Animals accumulate ammonia, urea, uric acid, carbon dioxide, water and ions like Na⁺, K⁺, Cl^–, phosphate, sulphate, etc., either by metabolic activities or by other means like excess ingestion. These substances have to be removed totally or partially. Excretion is the elimination of metabolic wastes from the body. Carbon dioxide is removed through the respiration process and other substances like mineral, salts, urea, water, pigments etc., are removed by excretion.