Phylum Nematoda – Classification, Definition, Characteristics, Examples

Definition of Phylum Nematoda

Nematodes (Gr. nema thread+ form) are often referred to by various names, such as threadworm, non-segmented roundworm or pinworm. They are distinct from flatworms and higher segmented anelids.

Characteristics of Phylum Nematoda 

  • They are widespread, terrestrial or aquatic, parasitic, or free-living.
  • Their bodies are cylindrical, unsegmented, worm-like and tapering at each end.
  • They are triloblastic animals that have a perivisceral cavity larger than the platyhelminths.
  • The organ-system grade organization of the body is called the body.
  • The body is covered in thick, flexible, multi-layered collagenous cuticle. It often has cuticle setae (hairs), spines and annulations.
  • Cuticle is removed periodically.
  • They are covered with syncytial epidermis (cellular or syncytial). The cell membranes do not separate the nuclei from one another.
  • They are made up of four bands and only longitudinal muscle fibers.
  • They lack true coelom. In most cases, the body cavity is either pseudocoel (or blastocoel) and not lined with mesoderm. It is usually filled with parenchyma.
  • They lack cilia.
  • The circulatory and respiratory system are not functioning. i.e. The general body surface is where respiration takes place.
  • Although internal cephalization exists, there is very little distinction between the anterior or posterior regions. i.e. The head is missing. The mouth is still present in the anterior.
  • A distinct mouth and anus complete the digestive system. The inner surface of the stomach and the muscular pharynx are not usually lined with cilia.
  • They also allow for extracellular digestion.
  • Six lips surround the mouth.
  • Excretory without flame cells and nephridia. Class Adenophorea glandular Renette cells with the Duct.
  • The nervous system is still not well developed. i.e. It consists of the circucumpharyngeal and longitudinal nerve cords.
  • The papillae are not well developed and are referred to as amphid (in the mouth) or plasmid (“anus”) respectively.
  • The sexes are distinct (gonochoristic). The male is usually smaller than the female.
  • They have tubular gonad. The cloaca is connected to the male genital tract. Female genital tracts have a separate opening.
  • Amoeboid Sperm Cells
  • No asexual reproduction.
  • Fertilization can be internal, cross- or self.
  • Development can be either direct or indirect, with or without an intermediary host.
  • There are many pores and lateral lines that run across the body’s surface.

Classification of Phylum Nematoda 

Nematoda is one of the most diverse phyla. At present, there are approximately 15,000 species. It is difficult to classify nematodes because of their diversity in form and structure. Chitwood (1933), based on presence or absence of phasmids, divided them into two classes, phasmidia (or Aphasmidia).

Class 1. Aphasmidia (Adenophorea)

  • Phasmid (causal sensor organs) is absent
  • Amphids are anterior sense organs of various types that are rarely pore-like.
  • Absence of excretory system. If not present, it is poorly developed.
  • Gut-developed mesenterial tissue.
  • Caudal adhesive glands present.

Order 1. Enoploidea

  • Anterior end has six labial and 10-12 sensory bristles.
  • Cuticle is usually made with bristles.
  • Cyanthiform amphids.
  • Primarily marine, free-living.
  • Examples: Enoplus, Metonchdiamus.

Order 2. Dorylaimoidea

  • Anterior end with 6-10papillae.
  • cuticle smooth, no bristles.
  • Cyanthiform amphids.
  • Buccal cavity with protruding spear
  • You can live naturally in soil or fresh water.
  • Examples: Dorylaimus, Tylencholaimus.

Order 3. Mermithoidea

  • Size large.
  • Anterior end has 16 labial and sensory bristles.
  • Cuticle smooth, no bristles.
  • Reduced amphids or cyanthiform
  • Long Oesophagus leading to the blind intestine.
  • Invertebrates are parasitic by Larva
  • You can live as an adult.
  • Examples: Mermis, Paramermis,

Order 4. Chromadoroidea

  • Small size
  • Cuticle smooth, ringed or rough; with thick bristles.
  • Spiral amphids
  • Teeth and buccal cavity
  • Pharynx with posterior bulbs
  • Most marine; non-living.
  • Examples: Halichoanolaimus.

Order 5. Monohysteroidea

  • Small size
  • Circular amphids
  • Cuticle can be smoothed or slightly ringed.
  • Anterior end with 4,6,8 or many sensory bristles.
  • Free-living; mostly marine, some freshwater, and some terrestrial.
  • Examples: Monohystera, Plectus.

