CONCEPT:Excretion is the process of removal of nitrogenous wastes from the body of an organism.Nitrogenous waste can be divided into three groups based on its toxicity.Ammonia – animals SECRETING ammonia are called AmmoneotelicUrea - animals secreting urea are called UreotelicUric acid - animals secreting uric acid are called UricotelicExplanation:BIRDS & reptiles secrete Uric acid as their main excretory product because Uric acid is a very less toxic waste and it does not require a lot of water for its elimination.Uric acid is excreted in the form of a semiliquid paste The other nitrogenous wastes excreted by various organisms are discussed in the following table Ammonia Urea Uric AcidHighly toxicModerate toxicNot very toxic Highly solubleModerately solubleVery less solubleExcreted mainly by AQUATIC invertebrates, Osteichthyes(BONY fishes and tadpole)Excreted by Chondrichthyes (Sharks and rays), terrestrial amphibians, and mammalsExcreted by insects, reptiles, and all birds Guanine: It is a nitrogenous waste excreted by the spider. It is the least soluble waste so does not require any water for its elimination