Caenorhabditis elegans

By: Julie Sun

Free Living Nematode
Free Living Nematode
Nematodes are classified under the kingdom Animalia and the phylum Nematoda. Nematodes are filed under ecdysozoa because of their ability to shed an outgrown exoskeleton. Nematodes live in aquatic environments, in wet soil, in the moist tissues of plants, and in the body fluids and tissues of animals. They also live in decomposing organic matter on the bottoms of lakes and oceans. About 90,000 species are known, and it is possible that 900,000 exist. at least 15,000 of these known species are known to be parasitic.

Diagnostic Characteristics of Nematodes

Nematodes are round worms that do not have segmented bodies. The cylindrical bodies range from less than 1 millimeter to more than a meter in length. They have a fine tip at the posterior end and a blunt tip at the anterior (head) end.
A tough exoskeleton, known as the cuticle, covers the worm’s body. As the worm grows, the exoskeleton periodically undergoes a process of molting and sheds off its old cuticle while secreting a new, larger one. The muscles of nematodes are all longitudinal which allow the worms to thrash about.
Nematode reproduction is usually sexual. Female nematodes are larger than male, and the sexes tend to live separately. Fertilization is internal, and a female may deposit 100,000 or more fertilized eggs per day. The zygotes of most nematode species are cells that are resistance enough to survive harsh conditions.
It has a simple, transparent body with only a few types of cells. The nematode grows from a zygote to a mature adult during the span of only three and a half days. Most nematodes are hermaphrodites, which are organisms that produce both eggs and sperm. It has 959 somatic cells, and these cells arise from the zygote virtually the same way for every individual

Acquiring and Digesting Food

Nematodes are either predators or parasites (SW 21). In order for a nematode to acquire nutrients, muscles move the stylet, a hard, sharp spear used for feeding, in and out and allow the nematode to puncture its prey and obtain necessary nutrients. Nematodes feed on bacteria, fungi, and other organisms in the soil, as well as plant and animal tissue.

Nematodes have a linear digestive system. Nutrients are absorbed by the thin wall of the intestines. The pathway of food consists of the mouth, pharynx, intestines, rectum, and anus. Digestion is rapid and waste is given off almost every minute. The pharynx acts as a pump that forces food into the intestines and can be used to suck up food through the mouth.

styletofnematode.pngstylet of a nematode
external image nematodexs.gif(Internal anatomy of nematode)
Sensing the Environment

Nematodes have two nerves that run the length of their body on both the dorsal (back) and ventral (belly) side. The muscle cells of a nematode branch toward the nerves, where as in most animals, it is the nerves that branch towards the muscles. The ventral nerve of a nematode has several nerve centers along its length and both the ventral and dorsal nerve cords connect to form a never ring and additional nerve centers in the head.


A Free-Living Nematode, the simplicity of a hydrostatic skeleton is evident (22RM)
A Free-Living Nematode, the simplicity of a hydrostatic skeleton is evident (22RM)
The nematode controls its form and movement by using its longitudinal muscles to change the shape of the fluid-filled compartment held under pressure in a closed body. These compartments are known as hydrostatic skeletons. Nematodes hold the fluid in the body cavity (a pseudocoelom) at a high pressure, and contractions of longitudinal muscles result in thrashing movements. Hydrostatic skeletons are well suited aquatic organisms because they provide cushion for the internal organs from shock and give support for crawling and burrowing. A hydrostatic skeleton cannot support the forms of terrestrial locomotion in which an animal’s body is held off the ground, such as walking or running.
Nematodes are restricted to undulation as a form of movement; they lack circular muscles that create peristaltic movement (similar to the way an animal's esophagus pushes down food), and they have no cilia to help move them through fluids.


Nematodes have no specialized organs for gas exchange and therefore must rely on gas exchange through the skin. Nematodes absorb oxygen directly through their skin by diffusion. Common in nematodes are both anaerobic respiration, lacking oxygen, and aerobic respiration, requiring oxygen. Most nematodes live in the interstitial spaces of aquatic sediments and soils, so they are essentially always in a moist environment. This is essential for diffusion.

Metabolic Waste Removal

Nematodes have a complete digestive tract. The extretion of metabolic waste is through two simple ducts which contain neither nephrodilia nor flame cells.


Body Cavity - Pseudocoelomate
Nematodes lack a circulatory system, but nutrients are able to be transported throughout the body using the fluid in the pseudocoelom, a body cavity only partially lined by mesoderm.

