A squid has two gills. Each gill is large and contains between 20 and 80 gill filaments on either side, which hang down into the mantle cavity where water passes over them for oxygen exchange

. These gills are the primary respiratory organs, extracting oxygen from seawater as it flows through the mantle cavity. In addition to the two gills, squids have a circulatory system with three hearts: two branchial hearts located at the base of each gill that pump blood through the gills, and one systemic heart that pumps oxygenated blood to the rest of the body