Phylum NematodaCori Plesko

Figure 1. Section of female nematode with eggs (RK)
Figure 1. Section of female nematode with eggs (RK)

What makes a Nematode a Nematode?
Nematodes, also known as roundworms, have non segmented bodies meaning that their bodies do not have lines dividing their bodies into segments. The head side of their body is rounded tip in comparison to their tail end which decreases to a narrow tip. Nematodes also have an exoskeleton or a cuticle that covers their body. Periodically, nematodes will shed this cuticle or molt in order to produce another tough cuticle.

Nematodes also do not have a circulatory system, a system that circulates nutrients throughout the body. Instead, their nutrients transport throughout the body by fluid in a body cavity that is lined by the mesoderm, called pseudocoelom. The mesoderm is the middle layer of an embryo that later develops into the nematode's muscles as well as other components. These muscles are longitudinal in a nematode and as they contract they help the nematode move. Even though they do not have a circulatory system, Nematodes do have a complete digestive system to absorb nutrients.

Nematodes reproduce sexually and a female nematode, often larger than males, can deliver up to 100,000 fertilized eggs in a single day. It only takes a couple of days for nematodes to fully mature from a zygote to an adult. Many are hermaphrodites meaning they produce sperm and eggs. It is these reasons that nematodes are often used in genetic studies because researchers can easily detect mutations when the nematodes reproduce as well as develop quickly.

In size, female nematodes are larger than male nematodes. (AR 2)

Nematodes also have much less cell multiplication than most other organisms and grow mainly by cell enlargement. For the most part, juvenile nematodes have the same number of cells as the adults. (AM source 8)

In addition, nematodes are known for a rather unique quality called eutely. In eutely, every single individual of a certain nematode species has the exact same number of cells, which is finally achieved once they reach developmental maturity. This helps to explain why growth after the end of the developmental period “involves increases in cell size rather than in cell number.” (MR; Source 11)
external image nematode.gifRJ


Most nematodes are dioecious which means that they is either male or female, and that they reproduce sexually. Male nematodes are typically smaller than females and have posterior ends that are shaped like hooks. However, other species of nematodes are hermaphroditic meaning that they have reproductive organs normally associated with both male and female sexes. Hermaphroditic nematodes are able to reduce via the process of self fertilization. (ZJ- source 3)

Male reproduce structures include one testis, a seminal vesicle, and a vas deferens. The female reproductive structure has one or two ovaries, seminal receptacles, a uteri, ovijector, and vuvla. (AR 2)


Nematode development involves numerous stages from egg to mature adult. The first stage is inside an egg. A nematode will begin the hatching process when environmental conditions - like moisture and temperature - favor its survival. At that point, nematodes secrete chemicals that will weaken the egg, until the organism finally ruptures it to exit. At that point, the larval (baby) nematode will feed and go through a number of larval stages. Each stage is separated by a point where the nematode has to molt, or shed its outer skin and grow another. These stages go from L1 (just after hatching), to L5, after which it is considered a sexually-mature adult. Nematodes often parasitize other organisms starting at stage L3. (Matt B - Source 2)

Locomotion: How do nematodes move?

Nematodes also have a hydrostatic skeleton. This means that there is fluid that is under pressure closed off underneath the skeleton in a special compartment. The nematodes muscles can change the shape of this compartment which allows them to control their movement.

Nematodes move by wave-like motions of their bodies which result in thrashing movements. The body's contractions are according to dorsal and ventral contractions of the body. While one segment of a nematode's body contracts, it pulls the rest of the body forward. As a result, a nematode can move by constantly contracting and relaxing muscles controlled by the pressure changes through the fluid skeletal space of the pseudocoel. (VN)

Nematode picture (CM)
Nematode picture (CM)

Acquiring and digesting food:
Nematodes have a complete digestive track. They acquire and digest food in four steps. First, Ingestion: this is the sage where the nematodes actually take in their food by eating it. Next, is digestion where the food is processes. During this step food is broken down into small molecules so that the body can later absorb it. This also allows the nematode to make ATP or energy. Nematodes then go through absorption where they absorb molecules of such as sugar that were broken down from the food in digestion. The final step is elimination where the food that was not absorbed or digested leaves the digestion track.
In animals with a digestive sac, such as a cnidarian, food enters and wastes exit through the same opening. In contrast, in animals with a complete digestive tract, like a nematode, food enters and wastes exit through different openings.
In animals with a digestive sac, such as a cnidarian, food enters and wastes exit through the same opening. In contrast, in animals with a complete digestive tract, like a nematode, food enters and wastes exit through different openings.

