The excretory organs are paired kidneys that serve for excretion of nitrogenous metabolic products (urea and uric acid) from the body into the external environment and regulation of the internal environment. Fish have a primary, or trunk kidney (mesonephros), and in the embryonic state - prenephros, or head kidney (pronephros), which in some species functions in adulthood (gobies, mullet, whitefish). Filtered metabolic products are collected in the ureters. The ureters drain into the bladder, which opens outward behind the anus.