 Like most other molluscs , the excretory organs of bivalves are a pair of nephridia . Each of these consists of a long , looped , glandular tube , which opens into the body cavity just beneath the heart , and a bladder to store urine . The pericardial glands either line the auricles of the heart or attach to the pericardium , and serve as extra filtration organs . Metabolic waste is voided from the bladders through a pair of openings near the front of the upper part of the mantle cavity , from where it joins the stream of exhalant water . 
