 Oxygen is absorbed into the hemolymph in the gills which provide the primary respiratory surface. The gills hang down into the mantle cavity, the wall of which provides a secondary respiratory surface being well supplied with capillaries. In species with no gills, such as the subclass Anomalodesmata, the wall of the mantle cavity is the only organ involved in respiration. Bivalves adapted to tidal environments can survive for several hours out of water by closing their shells tightly. Some freshwater species, when exposed to the air, can gape the shell slightly and gas exchange can take place.
