nephridium 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.

nephridium (pl. nephridia) The excretory organ of many invertebrates, consisting of a simple or branched tube, formed by the ingrowth of ectoderm, with cilia at the inner end. Excretory products diffuse into the nephridium and are wafted to the exterior by ciliary action. The most primitive type is known as a protonephridium and consists of a system of flame cells; it occurs in platyhelminths and rotifers. The metanephridium, which occurs in the earthworm and some other annelids, opens into the coelom by combining with a coelomoduct; the internal opening is known as the nephrostome and the external opening is the nephridiopore. Metanephridia superficially resemble kidney tubules.