This is my entry for the Photo Friday: Structure challenge. This is an interesting relationship between the coral and sipunculid worm. A coral larva settles on a shell already occupied by a sipunculid worm. The coral grows over the shell, leaving an opening on the bottom for the worm. The worm feeds through the hole and drags the coral structure around, righting it if it gets turned over in the soft sediment. If you are interested in reading more, take a look at my blog post from last year: Walking Coral.