The main variations in urinary system of vertebrate animals of different groups are associated with their habitat. In organisms that live in freshwater and the sea, the urinary system solves different problems of osmotic concentration. In freshwater organisms, kidneys have large well-developed glomeruli which produce large volumes of glomerular filtrate. Marine organisms face dehydration as the outside environment (sea water) has greater osmotic concentration. Terrestrial organisms also need to conserve water. Snakes use aglomerular kidneys for this purpose. In mammals, the Henle's loop of the nephron concentrates the urine by reabsorbing water from it. The vertebrate kidneys vary in structure. The ducts vary as does the urinary bladder. Also, these variations are due to two primary causes: the kidney must start functioning at an early stage of development to remove the metabolic wastes of the rapidly growing embryo. As a result, the urinary organs have been markedly modified in most vertebrate groups. However, the transition to the land has led to a similarity of the urinary system of vertebrates, as they have a common ancestor.