How does the atrium protect the gills of a cephalochordate?

In cephalochordates, the atrium serves as a protective chamber that encloses the pharynx and its associated gill slits. By housing these delicate respiratory structures within an internal cavity, the atrium shields the gills from the external environment while facilitating the flow of water necessary for filter feeding and gas exchange.12

Water is drawn into the mouth by the action of cilia and passes through the gill slits into the atrium before exiting the body through an opening known as the atriopore. This arrangement allows the animal to efficiently filter food particles from the water current while maintaining the integrity of the gill basket.123