What is a Nematode?
Nematodes are the most numerous multicellular animals on earth. A handful of soil will contain thousands of the microscopic worms, many of them parasites of insects, plants or animals. Free-living species are abundant, including nematodes that feed on bacteria, fungi, and other nematodes, yet the vast majority of species encountered are poorly understood biologically. There are nearly 20,000 described species classified in the phylum Nemata
Nematodes are structurally simple organisms. Adult nematodes are comprised of approximately 1,000 somatic cells, and potentially hundreds of cells associated with the reproductive system. Nematodes have been characterized as a tube within a tube ; referring to the alimentary canal which extends from the mouth on the anterior end, to the anus located near the tail. Nematodes possess digestive, nervous, excretory, and reproductive systems, but lack a discrete circulatory or respiratory system. In size they range from 0.3 mm to over 8 meters. 
Soil Biota
Soils are populated by a multitude of microbial and invertebrate organisms, in addition to more complex animal biota. Plant roots, seeds, and fungi, are a large part of this microhabitat. Soil microorganisms play an extensive role in the decomposition of organic matter and production of humus, cycling of nutrients and energy and elemental fixation, soil metabolism, and the production of compounds that cause soil aggregates to form. Many are in symbiotic relationships with plants and animals serving as nitrogen fixers and gut microbes. They function as a substantial part of the food web.
Among the microorganisms found in the soil are bacteria, actinomycetes, fungi, micro-algae, protozoa, nematodes, and other invertebrates (mostly arthropods).
It has been estimated that if you look at one “gram” of soil you will see the following numbers of organisms--bacteria 10 8-9 , actinomycetes 105-8 , fungi 105-6 , micro-algae 103-6 , protozoa 103-5 , nematodes 101-2 , other invertebrates 103-5 . A square meter of soil may contain 30-300 earthworms. There are more organisms in a gram of soil than there are human beings on this Earth!
References: Soil Biology Guide. Dindal (ed.) 1990 John Wiley & Sons and Soil Microbial Ecology, F. Blaine Metting Jr. (ed.) 1993
Phylum Nemata

The word Nematoda comes from the Greek words nematos, meaning thread, and eidos, meaning form. Over the years, nematodes have been classified in four different phyla, not always under the same name. There are two contending names for the phylum of nematodes.

Nathan Cobb
In 1919, Cobb named the study of nematodes nematology and therefore wished to rename nematodes nemata. Cobb also placed nematodes in their own phylum, the phylum Nemata. However, when nematodes were placed in the phylum Aschelminthes, they were classified as class Nematoda (along with class Rotifera, class Gastrotricha, class Kinorhyncha, class Priapulida and class Nematomorpha). In 1932, Potss elevated class Nematoda to the level of phylum, leaving the name the same. While both names have been used (and are still used today), many believe (including Maggenti, Luc, Raski, Fortuner and Geraert, 1987) that Nemata is a more precise name. When a reference is made to Nemata, there is no doubt that it is the phylum being referred to whereas when an author makes a reference to Nematoda, the author could mean either the phylum or the class. In addition, the name Nemata was used first and therefore should be given priority. While nematodes are generally accepted as being a phylum, debate is still ongoing concerning their relationship to other animals grouped together on the basis of the structure of the body cavity


Nematode Body Structure
The phrase tube-within-a-tube is a convenient way to think of nematode body structure, and also a term used to refer to a major trend in the evolution of triploblastic metazoa (Brusca and Brusca, 1990 Invertebrates). It refers to the development of a fluid-filled cavity between the outer body wall and the digestive tube. The nature of this body cavity has led to the grouping of metazoa into three grades, acoelomate, pseudocoelomate, and eucoelomate. Nematodes together with Rotifera, Gastrotricha, Kinorhyncha, Nematomorpha, Acanthocephala, and Entoprocta are traditionally grouped together as pseudocoelomates, on the basis of possessing a body cavity that is not formed from the mesoderm or fully lined by peritoneum. However, there are some problems in applying this concept to nematodes. Cell lineage studies on the nematode Caenorhabditis elegans have demonstrated that most tissues in the nematode are of mixed lineage, derived from several different sources of embryonic tissue (see Bird and Bird, 1991 The Structure of Nematodes). Also, not all nematodes retain a spacious fluid-filled cavity, as can be seen in a cross section of the esophageal region of the monohysterid marine nematode, Theristis.


