Liver synthesizes urea with the help of ammonia and carbon dioxide. Urea is passed to outside through excretory system. Kidney are the excretory organ in vertebrate. A pair of kidney is situated in the anterior part of the abdominal coelom one on either side of the median line. Urinary bladder lies in the pelvic cavity and is some what pear-shaped, muscular, sac-like structure. The urinary bladder stores urine temporarily. Gall bladder is pear-shaped, sac like structure attached to the posterior surface of liver by connective tissue. It stores bile secreted by the liver.