Nematodes: Characteristics and Classification

The nematodes are also known as roundworms. They belong to the phylum Nematoda. They are a diverse group of animal organisms found in a broad range of environment. Some are free living in soil, marine and freshwater; some make parasitic life in animals and plants body.

General Characteristics of Nematodes

They are unsegmented worms. They are elongated and cylindrical filiform. There is a body cavity in which alimentary and genital systems float. The alimentary canal is complete and consists of a oral cavity, oesophagus, intestine and anus. The oral cavity may have teeth or cutting plates. Sexes of nematodes found in man are separate. Generally, the female is larger than the male. Its posterior end is curved or coiled ventrally.

Taxonomic Classifiaction of Namatodes

Nematodes belong to the Phylum Nematoda. The phylum Nematoda is divided into the following three major classes:  

  1. Class Rhabditea
  2. Class Enoplea and
  3. Class Chromadorea

Class-1: Rhabditea

They include both free-living and parasitic nematodes. The parasitic nematodes are Ascaris, Enterobius (e.g. human pinworm), Necator species and Wuchereria species. They can cause various diseases in human beings. In this case, Wuchereria bancrofti can cause serious problem in human body, known as Lymphatic filariasis. It is characterized by the swelling of legs and arms. It makes painful to you and also creates the disfiguration of legs and arms. On the other hand Enterobius and Ascaris can also cause health problems in human body. The free-living nematode such as Caenorhabditis elegans is found in temperate region. They receive nutrition from the decaying matter such as rotting fruits.  

General Characteristics of Class Rhabditea

  • They have cylindrical body;
  • The majority species have unsegmented body;
  • The body has cuticle and hyperdermis;
  • The both end of the body are tapered;
  • The body contains an inner and outer tube; in this case, the inner tube consists of intestine, pharynx and gonads.

Class Rhabditea is further divided into the following two sub-classes :

Sub Class-1: Rhabditia

  • They have well-developed posterior sensory structures known as Phasmids;
  • They have poorly developed amphids;

Sub Class-2: Tylenchia

  • They are found as parasitic forms in plants.

Class-2: Enoplea

  • They have a cylindrical and bottle-shaped oesophagus;
  • They have well developed Amphipods;
  • They have a simple excretory system with no lateral canals;
  • They have no phasmids;

Examples: Trichuris, Diotyphyme, and Diotyphyme

Class Enoplea includes the following sub-classes:

Sub class-1: Enoplia

  • They have oval or pouch-like amphipods;
  • They have oval or pouch-like amphipods;

The sub class Enoplia includes the following two orders:

  • Order-1: Trefusiida
  • Order-2: Enoplida

Sub class-2: Dorylaimia

  • They live in different terrestrial and freshwater environments;
  • They have odontostyle or stoma which consists of teeth-like structures;
  • The posterior part of the oesophagus contains esophageal glands;

The subclass Dorylaimia includes the following orders:

  • Order 1: Dorylaimida 
  • Order 2: Mermithida
  • Order 3: Mononchida
  • Order 4: Dioctophymatida
  • Order 5: Trichinellida
  • Order 6: Isolaimida
  • Order 7: Muspiceida
  • Order 8: Marimermithida

Class-3: Chromadorea

  • They have three esophageal glands, spiral amphids with annules;
  • The amphids are pore-like that contain coil or spiral-like apertures;
  • They have annulated cuticles with setae;
  • The excretory system has tubular like appearance;

The class Chromadorea includes only one sub class, Chromadoria. It is further divided into the following orders:

  • Order 1: Rhabditida
  • Order 2 : Plectida
  • Order 3: Araeolaimida
  • Order 4: Monhysterida
  • Order 5: Desmodorida
  • Order 6: Desmoscolecida
  • Order 7: Chromadorida

Classification of Nematodes Based on Location in the Host

On habitat of Adult Worms

A. Intestinal Nematodes

Adults of these nematodes live in the intestinal lumen or in the intestinal mucosa of humans.

