This is a priapulid, otherwise known as a "penis worm," and if you click the video, you'll understand how it earned that phallic moniker. These creatures are currently shaking the branches of the animal taxonomy tree thanks to a mystery surrounding their mouths and anuses.

Priapulids have long been classed as protostomes ("mouth first"), a branch of animals whose embryos develop a mouth before developing an anus. Most invertebrates fall into the protostome category, while humans and other vertebrates, as well as a few lines of invertebrate animals are deuterostomes ("mouth second"), meaning our anuses develop before our mouths. The division between protostomes and deuterostomes, first established in 1908, is an evolutionarily significant one and has informed our understanding of how various types of animals developed.

A new study on priapulids may throw a wrench into that division, however. A team led by Andreas Hejnol, an evolutionary developmental biologist at the University of Bergen in Norway, examined the genes associated with mouth and anus formation in three-day-old priapulid embryos. When the first set of cells caved in, the genes associated with priapulid rears activated, suggesting those cells were caving in order to form an anus.

If it turns out that priapulid anuses do form before their mouths do, biologists can't simply declare them deuterostomes and call it a day. Priapulids are too closely related to other protostomes; the researchers suspect this means that early protostomes formed differently than their younger relatives do. It would also mean that the classifications of "protostome" and "deuterostome" would have to be renamed, forcing evolutionary biologists to rethink these divisions altogether.

You can read the findings from the penis worm study in the latest issue of Current Biology. And yes, today is apparently Penis Day.

'Penis worm' pokes holes in evolutionary dogma [Nature via Geekosystem]