Urinary system

It is now generally believed that the primitive vertebrate ancestor possessed an excretory organ referred to as an archinephros or holonephros ( Fig. 1 ). This probably consisted of a pair of dorsally located ducts extending the length of the body cavity. Each duct was joined by a series of segmentally arranged tubules, one pair to each segment. The other end of each tubule opened into the body cavity by a ciliated, funnel-shaped aperture. Close to each opening was a small knot of arterial blood vessels called an external glomerulus. From this type of kidney with its archinephric duct the various kidneys of forms living today may originally have been derived. The larval form of the hagfish and the larvae of certain amphibians, the caecilians, are present-day vertebrates possessing kidneys of this type.