself is the product of a specialized cell, the melanocyte, which is found in each hair follicle, from which the hair grows. As hair grows, the melanocyte injects melanin into the hair cells, which contain the s ...

Despite slang terms that imply otherwise, the human penis contains no bones. The same cannot be said for many of our closest evolutionary relatives: Chimpanzees and bonobos both have penis bones (a macaque one is pictured, left), also known as bacula.

To find out why some primates have the feature whereas others don’t, researchers traced the bone’s evolutionary history through time. The baculum first evolved between 145 million and 95 million years ago, as reported in the Proceedings of the Royal Society B. That means it was present in the most recent common ancestor of all primates and carnivores. Why some descendants, like humans, lost their bacula appears to be due to differences in mating practices: In primates, the presence of a penis ...

The harsh desert environment is known for its scorching temperatures and scarcity of water, making it a challenging place for most creatures to thrive. However, amidst this seemingly inhospitable landscape, the kangaroo rat stands out as a remarkable example of nature's ingenious adaptations. These small rodents have not only managed to survive but thrive in the desert with virtually no drinking water. The secret to their survival lies in their unique physiological features, particularly their kidneys' specialized structure.

Generally, the majority of water reabsorption in the kidneys occurs in the loop of Henle (see illustration). This structure is responsible for the reabsorption of water and essential ions from the urine back into the bl ...