In small animals like protozoans no special excretory organs are found, therefore, in them excretion occurs directly, e.g., metabolic wastes are directly removed into the surrounding environment as they are in direct contact with environment.
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In higher animals, on the other hand, each cell of the body is not in direct contact with the environment; therefore, in them these metabolic wastes are removed from the body with the help of specific excretory organs which are exclusively meant for the purpose of excretion of metabolic wastes. How excretion is affected in different animals that has been given below:
Excretion in Protozoa:
There is no uniformity in the production of nitrogenous wastes in protozoans. Amoeba forms uric acid, Glaucoma and Didinium ammonia and Paramecium and Spirostomum urea according to some workers and ammonia according to other workers.
In Protozoa no special organs of excretion are found. Excretion in most of the protozoans takes place by simple diffusion through the body surface.
Some protozoans have contractile vacuoles which are essentially the organs of water balance but these organs may also serve as excretory organs according to Shaw, 1960.
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The contractile vacuoles undoubtedly eliminate the excess of water, which probably contain waste products in solution, in this way to a certain extent they assist in excretion, but they cannot be responsible for the whole or even a major part of it.
Analysis of the fluid removed from the contractile vacuole of Spirostomum shows that it eliminates near about 1% of the total urea produced.
Excretioa in Porifera:
Like protozoans, porifers excrete their nitrogenous wastes in the form of ammonia or urea. In these animals the excretion is effected by direct diffusion from the tissues in the surrounding medium.
These animals also possess contractile vacuoles which are supposed to serve a part of excretion.
Excretion in Coelebferata:
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In coelenterates different types of nitrogenous wastes are met such as uric acid, it is found in Anemonia sulcata, urea, it is found in sea anemones and ammonia, it is chief product in actinians.
In coelenterates also, there are no specialized organs of excretion and it is assumed that waste products escape by simple diffusion through the cell membrane as they do in the Protozoa.
In actinians it has been shown that injected substances, such as, carmine, accumulate in certain regions and this may mean that these places are especially active in excretion.
Excretion in Platyhelminthes:
In Platyhelminthes the nitrogenous wastes, no doubt, are excreted by simple diffusion, yet a well-organized excretory system is found which consists of a series of tubes which branch and end into flame cells.
The lumen of each flame cell is continued with the excretory tube and contains cilia. The movement of these cilia sweeps water into the excretory tubes.
The chief nitrogenous waste product of planarians, Fasciola hepatica, Taenia pisiformia is ammonia, while Trichinella spiralis removes one third of its nitrogen as ammonia and rest as amino acids and peptides.
Excretion in Nematods:
In Nematoda ammonia is the chief nitrogenous excretory product, but fair quantity of urea is also produced. Some species produce amino acids and amines.
In Nematoda two types of tubular system are found for excretion. Cilia and flame cells are completely lacking.
In many free living species there are one or two ventral gland cell each with a terminal ampulla, which are said to be excretory in function.
In parasitic forms there is a canal formed from a single cell, usually shaped like a capital ‘H’ and opening ventrally. In some, one lateral link is lost, while in others such as Ascarii, the anterior portion is missing, giving a system like a capital of Rhabditis have H -shaped system and two glands as well.
Excretion in Annelida:
Polychaetes like Aphorodite and medicinal leech excrcte most of their nitrogen in the form of ammonia, while various species of Mmbricus and Pheretima produce both ammonia and urea.
Other substances like amino acids creatine and smell quantifies of purines are also produced.
Now it has- been established that nephridia are the chief excretory organs of the annelids. These are tubular structures and are found in pairs nearly in all the segments of the body.
Their walls are single-layered epithelia which often show evidence of being capable of active transport.
They may be assumed to secrete materials into the tubular fluids or extract materials from them.
The nephridia, which are closed at the inner end, are called protonephridia while those which open into the coelom by a ciliated funnel, nepbridfostome or nephrostome, and termed metanephridia.
Both types of nephridia open to the exterior through nephridiopores and frequently enlarge into a storage bladder just before they discharge.
The nephridia in most of the annelids, like Pheretima, are of three types;
(i) septal nephridia
(ii) pharyngeal nephridia and
(iii) integumentary nephridia.
Septal nephridia the so called typical metanephridia are found along the septa of the body.
Pharyngeal nephridia are found in the pharyngeal region of the alimentary canal. Integumentary nephridia are found attached to the integument.