(a) Small Intestine

  1.  Ascaris lumbrocioides (roundworm)
  2. Ancyclostoma doudenele (Hookworms)
  3. Necator americanus (Hookworms)
  4. Strongyloides stercoralis
  5. Trichinella spiralis

(b) Caecum and Appendix

  1. Enterobius vermicularis (threadworm or pinworm)
  2. Trichuris trichiura (Whipworm)

B. Tissue or Somatic Nematodes

Adults of these nematodes inhabit in tissues other than the intestinal mucosa of humans.

(a) Lymphatic system

  1. Wuchereria bancrofti
  2. Brugia malayi

(b) Conjuctiva-Loa loa

(c) Subcutaneous tissue

  1. Loa loa
  2. Onchocerca volvulus.
  3. Dracunculus medinensis (Guinea worm)

(d) Mesentery

  1. Mansonella ozzardi
  2. Dipetalonema perstans

(e) Lungs-Strongyloides stercoralis

C. Larva migrans:

These are larvae of nematodes that do not mature in the human host.

Reproductive System of Nematodes

Male genital system consists of testes, vas deference, seminal vesicle and ejaculatory duct. Cloaca is the common passage in the male nematode where the rectum and genital duct open. Spicule is rod-like protractile accessory copulatory organ. Gubernaculum is an elevation on the dorsal wall of cloaca which guides the spicule during copulation. Copulatory bursa is an umbrella-like expansion of the cuticle surrounding the cloaca.

Female genital system consists of ovary, oviduct and uterus, may be in a single or double tubes, and vagina and vulva. The female nematodes may be:


Oviparous are those that lay eggs or ova. Example: Ascaris lumbricoides (lay fertilized or unfertilized eggs). Ancyclostoma doudenele and Enterobius vermicularis(laying eggs containing larvae). Trichuris trichiura (laying unsegmented eggs).


Ovo-viviparous laying eggs containing larvae which immediately hatch out,  such as Strongyloides stercoralis.


Viviparous are those that give birth to larvae. Examples are Wuchereria bancrofti, Brugia malayi, Dracunculus medinensis and Trichinella spiralis.

Larva is the immature developmental form that hatches from the eggs. The larvae are of the following two types:

Rhabditiform larva

Rhabditiform larva is one which has a rhabditiform oesophagus, i.e. the oesophagus is short and dilated posteriorly into as bulb. Examples: Ascaris lumbricoides, Ancyclostoma doudenele, Necator americanus, Strongyloides stercoralis.

Filariform Larva

Filarform larva is one which has a filariform oesophagus.i.e. a long oesophagus without a bulb. Examples; Infective larval stages of Ancyclostoma doudenele, Necator americanus and Strongyloides stercoralis.

Moulting is referred to the changes in the larvae with casting of skin as it grows.

Nematode Transmission

Nematodes transmitted by ingestion of eggs are Ascaris lumbricoides, Enterobius vermicularis, Trichuris trichiura.

Nematodes transmitted by larvae are Wuchereria bancrofti and other filarial worms, Strongyloides stercoralis, retrograde infection by Enterobius vermicularis, Trichinella spiralis and Dracunculus medinensis.

Nematodes transmitted through skin are Ancyclostoma doudenele, Necator americanus, Strongyloides stercoralis, Wuchereria bancrofti and other filarial worms and nematodes of creeping eruption (Ancyclostoma braziliense, Ancyclostoma caninum, Necator americanus, Strongyloides stercoralis).

Benefits/Importance of Nematodes

Some species of nematodes are natural predators. They act as biological controllers and they control of different pest insects that can cause harmful effects for plants and human beings. Some nematodes live as endoparasites in some agricultural pests and kill them. Some nematodes provide beneficial effects to soil and they do decomposition without polluting the soil. They also kill various organisms and breakdown of organic matters which is further broken down by bacteria.