nephridium

:  an excretory organ that is characteristic of various coelomate invertebrates (as annelid worms, mollusks, brachiopods, and some arthropods), occurs paired in each body segment or as a single pair serving the whole body, typically consists of a tube opening at one end into the coelom and discharging at the other end on the exterior of the body, is often lengthened and convoluted, and has glandular walls