Kingdom: Animalia, Phylum: Nematoda
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The Nematoda, formally known as the Nemathelminthes, are unsegmented, cylindrical worms. Although over 1,200 species have been identified, it is widely believed that these are just a small fraction of the total species in existence. Taxonomy of nematodes has been very difficult due to the relatively few diagnostic characters available. Most species are free-living, and there are also many parasitic species, which are the best known. Nematodes occur in marine and freshwater, in the soil, and as parasites on a wide array of animal and plant species. The body cavity is a Pseudocoelom and their size varies from microscopic up to two meters in length.
Characteristics of The Phylum
Nematoda are uniform morphologically and there are many variations in the small size range. More detailed morphology is found on the head and tail ends, and thus the teeth and copulatory bursae are extensively used in identification. Although there are a few hermaphroditic forms, most species have separate sexes. Some species alternate as free-living and parasitic, some are facultative parasites and some obligate parasites. Most parasitic species do not have intermediate hosts. When there are intermediate hosts these are almost always arthropods.
Type Animal = Ascaris lumbricoides
Habitat.-- This species inhabits the intestine of humans or hogs, but it is not cross-infective between the two hosts.
Body Plan.-- There is a mouth and an anus. The female being larger and the male smaller with a curled tail express sexual dimorphism. Lateral lines are distinguished on the sides. The female's genital pore is inconspicuous on the ventral surface about one-third the way back from the anterior end. The male genital pore is located at the posterior end and is the same opening as the anus.
Body Surface.-- A heavy cuticle is present. The epidermis, which is often called a hypodermis because it is located under a cuticle, is a syncytium, which is not as prevalent as in the Acanthocephala.. There is a longitudinal muscle on the inside of the epidermis. It is shaped like a dugout canoe with the wood portion representing the contractile part and the hollow portion the cytoplasm. The cytoplasmic part projects into the body cavity, while the contractile part faces the epidermis.
There are no circulatory muscles. A pseudocoelom occurs inside the muscle layer, and suspended within are the digestive organs.
Food & Digestion.-- The food is the same food as that of the host, which is partially predigested. The mouth leads into a heavy muscular pharynx. The intestine is very flat and runs the entire length of the animal without muscular walls. There is no food absorbed through the body wall.
Circulation.-- No circulatory system is present.
Respiration.-- There are no special respiratory structures, and respiration occurs by diffusion. The roundworms are largely anaerobic.
Excretion.-- One pair of excretory canals lie on the lateral lines. The epidermis is thickened at the site of the lateral lines and bears a canal. Each canal consists of one single cell with a hollow cavity. A cross-connection runs between the lateral canals.
An excretory pore lies mid-ventrally just back of the mouth. Rossett cells (one per each canal) are associated with each tube in other species of nematodes. There is no sign of flame cells.
Cilia or Flagella.-- There are neither cilia nor flagella in any part of the body. Even the sperm lack flagellated tails and are amoeboid.
Support & Protection.-- The cuticle, which is constantly replaced as in the Cestoda, furnishes support and protection.
Locomotion .-- Movement is back and forth in a figure 8. The lack of circular muscles restricts movements greatly. Ascaris is not attached to the wall of the intestine, so it must remain constantly in motion forward in the gut.
Sensitivity.-- A few sense organs occur in parasitic nematodes and almost none in Ascaris. The fleshy lips are sense organs, and tactile and olfactory papillae occur at both ends. Free-living forms may possess eyes.
Nervous System.-- There is a continuous ring around the pharynx, which is analogous to the cerebral ganglion in flatworms. Six anterior and six posterior nerve cords run from the ring, but only two, the mid-dorsal and mid ventral, are of any consequence.
Reproduction.-- In the male the testes is a very long coiled tube. The vas deferens is the thicker portion of the tube, while the seminal vesicle is an even thicker part. The seminal vesicle opens into the gut at a cloaca. The anus serves for both the digestive and reproductive systems. Peneal spicules occur at the posterior end and serve as claspers. Some species have a copulatory bursa like the Acanthocephala.
The female's ovaries are two coiled tubes. Each oviduct enlarges abruptly to form a uterus. The uterus runs fairly straight to the oviduct. Two uteri units are located just before the genital pore.
During fertilization and development, the sperm are introduced into the genital pore of the female. The amoeboid sperm unite with eggs as they come down the oviduct. The fertilized egg forms a heavy proteinaceous shell and is warty or bumpy and extremely resistant. The eggs develop very little in the body of the female roundworm. They pass out of the body through the genital pore. There can be millions of eggs from one individual roundworm.
Life Cycle.-- Infection is direct and there is no intermediate host. The host swallows an egg and the juvenile worm hatches from the egg. The worms burrow to the vascular system, enter the blood and become stuck in capillaries of a hog's lungs. They burrow through the capillaries and into the air spaces of the lungs. Here they are coughed up and swallowed, entering the intestine for the second time where permanent residence is assumed. The process takes about ten days
Medical Importance.-- In the host there is an occlusion of the gut and the bile ducts become blocked. Secondary respiratory infections occur in the damaged lungs of pigs. However, Ascaris is generally not too important economically.
Several genera of Nematoda are of importance as disease incitants. Trichinella spinale attacks hogs and humans and is intertransferrable. The larvae burrow into the blood stream where they then encyst in the muscles. The disease is called Trichinosis.
The genus Wuchereria is transmitted by mosquitoes and causes the diseases called Filariasis. In extreme cases "Elephantiasis" can result. The adult parasites cause a stoppage of lymph flow while the larvae live in the host's blood.
The genus Dracunculus is the Guiana worm, which is known as the Fiery Serpent in the Bible. The worm is transmitted by the genus Cyclops. The adult female lives subcutaneously in the host where it may assume several feet in length. Body fluids of the worm are extremely caustic. Extraction is accomplished by winding the worm on a stick.
Plant Pests.-- Many soil-inhabiting nematode species are important in agriculture because they attack the roots of crop plants, causing death or deformation. Soil steam treatment or chemical fumigation has been widely used for control.
Please see following plates for Example Structures of the Nematoda:
Plate 36 = Phylum: Nematoda: Ascaris lumbricoides female
Plate 37 = Phylum: Nematoda: Ascaris lumbricoides
Plate 38 = Phylum: Nematoda: Dioctophyme venale, Dracunculus medinense, Enterobius
vermicularis, Ascaris sp.