Nematodes: "Roundworms"

Kingdom: Animalia

Phylum: Nematoda

-Yasheka Sharma


A. Diagnostic Characteristics
There are 90,000 nematodes known. Their bodies are not segmented, but are cylindrical bodies that range from
0.3 mm to lengths over 8 meters long! (Omer Zaidi) [3]They have a fine tip at the posterior (tail) end and rounded tip at anterior (head) end. They have a simple, transparent body with few types of cells with a bilateral symmetry, a mirror image produced when the animal is split longitudinally in the middle. Nematodes are triploblastic, which means they contain three different germ cell layers in their bodies. The endoderm is the innermost layer, the mesoderm is the middle layer, and ectoderm is the outermost layer that is skin.
The majority of nematodes are free living within the soil and are less than 2.5mm in length and can be microscopic. These free-living nematodes often live in the very slim and tiny spaces in between particles of soil and sediment and live in a film of water surrounding each soil particle. There are also many nematodes that are not free living but are instead parasites (they invade other organisms and obtain nourishment from it, often hurting or killing the host organism) (2) (RK). Humans host 50 nematode species (pinworms and hookworms).
They are part of the ecdysozoan phyla because they secrete external skeletons (exoskeletons). Other sources note that the covering of the outside of nematodes is not technically an exoskeleton, its known as a "cuticle." The epidermis layer of the nematodes secrets the material that becomes the flexible yet tough cuticle. Similar to an exoskeleton, the cuticle is shed multiple times during a nematodes' life (4 times usually) and gives support needed for movement (8 SV). Roundworms have bilateral symmetry and they do not have a complex body plan. They contain a simple nervous system and they have only longitudinal muscles. Nematodes do not have a digestive system or a respiratory system. (19)(BMB)

external image nematodexs.gif[8 Nangia]

B. Acquiring and Digesting food
They have a complete digestive tract containing a mouth and an anus. Nutrients are transported throughout body via fluid in pseudocoelom (a body cavity partially lined by mesoderm). Nematodes can either be carnivorous (“meat-eaters”), herbivorous (“plant-eaters”) or parasitic, which means that they get nutrients from the body of another organism. The digestive process of a nematode begins in the mouth, which eventually branches into a muscular pharynx. The muscular pharynx acts as a passageway by pushing and delivering food to other intestines of the nematode. The digestive process starts off externally (outside) in a nematode but then ends internally (inside) a nematode. (Jesse Carmen) [2]
Nematodes have a three part digestive system consisting of the stomodeum (mouth and lips), the intestine (digestive cavity), and proctodeum (excretory organs). (3)(RJS)

080221_sarid_nematode.jpg A SEM of the tip of a nematode. Nematodes cause over $70 billion per year in damage to crops. (18) (ER)

C. Sensing the environment
Many nematodes live in moist soil and decomposing organic matter on bottoms of lakes and oceans. They are found in most aquatic habitats, wet soil, moist tissues of plants, and in body fluids and tissues of animals. The predator nematodes attack roots of plants in agricultural fields. These free-living (non-parasitic) worms play an important role in decomposition and nutrient cycling.

Nematodes have a nerve ring made up of the largest ganglion, groups of nerves, in the organism. Two nerves, the ventral nerve and longitudinal nerve, run the entire length of the organism. Nerve impulses are conducted from all sensory structures on a nematode through nerves. Nematodes have a variety of different sense structures, from amphids, small pouches with neurons usually near the "lips" of a nematode, to papillae, small outwards projections (Denny Poliferno). [4]

D. Locomotion
Nematodes have longitudinal muscles and their contractions produce a thrashing motion.
This motion is made possible by a thick layer of muscle present under the skin. Due to their high internal pressure, the nematode's body flexes rather than flattening, causing back-and-forth thrashing(10 VM). As nematodes do not have circular muscles, they are not able to perform peristaltic motion. Instead, they are only able to move undulatorily, which is to say in a wave like fashion. Nematodes are poor swimmers and normally require a surface to adhere to in order to move effectively. (6) (KN) This undulating motion is not a side to side motion like that of the snake or eel; it is an up and down motion, made possible because of the presence of the exoskeleton (16 BL).

An illustration of nematoda locomotion (KN) (12)
An illustration of nematoda locomotion (KN) (12)

E. Respiration
Nematodes rely on their skin for respiration. There are no specialized organs for gas exchange, which occurs across the extracellular levels in nematode, but since they are thin, their cells individually get access to gas. It has been proposed that species are adapted to predominance in the Nematode community at different times of the year to reduce competition, usually by inhabiting different depths of soil. They also influence Nitrogen availability in the soil, which in turn affects agriculture. (15) (AR).

F. Metabolic Waste Removal
Wastes diffuse back across the body wall of nematodes.Some nematodes have specialized cells that excrete nitrogenous wastes, and others have canals in addition to these specialized cells(10 VM).

