nephridium(redirected from Protonephridium)
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One of the paired, segmentally arranged excretory tubules of invertebrates such as the annelids.
[G. nephros, kidney, + Mod. L. -idium, dim. suffix, fr. G. -idion]
n. pl. nephrid·ia (-ē-ə)
1. A tubular excretory organ in many invertebrates, such as mollusks and earthworms.
2. The excretory organ of a vertebrate embryo from which the kidney develops.