Nematode – Structure, Classification and Characteristics

Nature is a blessing to us. The biosphere is the area of the earth that contains both living and nonliving organisms. It is home to a wide range of species that all rely on one another. Biological diversity refers to the interdependence of species and their interaction with the environment to support life.

Nematode

• The soil nematodes feed on bacteria, fungi, and other nematodes and play an important role in nutrient recycling.
• They also attack insects and keep pests at bay. They do, however, cause significant damage to plants. They feed on the plant roots, reducing nutrient uptake and stress tolerance.
• The organisms of the phylum Nematoda are also referred to as “roundworms.” To date, 28000 Nematoda species have been identified. They are vermiform animals with no segments. Dorsal and ventral nerve cords run through the epidermis.
• A spadeful of soil contains a plethora of Nematodes. Human diseases caused by them include Ascariasis, Trichuriasis, Hookworm, Enterobiasis, Filariasis, and Angiostrongyliasis.
• Ascaris and Wuchereria are examples of animals from this group.

Characteristics of Nematode 

• They are abundant, aquatic or terrestrial, parasitic or free-living. They have cellular or syncytial epidermis, which means that cell membranes do not separate the nuclei.
• Their bodies are long, cylindrical, uncut, worm-like, bilaterally symmetrical, and tapering at both ends.
• They are triploblastic animals with a larger perivisceral cavity than platyhelminths.
• The body is organized by the organ system. The cuticle is molted on a regular basis.
• They are made up of only four bands of longitudinal muscle fibers.
• They don’t have a true coelom. In most cases, the body cavity is pseudocoel or blastocoel, not lined by mesoderm, and filled with parenchyma.
• The body is covered in a thick, flexible multi-layered collagenous cuticle that is often adorned with cuticle setae (hairs), spines, or annulations.
• They contain tubular gonads. The male genital duct enters the cloaca. Each female genital duct has its own opening.
• There are no asexual reproduction or amoeboid sperm cells.
• The nervous system is underdeveloped. The circumpharyngeal ring and the longitudinal nerve cord comprise it.
• Fertilization is internal, cross-fertilization, or self-fertilization.
• With a distinct mouth and anus, the digestive system is complete. Cilia do not normally line the muscular pharynx or the inner surface of the gut.
• Direct development, with or without an intermediate host, is possible.

Classification of Nematode

Nematoda is one of the most numerous phyla. Currently, approximately 15,000 nematode species are known. It is difficult to classify due to the wide range of forms and structures. Chitwood (1933) classified them as Phasmidia or Aphasmidia based on the presence or absence of phasmids.

Class Aphasmidia

• The majority of species have caudal adhesive glands and epidermal glands.
• Amphids are postliberal in shape and can be pouch-like, tube-like, or pore-like.
• Phasmids (caudal papillae with pores that connect to glandular pouches and are thought to be chemosensory in function) are absent.
• Excretory organs are renette cells that lack collecting tubules.
• Coelomocytes have matured.
• The majority of the species are marine and include both free-living and parasitic species. Terrestrial, freshwater, and major marine forms are among the free-living species.
• Males typically lack caudal alae. Males typically have two tests.
• Eg. Capillaria, Trichinella, etc.

Aphasmidia or adenophorea is classified into the following orders:

Dorylaimida

• The cuticle is smooth and free of bristles.
• These are mostly terrestrial in nature.
• A protruding spear forms the buccal cavity.
• It is made up of 6-10 labial papillae.
• Trichodoris, for example.

Chromedorida

• The cuticle is ringed or smooth.
• There are no bristles on the cuticle.
• The pharynx has a posterior bulb.
• They can be either free-living or marine.
• As an example, consider Paracanthonchus.

Monohysterida

• The cuticle is smooth, ringed, and bristly.
• Circular amphids are found on them.
• They can be marine, freshwater, or land-based.
• Monohystera, for example.

Enoplida

• They are mostly marine in nature.
• Bristles are present in the cuticle.
• Cyanthiform amphids are what they’re called.
• As an example, consider anticoma.

