Phylum Nematoda: Habitat, Structure and Development

In this article we will discuss about Phylum Nematoda:- 1. Habit and Habitat of Phylum Nematoda 2. Structure of Phylum Nematoda 3. Body Wall 4. Body Cavity 5. Digestive System 6. Excretory System 7. Respiratory and Circulatory Systems 8. Nervous System 9. Reproductive System 10. Development.

Contents:

1. Habit and Habitat of Phylum Nematoda
2. Structure of Phylum Nematoda
3. Body Wall of Phylum Nematoda
4. Body Cavity of Phylum Nematoda
5. Digestive System of Phylum Nematoda
6. Excretory System of Phylum Nematoda
7. Respiratory and Circulatory Systems of Phylum Nematoda
8. Nervous System of Phylum Nematoda
9. Reproductive System of Phylum Nematoda
10. Development of Phylum Nematoda

1. Habit and Habitat of Phylum Nematoda:

The Nematodes are popularly known as ‘Roundworms’ and sometimes called ‘Nemas’. They are among the most structur­ally simple of all worms because practically all of them depict materially the same basic body plan. Great number of nematodes is free-living and extends from north to South Pole and at the same time there exists a formidable array of parasitic forms living both on plants and animals.

In fact, every plant and metazoan animal has its quota of nematode parasites. The parasitic forms cause unimaginable damage to crop and domestic animals. So far as the absolute number of nematodes is concerned they are second to none than the insects and outnumber the insects in the variety of ecological nitches they occupy.

A survey of the abundance of nematodes in different sites gives the follow­ing figures:

(i) Roots of a single potato plant contain more than 40,000.

(ii) Intertidal muddy sand of sea has about 5 million/sq. metre.

(iii) Aerable land has up to 6 billion per acre.

Despite their superabundance in certain sites, the nematodes are never conspicuous and are not noticed for the reason that ma­jority of them are of microscopic sizes.

Free-living nematodes are saprozoic and feed on plant and animal remains. Some feed on yeast and bacteria. Few members prey on small protozoa and rotifers. Parasitic forms are provided with spines or teeth around the mouth which are used in piercing. None of the nematodes can engulf large particles. And in all essentialities they are microphagus or juice feeder.

The food habits of nematodes offer an opportunity to visualise evolution in action because with little imagination one can easily realise how saprophagus and her­bivorous forms have given rise to plant para­sites, and saprozoic types have been evolved into animal parasites.

The nematodes exhibit maximum tole­rance of environmental variations. They pos­sess the power to withstand extreme cold, heat and desiccation. The vinegar eel (Turbatrix aceti) living in vinegar (5% acetic acid) can thrive successfully up to a concen­tration of 14% acetic acid.

Living nematodes have emerged from mosses which have been rewetted after keeping them dried for about 5 years. The shelled eggs are much more resistant and remain viable for years. The eggs of Ascaris can withstand prolonged immersion in 12% formaldehyde, saturated solutions of mercuric chloride and in many toxic salts. Embryonic stages are usually less resistant.

2. Structure of Phylum Nematoda:

There exists a considerable similarity of organisation and shape in different nema­todes. General shape of the body as the name implies is round, cylindrical and tapering at both ends. The length usually varies from 0.4 m (Ascaris) to 1 m (Dracunculus).

The largest of all nematodes is Placentonema gigantissima. The females of this species attain a length of 8.5 m, the diameter being 2.5 cm and they parasitise the placenta of sperm whales. The females of all nematodes are generally larger than the male.

3. Body Wall of Phylum Nematoda:

On the outer surface of the body wall there is a cuticle which is hard and flexible. It is resistant to many solvents and gastric juices. Next to the cuticle lies the ectoderm. In some forms like ascaris the ectoderm is represented by a syncitial protoplasmic mass. Beneath the ectoderm only longitudinal muscles are found. The individual cells of the muscle fibres are very peculiar.

They are elongated and may reach a length of 10 mm. One end of the cell is contractile while the other end which houses the nucleus is non- contractile. The non-contractile part keeps contact with a nerve fibre. The longitudinal muscle layer is not continuous and is ar­ranged into four longitudinal bands. Two of these bands are dorso-lateral while the other two are ventrolateral in position.

In some free-living species the ectoderm bears unicellular glands. These glands help the animals to attach themselves to the sub­stratum.

4. Body Cavity of Phylum Nematoda:

Body cavity is not a true coelom because it is not lined by epithelial layer derived from mesoderm. Some workers have called it ‘Pseudocoelom’. According to them, the absence of mesenchyme in between the body wall and digestive tract has stood in a good way for the evolution of a more organised digestive system. The pseudocoelom is filled with a fluid and the fluid acts as a ‘hydro­static skeleton’.

5. Digestive System of Phylum Nematoda:

Digestive tract is complete. The mouth is situated at the anterior end of the body and remains surrounded by lips. In the basic plan there are six lips. But as seen in Ascaris the number of lips is reduced to three due to fusion. In some forms there may be many lips due to splitting. The mouth leads to a buccal capsule. The capsule is cuticular and the inner wall of the capsule in some cases forms plates.

