Excretory organs were acquired in the early phase of metazoan evolution, and they play a crucial role in the maintenance of homeostasis of body fluids. In general, these organs consist of two functional components, the primary-urine producing apparatus and the modulating tubule. This basic organization of the excretory organs is conserved among most metazoans. Herein, we present an overview of the morphological evolution of the primary urine-producing apparatus in metazoans and describe the acquisition of the renal glomerulus-a specialized primary urine-producing apparatus-in vertebrates. We also describe the advancement of the glomerular structure and function in higher vertebrates.
Keywords: Glomerulus; Metanephridium; Podocyte; Protonephridium; Terminal cell.