An Online Introduction to the Biology of Animals and Plants


Key Concepts


Section 3

Chapter 4

Roundworms / Nematodes


What Makes for a Successful Group?


If you were asked what the most successful group of living things on Earth was, you would probably be tempted to say, "Humans."  We have modified and controlled ecosystems and other organisms and like to think that we are in charge.  If asked for a more general group, "Mammals" would contain humans and many of the largest herbivores and top predators in most ecosystems, so that would only make for a stronger case.

But a biologist might say that the Monerans, the bacteria, were the most successful group - theyve been around the longest, they live virtually everywhere, they are notoriously difficult to kill, and as total biomass they probably outweigh all of the other Earthly organisms combined.  They could be said to control much more of the Biosphere than we humans do - we can barely control the handful of bacterial species that cause diseases.

Its all a matter of defining your terms - what amounts to success?  Are plants ability to support themselves with photosynthesis a critical detail?  Are fish not counted because they live in the water - that is, after all, where most life on Earth lives.

There are a couple of other animal groups that should be considered as well - one is obvious, one less so.  The obvious group, the arthropods, which will be covered in Chapter Seven, includes the insects, the spiders, and the crustaceans.  The less obvious group is the subject of this chapter.

There is a story zoologists tell:  if somehow one could make everything on Earth vanish but leave the Roundworms, also known as the Nematodes (Phylum Nematoda), behind, there would be a wormy "image" of almost everything that had made up the Biosphere (Earths ecosystem).  Roundworms live pretty much everywhere:  they live in soil, in oceans and freshwater, and inside and on most animals and plants above a certain fairly tiny size.  Their eggs fly through the air with the dust.  Some biologists estimate that for every five multicellular organisms (plant, animal, or fungus) in the world, four of them are roundworms.  This is an amazing feat for a group of animals that all look remarkably similar, except for size differences and an occasional difference in tail structure.  But, for all of their apparent physical similarities, roundworms have adapted chemically to a wide range of environments.  It is quite possible that there are more species of roundworms than species in any other phylum - no one knows, because identifying and cataloguing roundworm species is not easy, and usually not considered all that important.




There are a number of basic "body plans" that show up again and again in unrelated animal groups - one is the hydra-like body with a crown of tentacles, one is a head, neck, and body with bilaterally-symmetrical appendages, and another is the basic worm:  long, cylindrical, squirmy.  Roundworms have taken this last body plan and adapted to thousands of environments with it.

From microscopic to maybe a meter long, roundworms are just what they sound like - round worms.  At one end, they have a mouth, at the other, an anus, with a tube digestive system in between.  This type of system is a huge adaptive leap from the sac-type, single-opening system of the cnidarians and flatworms;  with a separate entrance and exit in the system, digestion can be much more of a step-by-step process, with new food coming in for processing while older food progresses through the system.  A tube system is like the opposite of an assembly line, its a disassembly line and is the basic pattern for digestion in all of the other animal groups from this  chapter on.

The space between the outside of the digestive system and the muscles and skin of the worms is filled with pressurized fluid.  Like a long balloon, this space tends to "rebound" back to a set shape when bent, or it changes shape in set ways when squeezed, giving some resistance and/or direction to the action of the muscles.  A muscle-resistance system based upon fluid-filled spaces is called a hydrostatic skeleton;  other animals use variations on them as well (including the extension of the human tongue, where the muscle bundles act as the fluid-filled space - we squeeze it and it gets longer).

The existence of an internal body cavity is also a huge evolutionary advance, allowing much greater independent movement of organs that are close to one another.  Most animal classification schemes even put a lot of importance to how the body cavities form in an embryo, giving different names to what wind up being effectively the same spaces.

Perhaps the most important feature of roundworms is their cuticle, a body covering produced by their skin.  This cuticle layer gives some physical protection, but it may be more important as a barrier to materials, limiting the exchange surfaces of a roundworm to a few areas and allowing them to adapt to some pretty nasty circumstances.  One type of roundworm, known as vinegar eels (Turbatrix aceti), can live happily in pure vinegar, a very acidic environment.  In order to grow, a roundworm must molt its cuticle and produce a new, larger one to "grow into."




