Apparently many polychete worms have a bit on their last segment called a pygidium, which contains stem cells that let the worm grow new body segments, making their bodies longer. This one in particular is just growing a few normally, and then two simultaneously, complete with the regular organs attached to them, and connected and extended gut and anus, and then each new butt has a pygidium that can grow and split again at random intervals. But theyll always be a butt, because only the last segment on its body, its butt, can grow more segments, and those segments will always be a butt, because that is all its designed to produce. Only one head, if you ignore the stolons, because the head cant grow or split.
Maybe in the distant future more highly derived forms will forego having any gut at all in adults, or more points on the body, like the sides of preexisting segments, where new limbs can grow.
To be fair, it would be no less horrifying if it grew thousands of butts and heads, and became a sessile predator that snaps little critters out of the water. But there would be one point on its body, the segments between the butts and the heads, of the original free-swimming juvenile, through which all that shit has to pass. Actually, that would be way more horrifying. Like, a lot.