Urinary systems are organ systems that produce and eliminate urine. Urine: is a watery fluid that contains a variety of substances that have been filtered from the blood or from the interstitial fluid that bathes all cells (in invertebrates). Excretion: is the general process by which wastes are eliminated from the body. Ammonia: the primary nitrogenous waste of fresh water organisms, which avoid harm by releasing the ammonia continually into their watery environments as it is formed. Urea, or uric acid: terrestrial animals collect and store their urine convert ammonia into less toxic urea. Homeostasis: the relatively constant internal environment that cells need in order to function properly. The urinary systems of freshwater flatworms consist protonephridia, which include tubules that branch throughout the body within the interstitial that surrounds the flatworm"s tissues. Insect urinary systems consist of malpighian tubules, small tubes that extend outward from the intestine and end blindly in the hemolymph.