An organ involved with the elimination of water and waste products from the body. In vertebrates the kidneys are paired organs located close to the spine dorsal to the body cavity and are covered ventrally by the coelomic epithelium. They consist of a number of smaller functional units which are called urinary tubules or nephrons. The nephrons open to larger ducts, the collecting ducts, which open into a ureter. The two ureters run backward to open into the cloaca or into a urinary bladder. The shape and the location of the kidneys varies in different animals. In fish, they are extremely elongated and may reach forward almost to the pericardium. In reptiles, the kidneys are smaller and are located in the pelvic region. In mammals, they are bean-shaped and found between the thorax and the pelvis. The number, structure, and function of the nephrons vary with evolution and, in certain significant ways, with the adaptation of the animals to their various habitats.