Each spiral is built from radioles arranged along a helical axis. Cilia on the radioles drive a water current and trap suspended particles, which are graded and moved down a groove to the mouth. The same surfaces serve respiration, so the crowns are simultaneously gills and a feeding net.
The visible part is a small fraction of the animal. The tube is a limestone burrow in the coral, and the worm can withdraw into it and seal the opening with a modified radiole called an operculum. Retraction is fast, triggered by shadow or water movement, which is why the crowns vanish as a diver approaches.
The relationship with the coral is often described as parasitic, but the evidence points the other way or to neutrality. Coral tissue around occupied tubes has been observed to suffer less predation by crown-of-thorns starfish, and the worm feeds on plankton rather than on coral. Whether larvae settle preferentially on particular coral species, and how the tube is initially bored, remain active questions.