Phylem Nematoda
By: Lauren Warshaw


(Campbell, N.C., Reece, J.R. (2002). Biology. (Sixth Edition). San Francisco: Benjamin Cummings.)

Nematodes, also known as roundworms, are among the most widespread of all animals. There are about 90,000 species currently known by scientists, but perhaps ten times that number actually exist. Nematodes are found in most aquatic environments, in wet soil, and in the body fluids and tissues of animals. Many also live in decomposing organic matter on the bottoms of lakes and oceans. The worms play an important role in decomposition and nutrient cycling, but little is known about most species. Nematodes are pseudocoelmates. The nematoda are the most diverse phylum of pseudocoelomates, and one of the most diverse of all animals. Nematode species are very difficult to distinguish; over 28,000 have been described, of which over 16,000 are parasitic. It has been estimated that the total number of nematode species might be approximately 1,000,000. (ZS)(11) There can be thousands of Nematoda in just a single handful of garden soil. Some species of round worm contain more the 27 million eggs at one time,and can lay 200,000 or more in one day.(MLK) (18)

Diagnostic Characteristics:
The bodies of nematodes are not segmented. Roundworms have cylindrical bodies that range from less than 1 mm to more than a meter in length. Roundworms normally have a fine tip at the posterior end and a more blunt tip at the anterior end. A tough exoskeleton called cuticle covers the body. As the roundworm grows, it periodically sheds its old cuticle in a process called molting, or ecdysis. It then secretes a new, larger one. Nematodes also have hydrostatic skeletons. These skeletons consist of fluid held under pressure in closed body compartments. Roundworms hold fluid in a body cavity called the pseudocoelem. The epidermis (skin) of a nematode is unusual. It is not composed of cells like other animals, but instead is a mass of cellular material and nuclei without separate membranes. (AK) (13)
external image NEMATODA%20B.GIF (6)(MF)
Nematodes are the most specious phylum after arthropods. They occur in almost every habitat in all sorts of plants and animals. However, they prefer moist habitats compared to dry ones. One species is known to live in vinegar (Turbatrix aceti), and another species has been found in German beer mats. Nematodes occur in dense numbers in the soil and in rotting vegetation. (DB) (21)

Acquiring and Digesting Food:
A stylet is a tool that is used by nematodes to acquire food. It has the same purpose as a spear and is controlled by muscles that allow it to move in and out to obtain nutrients primarily from the cells of plants. A median bulb allows food to move from the stylet to the stomach and is a part of the esophagus.The esophagus is the tube that carries the food and nutrients from the head to the stomach. The glands in the esophagus are called, shockingly, the esophageal glands and they secrete enzymes that help with the digesting of food. (NG)(1) Nematodes have complete digestive tracts. Nematodes have alimentary canals extending between the mouth and the anus. The intestines of the nematoda is a tube made from a single layer of cells. Food moves into the intestines from the esophagus and there in the intestines nutrients are absorbed. Then much like our own digestive system wastes are excreted through the anus. (RL)(19)

Circulatory System:
Nematodes lack circulatory systems. Nutrients are transported throughout the body via fluid in the pseudocoelem, a body cavity only partially lined by the mesoderm.

Respiratory System:
Nematodes lack a respiratory system. Their cuticle is permeable to water and gas, so both are just absorbed from the environment. (SM)(5)
Nematodes have to rely on their skin for respiration because there are no specialized organs that function to serve in gas exchange. 14 (MP)

The muscles of nematodes are all longitudinal. Muscles change the shape of the worms' fluid filled compartments. Roundworms hold the fluid in the pseudocoelem at a high pressure. The contraction of these muscles creates a thrasing motion. This is how they move and control their forms. These muscles used for locomotion are activated by two nerves that go down on the ventral (belly) and dorsal (back) sides of the nematode. The muscle cells stretch towards these nerves, which both connect to a nerve ring and nerve centers near the head. The ventral nerve also has nerve centers along it. Since these muscles are longitudinally aligned, the nematode can only bend its body from side to side- nematodes do not crawl or lift themselves. As a result, the motion of a normal nematode appears to just be mindless writhing. (TM)(8)

Life Cycle:
Life Cycle
Life Cycle

The nematodes penetrate into the insect body cavity, via natural body openings or areas of thin cuticle. Once in the body cavity, a symbiotic bacterium (Xenorhabdus for steinernematids, Photorhabdus for heterorhabditids) is released from the nematode gut, multiplies rapidly and causes rapid insect death. The nematodes feed upon the bacteria and liquefying host, and mature into adults. Steinernematid infective juveniles may become males or females, where as heterorhabditids develop into self-fertilizing hermaphrodites although subsequent generations within a host produce males and females as well. (MC)

