From ZooTerms (Dictionary of Invertebrate Zoology)
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nephridium (noun; plural -ia; Greek diminutive nephros, kidney; Latin ium, nature of): 1. In various invertebrates, simple or branched, tubular structures that function in excretion, opening to the outside through a nephridiopore

See also: protonephridium, metanephridium. 2. (Annelida: Oligochaeta) A segment essentially composed of excretory tubules that may discharge directly onto the body surface, or lead to a sinus discharging to the exterior through pores by the setal ring. 3. (Echiura) One to many organs used for the temporary storage of eggs and sperm.