How Do Invertebrates Remove Waste? Excretory Organs and Processes
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.
Marine Invertebrates
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.
FAQs on Invertebrate Excretory System: Overview & Key Functions
1. What are the primary excretory organs found in different invertebrate groups?
Invertebrates display a wide variety of excretory organs adapted to their specific phylum and habitat. The primary examples include:
- Protonephridia (Flame Cells): Found in Platyhelminthes (flatworms) and rotifers.
- Metanephridia: Segmentally arranged tubules found in Annelida (e.g., earthworms).
- Malpighian Tubules: Found in most insects and other terrestrial arthropods.
- Coxal Glands or Green Glands: Found in crustaceans like prawns and crabs.
- Renal Glands (Organ of Bojanus): Found in molluscs like snails and mussels.
2. What are Malpighian tubules and how do they function in insects?
Malpighian tubules are the main excretory and osmoregulatory organs in insects. They are thin, blind-ended tubules that arise from the junction of the midgut and hindgut and float freely in the body cavity (hemocoel). They function by absorbing nitrogenous wastes, water, and salts from the hemolymph. This fluid then passes into the hindgut, where essential salts and water are reabsorbed, and the waste, primarily uric acid, is excreted along with faeces. This system is highly efficient at conserving water, a key adaptation for terrestrial life.
3. What are the main types of nitrogenous waste excreted by invertebrates?
Invertebrates primarily excrete three types of nitrogenous waste, depending on their habitat and water availability:
- Ammonia: Highly toxic and requires large amounts of water for dilution. It is the main excretory product for most aquatic invertebrates (ammonotelism).
- Urea: Less toxic than ammonia and requires less water. It is found in some terrestrial invertebrates.
- Uric Acid: Least toxic and almost insoluble in water, excreted as a paste or solid. This is common in terrestrial invertebrates with a need to conserve water, like insects and land snails (uricotelism).
4. What are protonephridia or flame cells, and which animals use them for excretion?
Protonephridia are simple, tubular excretory structures found in acoelomate and pseudocoelomate invertebrates. They consist of a network of tubules that end blindly in specialized flame cells. Each flame cell has a tuft of cilia that beats, creating a negative pressure that draws interstitial fluid into the tubule system. Wastes are then filtered and expelled from the body through an excretory pore. These are primarily found in phyla like Platyhelminthes (flatworms), Rotifera, and some annelids.
5. How does the excretory system help a freshwater invertebrate survive differently from a marine one?
The excretory system's role in osmoregulation is critical for survival in different aquatic environments. A freshwater invertebrate lives in a hypotonic environment, meaning water constantly enters its body via osmosis. Its excretory system must work continuously to expel large amounts of dilute urine to prevent bloating and cell damage. Conversely, a marine invertebrate lives in a hypertonic environment and tends to lose water. Its excretory system is adapted to conserve water and excrete highly concentrated salts to maintain internal balance.
6. What is the key structural and functional difference between protonephridia and metanephridia?
The key difference lies in their structure and the fluid they filter. Protonephridia are closed internally, with flame cells filtering interstitial fluid from the body parenchyma. In contrast, metanephridia are open internally via a ciliated funnel called the nephrostome, which collects and filters fluid directly from the coelom (body cavity). This makes metanephridia, found in annelids, a more advanced system capable of processing a larger volume of body fluid more efficiently than the protonephridia of flatworms.
7. Why is the type of excretory product (ammonia vs. uric acid) directly related to an invertebrate's habitat?
The type of excretory product is a direct evolutionary adaptation to water availability in an organism's habitat.
- Ammonia is highly toxic and needs to be diluted with a lot of water to be safely removed. This is only feasible for aquatic animals that have constant access to water.
- Uric acid, being non-toxic and nearly insoluble, can be excreted as a semi-solid paste with very little water loss. This is a crucial adaptation for terrestrial invertebrates like insects, allowing them to thrive in dry environments where water conservation is paramount for survival.
8. How do the excretory structures in annelids, like the earthworm, show an advancement over those in flatworms?
The excretory structures in annelids show significant advancement. Flatworms possess simple protonephridia (flame cells) that filter interstitial fluid. Earthworms possess more complex, segmentally arranged metanephridia. Each metanephridium is a coiled tubule open at both ends: the internal nephrostome collects coelomic fluid, and the external nephridiopore expels waste. This system is more efficient as it filters fluid from a true coelom and allows for selective reabsorption of useful substances along the tubule, a feature less developed in protonephridia.