There are different types of nematodes that feed differently. Herbivores- This type of nematode feeds on plant roots. They can live in the soil or in the plant root itself. Bacteriavores- feed on bacteria, they are beneficial to the decomposition of organic matter. Nematodes can also feed on other nematodes depending on size, for example Mononchida is one of the big nematode predators. (AP)

The digestive system of a Nematode includes three parts: stomodeum, intestine, and proctodeum. The stomodeum includes the mouth, buccal cavity and pharynx. The buccal cavity is the area between the lip, cheeks, and gum. The three parts of the digestive system are used in the classification of nematodes. The digestive system also gives indication of what the specific nematode feeds on. One example is that plant parasitic nematodes have a modified buccal cavity in the form of a hollow spear to penetrate and withdraw materials from host cells. (AR 1)
The pharynx of a Nematode is an efficient pump and forces food into the intestines. There is a one-way valve between the intestines and the pharynx. The pharynx can, when this valve is closed, be used to suck liquid food into the mouth. After, digestion is quite rapid, so feces are expelled under pressure. (Fun fact: So much pressure is present that the parasitic nematode Ascaris lumbricoides shoots its feces two feet into the air. Eww...)(SP)

Most nematodes are parasitic to animals or even plants. Herbivore mouths (48% of nematodes) have needle-like aperatures in their mouths to puncture cells. Some feed at the roots, while others can enter the roots of plants and live inside. Others feed on bacteria and have a hollow, tube-like mouth for feeding. (LPE)

Sensing of environment:
Nematodes need a moist environment in order to be able to survive. This means that they must be able to detect moisture within their environment with the skin around them, their exoskeleton called the cuticle.

Papillae, tiny nipple-like structures, along with small sensory hairs cover a nematode's entire body giving it a sense of touch. A nematode also has a pair of amphids. Amphids are chemoreceptors that look like small cavities and allow nematodes to sense chemicals. (JS)

Eyes for nematodes are rare, and when they are present, they are often found inside the throat. The other sensory structures are located on bumps and pits in the cuticular surface; these structures are mostly innervated and presumed to be sensory although the stimuli that each responds to exactly is unknown, although it is assumed that they have tactile as well as chemosensory capabilities. The aforementioned amphids, located at either side of the body just at the back of the head, also have an unknown sensory modality. There may also be additional papillae (as defined above) in the "cervical region" for certain species. (IL- source 9)

The roundworm muscles are activated by two nerves that run along the length of the nematode both ventrally and dorsally. Whereas other animals' nerves continue on towards muscle cells, the nematode's muscle cells branch towards the nerves. Along its bottom, it has a series of nerve centers closer to the head. (JP- Source 4)

Nematodes take in oxygen from their environment, either from the water that they are in or in soil. Nematodes do gas exchange meaning they take in oxygen and give off carbon dioxide into the atmosphere. However, since do not have a circulatory system, Nematodes rely on the fluid within their body cavity, pseudocoelom, as mentioned above, to help get oxygen throughout their body.
Nematodes do not have a respiratory system either. (SP)

Free-living nematodes exist in the interstitial spaces of sediments and soils, living mainly in the films of water in soil, where they do much gas exchange and fluid exchange by diffusion. (PS Source 7)

Metabolic Waste Removal:
Since Nematodes do not have a circulatory system, they rely on their skin to take in oxygen as well as to diffuse metabolic waste out of. Waste is also removed via their digestive track.
The excretion of metabolic waste is via two simple tubules which have no nephridia or flame cells. (SP)

Self- Protection:
Nematodes protect themselves in several different ways. First, as mentioned above, their hard exoskeleton or cuticle protects nematodes by acting almost as armor for them and protecting them from outside elements. This cuticle sheds as the nematode grows in order for them to grow new larger ones that can better protect them. Nematodes also have self-protection traits when it comes to how they reproduce. Nematodes reproduction is internal which helps protect the gamete cells. A nematode's fertilized eggs, or zygotes, are capable of surviving very harsh conditions. These resistant traits in nematode zygotes help protect them and allow them to mature and survive to be adult nematodes.

