Phylum Nematoda-Characteristics, Classification, Examples

Phylum Nematoda Definition

As opposed to flatworms and higher segmented annelids, nematodes (Greek: nema thread+ eidos, form) are also known as thread worms or pinworms.

Phylum Nematoda Characteristics

  • They are abundant, aquatic or terrestrial, parasitic or nonparasitic, and ubiquitous.
  • This creature’s body is elongated, cylindrical, segmentless, worm-like, bilaterally symmetrical, and tapered at both ends.
  • They are triploblastic creatures, having a larger perivisceral cavity than platyhelminths.
  • The body is organised at the level of organ systems.
  • The body is often coated with a thick, flexible, multi-layered collagenous cuticle that frequently includes setae (hairs), spines, and annuli.
  • The cuticle is periodically shed.
  • The epidermis is cellular or syncytial. Therefore, there is no separation of nuclei by cell membranes.
  • There are only longitudinal muscular fibres with four bands.
  • They do not contain coelom. In the majority of cases, the body cavity is pseudocoel or blastocoel, which is unbounded by mesoderm and loaded with parenchyma.
  • They do not have cilia.
  • Not having a circulatory or respiratory system Therefore, respiration occurs via the body’s surface and is aerobic in free-living species but anaerobic in parasitic organisms.
  • There is internal cephalization, but there is minimal outward difference between the anterior and posterior regions. That is, no discernible head is present.
  • Nevertheless, The mouth exists in the frontal region.
  • The digestive system concludes with a mouth and an anus. Often, the muscular pharynx and the inner surface of the intestine lack cilia.
  • In them, extracellular digestion occurs.
  • There are six lips around the mouth.
  • The absence of flame cells and nephridia in the excretory. In the class Adenophorea, the duct contains glandular renette cells.
  • The nervous system lacks significant development. Thus, it is made up of the circucumpharyngeal ring and the longitudinal nerve cord.
  • Poorly developed sense organs take the shape of papillae, which are well-defined as amphid (in the mouth) and plasmad (anus).
  • Sexes are distinct (gonochoristic). Males are shorter than females.
  • There is a tubular gonad present in them. The male genital duct is connected to the cloaca. Female genital ducts have a distinct entrance.
  • Sperm cells that resemble amoebae.
  • There is no asexual reproduction.
  • Fertilization is internal or perhaps self or cross.
  • Development may be direct or indirect, with or without an intermediary host.
  • There are numerous lateral lines and pores on the surface of the body.

Phylum Nematoda Classification

Nematoda is one of the most abundant phyla. Approximately 15,000 species of nematodes are currently recognized. It is difficult to categorize due to the great variety of forms and structures. Chitwood (1933) separated them into two types based on the presence or lack of phasmids: phasmidia and aphasmidia.

Class 1. Aphasmidia (Adenophorea)

  • Absence of phasmid (causal sensory organs).
  • Rarely porous, amphids (anterior sensory organs) are of numerous sorts.
  • Absent excretory system. If present in a deficient state,
  • Strongly developed mesentery tissue
  • There are caudal adhesive glands.

Order 1. Enoploidea

  • There are six labial papillae and 10 to 12 sensory bristles at the front end of the tooth.
  • Cuticle typically has bristles.
  • Cyanomorph amphids
  • Primarily aquatic and free-living.
  • Examples: Enoplus, Metonchdiamus.

Order 2, Dorylaimoidea

  • The anterior end has six to ten papillae.
  • Cuticle without any bristles.
  • Cyanomorph amphids
  • Buccal cavity contains a protruding spear.
  • Free-living in fresh water and dirt.
  • Dorylaimus, Tylencholaimus, etc.

Order 3. Mermithoidea

  • Size big.
  • The anterior conclusion has sixteen labial papillae and no sensory bristles.
  • The cuticle has no bristles.
  • Cyanthiform or diminutive amphids
  • Long oesophagus that connects to the blind intestine.
  • Larvae are parasitic on invertebrates.
  • Adult independence
  • Examples: Mermis, Paramermis,

Order 4: Chromadoroidea

  • Size small.
  • Smooth or ringed cuticle; densely bristled.
  • Spiral amphids.
  • Oral cavity contains teeth.
  • Pharynx with bulbus posterior.
  • Primarily aquatic and free-living.
  • Examples: Halichoanolaimus.

