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an excretory organ that is characteristic of invertebrates. The nephridium regulates osmotic pressure, extracts and removes harmful metabolites from the body, and sometimes transports gonad products. The most primitive nephridia are those of flatworms, rotifers, gastrotrichs, nemerteans, and some polychaetes. These nephridia, which are called protonephridia, are branched tubules of ectodermal origin. They are closed at the internal ends by terminal cells, or solenocytes, and open to the exterior by means of excretory pores, or nephridiopores. Upon losing its solenocytes and opening into the coelom, the protonephridium is transformed into a metanephridium (primarily in annelids). In some annelids the nephridium coalesces with the mesodermal ciliated gonad funnels, or coelomo-ducts, forming an organ of mixed origin, the nephromyxium.
A. V. IVANOV