Osmotic Balance:
Osmosis is passive transport, transport not needing energy, of water. Since nematodes are composed of animal cells, their cells do not have cell walls. In order to stay in osmotic balance a nematode's cells must act depending on the environment it is in. If the cells are in an isotonic solution meaning that there is the same amount of water in the cell as outside of the cell, the water moves in both directions at the same rate and there is no change. However, if placed in an environment that is hypertonic or environment with less water, the water would want to leave the cells to the environment. And in a hypotonic environment with more water than the cells, the cells would want to take in more water. Hypertonic and hypotonic environments can be dangerous because they could cause cells to shrivel up and dry out or they can cause them to take in too much water. To prevent this from happening, nematodes have the special adaption of osmoregulation that helps control the water balance of the cells. This makes it possible for nematodes to live in water without their cells taking in too much water and bursting.

Temperature Balance:
Nematodes are invertebrates meaning that they do not have a back bone. Like other invertebrates, Nematodes are thermoconformers, this means that they have little control over their body temperature and instead their temperature depends on the environment around them.
As a result of this characteristic, nematodes need to be able to survive in both low and high temperatures because their environment can be either cold or hot and nematodes take on whatever temperature their environment is. Generally, nematodes do this by being tolerant of almost every climate possible or avoiding certain extreme climates. (MM)

Aquatic (water). Most nematodes are found in moist soil or plant tissue as well as in animal body fluids or tissues. Many live on the bottom of bodies of water such as lakes or oceans.
Nematodes can actually be found in soil habitats on land as well, including but not limited to fields, forests, grasslands, and ocean beaches. (MB)

In fact, nematodes are so wide-spread and plentiful that one square yard of soil can contain as many as several hundred different species of nematodes and as many as one million individual nematodes. (MR; Source 11)

Nematodes live in a vast variety of habitats, ecologically they can be divided into free living forms and parasitic forms. Free living forms have a simple life cycle involving 4 juvenile instars on the path from egg to adult. Parasitic species have developed a wide range of variations on this basic theme. The variations involve whether there is a secondary host and the amount of time spent in one or either hosts. There is also considerable variability in the way that they move from one host species to another. Thus while many species lay eggs that pass out of the primary host with the faeces where they are eaten by the secondary host which then gets eaten in turn by the primary host after the Nematodes have developed. Because it is not always totally reliable that the secondary host will be eaten just as the Nematode larvae have developed into the infective stage many species have the ability to encyst themselves in the muscle or cuticle of their secondary hosts. (RW)

Nematodes' length ranges from 1mm to as long as a meter in some cases. Because of their small size the nematodes , "
Their small size, resistant cuticle, and ability to adapt to severe and changing environments have made nematodes one of the most abundant types of animals on earth," (MP).

Nematodes can live in the bottom oceans. They are key components to decomposition
as well as nutrient cycles within their habitats as demonstrated in this video.
Photograph. Http:// Natural Resources Conservation Service. Web. 18 Oct. 2011. <>.
Photograph. Http:// Natural Resources Conservation Service. Web. 18 Oct. 2011. <>.

Video: Home Sweet Pineapple. Dir. Ennio Torresan., 1999.

Review Questions:

1. What biological adaptions have allowed nematodes to become so wide spread and a dominate parasitic threat? (MS)

Works Cited:
Campbell, Neil A., and Jane B. Reece. Biology. 6th ed. San Francisco: Benjamin Cummings, 2002. Print.
Pages 661,662, 405, 406, 1077, 857, 886, 146-147, 932 (TB) (MB)

"Soil-inhabiting Nematodes - Phylum Nematoda." UNIVERSITY OF FLORIDA ENTOMOLOGY AND NEMATOLOGY DEPARTMENT. Web. 30 Oct. 2011. <>. (AP) (AR 1) (AR 2)
Gaugler, Randy., and Anwar L. Bilgrami. Nematode Behaviour. (MM) (LPE) (JS)

Figure 1. (RK)

"The Nematodes (Phylum Nematoda)." The Earth Life Web. Web. 30 Oct. 2011. <>. (SP)

2. (Matt B)
3. Barnes, Robert. Invertebrate Zoology. 2nd. Philadelphia, PA: W.B. Saunders Company, 1969. Print.

6. "Nematode Locomotion Page." SU Classes Web Server. Web. 06 Nov. 2011. <>. (VN)

8. "Unique Features of Nematoda." Web. 3 Nov. 2011.
9. Shimek, Ronald L. "Nematodes." Reefkeeping... an Online Magazine for the Marine Aquarist. Reef Central. Web. 13 Nov. 2011. <>. (IL)
10. (RW)
11. "Nematoda." Animal Sciences. Ed. Allan B. Cobb. New York: Macmillan Reference USA, 2010. Gale Science In Context. Web. 29 Nov. 2011.