Phylum Nematoda: Have you heard of roundworms? Or doctors complaining about any disease known as Ascariasis? These roundworms are the worms that are cylindrical in shape and are members of the Phylum Nematoda. The Phylum Nematoda is a phylum that includes parasitic worms that cause deadly diseases in human beings and animals. Most species that belong to the phylum Nematoda are microscopic; however, a few are over a meter long.

The largest known nematode that infects sperm whales has been shown to grow up to \(9\) metres. This article helps you learn about Phylum Nematoda, classification and characteristics of Phylum Nematoda with examples and much more. To know more interesting facts about the Phylum Nematoda, scroll down the article.

Phylum Nematoda Definition

Nematodes (Gr., nema = thread + eidos = form), commonly referred to as non-segmented roundworms, threadworms or pinworms, constitute the largest phylum Nematoda of superphylum Aschelminthes.

Earlier, these roundworms were considered under the Phylum Aschelminthes. Now, Phylum Aschelminthes is considered as Superphylum Aschelminthes and divided into ten phyla.

Phylum Nematoda is one of the phyla under the Superphylum Aschelminthes. These are distinct forms like flatworms and tapeworms in having a cylindrical body, a pseudocoelom and a complete digestive tract lined by endodermal epithelium.

Fig: Nematoda

What are the Characteristics of Phylum Nematodes?

1. These are aquatic or terrestrial, parasitic or-living animals.
2. Their body is elongated, cylindrical, unsegmented and worm-like.
3. These are bilaterally symmetrical and tapering at both ends.
4. These are triploblastic animals with organ-system grade organization.
5. Their body wall is made up of thick cuticle, cellular and longitudinal muscle cells in four bands.
6. These nematodes lack true coelom. Their body cavity is pseudocoel or blastocoel, which is not lined by mesoderm and filled with parenchyma in most cases.
7. The nervous system is not much developed, and it is with the circumcenter ring and anterior and posterior nerves.
8. The circulatory and respiratory systems are absent, i.e., respiration occurs through the general body surface and is aerobic in-living form and anaerobic in parasitic form.
9. Internal cephalization is present, but externally, there is little differentiation between the anterior and posterior region. i.e., a distinct head is lacking. However, the mouth is present in the anterior region.
10. The digestive system is complete with a distinct mouth and anus. The muscular pharynx and the inner surface of the gut of nematodes are usually not lined by cilia.
11. Extracellular digestion occurs in them, and the mouth is surrounded by six lips.
12. An excretory system is without flame cells and nephridia.
13. Sense organs are poorly developed in papillae, which are well defined as amphid (in mouth) and plasmid (anus).
14. Sexes are separate (gonochoristic), and the male is smaller than females.
15. Tubular gonads are present in them. The male genital duct leads into the cloaca—female genital ducts with a separate opening.
16. Amoeboid sperm cells are in nematodes.
17. No asexual reproduction and fertilization are internal or maybe cross or self.
18. Development may be direct, with or without an intermediate host or indirect.

What are the Basic Classification of Nematodes?

The scientist named Chitwook in \(1933\) divided the Phylum Nematoda into two classes, namely, Class Phasmidia and Class Aphasmidia, on the basis of the presence or absence of phasmids (caudal sensory organs).

Fig: Classification of Phylum Nematoda

Class \(1.\) Aphasmidia

i. Phasmid, i.e., causal sensory organs, are absent in animals of this class.
ii. Amphids, i.e., anterior sense organs of various types and are rarely pore-like.
iii. The excretory system is absent if present poorly developed.
iv. Well-developed mesenterial tissue.
v. Caudal adhesive glands are present.
vi. The Class Aphasmidia is divided into six orders and is given below

Orders	Examples
Order \(1.\) Enoploidea	Enoplus, Metonchdiamus
Order \(2.\) Dorylaimoidea	Dorylaimus, Tylencholaimus
Order \(3.\) Mermithoidea	Mermis, Paramermis
Order \(4.\) Chromadoroidea	Halichoanolaimus
Order \(5.\) Monohysteroidea	Monohystera, Plectus
Order \(6.\) Desmoscolecoidea	Desmoscolex, Epsilonema

