General Characters of Phylum Nematoda
Posted on : 30-01-2017

Nematodes are generally referred to as non-segmented round worms, thread worms or pinworms. They are distinct from the lower flatworms and higher annelids. They constitute the largest phylum of pseudo coelomate group.

  1. Nematodes are free-living or parasitic. Free living forms inhabit marine, fresh water or terrestrial habitats. Terrestrial species live in thin film of water surrounding soil particles. Parasites attack plants and animals.
  2. Most of the free living species are microscopic while many of the parasitic forms are much larger.
  3. In Nematodes, body is elongated and cylindrical in shape. Tissues are syncytial or cellular. Nematodes are characterised by species-specific number of cells or nuclei.
  4. Locomotory cilia are absent. Some have ciliated gastrodermal cells.
  5. Body wall of nematodes consists of cuticle, epidermis and longitudinal muscles. Circular muscles are absent.
  6. In nematodes true coelom is absent so, body cavity is pseudocoelom. It is not lined by mesodermal peritoneum. Pseudocoleomic fluid acts as hydrostatic skeleton. It is also useful in circulation.
  7. In nematodes, alimentary canal is a straight tube with an anterior terminal mouth, pharynx, intestine and rectum. In males, rectum joins reproductive system to form a cloaca. In females, rectum directly opens to the exterior by anus.
  8. Digestive system is complicated. Digestion is both extracellular and intracellular. Intestine is non-muscular.
  9. Blood vascular system and special respiratory organs are absent. Excretory system includes excretory glands or H-shaped excretory canal or both.
  10. Nervous system in intraepithelial in the epidermis and gut. Nerves extend anteriorly and posteriorly from circum-pharyngeal ganglionated nerve ring. Sense organs include papillae, amphids and phasmids. Amphids are anterior and are mechano-sensory and chemo-sensory. Phasmids are posterior unicellular glands that may be chemosensory or secretory or excretory in function.
  11. Nematodes are unisexual. Many are sexually dimorphic. Males are smaller than females and have curved posterior end, cloaca and copulatory spicules. A few are hermaphrodite. Fertilization is internal. Development includes four moults.
  12. Nematodes exhibit eutely. This is the phenomenon in which the cell divisions (except in gonads) cease near the end of embryonic development so that the number of cells of the adults is constant which the characteristic feature for particular species. Growth results mainly by the increase in the cell size rather than cell number.

1. What is eutely? How is it relevant to Nematoda?

2. Mention few general characters of the phylum Nematode?

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