The freshwater flatworm Stenostomum brevipharyngium is, by all accounts, a simple fellow. The worm is small and entirely soft. It has no eyes, only sensory pits that control balance and orientation. This simplicity makes it easy for S. brevipharyngium to make more of itself in every way possible. The flatworm can regenerate major parts of its body, such as sensory organs and musculature, as well as make more worms via asexual reproduction.
The most famous form of asexual reproduction might be budding, in which a new organism emerges from a “bud” on a parent like a new polyp emerging from a coral. But the way S. brevipharyngium sees it—which here is a turn of phrase, as the worms have no eyes—budding is for squares. These flatworms reproduce through a process called paratomy, in which an individual forms new organs inside their original body before splitting into two. This happens through a different molecular process than mere regeneration, in which a split worm regrows a head or a tail. In paratomy, the worm must obey its existing body axis—understanding which end of the original worm is the head and which is the tail so they can put their new tail and new head in the right places.