Self Protection

Recently, scientists from the Max Planck Institute for Developmental Biology in Tinbergen, Germany, sequenced the genome of the Pristionchus pacificus nematode, a parasite infamous for being dependent on May, dung, and potato beetles for transport, shelter, and food. In comparing it to the genomes of Brugia malayi, a human parasite, and a model, Caenorhabditis elegans, they found a drastic difference in the number of protein-coding genes each contained. While the latter two had only 12,000-20,000 protein-coding genes, the Pristionchus pacificus contained more than 23,500 protein-coding genes. The scientists concluded that the increase in protein-coding genes was the nematode's adaptation to the complexities of the beetle ecosystem and could therefore aid in decomposing harmful substances within the beetle.
Nematodes are able to live in a huge variety of environments, from arctic ice to hot springs. They also parasitize nearly every kind of animal or plant.

Osmotic Balance

Osmotic balance is achieved by the nematode through the accumulation and loss of inorganic ions and organic osmolytes. Much of the research done on nematodes has helped scientists study osmoregulation in animals cells.


Nematodes reproduce sexually. Male nematodes have a bent tail for holding the female for copulation. During copulation, one or more spicules, or tiny spine-like structures, move out of the cloaca and are inserted into genital pore of the female. Amoeboid sperm crawl along the spicule into the female worm. Once fertalized, the eggs hatch into larva, which eventually grow into adults. The young nematode will undergo four molts (or shedding of the "skin") until they become adults and are able to reproduce.

Temperature Balance

Many nematodes are able to suspend their life processes completely when conditions become unfavorable. When their environments change dramatically, they can survive extreme drying, heat, or cold, and then return to life when favorable conditions return.

Biological Usefulness

Free-living marine nematodes are important and abundant members of the meiobenthos. They play an important role in the decomposition process, aid in recycling of nutrients in marine environments and are sensitive to changes in the environment caused by pollution.
Nematodes are essential to natural environments and useful as insecticides.
Artichoke plume moth
S. carpocapsae
Root weevils
H. bacteriophora
Root weevils
S. riobravis
Root weevils
H. bacteriophora, S. carpocapsae

Cranberry girdler
S. carpocapsae
S. feltiae
Root weevils
H. bacteriophora, H. megidis

Wood borers
S. carpocapsae, H. bacteriophora

Fungus gnats
S. feltiae
H. bacteriophora

Mole crickets
S. riobravis, S. scapterisci

H. bacteriophora, S. carpocapsae

Armyworm, Cutworm,
S. carpocapsae

Life cycle of a hookworm
Life cycle of a hookworm


About 15,000 species of nematodes are parasitic. Nearly every animal group has a parasitic nematode, and some plants have them, too. These nematodes acquire all their nutrition from their host, sometimes harming and even (in the case of insects) killing them. Nematodes that affect and cause disease in humans include ascarids, hookworms, pinworms, and whipworms.

This disturbing video above shows a tarantula's mouth parts infested with nematodes, thus exemplifying their parasitic nature.

Review Questions:
1) Compare and contrast nematodes with annelides.
2) Are nematodes parasitic and (if so), how much do they rely on their hosts?
1. Campbell, Neil A., and Jane B. Reece. Biology, Sixth Edition. Sixth ed. San Francisco: Benjamin Cummings, 2001. Print.
2. "Nematode Basics." 24 Oct. 2009 <>
3. Waggoner, Ben. "Introduction to the Nematoda: the roundworms." 21 October 2009 <>.
4. Ramel, Gordon. "The Phylum Nematodea." 21 Oct. 2009. The Earth-Life Web. 25 Oct. 2009 <>.
5. Science Daily. "Nematode Genome Provides Insight into the Evolution of Parasitism." Richard The Richard Dawkins Foundation for Reason and Science. Web. 25 Oct. 2009. <,3147,Nematode-Genome-Provides-Insight-Into-Evolution-Of-Parasitism,Science-Daily>.
6. "Introduction to the Nematoda." UCMP - University of California Museum of Paleontology. Web. 25 Oct. 2009. <>.
7. "Nematoda Outlines" 25 Oct. 2009. <>
8. Gaugler, Randy. "Nematodes" 26 Oct. 2009.
9. "Adaption of the Nematode Caenorhabditis elegans to Extreme Osmotic Stress." 3 Nov. 2009 <>
10. Dane, Sana. "Form and Function." Nematode / Priapulid Home Page. Seattle University. Web. 3 Nov. 2009
11. "Nematode Parasite Page." Seattle University. Web. 4 Nov. 2009. <>.
"Nematode." Web. 4 Nov. 2009. <>.
14. Waggoner, Ben. "Introduction to the Nematoda: the Roundworms." 5 Nov. 2009 <>.
Free Living Nematode:
Body Cavity - Pseudocoelomate:
Other Free-Living Nematode: