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nephridium ne·phrid·i·um (nə-frĭd'ē-əm)
n. pl. ne·phrid·i·a (-ē-ə)
The embryonic excretory organ from which the kidney develops.
unit of the excretory system in many primitive invertebrates and also in the amphioxus; it expels wastes from the body cavity to the (usually aquatic) exterior. The evolution of nephridia encouraged tissue specialization by eliminating the need for all cells of an organism to be in contact with seawater for diffusion of metabolic wastes.