Diagrams of Chaetopterus illustrating the extraordinary feeding process that this animal uses. The piston-like parapodia pump water through the tube from anterior to posterior. A mucus bag held between the aliform parapodia and connected to the food cup acts to filter the water passing through the tube. The food cup continually rolls up the mucus bag and and when the blus reaches a certain size it is is detached and passed forward to the mouth.