Order 6. Desmoscolecoidea

  • Small size
  • Amphids can be shaped in Cresent or Pump shapes.
  • The cuticle is heavily ringed.
  • Head armored, anterior end with 4 sensory bristles.
  • Marine; free-living.
  • Examples: Desmoscolex, Epsilonema.

Class 2. Phasmidia (secernentea)

  • Phasmids are present.
  • Pore-like amphids.
  • A well-developed excretory system.
  • Weekly mesenterial tissue is developed.
  • No caudal adhesive glands.

Order 1. Trichuroidea (Trichinelloidea)

  • Anteriorly, body filiform
  • Mouth without lips; pharynx slender.
  • Females with one ovary, males with one or none.
  • Parasites in vertebrates
  • Examples: Trichuris, Trichinella

Order 2. Dioctophymoidea

  • Large worm.
  • Mouth without lips; with 6,12 and 18 papillae.
  • A long pharynx without a bulb.
  • Female with 1 ovary, male with 1 Spicule and muscular Brüsa.
  • Parasites for birds and mammals
  • Examples: Dioctophyma, Hystrichis.

Order 3. Rhabditoidea

  • Size small or medium
  • The cuticle is either smooth or ringed.
  • Sensory bristles are papillae, which can be found in two rings: an inner ring with 6 and an outer ring with 4,6 or 10.
  • Copulatory spicules of males accompanied by the Gubernaculum
  • Animals and plants can be parasitic or free-living.
  • Examples: Rhabditis,

Order 4. Rhabdiasoidea

  • Size medium
  • Smooth cuticle
  • Absence of pharyngeal bulb
  • Vertebrates can either be parasitic at their hermaphroditic stage or parthenogenetic stage.
  • Examples: Rhabdias, Strongyloides.

Order 5. Oxyuroidea

  • Pin-shaped small worm.
  • A mouth surrounded by 3-6 small, slender lips.
  • Pharynx with posterior bulbs valvular to the ear.
  • A female with a long, pointed tail.
  • Parasitic in vertebrates and invertebrates.
  • Examples: Oxyuris, Enterobius.

Order 6. Ascaroidea

  • Large-sized worm.
  • Three lips surround the mouth.
  • Pharynx without bulb
  • Male with a ventrally coil tail.
  • Parasitic in vertebrates
  • Examples: Ascaris, Ascaridia.

Order 7. Strongyloidea

  • Mouth without lips, but leaf crown.
  • A well-designed buccal capsule.
  • Absence of pharyngeal bulb
  • Male with an expanded copulatory bone; females with an ovijector.
  • Parasites in vertebrates
  • Examples: Necator, Ancylostoma, Strongylus.

Order 8. Spiruroidea

  • Two lateral lips for the mouth
  • Pharynx with bulb and muscular anteriorly, glandular posteriorly.
  • Males have no copulatory brusa, but a spirally coiled tail.
  • Animals can be parasitized by parasites.
  • Examples: Thelazia, Gnathostoma, Spiroxys.

Order 9. Dracunculoidea

  • Lips and buccal capsule missing
  • Mouth enclosed by a ring or papillae.
  • Pharyngeal bulb absent. Pharynx muscular anteriorly, glandular posteriorly.
  • Males are not subject to copulatory brusa.
  • Parasites in vertebrates
  • Examples: Dracunculus, Philometra.

Order 10. Filarioidea

  • Filiform slender worm.
  • Lips and buccal capsule missing
  • 6 labial and 5 femoral papillae are present.
  • There is no pharyngeal bulb.
  • Males are small and have a coil tail.
  • Microfilariae are found in blood and skin, and were developed in blood-sucking insect.
  • Parasites in vertebrates
  • Examples: Wuchereria, Loa, Microfilaria.

Benefits/Importance of Nematodes

Natural predators are one of the most important characteristics of certain species of nematodes. This is an advantage because these species have been proven to kill over 200 pest species. These nematodes are biological controls for various pest insects that can affect humans and plants.

These beneficial nematodes are biologically effective and do not pose a threat to the health of animals or humans. Although they can infect some insects with bacteria, some nematodes live in agricultural pests as endoparasites and kill them.

They are beneficial for the soil because they help to decompose the soil and don’t pollute it.

Beneficial nematodes also play an important decomposer role. Because they are capable of attacking and killing many organisms as well as feeding on different plants, some nematodes can act as intermediate decomposers and are responsible for the reduction of organic material which is then further broken down and absorbed by smaller microorganisms like bacteria.

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