A nephridiopore is part of the nephridium, an excretory organ found in many organisms, such as flatworms and annelids.^[1] Polychaetes typically release their gametes into the water column using nephridiopores.^[2]

Nephridia are homologous to nephrons or uriniferous tubules found in the kidney of humans. Nephridiopores are present in ventral region. The nephridium consists of an opening called the nephrostome, a long convoluted tubule, and another opening called the nephridiopore. Body fluids are filtered in through the nephrostome and passed through the convoluted tubule system.^[3] Essential substances are reabsorbed through active mechanisms and waste products are secreted back into the lumen of the tube.^[4] The resulting excretory fluid or urine is passed out through the nephridiopore.^[3]