Last year, I saw a worm in my aquarium gravel in my 20 gallon. It was white, thin, wriggly, and around 1cm long, so I thought that it was a detritus worm, but then I saw that it lived in a tube that seemed to be a translucent brownish shell. It would move its head out of the tube but flinched back inside when I shined a flashlight at it. I did a water change and researched but I could not find a worm that matched the description. I ended up forgetting about it though I would see it from time to time. I did not see it for 6 months until today. I don't think it is harmful since it has been living in my tank for a year and no inhabitants or plants have died or gotten ill. Does anyone know what it is or do detritus worms make shells to live in?