PHYLUM NEMATODA (nemtuos, thread)
Nematodes are roundworms. They are some of the most abundant animals on earth. Number or round worm species is from 16, 000 to 500,000. There are following characteristic of these animals:
Roundworms feed on every source of organic matter. They feed on rotting. substances in the living tissues of other invertebrates, vertebrates and plants.
They are from microscopic to several meters long.
Many nematodes are parasites of plants or animals. Some are free- living in marine, freshwater, or soil habitats. Some nematodes play an important role in recycling nutrients in soils and bottom sediments.
There are two common characteristics between nematodes and arthropods:
(a) Nematodes and arthropods lack cilia.
(b) The sperm of nematodennd arthropods are amoeboid.
They are triploblastic, bilateral, vermiform (resembling a worm in shape; long and slender), unsegmented, pseudocoelomate.
Their body is rounded in cross section.
Body is covered by a cuticle. Growth takes place by molting.
They have complete digestive tract. Mouth is surrounded by lips Sense organs are present on lips.
They have unique excretory system. It is composed of one or two renctte cells or a set of collecting tubules Their body wall contains only longitudinal muscles.
The body nematode is slender, elongate, cylindrical, and tapered at both ends. It has following external structures:
1. Cuticle: The nematodes are most successful due to presence of cuticle on body wall. This cuticle is non cellular and collagenous.. Cuticle is also present in the foregut, hindgut, sense organs and parts of the female reproductive system. The cuticle may be smooth. Or it may contains spines, bristles, papillae, warts or smooth. Such structures have taxonomic importance. Cuticle is composed of three primary layers. These layers are cortex, matrix layer, and basal layer. The cuticle maintains internal hydrostatic pressure. It provides mechanical protection. It resists digestion by the host in parasitic species. The cuticle is molted four times during maturation.
Fig: Phylum Nematoda. Internal anatomical features of an (a) female and (b) male Rhabditis. (c) Section througb a nematode cuti showing the various layers. (d) Cross section through the region of the muscular pharynx of a nematode.
2. Epidermis: Epidermis or hypodermis is present beneath the cuticle. It surrounds the pseudocoelom. The epidermis may be syncytial. Its nuclei are present in four epidermal cords (one dorsal, one ventral, and two laterals). These nuclei project inward.
3. Muscles: They have only longitudinal muscles. They are the principal means of locomotion in nematodes. Contraction of these muscles produce undulatory waves. These waves pass from the anterior to posterior end of the animal. It causes characteristic thrashing movements. Nematodes lack circular muscles. Therefore, they cannot crawl.
3. Lips and skeleton: Some nematodes have lips surrounding the mouth. Some species develop spines or teeth on the lips. In others, the lips have disappeared. Some roundworms have head shields for protection.
4. Sense organs: Sensory organs are amphids. phasmids and ocelli. The nematods are lassi lied on the basis of presence or absence of these sense organs.
a) Amphids are anterior depressions in the cuticle. It contains modified cilia. It functions in chemoreception.
b) Phasmids are near the anus. It also functions in chemoreception.
c) Paired OceIli (eyes) are present in aquatic nematodes.
The pscudocoelom of nematode is a spacious, fluid-filled cavity. It contains visceral organs. It forms a hydrostatic skeleton. The body muscles contracts against the pseudocoelom. Thus fluid generates an equal outward force in all directions. Therefore all the nematodes are round.
FEEDING AND THE DIGESTIVE SYSTEM
Nematodes can feed on a wide variety of foods. ‘They may be carnivores, herbivores, omnivores, or saprobes (saprotrophs), or parasitic species. The parasitic species feed on blood and tissue fluids of their hosts.
Nematodes have a complete digestive system. It consists of a mouth. buccal cavity; muscular pharynx; long tubular intestine. Mouth may have teeth, jaws, or stylets (sharp, pointed structures). Digestion and absorption occur in intestine. They have short rectum and anus. Hydrostatic pressure in the pseudocoelom and the pumping action of the pharynx push food through the alimentary canal.
OTHER ORGAN SYSTEMS
The osmoregulation and exeretion of nitrogenous aste products (ammonia, urea) take place by two nique systems.
(a) Glandular system: It is present in aquatic species. It consists of ventral gland cells called renettes. It is present posterior to the pharynx. Each gland absorbs wastes from the pseudocoelom. It opens outside through an excretory pore.
(b) Tubular system: Parasitic nematodes have a more advanced system. It is called the tubular system. It is developed from the renette system. In this system, the renettes unite to form a large canal. This canal opens outside through an excretory pore.
The nervous system consists of an anterior nerve ring. Nerves extend anteriorly and posteriorly. They may connect to each other via commissures. Certain neuroendocrine secretions are involved in growth, molting, cuticle formation. arid metamorphosis.
REPRODUCTION AND DEVELOPMENT
Most nematodes are dioecious and dimorphic (different shapes). The males are smaller than the females. The gonads are long and coiled. They lie freely in the pseudocoelom.
