In invertebrates, the excretory system, according to research, follows the same rules as other species when it comes to detoxication mechanisms: aquatic forms get rid of ammonia by diffusion through the body surface, while terrestrial forms transform ammonia to uric acid. This indicates that in aquatic forms, the excretory organ is of the principal importance for their body fluid composition.
In general, the body fluids of marine invertebrates have a similar concentration as seawater; however, they usually differ, in the proportions of ions, with relatively less magnesium and more potassium than seawater. Moreover, their urine normally contains a similar concentration as seawater, but correspondingly it has more magnesium and less potassium. In freshwater invertebrates, commonly, though urine is not invariably, more dilute compared to the body fluids. A freshwater invertebrate conserves its body's salt content by removing water that enters via osmosis through its water-permeable surface by producing dilute urine.
Cnidarians, sponges, and echinoderms are among the few invertebrates without organs to which an excretory role can be confidently ascribed. Since all of these species are aquatic, it's more likely that they expel nitrogen (as ammonia) by simple diffusion. Their body fluids are closely the same as the seawater in composition, and it can be presumed that regulation only operates at the cellular level.
To know what organs are in the excretory system, they are the other invertebrates' excretory organs of various evolutionary origins. However, this is not to say that every invertebrate phylum has evolved its own specific type of excretory organ; rather, there appear to be 5 primary types of the invertebrate excretory organ: nephridium, renal gland, contractile vacuole, malpighian tubule, and coxal gland.
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Contractile Vacuoles of Protozoans
A vacuole, or internal sac, is an organelle found in a few protozoan species that enlarges as clear fluid accumulates and then discharges the contents to the outside. The cycle of emptying and filling can be repeated as frequently as each half minute. The contractile vacuole’s chief role appears to be in osmotic regulation but not in nitrogen excretion.
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Contractile vacuoles take place very frequently, and they are more active in freshwater species compared to closely related marine species. Whereas in freshwater, the concentration of dissolved substances in the cell is much greater than in the external medium, and also, the cell takes in water by osmosis. When the contractile vacuole is kept out of action, the cell increases in its volume. If the concentration of salts in the medium rises—which has the effect of lowering the osmosis rate—the rate of production, where the contractile vacuole is located, falls. The fluid eliminated by the vacuole is dilute compared to the cytoplasm.
Nephridia of Annelids, Flatworms, Nemertines and Rotifers
The term nephridium refers to the excretory organs of annelids in its strict sense, but it can be usefully extended to include excretory organs of other phyla with similar characteristics. Annelids are the segmented animals, which typically contain a pair of nephridia on every segment. Every nephridium contains the form of a fine tubule, often of considerable length; usually, one end opens into the body cavity, whereas the other to the exterior. In a few annelids, however, the tubule does not open into the body cavity but instead ends in a cell cluster of a unique kind known as solenocytes, or flame cells.
One of the characteristics that link them to the other non-segmented phyla that lack a true body cavity is the possession of solenocytes by a few annelids. They also have a system of tubules that start at the surface and end in flame cells embedded among the other body cells on the inside. In many cases, there exists no regular arrangement of different parts of the system. Animals that belong to all of these phyla are majorly aquatic, and, in some known cases, the primary excretory product is ammonia. But, it is not known that the quantity it leaves the body by the nephridia and the quantity through the body surface.
Some physiological studies have been made on the nephridia other than the earthworms. Although the earthworm is a terrestrial creature, its relationships, including its habitat, are more akin to those of a freshwater creature. The nephridium of the earthworm is more complex and longer compared to the marine annelids, 4 regions being variable. Body fluid enters the nephridium through an internal opening known as the nephridiostome.
As the fluid passes along the tubule, its composition is modified, probably driven by cilia. In the tubule’s two lower regions, the fluid becomes more dilute, progressively, presumably as a result of the salt reabsorption. Ultimately, dilute urine passes inside the bladder (the tubule’s enlarged portion) and then to the exterior via nephridiopore or external opening. The rate of urine flow for the earthworm can be as much as 60% of its body weight in a 24-hour period.