The capsule may house three or more teeth. In some cases a hollow ‘Stylet’ is formed inside the capsule by the fusion of these teeth. The buccal capsule leads to the pharynx. The pharynx, like the buccal capsule is also cuticular.

The lumen of the pharynx is triangular. The pharyngeal wall is a syncytium of radial muscle fibres and the wall contains many one-celled glands. In some, the pharynx acts as a sucking appara­tus. Pharynx leads to the intestine.

Intestine is straight and is made up of a single layer of epithelium. Rectum is short and opens into the anus. The anal opening is on the ventral surface of the posterior end of the body. The anus is cuticular and in some forms like Ascaris it acts as a cloaca in males only.

The intestine is much reduced in Mermis. Feeding habits of nematodes are variable. Free forms may be herbivorous, carnivorous or saprophagous. Parasitic forms live on the nutrients inside the host’s intestine or in the blood and disintegrated tissues of the host.

6. Excretory System of Phylum Nematoda:

Excretory system of nematodes is very different from other animals as it does not show any phylogenetic relationship to the protonephridial system of platyhelminthes or to the excretory system of any other higher phylum. The pseudocoelom in primitive forms houses a very peculiar cell called ‘renette cell’. It is a glandular cell with a tubular neck.

In primitive forms a pair of such cells open to the exterior through the excretory pore situated on the ventral sur­face of the anterior end. It is believed that the prevailing type of excretory system in ad­vanced nematodes is an evolutionary out­come out of the primitive renette cell.

Bilat­eral arrangement of these cells together with tubular outgrowths from the cells has given rise to a ‘H’-shaped system in some interme­diate forms like Oxyurida, Ascarida.

In most advanced forms anterior elongation of the excretory tubule has been lost resulting an inverted ‘U’-shaped system. The evolution of ‘U’-shaped excretory system from renette cells is encountered during the embryonic development of many parasitic nematodes (Fig. 15.24).

7. Respiratory and Circulatory Systems of Phylum Nematoda:

There is no special organ or organ system for respiration and circulation. The cuticle serves as the respiratory surface. Few intes­tinal parasites like Ascaris can live on oxy­gen in young stage but in adult stage they get oxygen by anaerobic splitting of nutrient materials present inside the intestine of the host.

To send the end-products of digestion to the cells of the body wall and other parts, there is no special organ for circulation. End- products of digestion are absorbed by the intestinal epithelium and from there they are passed onto the fluid of the pseudocoelom. From the fluid of the pseudocoelom nutrient materials reach the cells of the body wall.

8. Nervous System of Phylum Nematoda:

The nervous system is of simple type and consists of a ‘brain’ or nerve ring from which nerves extend to the anterior and posterior parts of the body. The nerve ring is present round the pharynx and is formed by two lateral pairs of ganglia.

From the ganglion a ventral nerve cord extends along the mid- ventral line and ends in a ganglion above the anus. Dorsal motor nerve and three pairs of lateral sensory nerves are also present.

9. Reproductive System of Phylum Nematoda:

In nematodes, sexes are separate. Adult males are smaller in size than the females, and in most males the posterior end of the body is curved. Male reproductive system consists of a single thread-like much coiled structure. The anterior part of the coil forms the testis, middle part forms the vas defer­ens and posterior part forms the seminal vesicle.

The testis may be monorchic (i.e. single testis, e.g., Ascaris) or diorchic means two testes when present in male reproduc­tive system in nematodes. The seminal vesi­cle continues as the ejaculatory duct and opens into the anus.

Inside the anus there is a pocket which contains a pair of eversible penial spicules. That means there is no male gonopore. The sperms are cone-shaped and have a broad base and a tapering apex. The sperms show amoeboid movement inside the body of the female.

The female reproductive structures con­sist of a pair of ovaries, a pair of oviducts and a pair of uteri. The two uteri unite to form a vagina which opens to the outside by a single female genital aperture situated on the ventral surface of the body.

If there is one tract containing single ovary, oviduct and uterus called monodelphic but didelphic and polydelphic also occur. In Ascaris there are two tracts containing paired ovaries, oviducts and uteri. In Trichinella, the female reproductive structure is single.

10. Development of Phylum Nematoda:

Eggs are fertilized in the vagina of the female. Soon after fertilization the eggs be­come enveloped by three membranes—an outer albuminous covering, a middle chori­onic covering of chitinous nature and an inner vitelling membrane. In hook-worms the outermost layer is absent. Cleavage is of determinate type.

In Ascaris the first cleav­age divides the egg into a somatic cell and a germ cell. Blastula is a coeloblastula. Epiboly is the usual mode of gastrulation. In Ascaris and Trichuris fertilized eggs leave the body of the mother and host before segmen­tation.

Segmentation starts outside the body and later on it becomes infective. In Enterobius the eggs leave the body of the mother and host in segmented condition. In Ancylostoma, the eggs leave the body of the mother and host in partially segmented condition.