Y-larvae or facetotectans were first discovered in 1899, but no one actually knew the adult form of the crustaceans ... until now. Scientists dosed the y-larvae with a hormone to force them into a growth spurt and discovered something strange: the bizarre crustaceans seem to undergo evolution, but in reverse!
The researchers next exposed y-larvae to a crustacean hormone that encouraged them to mature. The creatures metamorphosized into a juvenile form, dubbed "ypsigons," unexpectedly shedding their exoskeletons to become wriggling, eyeless, limbless creatures that resemble parasitic crustaceans.
At first the researchers thought their eyes were tricking them, but eventually "the juvenile literally crawled out of the old larval carapace," Glenner recalled. "It was only after several repeated experiments we actually believed what we saw. That feeling was a mind-blowing experience."
The fact that ypsigons are vastly different and far simpler than y-larvae might help explain why the adult versions of these creatures have escaped detection for so long. These are so simple compared with y-larvae that they even lack digestive tracts and nervous systems.
Link | The video of the metamorphosis