The hagfish is an evolutionary throwback that definitely lives up to its name. When the eel-like creature finds a carcass on the seafloor, it burrows inside the dead meat and starts eating...not just with its mouth, but with its skin.
The trick of absorbing nutrients not only through the mouth but through the skin and gills is practiced by a few other simple sea creatures, such as the mollusk. But all of those creatures were invertebrates - the hagfish is the first vertebrate ever discovered that eats in this way. That isn't exactly surprising, as the hagfish is essentially a living fossil, the oldest surviving link back to the first ever vertebrates.
Hagfish are extraordinarily simple, resembling little more than tubes. They are almost completely blind and, as University of British Columbia researcher Carol Bucking describes:
"Their most striking feature, besides the ability to exude thick, gel-like slime in copious amounts when disturbed, is the whisker-like appendages around their mouths that they use to explore the environment."
So, these aren't complicated creatures. But, as with most adaptations, their ability to process food through their skin and gills is rather ingenious and even elegant, despite the fact it seems pretty disgusting from our perspective. Carcasses are not often found, and when the hagfish do come across a food source they have to make the absolute most of it, particularly because other scavengers could show up at any moment. The ability to absorb nutrients all over its body allows the hagfish to keep going well into its species's 360,000,000th year on this planet.