These common under-rock inhabitants occur as large masses of intertwined tubes in the middle of the middle tide zone. A film of mucous secreted from the mouth to entrap small particles of food is periodically withdrawn for ingestion. Live tube snails can be distinguished from tube worms by the snail's rather slow withdrawal into its tube when disturbed. A blue colored operculum is also visible in the snail tube. Calcium carbonate is added by special glands to the shell as it increases in length.