 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 . 
