Either of the paired excretory organs that occur in the body segments of earthworms and other annelids. Each metanephridium consists of a ciliated funnel-shaped entrance (nephrostome) open to the coelom, a long convoluted tubule, a bladder, and an exit pore (nephridiopore) via which urine is discharged to the exterior. The nephrostome of each nephridium lies in the segment anterior to the tubule. Coelomic fluid containing nitrogenous waste in the form of ammonia is swept into the nephrostome by beating of the cilia and passes along the tubule, where substances are exchanged with the blood capillaries that closely associate with the tubule. The distal part of the tubule is enlarged to form a bladder, in which the highly diluted urine collects before discharge.