kidney opisthonephros 2020-04-03T13:52:02Z BTO:0006370 BrendaTissueOBO Marion The opisthonephros is the functional adult kidney in lampreys (cyclostomes), most fishes, and amphibians. It is formed from the extended mesonephros along with tubules from the posterior nephric ridge. The functional embryonic kidney in anamniotes is the pronephros.