|Earthworm - Excretion and Osmoregulation
All animals must maintain the chemical balance inside their bodies and cells. The first requirement is
maintain the volume of water in the body. Animals living in freshwater are often saltier than the
surrounding water and water will naturally move into their bodies (by a diffusion process called
osmosis) and if excess water is not removed from the body then the body fluids will become too dilute
and cells will expand and eventually burst. Note that salts will also tend to diffuse out of the animal into
the surrounding freshwater. An animal living in the sea may be more or less salty than the surrounding
salt water and if less salty it will tend to lose water to its surroundings and absorb salt. Terrestrial
animals face the challenge of conserving water unless they dry out and die. In all cases, both the
volume of water and the concentration of salts in the body fluids must be maintained. During growth,
an animal will need to take in more water to increase its volume, since living cells are some 90% water.
Salts re also vital to the functioning of cells. Electrical activity, involving the movements of salts across
cells, is a normal part of cell functioning and salts also serve many other functions as essential
nutrients. This regulation of the body fluids is called osmoregulation. Some animals are better
osmoregulators than others. Some exist in quite stable habitats where they do not face the added
burden of coping with changes in humidity or salinity and some are better able to tolerate a large
change in the saltiness and volume of their tissues. These more tolerant animals simply allow their
body fluids to change to a large extent and are called osmoconformers. They simply tolerate changes
that occur in their bodies as they are exposed to varying flows of fresh and salty water, such as may
occur near to a river estuary. In reality, most animals have a greater or lesser degree of
osmotolerance and osmoregulation.
In addition to regulating the salt and water content of their bodies, animals must eliminate waste
products from their tissues. Cells naturally produce waste chemicals as a product of their metabolism.
These materials may serve no further use to the cells and are often toxic and so they must be
removed. These waste products include carbon dioxide and lactic acid, which are both products of
respiration, and ammonia, which contains nitrogen surplus to requirements. Carbon dioxide and lactic
acid are both acidic in solution and will lower the pH of the body fluids. The cell machinery works best
within a narrow pH range and ceases to function outside a certain range of tolerance, and so pH must
be carefully controlled - excess acids can not be allowed to accumulate in the body! Ammonia is basic
and so raises pH, but it has other more toxic effects and must be rapidly removed from the system. In
animals which have an open circulation of water through their bodies, such as jellyfish which circulate
sea water around their bodies, the ammonia can be rapidly flushed out with few problems. In animals
with more tightly controlled environments, such as mammals, excreting ammonia would be problematic,
since it is very soluble in water and requires large amounts of water to be removed from the body to
carry it away - clearly this is not ideal in terrestrial animals. Instead mammals convert the ammonia into
urea (in the liver) which is less toxic and less soluble and can be filtered from the blood by the kidneys
and excreted in a modest amount of urine. Birds and insects convert the ammonia into uric acid, which
is of a much lower toxicity even than urea and which can be excreted dry as it is scarcely water
soluble. Thus these creatures can produce very dry urine (often mixed with the faeces) and so
conserve water better than mammals, which is a great advantage for such small bodies as insects,
which contain very little water.
Vertebrates, such as mammals have kidneys to perform all the following functions: osmoregulation, pH
(acid-base) regulation and excretion of nitrogenous wastes (though other organs in the mammal body
participate in these roles, the kidneys are the main excretory organs). Much carbon dioxide is also
removed across the lungs when the mammal breathes out and some materials are excreted in sweat.
Note that we must distinguish between excretion and defaecation. Excretion removes waste materials
that have been produced by or otherwise have travelled through the cells in the body. Defaecation is
the removal of faeces which comprise largely materials that never entered the cells of the body, but
remained undigested as food passed through the lumen of the gut. (Note, however, that some waste
materials may be excreted by cells into the gut and then voided with the faeces).
The mammalian kidney is made up of a million or so units, each unit is a series of tiny coiled tubules (a
tubule is a small tube) and is called a nephron. Earthworms do not have kidneys as such, but they do
possess single nephron-like units called nephridia (singular nephridium, literally 'little kidney').
