Opisthonephros
The opisthonephros is the functional adult kidney in lampreys (cyclostomes), most fishes, and amphibians.{{cite book |last= Juneja |first= Kavita |title= Animal Physiology | edition=1st| publisher= Anmol Publications |date=January 2002 |isbn= 81-261-1166-6}} It is formed from the extended mesonephros along with tubules from the posterior nephric ridge.{{cite book |last= Kardong|first= Kenneth |author2=Edward Zalisko |title= Comparative Vertebrate Anatomy: A Laboratory Dissection Guide | edition=5th| publisher= McGraw-Hill Science/Engineering/Math |date=October 2008 |isbn= 978-0-07-297008-1}} The functional embryonic kidney in anamniotes is the pronephros.
Kerr coined the term ‘opisthonephros’ in 1919.{{cite journal |author1=Ooi EC | author2=Youson JH | title=Morphogenesis and growth of the definitive opisthonephros during metamorphosis of anadromous sea lamprey, Petromyzon marinus L. |journal=J. Embryol. Exp. Morphol. |volume=42 |pages=219–235 |year=1977 }} In 1949, Hyman wrote the opisthonephros “has used up the mesomere tissue from which in amniotes both mesonephros and metanephros come.” Some accounts call opisthonephros the ‘mesonephros’, but the opisthonephros in anamniotes (lampreys, fish, and amphibians) differ considerably than the mesonephros in amniotes (reptiles, birds, and mammals). Thus, the term mesonephros is usually reserved for the embryonic kidney of amniotes.
The mesonephros and metanephros of amniotes are derived from different parts of the anamniote opisthonephros. The metanephros is derived from the posterior part of the opisthonephros. In amniotes, most of the former opisthonephros became the epididymis, and the archinephric duct became the vas deferens.{{cite book |last= Jurd |first= Richard |title= BIOS Instant Notes in Animal Biology | edition=2nd| publisher= Taylor & Francis |date=March 2004 |isbn= 1-85996-325-0}}