Nematoda

The Nematoda, like many other animal phylums, are generally triploblastic and possess an embryonic mesoderm that is compressed between the ectoderm and endoderm. They are bilaterally symmetrical, that is a longitudinal section can divide them into right and left sides that are symmetrical. Whereas, the nematodes, or roundworms, contain a pseudocoelom which comprises both the free-living and parasitic forms.

Phylum Nematoda contains more than 28,000 species with an approximately 16,000 being parasitic in nature. The name Nematoda was derived from the Greek word “Nemos,” which means “thread” ; it also includes roundworms. Nematodes are seen in all habitats that possess a large number of individuals of each species present in them. The free-living nematode Caenorhabditis elegans is majorly used as a model system in laboratories all over the world.

Characteristics of Nematoda

The different characteristics possessed by Nematodes are:

• These are widely distributed, aquatic or terrestrial, parasitic or free-living forms.

• Their body is usually elongated, cylindrical, unsegmented, worm-like, bilaterally symmetrical; it tapers at both ends.

• Nematodes are triploblastic animals with a perivisceral cavity which is more extensive than that of the platyhelminthes.

• Their body has an organ -system grade type of organization.

• A thick, flexible multi-layered collagenous cuticle covers the body of Nematoda and they often bear cuticle setae (hairs), spines or annulations.

• Cuticle is moulted periodically.

• They possess a cellular or syncytial epidermis, that is the nuclei are not separated from each other via cell membranes.

• They comprise only longitudinal muscle fibers having four bands.

• Nematodes lack true coelom. Their body cavity is pseudocoel or blastocoel and is not lined by mesoderm and is rather filled with parenchyma in many cases.

• Cilia is absent in them.

• Circulatory and respiratory systems are not present i.e. respiration occurs via the general body surface and are aerobic in free-living forms and anaerobic in parasitic form.

• Internal cephalisation is present but externally little differentiation can be seen between the anterior and posterior region. i.e. a distinct head is absent. Moreover, the mouth is situated in the anterior region.

• The digestive system is usually complete with a distinct mouth and anus. Muscular pharynx and the inner surface of the gut are not lined by cilia.

• In Nematodes extracellular digestion occurs.

• The mouth is usually surrounded by six lips.

• Excretory system lacks flame cells and nephridia. In the class, Adenophorea glandular renette cells with the duct are present instead.

• The nervous system is not much developed, that is it comprises a circumpharyngeal ring and a longitudinal nerve cord.

• Sense organs are poorly developed and are present in the form of papillae that can be well defined as amphid (in mouth) and plasmid (anus).

Classification of Nematoda

Kingdom: Animalia

Eumetazoa: Metazoans

Bilateria: Bilaterally symmetrical animals

Protostomia: Protostomes

Ecdysozoa 

Phylum: Nematoda, Roundworms

Class: Adenophorea 

Class: Secernentea

Class Adenophorea (Aphasmidia):

• The members of this class have differently shaped amphids behind the lips.

• Phasmids are absent in the caudal area.

• Cephalic setae and papillae are present. 

• Paired testes are present.

• Majority of the species living in this class are free- living (terrestrial and freshwater forms), few are parasitic also.

• This class further comprises twelve orders.

Examples: Mondnchus, Dorylaimus, Trichinella, Trichuris.

Class Secernentea (Phasmida):

• This class consists of nematodes that generally possess pore-like amphids in the lateral lips.

• At the caudal end there is the presence of a pair of phasmids.

• Only anterior testis are present.

• Most of the parasitic forms are members of this class whereas the free-living species greatly inhabit soil.

• This class consists of eight orders.

Examples: Ascaris, Wuchereria, Ancylostoma, Rhabditis, Loa loa (eye-worm).

Examples of Dangerous Nematodes in Agriculture

Nematodes are almost immeasurably abundant in soil. A single handful of soil may contain thousands of different types of nematodes, some may be too small to be visible by naked eyes and others can be big enough to see and feel their wriggling, around one’s fingertips. 

Another handful of soil from a totally different landscape can yield a totally different thousand types of roundworms. Microscopic parasitic root knot nematodes can spread easily – they can be blown away in the wind or remain attached to a bird that flies over a vulnerable crop. Then, they generally attack the roots of a plant, leading it to slowly wilt and die.

Root crops such as potatoes and radishes, as well as few fruits that grow in orchards, like peaches, are highly vulnerable. In order to keep their plants healthy, farmers should follow government’s rules and regulations about the nematode-free fertilizer and soil and use methods such as crop rotation, cover crops and pesticides thus preventing the spread of root knot nematodes.

Conclusion

Free-living nematodes provide various benefits to the ecosystem, even though we can’t see it. When these microscopic organisms feed upon bacteria or fungi they release certain nutrients into the soil that plants may utilise for its survival and circulate into the rest of the food web. They also serve as a food source for other creatures, like those of other nematodes and invertebrates living in the soil.

Many researchers have also started utilising nematodes for use in farming and agriculture. Different nematode species feed on insects or insect larvae that cause direct harm to crops, lawns and garden plants. After utilizing these beneficial nematodes, farmers and homeowners can now naturally control their pest populations. Here we come to an end of this topic. We hope that you were able to grasp a clear concept of the topic.