Nematodes – Phylum Nematoda – Examples, Classification/Characteristics

Nematodes are also known as roundworms.

Nematodes - Phylum Nematoda - Examples, Classification/Characteristics

It isn’t known exactly how many species of nematode there are, but it’s certain that they are one of the most (if not the single most) ubiquitous forms of life on the planet.

Nematodes are absolutely everywhere.

This article will tell you about the three currently accepted classes of nematode, as well as explaining the characteristics of nematodes.

About Nematodes

Nematodes are perhaps the most ubiquitous form of life on planet Earth.

A single meter square of soil can contain up to a million of them, and it’s estimated that there are something in the region of 55 billion nematodes for every single person on earth.

Combined together, the weight of all of the nematodes on the planet would add up to around 80% the weight of all the humans.

Nematodes are more commonly known as roundworms. They are distinguished from other types of worm by their general shape and size.

The vast majority of nematodes cannot be seen by the naked eye, although there are of course some species of nematode that are much larger.

In fact, the largest known nematode, a parasitic kind that lives in whales, can reach several meters in length.

They are an extremely diverse phylum, even when considering that nematodes are all essentially very simple organisms.

It’s not even precisely known how many nematode species there are.

There are somewhere between 15,000 and 40,000 known species, but some estimates put the possible number of extant species at a million.

It has been said (by famed nematologist Nathan Cobb) that if every form of matter apart from nematodes would vanish, we could still see the remnants of every thing on planet Earth -mountains, trees, animals, people, cities – in the form of the film of nematodes that covers everything.

There are of course some more famous examples of nematode that many people know about.

The ones known to humans are primarily those which cause us harm, such as Trichinella – a type of nematode that lives parasitically in animals, and can infect humans through improperly cooked food.

Trichinella causes an infection called trichinosis, which causes diarrhea, vomiting, and intense abdominal pain.

Untreated, it can lead to facial swelling, fever, inflammation of the heart and lungs, and complications with the central nervous system.

Nematodes have been found in the most inhospitable places on earth. They can be found in places that no other animal can survive.

It has been said that nematodes essentially cover every surface and living thing on the planet – and of course, some forms of nematode even inhabit living things.


The name “nematode” is, unsurprisingly, of Ancient Greek origin. It originally came from the grouping Nematoidea.

The word “nematoidea” derives its meaning from two Ancient Greek words, and essentially translates to “thread species”.

Thus, we can see how the word nematoidea, and, by extension, nematode, describes the phylum well.


Nematodes are eukaryotic, multicellular organisms, just like the other inhabitants of the kingdom Animalia.

The name for the phylum that nematodes inhabit is called Nematoda.

While the inhabitants of Nematoda are remarkably diverse, they all share some common characteristics.

They are thin, tube-like organisms – much like other worms. They are often extremely thin, and have many of the characteristic features of other worms.

For example, it can be difficult to tell one end of a nematode from the other on occasion – although, broadly, nematodes have a distinct head end that can be seen by a trained observer.

The majority of them seem to move in much the same way as more commonly known worms.

However, the vast majority of nematodes are invisible to the naked eye, and can only be seen by a microscope.

This of course is not universal, but the majority of nematode species are unknown to most people simply because they cannot be readily seen without equipment – and knowing what to look for.


The study of nematodes is ongoing, and the exact classification of Nematoda changes as more research is done and more about the phylum is understood.

Therefore, it’s possible that some of the information here won’t fit with updated research.

As more is understood about nematodes, members of classes can shift, and classes themselves can disappear or emerge as more about nematodes is discovered.

Currently, nematoda is considered to be broken down into three extremely broad classes – Dorylaimea, Enoplea, and Chromadorea.

Each of these classes has various subclasses, orders, families, genera, and so on underneath it – and, of course, as scientific knowledge is always in a state of flux, these classifications may well be outdated some day.


Dorylaimea is a class of nematode that is currently recognized as one of the three known classes.

There are an estimated 4-5000 species of nematode in the class Dorylaimea, under 3 subclasses and 3 superorders.

Dorylaimea was previously considered to be a subset of Enoplea due to the presence of primarily non spiral amphids.

However, under molecular analysis, Dorylaimea seems to be a separate branch in its own right. This is despite similar morphological characteristics to Enoplia.

The name “dorylaimea” comes from two Greek words – laimos, meaning “throat”, and dory, meaning “spear”.

This name was chosen because of a common characteristic shared between these nematodes – a movable tooth as part of the digestive system.

Examples of nematodes in the class Dorylaimea can be found in both soil and freshwater. They are often to be found in moist soils near to plant roots.

They commonly sustain themselves by consuming other microorganisms such as bacteria.

There is a known family of Dorylaimea called Longidorae that is known to be parasitic to plants.

Characteristics & Facts

There are of course some characteristics that are shared between members of the class Dorylaimea.

The vast majority of known members of the class Dorylaimea posess a hollow spear-like tooth, sometimes called an odontostyle (which gives Dorylaimea its name) and a stoma, both of which are of course for the purposes of digestion.

