All living organisms, plants and animals, are organized into a hierarchy of groups
called taxa. This structure is based primarily on the degrees of similarity among
members of the same group and also shows the contrasts among members of different taxa.
The highest level of classification considered here is the Phylum and the lowest is
the Species. Nematodes belong to the Animal Kingdom and their taxonomic hierarchy
is expressed as follows:
Species are named using the binomial system of Linnaeus (1753) and are written in
italics. A species is defined as a collection of similar organisms that will only
interbreed among themselves. Examples of nematode species include Strongylus vulgaris
in horses, Toxocara canis in dogs, and Haemonchus contortus in sheep. Even
though many different species are morphologically similar and may share the same habitat
in the same host they are still distinct species because they do not interbreed.
For example, three members of the genus Strongylus inhabit the large intestines of
horses: Strongylus vulgaris, Strongylus equinus, and Strongylus edentatus.
They are similar in size and appearance and without the aid of a microscope they are not
readily distinguishable from each other. Although they share the same host and the same
habitat within that host, they do not breed with each other.
However, their similarities (as species) allow us to group them in the same
hierarchical tree of classification. In addition, these similarities allow us to conclude
that the three Strongylus species share common ancestors and this phylogenetic
relationship is also recognized by placing them in the same taxonomic hierarchical tree.
Within the nematodes, some of the taxonomic groups have suffixes (endings) that are
specific for the particular group. Examples are shown in the following table.
Most of the nematodes of veterinary importance are found in the six orders
and thirteen superfamilies listed in the table below.
Nematodes in the order Strongylida are also called "bursate
nematodes", a descriptive term referring to the fact that each male has a pronounced
copulatory bursa at the tail (posterior) end. The two images below show nematodes with and
without a copulatory bursa.
|Fig. 1 Cooperia (Order Strongylida, Superfamily
Trichostrongyloidea). Tail end of male showing copulatory bursa, spicules and bursal rays.
||Fig. 2 Heterakis (Order Ascaridida, Superfamily
Ascaridoidea). Male without a copulatory bursa showing spicule (A),
caudal ala (B) and pre-cloacal sucker (C).
Click here to view a table
showing a more complete classification of nematodes of veterinary importance.
There are several good Web sources that describe taxonomic hierarchy. One can be found
on the Web site maintained by the Royal (Dick)
School of Veterinary Medicine at the University of Edinburgh in Scotland. A second,
more comprehensive, taxonomy database is maintained by "NCBI/Gen Bank" and
includes a searchable index.