Nematodes belong to the Animal Kingdom--Subkingdom Metazoa (many-celled).
In some systems of classification (e.g. Hyman) nematodes are considered to be a class in a phylum called Aschelminthes grouped with other closely related organisms as follows:
The group of classes placed in the Aschelminthes are sometimes classed as the protonephridia or as pseudocoelomate animals. They have also been grouped together as phyla under the heading of "Acoelomate minor phyla".
A classification scheme can be a useful and valuable way to group organisms in order to visualize the similarities and differences between them. If one knows the characteristics of one species, they also know a considerable amount about other nematodes in the same taxonomic grouping. Perusing a number of textbooks on parasitology and nematology, considerable agreement can be found as to the nematode species which are the most important parasites. Most are also in agreement as to the highest (Class) and lowest (Genus and species) levels of classification. However, when it comes to intermediate levels there is very little consistency. There is generally agreement as to which nematodes should be grouped together, but not to the level to which to assign the group or to the number of subgroups into which they should be divided. Some of this results from the previously mentioned division between helminthology and nematology and some from the fact that this is an active area of research. In this course we will utilize a classification proposed by A. R. Maggenti (one of the originators of this course) to encompass all nematodes. It considers the nematodes as a separate and distinct phylum the Nemata composed of two classes the Secernentea and the Adenophorea.
The development of the light microscope made it possible to clasify nematodes on the basis of morphology. Two recent scientific developments have changed or show promise for changing or further defining taxonomic groupings. The first was the development of the Scanning Electron Microscope (SEM) which made it possible to more clearly view external morphological characters. The second is a variety of recently developed techniques useful for examining the biochemistry and DNA of nematodes.
The classification we will be using is a summary of the current state of knowlege with respect to morphology, phylogenetic relationships (descent from a common ancestor), and life history habits and will serve as a guide or a roadmap for our survey of parasitic nematodes. The biological definition of species has seen very little application in the classification of nematodes. Classification of the thousands of species that have been described is based largely on morphology, life history, and habitat. The life cycles and breeding habits of many nematodes are unknown.
In addition to scientific names, many nematodes have been given common names (e.g. pinworm, pin nematode, root-knot nematode, hookworm) which are frequently used interchangeably with their scientific names. These are usually derived from a morphological characteristic of the nematode or from the symptoms it causes.
Within the major taxonomic categories containing parasites, we will examine some of the most important examples of parasites of plants, insects, and vertebrates. For each example we will review points of historical interest, hosts, distribution, life cycle, symptoms/pathogenicity, management, and the morphological characteristics used in diagnosis. Both classes of nematodes contain free-living nematodes and parasites of plants, invertebrates, and vertebrates. This indictes that some parasites of plants are more similar to some parasites of invertebrates and vertebrates than they are to other parasites of plants.