Araeolaimida

• Smooth cuticle, sometimes with bristles.
• The presence of labial papillae.
• Four cephalic bristles on the front.
• Amphids spiral inward.
• Examples: Wilsonema, Plectus, and Odontophora

Desmoscolecida

• A thick ring of prominent bristles around the entire body or in specific areas.
• Four sensory bristles on the anterior end.
• Amphids with crescent or pump-shaped bodies.
• Nematodes that live in the sea and in the open.
• Examples: Epsilonema, Desmoscolex, Greefiella

Mermithida

• A smooth, bristle-free cuticle.
• There are 16 labial papillae.
• Cyanthiform or reduced amphids
• Invertebrate parasites (larval stage); free-living adult stage
• Examples:
• Mermis, Agamermis, and Paramermis

Class Phasmidia

• These are mostly parasitic in nature.
• There are no caudal glands.
• There are plasmids, which are unicellular, pouch-like sense organs.
• The excretory system is comprised of paired lateral canals.
• For example, Ascaris and Enterobius

The Phasmidia class is divided into the following orders:

Strongylida

• They are lipless vertebrate parasites.
• There is no bulb in the pharynx.
• They have an established buccal capsule.
• They have a genuine copulatory bursa.
• Strongylus, for example.

Ascaridida

• These are oviparous, large, stout nematodes that live as parasites in vertebrate intestines.
• A posterior bulb may or may not exist in the pharynx.
• The mouth has three prominent lips.
• The buccal capsule does not exist.
• For instance, Ascaris

Trichuroida

• These are commonly referred to as whip-worms.
• They have a thin pharynx.
• Lips are missing from the mouth.
• Trichuris, for example.

Spirurida

• Thread-like, medium to large in size.
• Males are larger than females.
• The mouth usually has two lateral lips and a bulbless pharynx.
• Males who have two unequal copulatory spicules but no bursa or caudal alae.
• viviparous or oviparous
• Vertebrate parasites with a blood-sucking invertebrate as an intermediate host
• Spirura, Wuchereria bancrofti (Filaria), Loa loa (Eye worm), Brugia, Onchocerca, and other parasites

Camallanida

• Thread-like, with females being larger than males.
• Males have no lips and no bursa.
• The buccal capsule is either large or absent.
• Spicules of males of the same size and vertebrate parasites
• Adult females with degenerated bursa and oviparity.
• Camallanus, Procamallanus, Dracunculus medinensis (Guinea-worm), and Philometra are a few examples.

Rhabditida

• Smooth and ringed cuticle
• Sensory bristles in two rings: the outer ring has four, six, or ten bristles, and the inner ring has six bristles.
• A posterior bulb in the pharynx.
• Male copulatory spicules with gubernaculum.
• Nematodes are both free-living and parasitic.
• Examples: Heterodera, Bunonema, Rhabditis

Oxyurida

• small to medium in size.
• Males have copulatory spicules.
• Females have a long, narrow, and pointed tail.
• 3-6 simple lips are provided for the mouth.
• A valvular posterior bulb in the pharynx.
• There are caudal alae present.
• Parasites of both invertebrates and vertebrates
• Examples: Oxyuris, Enterobius vermicularis (Pinworm), Heterakis gallinae, Thelastoma, and Aspiculuris are all examples of parasites.

Frequently Asked Questions 

Question 1: What is the Class Rhabditea?

Answer: 

Both parasitic and free-living nematodes belong to the class Rhabditea. In this class, parasitic nematodes make up the majority. Rhabditea Free-living obtains its energy from bacteria. Both in water and between soil granules, they are present.

Question 2: What function do nematodes provide in the soil?

Answer:

By driving live and dormant microbes on their surfaces and in their digestive systems, nematodes aid in the distribution of bacteria and fungi through the soil and along with roots. Higher-level predators, such as predatory nematodes, soil microarthropods, and soil insects, consume nematodes. Soil nematodes can also help farmers maintain adequate soil nitrogen levels. They mineralize critical soil processes into inorganic forms that plants can use.

Question 3: What is the anatomy of a nematode?

Answer:

Nematodes range in thickness from 5 to 100 m and length from 0.1 to 2.5 mm. They can range in size from microscopic to 5 cm in length, with some reaching more than 1m in length. The body is characterized by ridges, rings, bristles, or other distinguishing features.

Question 4: Is Nematoda useful in agriculture?

Answer:

A nematode can be beneficial or harmful to plant health depending on its species. There are two types of nematodes: predatory nematodes that kill garden pests and pest nematodes that attack plants (root-knot nematodes).

Plant viruses are spread between crop plants by vectors. Plant-parasitic nematodes, such as eelworms, frequently attack leaves and buds. Plant rotation of nematode-resistant species can be used to manage parasitic nematodes. As a treatment method, natural antagonists such as the fungus Gliocladium roseum are used.

Question 5: What are the benefits of Nematoda?

Answer:

• The beneficial nematodes are microscopic, non-segmented families that occur naturally in soil all over the world.
• Nematodes use carbon dioxide and other waste-derived substances as chemical markers to locate their hosts.
• When beneficial bacteria are released into an insect’s gut, they kill it within 24 to 48 hours.