Phylum Nematoda

Phylum Nematoda- characteristics, classification, examples

Last Updated on March 19, 2020 by Sagar Aryal

Nematoda Definition

Nematodes (Gr., nema thread+ eidos, form) are commonly referred to as non-segmented roundworm, threadworm or pinworm, as distinct from flatworm and higher segmented annelids.

Phylum Nematoda

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Phylum Nematoda Characteristics

  • They are widely distributed, aquatic or terrestrial, parasitic or free-living.
  • Their body is elongated, cylindrical, unsegmented, worm-like, bilaterally symmetrical and tapering at both ends.
  • They are triploblastic animals with perivisceral cavity more extensive than that of platyhelminths.
  • The body is of organ -system grade organization.
  • The body is generally covered with thick, flexible multi-layered collagenous cuticle and often bears cuticle setae (hairs), spines or annulations.
  • Cuticle moulted periodically.
  • They have cellular or syncytial epidermis I.e. the nuclei are not separated from each other by cell membranes.
  • They consist of only longitudinal muscle fibers with four bands.
  • They lack true coelom. The body cavity is pseudocoel or blastocoel not lined by mesoderm and filled with parenchyma in most cases.
  • They lack cilia.
  • circulatory and respiratory systems are absent. i.e. respiration occurs through general body surface and aerobic in free-living form and anaerobic in parasitic form.
  • Internal cephalization is present but externally there is little differentiation between the anterior and posterior region. i.e. distinct head is lacking. However, the mouth is present in the anterior region.
  • The digestive system is complete with a distinct mouth and anus. Muscular pharynx and the inner surface of the gut usually not lined by cilia.
  • Extracellular digestion occurs in them.
  • The mouth is surrounded by six lips.
  • Excretory without flame cell and nephridia. In the class Adenophorea glandular renette cells with the duct.
  • The nervous system is not much developed. i.e. consists of circucumpharyngeal ring and longitudinal nerve cord.
  • Sense organ are poorly developed in the form of papillae, which are well defined as amphid (in mouth) and plasmid (anus).
  • Sexes are separate (gonochoristic). the male is smaller than females.
  • Tubular gonad is present in them. Male genital duct leads into the cloaca. Female genital ducts with a separate opening.
  • Amoeboid sperm cells.
  • No asexual reproduction.
  • Fertilization is internal or maybe cross or self.
  • Development may be direct, with or without an intermediate host or indirect.
  • Various lateral lines and pores are present on the surface of the body.

Phylum Nematoda Classification

Nematoda are among the most numerous of any phylum. About 15,000 species of nematodes are known at present. Due to much diversity in form and structures difficult to classify. Chitwood (1933) divided them into 2 classes phasmidia and Aphasmidia on the basis of presence and absence of phasmids.

Class 1. Aphasmidia (Adenophorea)

  • Phasmid (causal sensory organs) absent.
  • Amphids (anterior sense organs) of various types, rarely pore-like.
  • Excretory system absent. If present poorly developed.
  • Well-developed mesenterial tissue.
  • Caudal adhesive glands present.

   Order 1. Enoploidea

  • Anterior end with six labial papillae and 10-12 sensory bristles.
  • cuticle usually with bristles.
  • Cyanthiform amphids.
  • Chiefly marine, free-living.
  • Examples: Enoplus, Metonchdiamus.

Order 2. Dorylaimoidea

  • Anterior end with 6-10 papillae.
  • cuticle smooth, no bristles.
  • Cyanthiform amphids.
  • Buccal cavity with the protrusible spear.
  • Free-living in soil and fresh-water.
  • Examples: Dorylaimus, Tylencholaimus.

Order 3. Mermithoidea

  • Size large.
  • Anterior end with 16 labial papillae and no sensory bristles.
  • Cuticle smooth, no bristles.
  • Cyanthiform or reduced amphids.
  • Oesophagus long leading into the blind intestine.
  • Larva parasitic in invertebrates.
  • Adult free-living.
  • Examples: Mermis, Paramermis,

Order 4. Chromadoroidea

  • Size small.
  • Cuticle smooth or ringed; with heavy bristles.
  • Spiral amphids.
  • Buccal cavity with teeth.
  • Pharynx with posterior bulb.
  • Mostly marine; free-living.
  • Examples: Halichoanolaimus.