Nematode reproduction is usually sexual. The sexes are separate in most species. Females are generally larger than males. Fertilization is internal, and a female may deposit 100,000 or more fertilized eggs per day. Roundworms have vulva, opening where they can lay their eggs. The embryos in the eggs then develop into the first larval stage.Through molting the larva go through three other stages before reaching the adult stage. Most growth in nematode development is not through cell division, but through cell growth, with the early stages having about the same number of cells as the adults. nematode reproduction can vary with species; parasitic nematodes develop differently, and from species to species, there can be a number of accessory sex organs for males.(ORS 7)


The zygotes of most nematode species are resistant cells capable of surviving harsh conditions. Dehydrated roundworms contain large amounts of sugars, especially disaccharides of trehalose. Consisting of two glucose units, trehalose seems to protect the cells by replacing the water associated with membranes and proteins. Trehalose is the main circulatory sugar in nematodes, exceeding even glucose. It acts as an energy reserve in some nematodes and their eggs, but also serves as protection. It is a biological stress protectant, and works directly with lipid membranes and proteins to protect them from such environmental stresses as freezing. (ZXU)(9) Nematodes have the capability to survive in a large array of environments & temperatures. Nematodes can parasitize almost any kind of plant or animal. (SR) (20)

Sensing the Environment:
Nematodes have several small sensory organs, mostly located on their heads. Amphids are chemosensory organs located on the sides of the heads of Nematodes in pairs. As sensory organs, amphids respond to pheromones. (They also double as secretory organs). Some nematodes also have a pair of phasmids on their tails, which are smaller sensory organs with similar functions to amphids. In addition, nematodes have deirids, which respond to strain on the exoskeleton. Some species of nematodes have eye-like spots. (CSR, 3) “Their nervous system starts at their pharynx and forms a dorsal and a ventral nerve cord that runs the entire length of the body. These will carry stimulus from througout their body and send out responses to stimulus as well.” (SD) (16)

external image amphid.jpg

An amphid. 12 (NI)

The Nematode excretory system consists of an excretory gland and a pore located in the mid-esophageal region. The gland cells and tubes serve as absorptive bodies. They collect wastes from the pseudocoelem and function in osmoregulation.

Temperature Balance:
When temperature conditions are unfavorable for Nematodes, they halt their normal functions. They can survive during these periods, however. When conditions become optimal again, Nematodes can continue their normal processes.

Environmental Adaptations:
Nematodes have the ability to go into temporary dormancy or diapause, which is a great survival adaptation when the environment reaches a point were the nematode may be unable to survive. Dormancy is also key if there is no host for the nematode to live off of, the nematode then has the ability to remain temporarily dormant.(CW)(17)

Phylem Nematoda also includes many important agricultural pests that attack the roots of plants. Other species parasitize animals. Humans host at least 50 nematode species.

Caenorhabitis elegans: a widely cultured species of nematode that has become a model research organism in developmental biology. This species has exactly 959 cells within the body.

Trichinella spiralis: the worm that causes trichinosis. Humans acquire this nematode by eating undercooked pork or other meat with juvenile worms encysted in the muscle tissue. The juvenilea develop into sexually mature adults in the intestine. Females burrow into the intestinal muscles and produce more juveniles, which bore through the body or travel in lymph vessels to encyst in other organs, including skeletal muscles.

Many nematodes can suspend their life processes and enter a kind of hibernation when conditions become unfavorable. This is called cryptobiosis and in this state, nematodes can survive extreme heat, cold, or aridity and return to normal life when conditions get better.(KL)(2)

Nematode parasites affect human beings among other species. The most common parasite to live off of humans is the Ascaris lumbricoides, the giant roundworm. This Nematode affects approximately 700 million people worldwide, more commonly children than adults. The consequences are not fatal, but if the person is affected by numerous worms at once, the body will have a weakening response.(MT)(3)

The size of Nematodes vary quite dramatically. If you pick up a handful of dirt, you are probably holding around a thousand microscopic Nematodes, most of them parasitic. However, Nematodes can be very large as well, some of them larger than a meters in length. The largest Nematode ever recorded was named Placentonema gigantisma, and as it is suggested by its name, it was found in the placentae of a whale. It was eight meters long! (RG) (15)

Review Questions:

1. Describe the structure of Nematoda and its relation to their locomotion. (SI)
2. How does the circulatory system function in the Nematoda? (AC)
3. Explain how the Nematoda's ability to enter a state of dormancy increases its fitness. (LJ)
4. What makes Nematoda pseudocoelomates instead of coelomates? (JS)

Campbell, N.C., Reece, J.R. (2002). Biology. (Sixth Edition). San Francisco: Benjamin Cummings.
Picture on Page 662
1. (1)(NG)
2. (2)(KL)
5. (5)(SM)
6. B.GIF (6)(MF)
7. (ORS 7)
8. (8)(TM)
9. (9)(ZXU)
10. (10)(MC)
13. (13) (AK)
18. (MLK)
19. (RL)