ABSTRACT
Excretion (L. ex, out; cretus, separated) is a process that is concerned with elimination of nitrogenous metabolic wastes from the body whereas osmoregulation is the active maintenance of the normal osmotic pressure of body fluids so that the fluids are neither too concentrated nor too dilute. Whereas the extra-cellular fluid of invertebrates has an osmotic composition that is similar to that of seawater, the same fluid in vertebrates is about a third the concentration of some ions in seawater. Marine invertebrates are thus osmoconformers. Terrestrial vertebrates have a lower concentration of ions in their extra-cellular fluid than marine vertebrates, a fact that could be attributed to the freshwater evolution of many vertebrates. The maintenance of the proper ionic composition in the body is vital to the maintenance of the homeostasis of the body’s water levels.
