17:55 04 February 2016
For more than 60 years, researchers could not determine the make up of a creature that resembles a purple sock or how it fitted into the family tree. However, following the discovery of four new species in the Pacific, the researchers have concluded that the animal, called Xenoturbella, belongs to one of the earliest branches of life.
Lead researcher Prof Greg Rouse, from the Scripps Institution of Oceanography in the US, said: "Our nickname for them was purple socks.
"So if you think of a sock that you have taken off and thrown on the floor - they literally look like that.
"Or a deflated balloon."
The animal was first discovered in 1949. It has no eyes, no brain and not gut - just a small opening where the food goes in and the waste come out.
"We've never seen it feeding," said Prof Rouse.
"We find it where these molluscs are, and when we sequence it, we find these molluscs, their DNA, is inside. But when we open them up, we find their gut is empty.
"And they just have a tiny little mouth opening. They don't have teeth, they don't have any sucking proboscis structure that could tear off a piece of some bivalve.
"It is a great unsolved mystery as to how Xenoturbella eats."