Amphioxus is a filter feeder. During ingestion, a continual stream of water having food enters the mouth and passes out of the pharynx into the atrium through the gill slits and then expels from the body through the atriopore. Before entering the mouth larger particles in the water are sieved off by the oral cirri, which form a mesh. A currant of water passes outwards from the pharynx through the gill slits into the atrium by the activity of lateral cilia, which are present at the front and back faces of each gill bar. This cause water bearing food particles to enter the mouth. This process is assisted by arterial cilia i.e., cilia present on the side of gill bar that is toward the atrium. Since fine food particles fall out of the main stream they are collected in the grooves between the lobes of the wheel organ due to its ciliary's action. These food particles become mixed with mucosa secreted by wheel organ and then drawn into the mouth ultimately pharynx when they again enter the main stream.