Introduction to Excretion in Invertebrates:
The tissues or organs liable for elimination of waste products are known as excretory organs. These organs are involved in elimination of waste products of metabolism, like breakdown products of nitrogen metabolism and elimination of exogenous substances. Excretory organs also partake in osmoregulatory mechanism; making sure equilibrium between gains and loss of substances. If particular substance is in excess in body fluids, the excretion is increased through excretory organs and vice versa.
Excretion in Lower Animals:
These are the most ancient invertebrate animals. Though they don't have specialized excretory organs, waste are discharged by cellular membrane. Many mechanisms such as osmosis, diffusion, etc. are liable for waste elimination through membrane. Though, in number of species, contractile vacuole acts as excretory organelles. Contractile vacuole is most appropriate for protozoans as they are present in freshwater environment where there is constant osmotic gain of water. They are less present in marine species. Contractile vacuoles are circular shaped organelles which water enters. Vacuole then fuses with membrane of protozoan and water is expelled to environment. Rate of water extrusion is linked to external osmotic concentration of environment, and, thus, water influx. As osmotic concentration of external fluid surrounding protozoan decreases (that is becomes more dilute), rate of water entering protozoan and, thus, amount of water which requires to be expelled increases.
Colenterates also don't have specialized excretory organs and procedures such as diffusion osmosis and active transport to regulate fluids in body. Requirement for organs of excretion is really limited. Though, contractile vacuoles, or structures serving related functions, are also found in coelenterates but much less is known about physiology of the structures compared to contractile vacuoles in protozoans.
Animals which belong to group have characteristic specialized excretory structure called as flame cell system. Flame cell is large cell blinded at one end and bearing several cytoplasmic processes. There are series of such cells that open in excretory duct. Nucleus is displaced usually towards blind end side and cytoplasm bears several secretory droplets. The bunch of cilia arises in hollowed out cytoplasmic region that keeps on moving to produce directed flow of fluids. Excretory products enter flame cells in fluid state from parenchymatous cell by diffusion. Excess of water along with metabolic waste are therefore discharged by flame cells. True excretory organs are found in animals above levels of coelenterates. Simplest of the excretory organs are found in platyhelminthes and are known as protonephridia. Protonephridia are excretory structure that exist as closed or blind-ended, tubules and that don't connect with coelomic cavity. Cell that forms tips of blind-ended tube is ciliated. If it has a single cilia it is known as solenocyte. In Platyhelminthes, where cell that forms tips of blind-ended tube is composed of many cilia, protonephridia is called as flame cell.
Excretory organs are in form of tubular and coiled structures known as nephridia or metanephridia. These are excretory organs with ciliated opening to coelom known as nephridiostome and which end in pores that open to external environment, known as nephridiophores. Blood is filtered across membranes of capillaries and fluid produced enters coelomic space. This is good example of ultrafiltration, as only water and molecules of small molecular weight enter coelomic fluid, whilst larger molecules, like proteins, remain in vascular system. Fluid in coelom then enters metanephridia through nephridiostome. Initial urine that is formed passes along metanephridia where its composition is modified by processes of reabsorption and secretion. Result is creation of urine that is hypoosmotic (that is less concentrated) to body fluids from which it was formed. These metanephridia finally eradicate urine rich in urea and ammonia.
In molluscs, excretory organs are in form of kidneys and pericardial glands. Kidneys are mesodermal organs that communicate with coelom, while epithelial lining of pericardium containing glandular tissues act as pericardial gland. In cephalopods, nitrogenous wastes are eliminated in form of guanine, while uric acid and urea in opisthobranchs and bivalves respectively.
Excretory organs in arthropods are of many kinds and comprise coxal gland, nephridia, shell gland, Malpighian tubules and green gland. Of all these organs, Malpighian tubules have proved to most proficient organs of excretion in terrestrial arthropods. Malpighian tubules are excretory organs of insects. Precise number of the structures present in insect will differ from just few too many hundreds. Malpighian tubules contain a closed end that lies in fluid-filled cavity called as haemocoel, and open end that opens in gut between midgut and rectum. As insects have open circulatory system which operates at low pressure, there is no driving force for ultrafiltration of body fluids. The Malpighian tubule operates little in a different way to all other excretory organs. To form urine, potassium ion is vigorously transported from haemocoel in lumen of Malpighian tubules. Because of this, chloride ions follow, using diffusion and, eventually, water due to osmotic potential differences. Other substances enter tubule, including sodium ions, urates and nutrients, like amino acids. Urine then flows down tubule and enters midgut. It is possible that there is some alteration of composition of urine as it passes down tubule, but vast majority of alteration happens in rectum before its discharge to environment. In rectum, both nature and composition of urine changes noticeably. Concentrations of ions, like K+, are severely reduced - in few cases by as much as 75%. More significantly, there is tremendous amount of water reabsorption and urates precipitate out as uric acid
Though, in crustaceans like crabs and lobsters excretory organs are green, or antennal glands that is located in head region. Green gland comprises of blind-ending sac called end sac connected to tubule, nephridial canal that terminates in region known as bladder. Bladder exits to external environment through excretory pore that is located near to base of antenna. End sac is enclosed by coelomic fluid that is filtered to produce initial urine that lies within gland.
As with all excretory structures, composition of urine at the stage is like that of body fluid (hemolymph) from that it was formed, with exception which it has no substances of high molecular weight, such as proteins. As this fluid passes along nephridial canal, water and other solutes are reabsorbed. Not like bladder in mammals, for instance, bladder related with antennal gland is also able of reabsorbing substances.
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