This “spear” is known to originate from something called the Esophostome, an esophageal cell.

Members of Dorylaimea have various different tail shapes – tail shape can even vary between different sexes of the same species.

In some members of the class Dorylaimea, the spear-like tooth is hollow, which enables food to pass through after the tooth pierces the food.

This occurs in both plant parasitic members of Dorylaimea, and in predatory variants too.



Another currently accepted taxonomic class of Nematoda is Enoplea.

There are over 2000 species of Enoplea under current taxonomy, divided into 3 subclasses and 4 superorders.

This class is considered to be a comparativeyu more ancestral class than Chromadorea.

As such, it is sometimes called “ancestrally diverged” – in comparison to Chromadorea, which is said to be “more recently diverged”.

Members of the class Enoplea are known, like many other organisms, to have no bilateral symmetry during the early developmental stages of their embryo.

Characteristics & Facts

Members of the class Enoplea are of course as diverse as those of other classes of nematode, but they do share some characteristics in common.

Compared to the larger, rounder esophagus found on members of Chromadorea, the esophagus of enopleans are comparatively thinner – commonly described as either cylindrical or bottle shaped.

The amphids on enopleans are often described as being shaped like a pocket.

This is in comparison to the amphids found on members of other classes of nematode, which commonly have much different shapes.

Members of this class are known to have either a rather smooth surface, or one that has fine lines across it.

The excretory system of an enoplean is characterized by being extremely simple – in fact, it is commonly made up of only a single cell in many enopleans.

The class is the home for one of the more well-known parasitic nematodes, Trichuris trichiura, which is responsible for a tropical disease called trichuriasis.

This is an infection of the large intestine in humans.

The nematode that causes this infection commonly passes to humans through infected soil – which is both caused and exacerbated in farming regions that use human excrement as fertilizer.

The infection is also called whipworm infection, after the common name for the nematode which causes it.


Chromadorea was formerly considered to be a subclass under the now-abandoned class Adenophora.

Under current taxonomy, they are considered to be a class in their own right containing over 21000 known species under 2 subclasses and 5 superorders.

Currently, they are considered to be a separate class from Dorylaimea and Enoplea due to research into their spermatogenesis and development.

Of course, as the research into nematodes is constant and ongoing, it is possible that in the future any or all of these classifications, whether for chromadorea, dorylaimea, or enoplea, may be revisited and revised.

Characteristics & Facts

Chromadoreans of course can share many common characteristics. They normally have annules, and spiral shaped amphids.

One common feature amongst chromadoreans is that they have three esophageal glands.

Chromadoreans, just like the other current classes of nematode, can be found in practically every environment on earth.

They are, however, commonly found in marine sediments.

Just as with the other two currently accepted classes of nematode, many of the inhabitants of Chromadorea are parasitic worms.

One example of these worms is the parasite Anisakis, which causes the infection Anisakiasis.

This is a parasitic infection of the gastrointestinal tract in humans, which can be identified by a strong pain in the abdomen of the infected person.

This infectious parasitic worm is passed to humans through the consumption of raw fish.

Accordingly, it is most common in places with a strong culture of eating uncooked fish, such as Japan, Scandinavia, the Netherlands, Spain, and South America.

To avoid infection with this parasite, fish must be either frozen for about a week, or cooked to a safe temperature before consumption.

Both of these are the only way to be sure to kill the nematodes that reside within – although many local myths abound regarding the use of salt and marinades in order to render the fish safe to eat.

Characteristics Of Nematodes

Nematodes are of course an extremely diverse and broad class of organism.

However, even accounting for the fact that they are currently divided into three classes, each nematode species shares certain characteristics with all nematodes.


One key characteristic that is shared by all nematodes is the body shape.

Although during different developmental stages nematodes can indeed take on different forms, the class is broadly characterized by the shape and structure of the body of the organisms.

They are all thin worms, covered in an epidermis. This epidermis is itself covered in a thick layer (or often, multiple layers) of cuticle made from a substance called collagen.

Most commonly, nematodes are extremely small. In fact, the vast majority of nematodes are difficult to completely impossible to see with the naked eye.

This is, of course, how there can be so many of them on the planet.

And, the vast majority of nematodes are completely harmless – however, there are of course more than enough nematodes that are extremely harmful to us, that we can’t see without a microscope.

There are some nematodes that are far easier to see, existing on a macroscopic scale. Nematodes can be broadly divided into two types – free-living and parasitic.

Free-living nematodes live like the majority of animals – as independent travelers, sustaining themselves on food that they either find or kill.

Parasitic nematodes, on the other hand, live as unwelcome tenants in the body of another organism.

They sustain themselves by infecting a host and essentially draining it of resources.

Not just that, but they also harm organisms by their very presence, causing damage to the vital structures and organs which they infect.

These parasites are responsible for some terrible and debilitating infections.

Many of the larger nematodes are parasitic, and can very easily be visible with the naked eye. The largest parasitic nematodes can be several meters long!


Nematodes are of course fairly simple organisms by comparison with larger animals.

They are essentially little more than creatures solely intended to digest and reproduce. As such, they are extremely proficient at both of these tasks.