Order 5. Monohysteroidea

  • Size small.
  • Circular amphids.
  • smooth or slightly ringed cuticle, frequently with bristles.
  • An anterior end with four, six, eight, or more sensory bristles.
  • Primarily marine, with some freshwater and terrestrial species.
  • Examples: Monohystera, Plectus.

Order 6. Desmoscolecoidea

  • Size small.
  • Amphibians with a cresent or pump form.
  • The densely encircled cuticle
  • The anterior end has four sensory bristles, and the head is armoured.
  • Marine; independent.
  • Examples: Desmoscolex, Epsilonema.

Class 2. Phasmidia (secernentea)

  • Phasmids present
  • Pore-like amphids
  • Excretory system in good condition
  • The weekly development of mesentery tissue
  • No caudal glands of adhesion

Order 1.Trichuroidea (Trichinelloidea)

  • An anteriorly filiform body shape.
  • Mouth devoid of lips; pharynx thin.
  • Females have one ovary, whereas males have one or none.
  • Vertebrate hosts for parasites.
  • Trichuris and Trichinella serve as examples.

Order 2. Dioctophymoidea

  • Large worm.
  • Without lips, containing 6, 12, or 18 papillae.
  • Long pharynx without a bulb.
  • Females have one ovary, whereas males have one spicule and a muscular brusa.
  • Parasites of birds and mammals
  • Examples include Dioctophyma and Hystrichis.

Order 3. Rhabditoidea

  • Small or average size.
  • The cuticle’s smooth or ringed surface.
  • Sensory bristles as papillae arranged in two rings, an inner ring of six and an outer ring of four, six, or ten.
  • Male copulatory spicules accompanying the gubernaculum
  • Free-living or parasitic in plants and animals.
  • Examples: Rhabditis,

Order 4. Rhabdiasoidea

  • Size medium.
  • Textured cuticle
  • Absent pharyngeal bulb.
  • The parasitic stage of vertebrates may be hermaphrodite or parthenogenetic.
  • Examples include Rhabdias and Strongyloides.

Order 5. Oxyuroidea

  • Small, pin-shaped worm.
  • A mouth encircled by three to six tiny lips.
  • Pharynx has posterior bulbs that are valvular.
  • Long-tailed female with a pointed tail.
  • Both invertebrates and vertebrates harbour parasites.
  • Examples: Oxyuris, Enterobius.

Order 6: Ascaroidea

  • Large-sized maggot
  • The mouth is encircled by three lips.
  • Pharynx lacks a bulb.
  • Male having a tail that is curled ventrally.
  • Parasitism in vertebrates
  • Examples: Ascaris, Ascaridia.

Order 7: Strongyloidea

  • Mouth with no lips and a crown of leaves.
  • Well-developed buccal capsule.
  • Absent pharyngeal bulb.
  • Males have an enlarged copulatory brusa, whereas females typically have an ovijector.
  • Vertebrate hosts for parasites.
  • Necator, Ancylostoma, and Strongylus are examples.

Order 8: Spiruroidea

  • The mouth has two lateral lips.
  • The pharynx has a bulb. It is muscular anteriorly and glandular posteriorly.
  • Males lack copulatory brusa, whereas their tails are spirally coiled.
  • The presence of parasites in animals
  • Thelazia, Gnathostoma, and Spiroxys are examples.

Order 9. Dracunculoidea

  • Lips and buccal capsule lacking.
  • The mouth is encircled by a ring, or papillae.
  • Absent pharyngeal bulb; anteriorly muscular and posteriorly glandular pharynx.
  • No copulatory brusa in male.
  • Vertebrate hosts for parasites.
  • Examples: Dracunculus, Philometra.

Order 10. Filarioidea

  • Filiform slender worm.
  • Lips and buccal capsule lacking.
  • 6 labial papillae are present.
  • No pharyngeal bulb.
  • Male with a tiny, curled tail.
  • Blood or skin-dwelling microfilariae that are transmitted by bloodsucking insects.
  • Vertebrate hosts for parasites.
  • Examples: Wuchereria, Loa, Microfilaria.


  1. Kotpal RL. 2017. Modern Text Book of Zoology- Invertebrates. 11th Edition. Rastogi Publications.
  2. Jordan EL and Verma PS. 2018. Invertebrate Zoology. 14th Edition. S Chand Publishing.
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