Histopathologic studies at ERL,N of various marine molluscs have shown that their excretory processes often consist of two separate components, the urinary system and the macrophage system. The anatomical structure of the urinary system varies from species to species. Oyster and mussel kidneys consist of tubules, but their response to metal exposure is entirely different. Soft shell clam and quahog kidneys both consist of folds rather than tubules. The quahog kidney normally forms solid urine called concretions. The soft shell clam kidney forms concretions only under various conditions of stress. The macrophage system is considered by the author as part of the excretory system because of its ability to ingest and accumulate unwanted metabolic by-products and toxicants. The jammed macrophage moves to the body epithelium where it is excreted. Oysters that have been exposed to copper show this activity particularly well.