Nematode Digestive System
The nematode digestive system is generally divided into three parts, the stomodeum, intestine, and proctodeum. The stomodeum consists of the “mouth and lips”, buccal cavity, and the pharynx (esophagus). Each of these regions are used extensively in taxonomy and classification of nematodes, as well as providing as indication of feeding habit or trophic group. For example, the buccal cavity of plant parasitic nematodes (and some insect parasites ) is modified in the form of a hollow spear, adapted to penetrate and withdraw the contents of host cells. Predaceous nematodes often have a buccal cavity characterized by teeth or hook-like projections. The buccal cavity of bacterial feeding nematodes is relatively unadorned. 
Bacterial feeding nematodes have the least modified or diversified stomodoeum structures. The basic plan is a circular opening surrounded by six ‘lips’, sometimes fused into three or less ‘lips’. The structures called ‘lips’ are actually cuticularly lined area of the mouth that are exposed to the outside. This opens into the buccal cavity, a triangular or cylindrical tube that can contain small ‘teeth’. Muscles extending from the body wall to the cuticular lining expand the lumen and suck food through the mouth into the buccal cavity. The buccal cavity terminates in a valve-like glottoid apparatus leading to the pharynx, also referred to as the oesophagus.
Nematode Reproductive System
While there is much diversity represented in the reproductive structures of the Phylum Nemata, there are many features that are typical of the phylum. Male nematodes are usually smaller than their female counterparts. Basic male reproductive structures include: one testis, a seminal vesicle and a vas deferens opening into a cloaca. One testis is most common, but two testis are found in some species, while in others one testis is reduced. Spermatogonia are produced in the testis and stored in the seminal vesicle until the nematode mates. The presence of one or two copulatory spicules help dialate the vulva and can also serve as a canal for the spermatozoa. The spicules are made from hardened cuticle, terminating in sensory dendrites near the tip. Often the body wall around the cloaca is modified into a bursa, which helps orient the male nematode and then helps hold the two nematodes together.

 Spermatozoa are amaeboid, and can have many different modifications. Some spermatozoa are round to ovoid in shape while others bear a resemblance to flagellated sperm. Different types of spermatozoa characterize different taxonomic groups of nematodes.

 Basic female structures include: one or two ovaries, seminal receptacles, uteri, ovijector and a vuvla. The ovary produces oogonia, which later develop into oocytes. The seminal receptacles, sometimes developed into a spermathecea, stores the spermatozoa until they are needed to fertilize an ooctye. The fertilized oocyte then develops into an egg in the uterus. The uteri often ends in an ovijector. The ovijector is very muscular and uses body movement combined with the high internal body pressure of the nematode to expel the egg through the vagina. All nematodes lay eggs. Syngamy, or cross fertilization, is common in most nematodes. Hermaphroditism also occurs, with the nematode gonads producing spermatozoa first and storing them until the eggs are produced. Parthenogenesis is also a normal means of reproduction in some nematodes.


Placentonema gigantisma- The Largest Nematode
The largest nematode ever observed is Placentonema gigantisma, discovered in the placenta of a sperm whale. This 8 meter long nematode is said to have 32 ovaries. The original reference (which I’ve never seen) is Gubanov, N.M. 1951. Compt. Rend. Acad. Sci. URSS 77, 1123.

Other big verebrate parasites include Dioctophyma renale, the giant kidney worm (1m x 1.5cm). Most plant parasites are considerably smaller, usually measuring around 1 mm or less. Several Longidorus species exceed 10mm in length.