The structure and function of vertebrate kidneys differ depending on vertebrate groups and developmental stage. The protonephridial excretory system is composed of a network of excretory canals that open to the outside of the body through excretory pores e.g., vertebrate embryos. Mesonephros is the functional embryonic kidney of many vertebrates and also adult fishes and amphibians. Metanephridia is the advance type of kidney. The mesonephros, during embryonic development gives away metanephros in adult reptiles, birds and mammals.