The excretory system of Unio includes a pair of kidneys (organs of Bojanus) a pericardial gland (Keber's organ). The kidneys include the coelomic cavities.
Organ of Bojanus: These are present below pericardium on either side of the venacava. Each kidney has two parts.
a) The ventral part is brown, thick walled, and glandular It opens into the pericardial cavity through renopericardial aperture.
b) The dorsal part of the kidney is thin walled and non-glandular. It is called the urinary bladder. It opens into the suprabranchial chamber of the inner gill lamina through a small renal pore. The urinary bladders of the both sides are connected by a large opening. The inner side of the bladder has cilia. The kidneys include the true coelom.
Process of Excretion : In the pericardial fluid excretory matter is present. It enters the kidney through the renopericardial opening. From the blood kidney proper collects wastes. By the action of cilia of the bladder excretory substance are sent outside. Ammonia is the important nitrogenous waste.
Kidneys also carry on osmoregulation.
Keber's Organ : Keber's organ is a large reddish brown gland in front of the pericardium. It sends waste matter into the pericardium.