Class \(2.\) Phasmidia

i. Phasmids are present in animals that belong to the class Phasmidia.
ii. Pore-like amphids are present.
iii. These have a well-developed excretory system.
iv. Weekly developed mesenterial tissue.
v. No caudal adhesive glands.
vi. The Class Phasmidia is divided into ten orders and is given below:

Orders	Examples
Order \(1.\) Trichuroidea (Trichinelloidea)	Trichuris, Trichinella
Order \(2.\) Dioctophymoidea	Dioctophyma, Hystrichis
Order \(3.\) Rhabditoidea	Rhabditis
Order \(4.\) Rhabdiasoidea	Rhabdias, Strongyloides
Order \(5.\) Oxyuroidea	Oxyuris, Enterobius
Order \(6\). Ascaroidea	Ascaris, Ascaridia
Order \(7.\) Strongyloidea	Necator, Ancylostoma, Strongylus
Order \(8.\) Spiruroidea	Thelazia, Gnathostoma, Spiroxys
Order \(9.\) Dracunculoidea	Dracunculus, Philometra
Order \(10.\) Filarioidea	Wuchereria, Loa, Microfilaria

Phylum Nematoda Examples

Some of the examples of the phylum Nematoda are as follows:

Ascaris

Classification of Ascaris–
Kingdom: Animalia
Phylum: Nematoda
Class: Chromadorea
Order: Ascaridida
Family: Ascarididae
Genus:Ascaris

Fig: Ascaris

Wuchereria bancrofti 

Classification of Wuchereria bancrofti –
Kingdom: Animalia
Phylum: Nematoda
Class: Secernentea
Order: Spirurida
Family: Onchocercidae
Genus: Wuchereria
Species: bancrofti

Fig: Wuchereria bancrofti

Adenophorea

Classification of Adenophorea–
Kingdom: Animalia
Phylum: Nematoda
Class: Adenophorea
Genus: Adenophorea

Fig: Adenophorea

Enoplea

Classification of Enoplea–
Kingdom: Animalia
Phylum: Nematoda
Class: Enoplea
Genus: Enoplea

Fig: Enoplea

Economic Importance of Phylum Nematoda

The economic importance of the Phylum Nematoda are as follows:

i. Important in aerating the soil and distributing organic materials and minerals in the soil.
ii. Infest crops, livestock and humans causing disease – Hookworm, trichinosis, crop damage.
iii. These feed on microorganisms such as fungi, protozoans, bacteria and even other nematodes and play a very important role in the nutrient cycling process and release of nutrients for plant growth.
iv. They also play an important role in pest control by attacking insects.
v. Nematodes are considered primary producers and play an important role in breaking down organic matter in their ecosystem.

Summary

The members of the phylum Nematoda are commonly known as roundworms. These are aquatic,-living and also parasitic animals. These are elongated, unsegmented worm-like with bilateral symmetry and triploblastic animals. The members of this phylum infect plants, animals and human beings. Organisms under this phylum can cause various diseases such as ascariasis, enterobiasis, filariasis, etc. Some of the nematodes in human beings may enter the bloodstream, where they cause tissue damage, and continue spreading to other parts of the body. They can occur in almost all organs of the body, but the most common sites are in the alimentary, circulatory, and respiratory systems.

FAQs on Phylum Nematoda

Q.1. What is Nematoda also called?
Ans: Nematoda is also called roundworms.

Q.2. What are the \(2\) classes of the phylum Nematoda?
Ans: The \(2\) classes of the Phylum Nematoda are Class Phasmidia and Class Aphasmidia.

Q.3. What animals are in the Phylum Nematoda?
Ans: The animals that belong to the Phylum Nematoda are Ascaris, Wuchereria bancrofti, Enoplea, etc.

Q.4. What are the five characteristics of Phylum Nematoda?
Ans: 1. These are aquatic or terrestrial, parasitic or-living animals.
2. Their body is elongated, cylindrical, unsegmented and worm-like.
3. These are bilaterally symmetrical and tapering at both ends.
4. These are triploblastic animals with organ-system grade organization.
5. The nervous system is not much developed, and it is with the circumcenter ring and anterior and posterior nerves.

Q.5. Are nematodes visible to the human eye?
Ans: Most nematodes are so tiny that one needs a microscope to see them, but some are large enough to be seen easily with the naked eye, such as Placentonema gigantissima, which can reach around \(30\) feet long.

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