There are two types of nephridia found in worms, the protonephridium and the metanephridium. The
earthworm type is the metanephridium. The common earthworm, Lumbricus terrestris, has a pair of
metanephridia in each body segment.
The circulatory system of a jellyfish consists of a series of ciliated canals that pump sea water from
the gut around the body. If a worm has a similar system in which sea water flushes through the body
cavity then it can simply pump seawater in and out of its body. This type of worm might have a pair of
coelomoducts in each body segment. A coelomoduct is simply a ciliated funnel and duct which sucks a
current of water out of the coelom (fluid-dilled body cavity) and into the surrounding sea water
through the coelomopores and sea water might be replenished from the gut or through some other
opening. However, most animals will modify their body fluids, such that they differ from sea water and
this system then has the drawback that those materials added to the circulating fluid will be wasted as
they are continually pumped out of the body, so many animals have a separate circulatory system,
the blood system, which is closed off from the coelom. Thus, they have two circulatory systems, the
blood system and the coelomic system. In mammals, the blood (and lymph) system is by far the most
important but a coelomic circulation still exists, which employs cilia to propel a small volume of fluid
around the coelom (peritoneal cavity) and there is a separate system of fluid-filled cavities within the
brain and spinal cord in which cerebrospinal fluid (CSF) is circulated by cilia.
With a separate blood system, the blood can contain oxygen carrying pigments or red blood cells and
other materials, such as iron-carrying proteins or immune proteins, which are retained within the body.
However, both the blood and coelomic fluid needs to be sorted to remove toxins and excess salts and
other unwanted materials that may otherwise accumulate within it. In both earthworms and mammals
the blood is filtered under pressure. In the mammalian kidney, blood is forced through a series of
sieve-like membranes which allow water and small materials, such as salts and nitrogenous wastes,
dissolved in the water (solutes) to pass through. This filtered fluid extracted from the blood (filtrate)
passes down nephrons which are tiny coiled tubules, much like those in the earthworm. During the
passage of the filtrate along these coiled tubules, various materials are re-absorbed from the filtrate,
these are materials that the animal wants to keep, such as glucose and amino acids and much of the
water, and some materials are secreted into the filtrate (such as bicarbonate ions). Essentially then,
the blood is filtered and this filtrate is modified into urine by adding additional materials to be removed
from the body and by re-absorbing useful materials to be kept by the body. This process is strictly
controlled - the body may need to remove more salt and water one day, whilst needing to retain more
of these chemicals on another day. The filtrate has now become urine and is stored in the bladder
prior to urination.
Along with the unwanted materials that are excreted in the urine are nitrogenous wastes - chemicals
that carry excess nitrogen to be removed from the body. In mammals this nitrogen is mostly in the
form of urea, though some ammonia nd uric acid may be present. Ammonia is very toxic and so must
be eliminated very quickly, and it is also very soluble in water, and so its removal requires more water
to be excreted. Water is a valuable resource and so mammals have adapted to life on land by
secreting a less toxic nitrogen-containing material, urea. Urea is produced from ammonia in the liver.
The ammonia itself remains the primary nitrogenous waste produced by the cells, but this ammonia is
carried to the liver in the blood and quickly converted into ammonia. Birds and insects have adapted
better to life on dry land by converting the ammonia into uric acid. Uric acid has a very low toxicity and
is almost insoluble in water and so can be excreted with the minimum amount of water. The watery
faeces of birds and the yellow drops that flies may excrete onto your window pane (!) are actually a
combination of faecal matter and uric-acid rich urine - clearly these animals waste much less water in
excreting nitrogenous waste than most mammals do.
The nephridium of the earthworm Lumbricus terrestris works in a similar way to the mammalian
nephon. The main difference is that in the mammal the nephrons are grouped into one pair of
kidneys, about one million of them per kidney in a human. In earthworms, nephridia are found in each
body segment, with anything from 2 to 250 per segment. Also, in the mammalian kidney the filtration
occurs immediately next to the collecting funnel of each nephridium, whereas in the earthworm the
filtration occurs across the walls of major blood vessels, such as the ventral vessel and the
peri-intestinal blood sinus of the typhlosole (and possibly the sub-neural vessel). The walls of these
vessels contain podocytes - cells with long slender processes that form a porous mesh across which
fluid passes from the blood, under elevated pressure, into the coelom, to form the coelomic fluid. This
coelomic fluid gets sucked into the metanephridia through the nephrostomes, which are open ciliated
funnels. The cilia beat to drive currents of coelomic fluid into the nephridium. This coelomic fluid (the
filtrate) passes along the coiled tubule of the metanephridium, first along a long narrow ciliated tubule.
