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Drawing of typical plant-parasitic
nematode (female).
(from head to tail) - Stylet, Esophagus, Median Bulb,
Nerve Ring, Esophageal Glands,
Intestine, Ovary,
Eggs, Vagina,
Anus, Tail, Cuticle
Figure is courtesy of Dr. R. P. Esser, University of Florida.
Nematodes are simple worms consisting of an elongate stomach and reproduction system inside a resistant outer cuticle (outer skin). Most nematodes are so small, between 400 micrometers to 5 mm long, that a microscope is needed to see them. 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. As Nathan Cobb explained in 1914,
"If all the matter in the universe except nematodes were
swept away, our world would still be recognizable, and if, as
disembodied spirits, we could then investigate it, we should find its mountains, hills, vales, rivers, lakes, and oceans represented by a film of nematodes. The location of towns would be decipherable, since for every massing of human beings there would be a corresponding massing of certain nematodes. Trees would still stand in ghostly rows representing our streets and highways. The location of the various plants and animals would still be decipherable, and had we sufficient knowledge, in many cases even their species could be determined by an examination of their erstwhile nematode parasites."
Most nematodes feed on bacteria, fungi, and other soil organisms. Others are parasitic, obtaining their food from animals (such as the dog heartworm), humans (such as the pinworm), and plants. Agricultural cultivation encourages an increase in parasitic nematodes that feed on the crops being grown. Occasionally, new kinds of plant parasitic nematodes may be introduced into a field by contaminated plant parts, soil on farm equipment and irrigation water. Nematodes which parasitize plants may cause yield losses by themselves or they may join with other soilborne organisms such as viruses, fungi, and bacteria, to promote disease development in plants. Most often, nematode feeding reduces the flow of water and nutrients into the plant, increasing the plant's susceptibility to other stress factors such as heat, water, and nutritional deficiencies.
Stylet
 The stylet is a hard, sharp spear used for
feeding. Muscles move the stylet in and out and allow the nematode
to puncture plant cells. Once plant cells are opened nematodes
empty the contents of the cell.
Esophagus
 The esophagus is a tube where food moves
from the head to the intestine/stomach. Glands empty their contents into the esophagus to help digestion. 
The median bulb is a circular muscle which pumps food through the esophagus. In the center of the median bulb is a valve which only allows contents to move from the stylet to the intestine.
 The largest glands in nematodes are the esophageal glands. These glands are made of large cells with large nuclei and are thought to release substances which help with digestion and feeding.
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Nerve Ring
 The nerve ring is the largest group of nerve cells in nematodes. Six bundles of nerve cells extend from the nerve ring to sensory receptors in the nematode head. Other nerve cells extend from the nerve ring towards the tail.
Digestive System (Intestine, Anus)
 Nematode digestive systems are made of a tube of a single layer of cells - the intestine. Food enters the intestine from the esophagus, nutrients are absorbed, and wastes are released through the anus.
Reproductive System 
The ovary is where germ cells give rise to eggs. Fertilization, by the sperm, takes place in the uterus and eggs are released through the vagina. Even though the size of nematodes varies greatly, most nematode eggs are about the same size and shape.
eggs
Most nematode species produce males and females, but some species only produce females.
 Male
nematodes produce sperm in the testes which are shaped similar
to the female ovary. Sperm accumulate in the seminal vesicle
and exit through the anus. During mating rigid spicules insert into the vagina and form a passageway for the sperm. Some males have thin cuticle extensions on both sides of the anus called bursae. A few species produce male and female structures within the same individual.
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Cuticle
The cuticle is the flexible coating around the nematode
[its skin], which protects the nematode from physical and chemical dangers. The most noticeable feature of the cuticle is the system of grooves across the body from head to tail. As nematodes grow they usually shed their cuticle four times.
Tail
 The tail is the region between the anus and the back tip of the nematode.
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Some nematodes appear very different from the above drawing. Nematode species can be very difficult to tell apart so differences in the size, shape, and position of structures are used to identify nematodes.
Content updated August 6, 2002
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