Blue christmas worm
      in a hole of white coral

Fire Coral
      and a Christmas tree worm
Christmas tree worm

Blue coral worm

( Spirobranchus giganteus )

The spiral worm lives in the cave on the surface of a coral with small holes.

The photo shows two blue spiral tentacles and a lid of the worm's house. The house is a long tube embedded into coral. Coral grows around the tube. In principle, a worm can run out of its house if the house is destroyed. It is dangerous for worm, and the worm never runs out. The worm does not alter his shell like crabs and hermit crabs do. It merely grows a little extra length of the tube.

Spirobranchus Wiki