Roundworm parasites affect most species of animals and plants, making them important agricultural pests.  There are also several species of roundworms that live in humans, some nasty, some not.  Here are a few...

Trichinella spiralis  is also a parasite that has been around for a while, since it has probably been responsible for several cultures long-standing dietary laws.  Trichinella can be found around the world, more in temperate zones than the tropics, mostly in various animals that eat meat, from rats to bears.  Humans most common exposure comes from pork, and pigs commonly pick it up from eating rats.  These worms can live as juveniles in muscle and other tissues while adults occupy support tissues and the lymphatic system.  A new host becomes infected by eating tissue containing juveniles.  Juveniles become adults and mate in the new hosts intestines, then females bore out of the intestines, which can cause a wide range of serious symptoms, settle someplace and begin to release juveniles, which migrate all over the body, causing damage as they go, until they mostly coil up in muscle tissues and "wait" for the host to be eaten by a new host.

Hookworms infect a variety of mammals, with species in cows, dogs, cats, and others, as well as humans.  Hookworms are fairly host-specific - worms of non-human hosts cant live long in a human.   You definitely dont want to catch one of the hookworms specific to humans, though.   They have a very unusual habit for worms that live in the intestine:  instead of living on all of the food around them, they bite through the intestinal lining and live on blood.  Serious infections occur when bacteria from the intestines get into the surrounding tissues and/or the blood, and heavy infections can produce enough blood loss to cause anemia.  Its no wonder that one genus is called Necator, or "killer"!  Hookworm eggs pass in feces, and juveniles live for a while in the soil if its nice and wet.  The worms get into the next host either by latching on and boring through their skin, or sticking to paws and getting licked off.  If they come in through the skin, they get into the blood and migrate to the intestines, usually by way of the lungs, sometimes causing tissue damage as they go.  Hookworms for non-human hosts that penetrate human skin by mistake can wander under the skin, unable to penetrate further, but the bodys reaction to them can cause a condition sometimes called creeping eruption.

Ascaris is an impressively large worm, up to 50 centimeters long and about as thick as a pencil, that lives in human intestines, with maybe as much as a quarter of the worlds population infected.  They are taken in as accidentally-swallowed eggs, hatch in the intestine, and the juveniles bore out, get into the blood, wander the body (where they can cause problems), emerge in the lungs, grow there for a while (and possibly cause problems), then migrate up to be swallowed and get back to the intestines again, where they mate.  Females find males by touch (its dark in an intestine) and crawl into the males hooked tail for mating;  sometimes they mistake the opening of the ducts from the liver or pancreas for a males tail and get caught, blocking the flow of digestive juices.  A heavy infection can produce a knot of worms that blocks movement of materials through the intestine.  Females that cant find males have been known to migrate up or down the canal, reaching the nose or anus in some cases - quite a surprise for the host!  Females lay eggs that pass in feces.  The eggs can remain infective in the environment for years, long after that fecal material has been broken down.  Dirty hands in the mouth explains why children are the most common hosts for these worms.

Filarial worms are a group of roundworms that commonly use biting insects to get juveniles from host to host, then the adults live in the fluid systems - blood or lymph systems - of the final host.  There are several filarial worms that infect humans, including Wuchereria, which can block fluid drainage through the lymph system, causing gross swelling of tissues and a form of elephantiasis Onchocerca causes a disease called river blindness when juvenile worms enter and gradually damage the eyes (the "river" part is due to the biting fly carriers being tied to rivers for breeding, restricting the geographical range of the disease).  Heartworms are filarial parasites of dogs and cats.  The juvenile worms are carried by mosquitos, and the adults settle in the chambers and major vessels of the heart.  Heartworms do not generally infect humans.

