Previously you studying about excretory products and their formation. In this you will study about the organs that eliminate the excretory products. - Excretory organs serve not only to eliminate the nitrogenous waste products, but also to remove accurately regulated amounts of all substances present in excess in the body. Thus they contribute to maintain a steady state or homeostasis in animals to over-ride the influence of all those factors in the environment that tend to impose a no special organs of excretion are present in many marine protozoans and sponge.
Similarly, the coelenterates, which are mostly marine, and the echinoderms, which are restricted to the sea, are devoid of excretory organs. These marine invertebrates, being isosmotic to their habitat, do not need organs of water and ionic balance. Discharge of nitrogenous wastes and ionic regulation are carried out through the general body surface.