Nematodes have an oral cavity, which is commonly lined with ridges or teeth.

These are most commonly found in species that are carnivorous – some nematodes, for example, sustain themselves by consuming other nematodes.

These teeth are commonly accompanied by a single sharp structure called a stylet, which is used for piercing food.

In carnivorous species, the stylet is used to pierce prey for aid in consumption. In some species, this tooth is hollow, and sometimes called an odontostyle.

The hollow nature of this structure allows food to be sucked into the pharynx – a muscular structure that is lined with cuticles, and that contains enzyme-producing glands that begin the chemical digestion process.

Nematodes do not possess stomachs. Instead, the pharynx connects directly to an intestine that is the main structure forming the gut of the nematode.

More enzymes are produced here, which further break down the food.

In this muscleless intestine, the nutrients from the digested food are absorbed too, through the extremely thin (one cell) lining of the intestine.



Just as with other forms of life, nematodes must eventually excrete waste products as a part of the digestive process.

Their excretion processes are similar in many ways to those which humans experience, but of course with some key differences.

In many animals, the intestine is a structure that is at least in part made of layers of muscle.

This muscle tissue works to move the food through the body, as well as allowing for the absorption of nutrients from digested food.

As the intestine of the nematode has no muscles, the movement of food and waste through the digestive system comes through the aid of the movement of the nematode itself.

As the digestion process ends, food waste passes through the final portion of the intestine, a cuticle lined rectum which leads into the anus.

Sphincters and valves assist in the movement of food.

Many animals store some unwanted byproducts of the digestive process in a bladder.

Unlike many other animals which excrete their nitrogenous wastes through urination, nematodes instead excrete them as ammonia directly through the cellular wall of their bodies.

They must also excrete other unwanted substances – for instance, they also have complex mechanisms to regulate the osmotic excretion of salt from their bodies.

These mechanisms ensure that the nematode’s salt content is controlled, and make sure that the nematode excretes only the correct amount of salt.

Nervous System

Nematodes have a nervous system. Along the entire length of their body, they have four main nerves.

These nerves lie between the muscle cells of the nematode, beneath the cuticle. These nerves all have different functions.

The largest nerve on a nematode is the ventral nerve, which of course runs across the ventral surface of the organism.

It functions as both a sensory nerve and a motor control nerve for the nematode.

The dorsal nerve handles only motor control in the nematode, and the two nerves on the lateral surfaces are both sensory nerves.

At the front end of the nematode, the nerves cluster into a ring structure that surrounds the pharynx. This dense nerve ring functions as the brain of the nematode.

From this larger ring, smaller nerves branch for the sensory functions of the nematode’s head.

Nematodes don’t normally have eyes as we’d normally think of them, but some researchers have actually observed the presence of photoreceptors in nematodes.

They have also been observed moving to avoid UV light. Research has also shown that they can detect color.

Sensory structures on the surface of nematode bodies help the nematode sense touch.

The amphids are supplied by nerves which are thought to be used to detect chemicals, aiding the nematode in navigating and “understanding” its environment.


Just as with digestion, reproduction is of course extremely important for nematodes, and is one of the most important functions of the nematode body.

In many nematodes, nearly half of the cells of the organism are dedicated to reproduction.

The majority of species of nematode have both male and female individuals, although some species have hermaphrodites.

In species that exhibit sexual dimorphism, males are typically smaller than their female or hermaphroditic counterparts.

Nematode reproduction is normally done sexually. However, in the case of hermaphroditic nematodes, self-fertilization occurs.

Nematodes sometimes deposit fertilized eggs that aren’t developed, and that embryonate outside of the body of the female.

In free-living (non-parasitic) nematodes, eggs hatch into a larval stage once developed.

These larvae are mostly identical in appearance to the adult form of the nematode, but their reproductive system is not yet fully developed.

Parasitic nematodes can have far more diverse and complex development stages.

Some species of nematode have a process called endotokia matricidia, which refers to the death of the mother during the birth process.

For instance, in C. Elegans, the eggs hatch while still inside the female parent. They consume the parent from within in the process of emerging.

In C. Elegans, this is an abnormal process – but some parasitic nematodes, for instance, consume their host from within.


There are many types of parasitic nematode. These nematodes live on or in another living organism, whether plant or animal.

Parasitic nematode infections are the cause of many infections in humans, such as trichinosis, elephantiasis, and river blindness.

The presence of parasitic nematodes in an organism can often be debilitating, and in many cases can result in the death of the infected.

For instance, the fig wasp. The fig tree is only fertilized by fig wasps, which have a nematode that has evolved to prey solely on them.

The nematode hitches a ride on these wasps, killing it when it lands on an unripened fig flower.

Offspring of these nematodes then repeat the process, hitching a ride on a wasp that lands on the ripening fig.


Nematodes are a surprisingly diverse lifeform, considering the fact that they are all simple forms of worm.

They can be found all across the globe, even in places where little or nothing else can survive. Hopefully, this guide has helped you to learn about them!

Jennifer Dawkins