nephridium n.; pl. -ia [Gr. dim. nephros, kidney; L. ium, nature of] 1. In various invertebrates, simple or branched, tubular structures that function in excretion, opening to the outside through a nephridiopore. see protonephridium, metanephridium. 2. (ANN: 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. (ECHI) One to many organs used for the temporary storage of eggs and sperm.
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