Ponz-Segrelles, Aguado & Glasby
In Darwin, Australia, a species of worm called Ramisyllis multicaudata makes its home inside the canals of a sponge. This worm is special — it can split and branch its posterior end. It is one of only two known species with this awesome adaptation. These worms might help us understand how different body parts grow and differentiate.
A by researchers at the Universities of Göttingen and Madrid describe the worm's weird anatomy. Its head sits deep within a coral. Its various posterior ends — in other words, its many butts — extend throughout the rest of the coral's canals. What always perplexed researchers was how the worm's body, especially its internal anatomy, could adapt to fit the coral.
It turns out that every time that the worm's posterior splits into two (worms don't have butts, technically), its organs also split in half, and a "muscle bridge" forms to connect the split organs. The anatomy of these bridges suggested to the researchers that the worm's butt only begins splitting in adulthood. Its unique characteristics also allowed them to distinguish main branches from side branches.
They also figured out how this worm reproduces: from its butt, of course! The posterior ends begin to form reproductive organs, a nervous system, and eyes. The nervous system develops a brain-like ring around the intestines. It can then separate and search for a mate. While this study provides a lot more insight on this weird worm, it also shows that there's still a lot we don't know about its anatomy.