Many large coral heads in tropical waters are decorated with Christmas tree worms (Spirobranchus giganteus), which occur in a huge variety of colors. The worm lives in a calcareous tube buried in the coral and extends neat, twin spirals of feeding tentacles above the coral surface. If disturbed, the worm pulls back into its tube in a fraction of a second. For added safety, the worm can also plug its tube with a small plate called an operculum.
(Photo source 1, 2, 3, 4)