Definition of: nephridium

(ni·frid′ē·əm) noun plural ·phrid·i·a (-frid′ē·ə) Biol.
1. One of the series of primitive excretory organs that afterward develop into uriniferous tubules, as in annelid worms, mollusks, and other invertebrates.
2. The embryonic tube which develops into the kidney in vertebrates. [<NL <Gk. nephridios pertaining to the kidneys]
—ne·phrid′i·al adjective