'Penis worms” anuses shake the ground of evolutionary biology
Deuterostomic Development in the Protostome 'Penis worm'
The ‘Penis worm’ is a member of the Priapulids which 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 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 probably have to be revised, forcing evolutionary biologists to rethink these divisions altogether.