The Nematodes

Classification

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:

KINGDOM

PHYLUM

                CLASS

            ORDER

SUPERFAMILY

FAMILY

SUBFAMILY

GENUS

SPECIES

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.

CLASSIFICATION
Strongylus species

Phylum: Nematoda Family: Strongylidae
Order: Strongylida Subfamily: Strongylinae
Superfamily: Strongyloidea Genus: Strongylus

 

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.

SUFFIX TAXONOMIC GROUP
ida ORDER
oidea SUPERFAMILY
idae FAMILY
inae SUBFAMILY

 

Most of the nematodes of veterinary importance are found in the six orders and thirteen superfamilies listed in the table below.

 

Order Superfamily Comments
Strongylida Trichostrongyloidea
Strongyloidea
Ancylostomatoidea
Metastrongyloidea
"Bursate" nematodes
Ascaridida Ascaridoidea "Non-bursate" nematodes
Oxyurida Oxyuroidea
Rhabditida Rhabditoidea
Spirurida Spiruroidea
Thelazioidea
Filarioidea
Habronematoidea
Enoplida Trichuroidea
(Trichinelloidea)
Dioctophymatoidea

 

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.

bursasma.JPG (33484 bytes) nobursaA.JPG (31203 bytes)
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.

Sources

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.

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Parasites and Parasitic Diseases of Domestic Animals
Dr. Colin Johnstone (principal author)
Copyright 1998 University of Pennsylvania
This page was last modified on January 24, 2000