In many invertebrates, an organ probably concerned with excretion and the regulation of the water content of the body. It consists of a simple or branched tubule, lined with cilia. Excretory products diffuse into the tubule and are wafted by the cilia through a nephridiopore leading to the exterior of the body. There are several types of nephridia. A protonephridium is a tubule with a blind inner end bearing a flagellum or tuft of cilia, found in all acoelomates and pseudocoelomates, and in some coelomates. A metanephridium is an unbranched tubule open at the inner end through a funnel (the nephrostome), found in some coelomates.
Subjects: Zoology and Animal Sciences.