The elimination of water, minerals and traces of the metabolism are such primitive functions as nutrition or reproduction.

In unicellular animals, we cannot find specific organelle destined to this excretory function, while in organisms more complex, they have needed to develop systems to expel residues outside by keeping inside those essential substances to keep stable the internal environment as well as the basic functions of the organism.

In both systems we can find the first mechanism in common: Osmoregulation.

The fresh water protozoa have contractile vacuole which pumps permanently the excess of water that penetrate into the cell via osmosis.

The elimination of residues is done for the surface of the body through the vacuole, whose function is expelling the absorbed water of a hypotonic medium. For this reason we can find protozoa in fresh water but not in salt water as they have an anal pore which excretes nitrogen residues such as ammonia and urea.

Among the protozoa we can underline the Choanoflagellate which are formed by a cell with some small villi called choanocyte, responsible of filtering food and excreting the residues. This is the first step of cellular differentiation intended, in the future, to form a specialized excreting organ.

In sponges (one of the oldest animals with more than 650 million years old) can be observed many orifices from water penetrate with food particles to the atrial cavity, which is formed by choanocytes that absorb the food, and eliminate the water for an orifice called oculus. We could find there the first group of specialized cells although we cannot consider them yet as an organ.

From this point on, it is produced an important methabolic differentiation linked to the metabolic path whose metabolites are toxic. One group of animals excretes uric acid (mentioned uricotelic), and the other evolutionary line excretes urea (ureotelic) producing a differentiation of specialised mechanisms.

The uricotelic are those animals which excrete uric acid as the main nitrogenised catabolite (they excrete the excess of nitrogen as uric acid). As uricotelic we can mention gastropod molluscs, insects, all reptiles (except chelonian) and birds.

The ureotelic are those animals that excrete urea as main nitrogenised catabolite (they excrete the excess of nitrogen as urea. As ureotelic we can mention elasmobranchii fish, amphibious, chelonian reptiles and all the mammals.

Cnidarians (jelly fishes and anemone) are uricotelic animals that present a kind of excretor organ where they release uric acid, creatinine, xanthine and guanidine. Its capacity to eliminate salts and toxics is reduced they only develop in areas where the quality of the water has tolerable limits for them.

50 years ago (Cambrian) appeared the first beings with lateral symmetry. From them, primitive plathyhelminthes are surviving as they do not have a developed nephritic system.

Other plathyhelminthes which appeared later (maybe evolved of some primitive insects explained by its parasite character) have a protonephridial system with protonephridial tubule and bulbs. Some species have very large cells (astrocytes) which wrap the protonephridial tubule and fix the colourings, principle that makes suggest that its excreting function has been done at the level of walls of protonephridial and astrocytes.

Nematodes expel its nitrogenised wastes as ammonium ions, which are spreaded through the body wall with excreting structures linked to a system of excreting channels.

In the class Adephorea which includes marine and fresh water can be found a excretor cell in the ventral face and in the class secernentea that includes terrestrial nematodes. This cell concludes in a pseudocoelom (body cavity not proceeding from gastrulation) which finally flows to the outside through a pore.

These structures can be considered beside the kidney, ureters and bladder as the most primitive structures in terrestrial animals (gastrotrichia).

Molluscs have excretor organs, called kidneys composed by tubule metanephridia which connects the pericardial cavity (renopericardial tube) and nephridiopore that flows to the outside through excretor pores.

Here we find a new element as the sanguineous system connects with the kidney so that the filtration of the blood becomes the mechanism of purification of the internal environment.

In insects, the apparatus of excretion is sophisticated at the same time than animal metabolism with nitrogenised wastes resulting from the processing of proteins, the regulation of the composition and concentration of ions from hemolymph, as well as the degree of hydration of tissues. In insects they work or can work as excretor organs the Malpighian tubule system, nephrocytes, several parts of the digestive tubule, specialized cells of the adipose tissue and eventually some other organs less common.

The next step is amphioxus (Branchiostma lanceolatum) which is considered an evolutionary step from invertebrates to vertebrates. Its excretor system shows an evolution regarding the insects and this would prove the mentioned differentiation between ureotelics (fish, amphibious, chelonian and mammals) and uricotelics (molluscs, insects, reptiles and birds) so that the kidney of ureotelics evolved in the line of amphioxus despite of having carried out previously some assays in the line of uricotelics.