G. Circulation
Nematodes lack a circulatory system (no blood system). There is no heart or blood vessels, however, nutrients are transported throughout the body via fluid in the Pseudocoelom (a body cavity partially lined by mesoderm)

H. Self Protection
Nematodes are called Pseudocoelom, since the body cavity (a space filled with fluid that separates the digestive tract from the outer body wall and something that cushions organs to protect internal injury)
is not enclosed by the mesoderm.Also, nematodes have an exoskeleton, called cuticle, which covers the worms body. The cuticle is a rigid shell that does not grow as the nematode grows. Therefore, a nematode periodically sheds its old cuticle (ecdysis) and secrets a new and larger one. A nematode may undergo ecdysis about four times before it reaches its adult stage. The cuticle is the closest thing a nematode has for a skeleton, and, in fact, it uses the cuticle as a support and leverage point for movement. [8 Nangia]

Nematodes are able to survive is extreme conditions such as intense heat or cold or in an extremely dry environment. They are able to do so through a process called cryptobiosis. During this process the organism stops almost all of its bodily functions (circulation, respiration, movement, etc.) and continues just enough to stay alive. Then when the environment is back to a positive condition the nematode “restarts” all of its functions and goes back to its normal life. (13 HS)
external image 026B.GIFpictured to the left is a nematode starting to shed its cuticle.(SJB)(9)
I. Osmotic Balance
Each cell makes sure of the osmotic balance individually. Cells control themselves because there are not enough cells for a set process to occur.

J. Temperature Balance
The surroundings of the nematodes define their temperature. However, this is not to say that nematodes cannot control their survival in cold temperature (11, DJ). A certain type of nematode, Panagrolaimus davidi, resides in Antartica (11, DJ). The optimum temperature for growth for the P. Davidi is 25 degrees Celsius, but will survive temperatures as low as -80 degrees celsius (11, DJ).

K. Reproduction
Reproduction for nematodes is sexual. Females are mostly larger than males. There is internal fertilization and the female may release 100,000 or more fertilized eggs per day.
The eggs grow from zygote (two haploid cells) to mature adults in three and a half days.
Most nematodes are hermaphrodites (produce eggs and sperm). Every adult hermaphrodite has 959 somatic cells (cells forming the body of the organism). Through the vulva, a tiny opening, the worms lays its eggs. Biologists have been able to construct a cell lineage diagram, a fate map of the different parts of an embryo.
Furthermore, nematodes have both male and female organs. However, most nematodes have separate sexes, where they are either male or female at a given time, but never both at the same time. This characteristic is called gonochronistic. This feature allows cross-fertilization (mating by aligning themselves so that they exchange sperm and then separate) to occur. During reproduction, with most freshwater species, the male nematodes have spicules, which is a small needle-like structure, that enter the female's vagina.Through these spines the males inject sperm into the females(10 VM). Other species specialize in "traumatic fertilization," in which the male simply punctures the cuticle, or outer covering, of the female with his spicules and releases the sperm directly into her body cavity. (5) (SS)

(NK, 14)

A common parasitic nematode, Trichinella spiralis, is the worm that causes trichinosis by eating undercooked infected pork or other meat with worms in the muscle tissue. They develop into sexually mature adults. Females burrow into intestinal muscles and reproduce. They travel through the body and in lymph vessels to encyst in other organs, including skeletal muscles. Sometimes nematodes can go unnoticed in the body if there are not enough eggs to induce infection. Some nematodes have the capability of living in human intestines for 12-18 months and can produce of 25 millions eggs! The presence of eggs in human bodies can cause vomiting, loose stool, abdomen pain, weight loss and anorexia. The best way to stay clear of a worm infection is keeping clean. (GR, 17).

A picture of Trichinella spiralis within a human muscle.

(7 SES)

L. Review Questions

1. Explain how Nematodes protect themselves from the environment. (JAC)

Edits By:
Jesse Carmen
Omer Zaidi
Denny Poliferno
Kevin Nayer
Sarah Schwarzschild
Meru Nangia
Sam Blatchford
Vonai Moyo
Daisy Joo
Sarah Vlach
Rachel Kornetsky
Hilary Stepansky
Josh Czik
Arielle Reiter
Becca Levenson
Ethan Richman
Brittany Marcus-Blank

Work cited:
Campbell, Neil A., and Jane B. Reece. Biology. 6th ed. Boston: Benjamin-Cummings Company, 2002
[2] “Phylum Nematoda.” University of Winnipeg. Winter 2006. 1 December 2008. <>.
[3]"Nematodes in General." University of Nebraska-Lincoln. 2 December 2008. <>
[4]"Sensory/Nervous System." University of California. 2 Dec. 2008 <>.
[5] Hebert, Paul D. "Nematoda." The Encyclopedia of Earth. 17 Nov. 2008. Boston University. 7 Dec. 2008 <>.
(6) "Outlines." Nematoda. Vassar. <>.
[7] "Trichinella spiralis in Human Muscle." Medical Encyclopedia. 9 Oct. 2006. US National Library of Medicine. 7 Dec. 2008. <>
[8]Waggoner, B. M. "Introduction to the Nematoda." 2005. University of California Museum of Paleontology. 7 Dec. 2008 <>.
(10) Myers, P. 2001. "Nematoda" Animal Diversity Web. 7 Dec.2008 <>
[11] Wharton, David A. Life at the Limits : Organisms in Extreme Environments. New York: Cambridge UP, 2002. 177-78.
(12) Seattle University. <>.
(13) Waggoner, Ben. "Introduction to the Nematoda." 2008. University of California. 14 Dec. 2008 <>.
(15). Respiration in Nematodes. 14 Dec 2008.

(16) "muscle." Encyclopædia Britannica. 2008. Encyclopædia Britannica Online. 16 Dec. 2008 <>.
(17) Hunt, Richard. "NEMATODES (Round Worms)." Microbiology and Immunology. 24 Sept. 2008. University of South Carolina School of Medicine. 18 Dec. 2008 <>.

(18) 080221_sarid_nematode.jpg
(19) "Nematoda." 18 Dec. 2008 <>.