These water filled bags surrounded by muscles will contract to elongate the proboscis, thus forcing the animal forward. Minute whip like protoplasmic hairs pump water through openings in the walls of the proboscis and collar which causes them to swell. This swelling anchors the front part of the animal while the rest is dragged forward by muscle contractions. While the acorn worm moves around mud and sand is forced into its mouth. Water is filtered out through the gill slits and solid materials are passed down where any organic matter is digested.
Undigested sand is bound in mucus and ejected.
Reproductive organs of the acorn worm lie in pairs beside the gills. Eggs are laid along the same sides of the parent's tunnor or directly into the surrounding water. Species living in deep or cold water will lay a few large eggs with a rich yolk that develop directly into baby acorn worms. Others that live in warm or shallow water lay a large number of small eggs that develop into larvae that swim on the surface of the water before settling to the bottom to become adult and worm like. On the surface acorn worms look a lot like earthworms, but the internal structure of their body sets them apart.