(Warning: the rest of the story is not for the squeamish. Ma Nature is pitiless. Skip the rest, and come back tomorrow, ok? Or scroll down to the dividing line; after that, things improve.)
At least the victim had left its brain behind, in the head, so it wasn't really aware of its condition. It lay on the sand, twitching feebly, then swam about until it found a shell to hide under. There it rested.
But the flatworms awoke and began to prowl. By morning, one had wrapped itself completely around the undamaged tail. A few hours later, the second one enclosed the broken end, and then they started a slow-motion tug-of-war. Which neither won, but the polychaete lost. They dragged it up over the edge of the shell, then off into a bit of sea lettuce, one on each end with the worm stretched in between, still making swimming motions. In the afternoon, both flatworms had eaten enough, and oozed away to sleep and digest.
Both ends were now demolished, and the polychaete seemed dead. So that was that. Maybe it would have healed alone in solitary. Maybe not.
(My old invertebrates textbook has an entire chapter devoted to the author's experiments at chopping up live planaria, and watching them develop new heads, new tails, multiple heads, etc. Interesting, but I don't think I will repeat this episode. Next time, if there is a next time, the patient will be put out of its misery.)
I decided that the dead worm would probably serve as food for the hermits, a bit each day; some variety in their diet would be good. I cut one end off with a pair of scissors, and poor poly started to try to swim again. It had no brain, but the nerves were still alive. I killed it quickly by putting it in the freezer.
(Ok, end of gruesomeness. You can open your eyes now.)
I dropped the bit of dead worm into the aquarium. The hermits congregated around it, tugging it this way and that, tearing off pieces and eating them. Then suddenly, from the far end of the tank, a big shore crab raced up, scrambling over the feasting hermits, tumbling them out of her way. She yanked the worm out of their pincers, took a few quick bites, then hauled her trophy off into a corner to eat it in private.
Greedy crab and befuddled hermits.
I hadn't seen her move so quickly before. No other food seems to be worth it. So I think it was one of the crabs that cut the polychaete in half. No wonder the worms slither back under the sand so quickly when anything comes near!
The photo turned out clear enough to see her mouthparts in action. The white squares on the sides are maxillipeds. They work as "doors", much like our lips, except that she's always moving them, fanning water around her face. Behind them are two pairs of green maxillipeds; these are used to manipulate the food, which the crab stuffs into the mess of jaws and feathery appendages beyond them. The four short, green antennae are also visible, between the eye stalks.
The hermits didn't go hungry; I gave them an extra serving of dried fish, to compensate for their loss.