Nephridium

Nephridion (nef-rid'-e-um, nef-rid'-eon). In biology: I. The fat about the kidneys. 2. The renal sexual organ (organ of Bojanus) in certain invertebrate animals. Much convoluted cellular tubes functioning as kidneys in the a nnelida and certain mollusca. They occasionally act as ducts for the transmi ion of the sexual products. Cf. A'ephrogonaduct. I. Provisional or Embrydnic Nephridia, those that temporarily appear in the ontogenetic development; (a) those that appear in that region of the embryo or larva that corresponds with the subsequent head-segment and which lie at the anterior extremity of the cell-ma (mesoderm streaks) are called embryonic head nephridia or head kidneys; (b) those that appear in the trunk-segments are called embryonic or provision a trunk nephridia or kidneys. II. Permanent Nephridia, on account of their frequently strict segmental arrangement, are often called segmental organs, or on account of their looped or winding course (in Oligocholia and Hirndinea) looped canals. The anal tubes of the Echiurida are sometimes called nephridia.