Order 5. Monohysteroidea

  • Size small.
  • Circular amphids.
  • Cuticle smooth or slightly ringed, often with bristles.
  • Anterior end with 4,6,8, or many sensory bristles.
  • Free-living; mostly marine, some fresh-water, some terrestrial.
  • Examples: Monohystera, Plectus.

Order 6. Desmoscolecoidea

  • Size small.
  • Cresent-shaped or pump-shaped amphids.
  • The heavily ringed cuticle.
  • Anterior end with 4 sensory bristles, head armored.
  • Marine; free-living.
  • Examples: Desmoscolex, Epsilonema.

Class 2. Phasmidia (secernentea)

  • Phasmids present.
  • Pore-like amphids.
  • Well-developed excretory system.
  • Weekly developed mesenterial tissue.
  • No caudal adhesive glands.

Order 1. Trichuroidea (Trichinelloidea)

  • Body filiform anteriorly.
  • Mouth without lips; pharynx slender.
  • Female with 1 ovary and male with 1 or none spicule.
  • Parasites of vertebrates.
  • Examples: Trichuris, Trichinella

Order 2. Dioctophymoidea

  • Large worm.
  • Mouth without lips; with 6,12 or 18 papillae.
  • Elongated pharynx with no bulb.
  • Female with 1 ovary and male with 1 spicule and muscular brusa.
  • Parasites of mammals and birds.
  • Examples: Dioctophyma, Hystrichis.

Order 3. Rhabditoidea

  • Size small or moderate.
  • The smooth or ringed cuticle.
  • Sensory bristles as papillae in 2 rings, an inner ring of 6 and outer ring of 4,6 or 10.
  • Copulatory spicules in male accompanied by the gubernaculum
  • Free-living or parasitic in animals and plants.
  • Examples: Rhabditis,

Order 4. Rhabdiasoidea

  • Size medium.
  • Smooth cuticle.
  • Pharyngeal bulb absent.
  • The parasitic stage in vertebrates is either hermaphroditic or parthenogenetic.
  • Examples: Rhabdias, Strongyloides.

Order 5. Oxyuroidea

  • Pin-shaped small worm.
  • Mouth surrounded by 3-6 small lips.
  • Pharynx with valvular posterior bulbs.
  • Female with a long pointed tail.
  • Parasitic in invertebrates and vertebrates.
  • Examples: Oxyuris, Enterobius.

Order 6. Ascaroidea

  • Large-sized worm.
  • Mouth surrounded by 3 lips.
  • Pharynx without bulb.
  • Male with a ventrally coiled tail.
  • Parasitic in the vertebrates.
  • Examples: Ascaris, Ascaridia.

Order 7. Strongyloidea

  • Mouth without lips but leaf crown.
  • Well-developed buccal capsule.
  • Pharyngeal bulb absent.
  • Male with expanded copulatory brusa; female usually with ovijector.
  • Parasites of vertebrates.
  • Examples: Necator, Ancylostoma, Strongylus.

Order 8. Spiruroidea

  • Mouth with 2 lateral lips.
  • Pharynx with bulb, muscular anteriorly and glandular posteriorly.
  • Copulatory brusa absent in male but spirally coiled tail.
  • Parasites in animals.
  • Examples: Thelazia, Gnathostoma, Spiroxys.

Order 9. Dracunculoidea

  • Lips and buccal capsule absent.
  • Mouth surrounded by ring or papillae.
  • Pharyngeal bulb absent; pharynx muscular anteriorly and glandular posteriorly.
  • No copulatory brusa in male.
  • Parasites of vertebrates.
  • Examples: Dracunculus, Philometra.

Order 10. Filarioidea

  • Filiform slender worm.
  • Lips and buccal capsule absent.
  • 6 labial papillae present.
  • No pharyngeal bulb.
  • Male small with a coiled tail.
  • Microfilariae in blood or skin and developed in blood-sucking insects.
  • Parasites of vertebrates.
  • Examples: Wuchereria, Loa, Microfilaria.

References

  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.

Phylum Nematoda

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