Their number varies from 200 to 250 in all the segments of the body. These are exonephric as they are directly communicated with the exterior.
These nephridia pick up the nitrogenous wastes from the fluid of coelomic cavity through the nephrostomes and carry them outside the body.
Excretion in Arthropoda:
In some arthropods such as crustaceans, the excretory products are ammonia, while in others like insects the excretory products are uric acid.
Clothes moths excrete most of their nitrogen in the form of ammonia but rest in the form of urea and amino-compounds.
In arthropods various types of excretory organs are found to remove the nitrogenous wastes from the body.
The antennary gland and maxillary gland are said to be the chief excretory organs of Malacostraca and Entomostraca respectively.
The Malpighian tubules are the chief excretory organs of most insects and also of Myriapoda and Arachnida. Their morphological arrangement and histological structure vary in different animal.
At one end they are communicated with the gut and at the other end they lie in the blood sinuses from where they extract the wastes.
They excrete both wastes and carbon dioxide which they receive from the blood in solution. In insects there are subsidiary organs of which the chief is the far body, the parietal layer of which stores a sufficient amount of uric acid.
Excretion in Molluscs:
The lamellibranchs excrete most of their nitrogen in the form of ammonia and amino-compounds.
Urea and purines are also excreted in fair quantities. The cephalopods excrete ammonia, trimethylamine-oxide, amino compounds and smaller quantities of urea and purines.
Helix pomtia, eliminates ammonia, ammo compound, urea, uric acid and other purines where as Umax agrestes excretes chiefly urea and Aplysia linvacina excretes ammonia, amino- compounds, purines and urea.
The excretory organs of molluscs are coelomoducts opening from the pericardium to the exterior.
The cells of the inner portions generally have a brush border, while of the distal parts are ciliated.
In lamellibranchs, snails and cephalopods all the three physiological processes, e.g., filtration, reabsorption and secretion take place simultaneously during the course of excretion of waste products but the sites of these processes vary.
In cephalopods filtration takes place in branchial heart appendage, where as in snails probably in kidney sac. The cilia and body contractions help to eliminate the waste fluid.
Excretion in Echinodermata:
The nitrogenous wastes of echinoderms are ammonia, amino-compounds, urea and uric acid. In echinoderms no specialized organs of excretion are found.
Excretion takes place through wandering phagocytic cells, the so called amoeboid cells or amoebocytes.
Excretion in Vertebrates:
The nitrogenous wastes that are excreted in vertebrates vary from animal to animal. Certain teleosts, tadpoles of amphibia and crocodiles excrete most of their nitrogen in the form of ammonia.
Elasmobranchs, amphibians and mammals produce urea as their chief excretory product where as in lizards, snakes and birds uric acid is eliminated as their chief nitrogenous compound.
Other products like creatine, creatinine, trimethylamine oxide, guanine and inorganic ions are also excreted in vertebrates.
The chief excretory organs of the vertebrates are the kidneys which are of three types depending upon their position and stage of development in the life cycle of the particutar animal.
These kidneys are known as pronephros, mesonephros and the metanepbros according to the English embryologist Francis Bolfaur.
Pronephros is embryonic kidney in all vertebrates except in Bdellostoma and Myxine, where it is the functional excretory organ in the adult.
The mesonephros is the functional kidney of the Petromyzon, fishes and the amphibians. It is functional excretory organ during the embryonic stage of the reptiles, birds and the mammals, while the metanepbros is the functional excretory organ of the adult reptiles, birds and the mammals.
The kidney of adult vertebrate is made up of a large number of structural and functional units what are known as uriniferous tubules or renal tubules or nephrons, each consisting of a Malpighian body and a tubule.
The Bowman’s capsule enclosing the glomeruli forms a malpighian body and there are about a million of such bodies in each kidney.
The uriniferous tubule or nephron resembles to a great extent in structure and physiology with the nephridium of the invertebrates, therefore, nephridia are supposed to be the forerunners of the vertebrate kidneys.
Other structures like integument and gills may also be involved in excretion. In frog a negligible quantity of nitrogenous waste, carbon dioxide and water goes through integument.
In the teleosts most of the ammonia and urea that are formed in the metabolism of foodstuffs are lost through the gills but in elasmobranchs the gills are not permeable to urea.
Most of the knowledge about excretion has come from the mammals, therefore, we will discuss the excretion in mammals in detail so that one may able to get the real idea of the excretion.