The taxum, or named taxonomic unit, Nematoda, is part of the Kingdom Animalia. Currently, there are 90,000 known species of Nematodes; however, there are probably many more species that scientists have yet to discover. Nematoes ( categorized in the phylum Nematoda) are the most diverse phylum of pseudocoelomates. Nematoda comes from the Greek word Nema meaning thread and Edios for form. They are usually bilaterally symmetrical. Nematodes are found in aquatic habitats, especially at the bottom of lakes and oceans, in wet soil, in damp plant tissues, and in the bodily fluids of animals. Among nematodes are the typcial garden earthworms. While many of these earthworms are beneficial, there are some which damage desirable plants. Many nematodes are free living as decomposers (these are the good ones in the garden) and predators to microorganisms, but some nematode species are also parasites that can cause diseases such as trichonosis , filariasis , and onchocerciasis
.The presence and feeding of these nematodes accelerate the decomposition process. Their feeding recycles minerals and other nutrients from bacteria, fungi, and other substrates and returns them to the soil where they are accessible to plant roots.Human beings, along with all other living things are host to numerous Nematode parasites. The most common of these is Ascaris lumbricoides with an estimated 700 million people effected globally, this Nematode is not normally fatal and in low numbers may have very little effect on adults, however in heavy doses it can be quite debilitating, especially for children. The Nematodes infecting mankind include several species of filarial worms.
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In this phylogenetic tree, the nematoda are placed in the pseudocoelomates category meaning they have a body cavity not enclosed by their mesoderm. (Walker K.)
In this phylogenetic tree, the nematoda are placed in the pseudocoelomates category meaning they have a body cavity not enclosed by their mesoderm. (Walker K.)

Structure and Locomotion:
Nematodes are very small creatures; some are less than one millimeter in length! Some nematodes have circular bodies, such as the roundworm, and others have translucent or see-through bodies, such as the Caenorhabditis elegans. Also, nematodes are known for having long and skinny bodies with a “fatter” poorly developed head, and a thinner end at the rear. Nematodes have a "pseudocoel", an inner body cavity similar to a coelem in more complicated organisms. The pseudocoel is formed right from the blastula not the mesoderm. It houses the intestines and reproductive organs, so the cavity itself is quite small . These animals, similar in a way to humans, have a type of “skin” called an exoskeleton, which is a hard cover on the surface of animal that offers protection and points of attachment for muscles that extend into the interior of the body cavity. The nematode’s exoskeleton is known as a cuticle. The cuticle is both strong and flexible. As the organism grows, it regularly casts off its old cuticle and grows a new one. In addition to the nematode’s exoskeleton is a hydrostatic skeleton. A hydrostatic skeleton is one that consists of liquid held under pressure in a sealed body compartment. In the case of a nematode, the pseudocoelom, the nematodes body cavity, holds the liquid at a high pressure, and the tightening of the longitudinal muscles (nematodes have longitudinal muscles which means long and straight muscles) results in beating movements that causes the organism to move. Because the nematode only has straight muscles, it can only move its body from side to side. In many species there are male and female worms, but some species are hermaphrodic, meaning that both the male and female reproductive organs are contained in the same individual.

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Internal anatomy of a roundworm (JM)
Internal anatomy of a roundworm (JM)

Digestive System:

Nematodes are triploblastic, meaning that they consist of three germ layers: the endoderm, the mesoderm, and the ectoderm. This fact explains the formation of pseudocoelom when mesoderm tissue only partly lines the the inside of the nematode. Nematodes are pseudocoels, meaning they don't have the muscles to force food through their digestive system, so they rely on internal pressure. Nematodes have a full digestive tract. This means that nematodes consume food through their mouths (located in the poorly developed head), which contain teeth to chew the plant or animal tissue. The chewed food then leads to the pharynx (the throat), which pumps (this is caused by the high pressure within the body) the chewed food to a long and uncomplecated gut cavity without any muscles. This is where the food is digested and broken down into simpler substances. Undigested food travels to the anus where it is excreted (because of the high pressure) as feces.

A Diagram of the Digestive System of Nematodes (SW)

Circulatory System and Respiratory System:
Nematodes lack a circulatory system which is responsible for transporting nutrients through the body, and a respiratory system, which is responsible for taking in oxygen. Because of this lack of a circulatory system, nutrients are transported through the bodily fluid in the nematode’s body, which is called the pseudocoelom. As a result of not having a respiratory system, the cuticle of a nematode is porous to water and gasses which allows the animal to respire and rid itself of metabolic wastes. The metabolic wastes are removed by an excretory canal which runs along each side of the nematode’s body.

Nervous System:
Nematodes have a fairly advanced nervous system which helps them respond to stimuli. Their nervous system is composed of a circum-pharyngeal nerve ring that is made up of four nerve ganglia from which six longitudinal nerves extend down through the body to the various parts of the gut and the reproductive organs. An additional six shorter nerves extend forwards from the circum-pharyngeal ganglia towards the mouth.

Water/Temperature Balance:
Osmosis is the diffusion of water across a membrane. Water is vital to the survival of organisms, and the amount of water within an organism needs to be regulated. If a nematode where to have to much water, it would shrink rapidly, and potentially die. On the other hand, nematodes do not have a preferable temperature. Nematodes live in practically every geographic location on earth, and they live in very extreme habitats, such as ice and hot springs, as well as on plants and animals, including humans. Nemotodes are unique in that they can live through extreme osmotic stress. In other words, their threshold for hypertonicity (low water, high solute, cause to shrivel) and hypotonicity (high water, low solute, cause to expand) is relatively high.

Most species of roundworms are dioecious, meaning that there are separate sexes of worms (Male and Female) containing different reproductive organs. Fertilization is initiated by males use a spine they possess to open the females reproductive tract and deposit their sperm in them. Roundworms sperm do not have flagella, and instead move with the help of pseudopodia. Many nematodes are able to suspend their life processes completely when conditions become unfavorable; in these resistant states they can survive extreme drying, heat, or cold, and then return to life when favorable conditions return. This is known as cryptobiosis, and is a feature nematodes share with phyla rotifera and tardigrades.
Cryptobiosis is the animal equivalent to being freeze-dried. "Sea Monkeys" that arrive in the mail are the most popular example of cryptobiosis, because, like the nematodes and other organisms that practice this, a reintroduction of water will bring the animal "back to life" pretty quickly.

Review Question:
1) Discuss the nervous system of Nematodes and how it enables them to respond to their environment.
2) Why are nematodes referred to as pseudocoelomates? What does this term mean?
3) How do Nematodes make up for not having a circulatory system?

1. Campbell, Neil and Jane Reece. Biology Sixth Edition. San Francisco: Benjamin Cummings 2002.
2. Waggoner, Ben. "Introduction to the Nematoda: the roundworms." 15 October 2009
3. Ramel, Gordon. "The Phylum Nematoda." 4 May 2009. 15 October 2009