The nephridium is richly supplied by blood vessels that branch from the ventral vessel, carrying richly
oxygenated blood that has just been pumped backwards by the pseudohearts. Some filtration occurs
across these nephridial blood vessels into the narrow ciliated tubule, carrying chloride, sodium and
potassium salts and more nitrogenous wastes in the form of ammonia, urea and uric acid.
The narrow tubule (also called the proximal tubule since it is nearest to the nephrostome) passes into
the brown ciliated tubule (middle tubule) and then into the wide non-ciliated tubule, which is richly
supplied by blood vessels. In these more distal tubules, selective re-absorption occurs, with proteins,
water and some of the salts being re-absorbed into the worm's blood, to prevent their loss from the
body and their wastage. The wide tubule opens into the bladder, a wide muscular tube which
forcefully expels the filtrate, now urine, to the outside through the nephridiopore.
Presumably, the earthworm, just like the mammal, can regulate the concentration of its urine and thus
limit the amount of water that is lost and how much salt is retained - osmoregulation. About 80% of the
salts that pass into the nephridium are re-absorbed, conserving these valuable nutrients. The urine is
also hyper-osmotic (more concentrated) than the blood and coelomic fluid, even though the fluid
entering the open nephrostome is isotonic (just as concentrated) as the coelomic fluid since that is
what it is. Thus, the earthworm clearly re-absorbs a lot of water in the nephridium - a good adaptation
to life on land. However, most of the nitrogenous waste is in the form of urea and ammonia, so
significant amounts of water are lost in the urine. The skin of the earthworm is also not as well
waterproofed as that of many land-dwelling animals and the worm secretes copious amounts of mucus
(which also removes some excess nitrogen as mucoproteins) and coelomic fluid is also excreted
through the dorsal pores. For these reasons, earthworms prefer moist conditions, though they will
drown if they remain in their burrows when they flood after heavy rain, which is why earthworms like
Lumbricus terrestris can be seen on the surface after heavy rain. If these worms remain exposed to
direct sunlight for too long then they rapidly dry, both as a result of drying up and from the ultraviolet
radiation, against which they have little protection. In dry conditions, earthworms in the soil enter a
dormant state, a process called aestivation.
Earthworms other than Lumbricus terrestris
The nephridial system of Lumbricus terrestris is actually quite atypical of earthworms in which the
metanephridia are often grouped together in some way. In Pheretima (an earthworm found in New
Guinea and parts of Southeast Asia) there are three types of metanephridia. One type forms an
anterior group in segments 4,5 and 6. The nephridia of this group (called pharyngeal nephridia) are
arranged into one pair of bushy tufts per segment. Each nephridium gives off one excretory ductule
and the ductules of each tuft unite into one common muscular excretory duct, making one pair of
ducts in each of the three segments, or three pairs in total. Each of these ducts empties into the
anterior region of the gut. The nephridia of segments 4 and 5 send their ducts forwards to open into
the pharynx in segment 4 and those in segment 6 travel forward to open in the buccal cavity in
segment 2 and then the waste is removed through the mouth.
A posterior group of nephridia begins at the intersegmental septum between segments 15 and 16 and
these nephridia occur on every intersegmental septum posterior to this. There are about 80-100 of
these septal nephridia per segment. This group also empties into a pair of common excretory ducts
that run along the dorsal surface of the intestine. From this pair of ducts, pairs of branches open into
the intestine and the urine passes into the intestine and is voided through the anus. These two
systems are called enteronephric, since they open into the gut ('enteric' pertains to the gut).
