EXCRETORY SYSTEM, Colt PA RATI VE ANATOMY OF THE. The organs whose function it is to remove from the animal body both the waste products of metabolism and the excess of other substances that occur in the blood. Not only are the products resulting from the metabolism of proteids separated from the blood, but the blood is kept at its normal standard by the ex creting organs. They re alive both the qualitative and quantitative excess of any substance in the blood. The excretory organs may be likened to discriminating strainers. Of two substances equally soluble in the blood, such as sugar and urea, the urea is extruded by them and the sugar retained, while such insoluble substances as resin are removed by the vertebrate kidneys. The excretory organs of vertebrates are primarily the kidneys. The skin and lungs likewise func tion to some extent as eliminating organs. The lungs, however, are both organs of nutrition and excretion, for while they take in oxygen. so necessary for the animal welfare, they throw out carbonic-acid gas, which is a poison to the ani mal body, and free the body from other exere mentitious substances. The whole surface of the
skin functions to some extent as an excreting organ, for considerable water, salts, and fats are expelled by the skin. The waste products pro duced by the breaking down of nitrogenous sub stances, and whieh contain nitrogen, such as uric acid and urea, are separated from the blood and discharged from the body by another set of organs. known as the renal organs or kidneys. These organs vary greatly in form and function throughout the animal kingdom.
PaurozoA. The simplest excretory organ is the contractile vacuole found in the Protozoa, hut little is known eoneerning its function. In cer tain Infusoria fine lines or spaces radiate out from the central part of the vacuole into the substance of the organism, and through them the waste substances are drained into the vacuole. The contents of the vacuole either burst to the exterior or are cast out forcibly through the by the contraction of the walls of the vacuole.