The elimination of metabolic waste products like ammonia, urea, uric acid, organic and inorganic salts, water and CO2 from body is known as excretion. In vertebrates, excretion mainly occurs through special excretory organs called kidney. Besides these the organs that help in excretion are skin, lungs, liver, gills, intestine and salt glands. Kidney is of following types:
(i) Pronephros Kidney that remove excretory products from coelom, found in all vertebrates embryos.
(ii) Mesonephros/opisthonephros Kidney that have nephrostome (funnel shaped) to filter coelomic fluid and also Bowman's capsule to filter blood, eg, amphibians and reptiles.
(iii) Metanephros Kidney that remove excretory substances from blood alone, eg, birds and mammals.
(iv) Archinephros This is primitive kidney consists of all above found in larva of hag fishes (Myxine).