This worm also has a third group of exonephric metanephridia, which resemble those we have already
seen and do not open into the gut. However, these nephridia are microscopic and are called
micronephridia, whereas the large nephridia of Lumbricus terrestris are called meganephridia. These
nephridia occur in all segments except the first two and are especially numerous in segments 7 to 15
and are densest in the region of the clitellum (segments 13 to 15). There are 200-250 of these
exonephric metanephridia per segment in segments 7 to 15 inclusive. These micronephridia are
one-third to half the size of the septal nephridia. Each of these integumentary nephridia is a separate
unit and each opens to the exterior via its own pore in the body wall. Thus, there are numerous
minute nephridiopores in the body walls of these worms.
The reason why Pheretima needs such an extensive and well-developed excretory system is because
it prefers soils that are almost water-logged and so it needs to excrete large amounts of excess water.
So far we have seen three different types of (meta)nephridia in earthworms: exonephric
meganephridia, as in Lumbricus terrestris; exonephric micronephridia, as in the integumentary
nephridia of Pheretima and the enteronephric micronephridia that open into the gut in Pheretima.
A fourth metanephridial type, enteronephric meganephridia, or large nephridia that open into the gut,
is found in the Indian earthworm Lampito. Like Pheretima, Lampito has integumentary micronephridia
a pair of which is located in the front of each coelomic chamber (the coelom of each segment is
divided into left and right chambers) of every segment from 15 rearward. Each opens externally via its
own nephridiopore. Lampito also has pharyngeal nephridia, comprising a pair of tufted masses in
each of segments 5, 6, 7, 8 and 9. The individual ductules coalesce into bundles which project
forward and open into the pharynx at segments 2, 3 and 4. In addition to these systems, however,
Lampito has one pair of enteronephric meganephridia in each segment from 20 rearward. These
resemble the nephridia of Lumbricus terrestris in that they have pre-septal nephrostomes (funnels
opening in front of each intersegmental septum), each is about six times larger than an integumentary
micronephridium. However, these do not open to the exterior via nephridiopores, but instead each
gives off a duct which joins to a longitudinal excretory duct that runs along the worm above the
intestine and opens into the intestine in each segment from segment 20.
Many other species have three nephridial systems: the exonephric integumentary micronephridia,
which are most abundant in the clitellar region, pharyngeal enteronephric micronephridia and septal
nephridia, which are often meganephric as in Lumbricus but often enteronephric, opening into the
intestine. The numbers and types of these nephridia vary according to species. Megascolex has
these three systems, but its nephridia have multiple nephrostomes (as if each is several nephridia
connected together by a common duct). Nephridia of this type are called meronephridia.
It is possible that each of these nephridial subsytems functions under different conditions or perhaps
each has additional functions other than excretion.
Above: diagrams of the nephridium of the night-crawler earthworm Lumbricus terrestris. There is one
pair of these nephridia (literally 'little kidneys') in each segment. The diagrams below show the
nephrostome in detail. The head or nephrostome of each nephridium passes through the
intersegmental septum to project into the coelom of the preceding segment. The nephrostome is an
open funnel which uses tiny beating hair-like cilia to draw coelomic fluid inside it. This coelomic fluid is
pumped through a coi8led tube, by means of cilia that line the tube. As it travels through this coiled
nephridial tube it is modified and converted into urine which is stored in a muscular bladder prior to
release to the outside via a small nephridiopore in the worm's under-surface.
Left: a cross-section through Lumbricus terrestris, showing
Left: aquatic worms, such as
flatworms and polychaetes, have
protonephridia rather than
metanephridia (although many
polychaetes have both). The
protonephridium opens into tissues
rather than into the coelom as a
series of branching porous
capillaries. Terminal cells bear cilia
(flagella?) that beat, appearing to
flicker like a flame under the light
microscope. These cilia drive fluid
along the protonephridial capillaries,
creating a suction pressure that
draws in fluid through porous filters
in the walls of the protonephridial
Left: there is one pair of metanephridia in each segment.
The arrangement shown is that in Lumbricus terrestris,
which has one pair of metanephridia per segment. Other
earthworm species have more complex arrangements
involving the grouping together of several metanephridia
into more complex arrays in which the nephridia are
connected by common ducts.