Female reproductive organs
The female system consists of a pair of convoluted ovaries. Each ovary is continuous with an oviduct. The proximal end of oviduct is swollen to form a seminal receptacle. Each oviduct becomes a tubular uterus. The two uteri unite to form a vagina. Vagina opens to the outside through a genital pore.
Male reproductive system
The male system consists of a single testis. It is continuous with a vas deferens. Vas deferens opens into a seminal vesicle. The seminal vesicle connects to the cloaca. Males contain a posterior flap of tissue called a bursa. The bursa helps in the transfer of sperm to the female genital pore during copulation.
Fig: Nematode Reproductive Systems The reproductive systems of (a) female and (b) male nematodes, such as Ascaris.
Fertilization and Development
Fertilization takes place during copulation. The hydrostatic forces in the pseudocoelom move each fertilized egg to the gonopore. The number of eggs produced varies with the species. Some nematodes produce only several hundred eggs. But others may produce h indreds of thousands daily. Some nematodes give birth to larvae (ovoviviparity). External factors like temperature and moisture influence the development and hatching of the eggs. Hatching produces a larva. The larva has most adult structures. The larva (juvenile) undergoes four molts. In some species, the first one or two molts may occur before the eggs hatch.
SOME IMPORTANT NEMATODES PARASITES OF HUMANS
Parasitic nematodes have a number ‘of evolutionary adaptations. These adaptations are:
1. They have high reproductive potential.
2. Their life cycles increase the chance of transmission from one host to another.
3. They develop enzyme resistant cuticle, resistant eggs. and encysted larvae.
4. Only one host is involved in the life cycle Of nematods. Therefore Nematode life cycles are not as complicated like cestodes or tremoteds.
Ascris lumbricoides: The Giant Intestinal Roundworm of Humans
Approximately 800 million people are infected with Ascaris throughout the world. Ascaris live in the small intestine of humans. They produce large numbers of eggs. These eggs pass out with feces. A first-stage larva develops rapidly in the egg. It molts and forms second-stage larva. Second larva is the infective stage. Human may ingests embryonated eggs. They are hatched in the intestine. The larvae penetrate the intestinal wall. Blood carries it to the lungs. They molt twice in the lungs and move up into the trachea, and are swallowed. The worms become sexually mature in the intestine. It mates and begins egg production.
Enterobius vermicularis: The Human Pinworm (enteron, intestine + bios, life)
Pinworms are the most common roundworm parasites in the United States. Adult Enterobius present in the lower region of the large intestine. The gravid females move out of the caecum at night. It reaches into the perianal area (area around anus). They deposit egg there. These eggs develop first stage larva. The eggs then fall. The human ingest the eggs and they are hatched. The larva molt four times in the small intestine. It moves into large intestine. Mating takes place between the male and female and again agg production starts.
Necator americanus:The New World or American hookworm, Necator americanus is found in the southern United States. The adults live in the small ntestine. They hold the intestinal wall with teeth and feed on blood and tissue fluids.
Female may produce as many as 10,000 eggs daily. These eggs pass out of the body with feces. An egg hatches on warm and moist soil. It releases a small rhabditiform larva. It molts and becomes the infective filariform larva. Filariform larva penetrates the human’s skin between the toes. The larva burrows through the skin. It enters into the rculatory system. It reaches the small intestine and becomes adult.
Trichinella spiralls: The Porkworm (Or. trichinos, hair)
Adult Trichinella live in the mucosa of the small intestine of humans and other omnivores Lille pigs. The adult females produce young larvae in the intestine. It then enters into the circulatory system. It then reaches in skeletal muscles. The young larva forms cyst in the skeletal muscles. The muscle remains infective for many years. It causes disease called trichinosis. Another host ingests infective meat (muscle). Humans are infected by eating improperly cooked pork (flesh of pig) products. After ingestion the larvae form cyst in the stomach. It reaches in to the small intestine. They molt four times in intestine and develop into adults.
Wuchereria spp: The Filarial Worms(L fihium, thread)
Over 250 million humans are infected with filarial worms in tropicalcountries. Two species of human filarial worms are W bancrofti and W. malayi. These are elongated thread like nematodes. They live in the lymphatic system and block the lymph vessels. The lymphatic vessels return tissue fluids to the circulatory system. Therefore, fluids and connective tissue accumulate in peripheral tissues. This accumulation causes the enlargement of various appendages. This condition is called elephantiasis.
Life cycle of Wuchereria
1. The adult filarial nematodes copulate in the lymphatic vessels. They produce larvae called microfilarie. The microfilariae are released into the bloodstream of the human host. They migrate to the peripheral circulation at night.
2. A mosquito feeds on a human and ingests the microfilariae. The microfilariae migrate to the thoracic muscles of mosquito. There they molt twice and become infective.
3. The mosquito injects it proboscis into a healthy person. It transfers the infective third-stage larvae into the blood of the healthy human. The larvae enter the lymphatic vessels. Final two molts take place in it and it becomes adult.
A filarial worm Dirofilaria immitis is common in the United States. It is a parasite of dogs. The adult worms live in the heart and large arteries of the lungs. Its infection is called heartworm disease. Filarial worms are difficult to eliminate and they can be fatal. All dogs should be given heart- worm medicine.