The Collaborative International Dictionary of English v.0.48:

Nephridium \Ne*phrid"i*um\, n.; pl. Nephridia. [NL., fr. Gr. ?
   of the kidneys.] (Zool. & Anat.)
   A segmental tubule; one of the tubules of the primitive
   urinogenital organs; a segmental organ. See Illust. under
   Loeven's larva.
   [1913 Webster]