Dracunculus medinensis, also called guinea worms, have been known and written about for centuries (although often called "serpents" in modern translations), including passages from Ancient Greek scholars and from the Bible.  Adult worms can be as long as a meter, although they are very thin.  As adults, they live in the tissues under the skin, usually somewhere at and below the hips, where they may be visible as a white line.  After mating, a female produces huge numbers of eggs that hatch inside her and begin to migrate out into the surrounding tissue, often causing an allergic reaction with inflammation and ulceration of the skin (some ancient texts call them the "fiery serpents" from their effects on the skin).  When the skin breaks, many many tiny juvenile worms may emerge.  An opening remains in the skin through which the female will continue to release young.  To continue their life cycle, the juvenile worms must get into open water and infect a tiny crustacean;  for this reason, worms are most active when the skin is wet.  Ancient treatments, still used in some places, involve cutting a thin slot in a stick, wetting the skin so the worm sticks out, catching the writhing worm in the sticks slot, then winding it slowly out from under the skin.  The medical symbol, the caduceus, of a snake or snakes wrapped around a pole, most likely is taken from one of the few effective devices ancient doctors had, a worm-removal stick (worm, snake;  remember, in ancient classification schemes not much distinction was made among long wriggly things).  Worms infect the next host when water containing infected crustaceans is drunk;  the juveniles leave their carriers in the intestine, bore out of the intestine and migrate to their position under the skin.  Humans are affected both by their allergic reactions to the released juveniles, infections from bacteria that enter through the broken skin, and worms that stall in deeper tissues, where they may cause serious damage.  Because there is a fairly simple preventative measure - physically filtering drinking water - this parasite is very close to being eliminated.

Pinworms are an interesting case for a few reasons.  First, although they live inside animals, they may not be parasites, living instead on cells and materials that would be discarded anyway (this, if true, makes them commensals, benefiting from another organism but neither harming nor helping the host).  They also live in the large intestine or colon, which animal "hitchhikers" almost never do (its not the most nutrient-rich environment as well as being very oxygen-poor).  They are by far the most common human roundworm in North America and Europe - some estimates put the infection rate at as much as 75%, and upper class folks are more likely to be infected, another unusual feature or these worms.  Females ready to lay eggs migrate to the anus, crawl out, and deposit eggs behind them as they crawl around on the surface around the opening.  The most common symptom of pinworms is itching  - although these worms are probably not more common in children, infected children are often are more likely to show symptoms because they are willing to scratch that area in the presence of others when it itches.  Pinworm eggs are incredibly tiny - regular vacuum cleaner bags wont generally catch them - and they float off from the skin surface, even through clothing, as dust motes and spread everywhere.  Picking up eggs, which remain infective for from one to two weeks, from where they have settled on a surface and accidentally swallowing them will infect the next host.  This worm is almost impossible to get rid of - even if you were to be treated, thoroughly cleaned your clothes and left your house until the eggs there were not viable, you would probably soon pick more eggs up from somewhere else.

This was only a partial list of roundworms that live in humans.  

There is another roundworm with a more pleasant pedigree:  Caenorhabditis elegans is one of the most extensively-studied animals ever.  It, like fruit flies, lab mice, yeast, and mustard plants, is used as a model organism, which means scientists study it to answer "Big Questions" about life in general.  C. elegans (thats how a species is abbreviated) has been used to study many processes, but probably its most important role has been in the study of embryo development, especially HOX genes or homeogenes, which are responsible for determining the basic layout of an organism, such as where the front end will be, how the right side will be different from the left side, how, when, and where organs will develop, et cetera.  C. elegans, like several roundworm species, has adults with a set number of cells - in C. elegans the number is 1090 - which means tracking development from the very first cell and following each new cells track to the final number is simplified, and can lead to figuring out the genetics of the developmental process.


Informational Links


A list of common nematodes, matched to their common names.  Impressive just on the size of the list.

A page linking to a flash-based film of pinworms taken inside someones colon (warning - likely to make you queasy!).

Roundworms have a large web presence, maybe because the researchers are spread so far and wide - the internet gives the people who work with nematodes a connection that barely existed before.


Click on term to go to it in the text.
Terms are in the order they appear.


Success in Living Groups 
Roundworm Success 
Tube Digestive System 
Hydrostatic Skeleton 
 Body Cavities 
Parasites of Humans 
Creeping Eruption 
Filarial Worms 
River Blindness 
Dracunculus medinensis / "Guinea Worms" 
Caenorhabditis elegans 
Hox / Homeogenes 


Go On to Chapter Five - Mollusks



Online Introduction to the Biology of Animals and Plants.

Copyright 2001-2014, Michael McDarby.   e-mail Contact.

Reproduction and/or dissemination without permission is prohibited.


Hit Counter