Dictionary:

nephridium

  (nə-frĭd'ē-əm)

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.

[NEPHR(O)– + Latin -idium, diminutive suff. (from Greek -idion).]

The embryonic excretory organ from which the kidney develops.

Pl. nephridia [L.]
1. either of the paired excretory organs in each body segment of certain invertebrates, having the inner end of the tubule opening into the coelomic cavity.
2. various excretory structures such as the nephron of the embryo.