Roundworms is a common or semi-scientific designation for certain slender cylindrical or spindle-shaped worms mostly parasitic. The term is sometimes applied to the phylum Nemathelminthes and again restricted to the class Nematoda, also called the true roundworms. Some of the species included in these subdivisions are very slender and to such the term threadworms is applied though this designation is regarded by some as equivalent to roundworms. In medical usage the term roundworm is employed to designate especially a few well-known human parasites that fall within the general group of Nematoda. The most famous is the stomach worm or common roundworm, Ascaris lumbricoides, which as the name suggests has a certain external likeness to the earthworm.

It has been known since the days of Egyptian and Greek medicine as a frequent guest in the human intestine, especially in children. The parasite is 15 to 40 centimeters long by three to five millimeters thick, the male being much smaller than the female and showing two projecting spicules near the posterior end. It is distributed over the entire world, though more abundantly in the warmer regions and in the country than in colder latitudes and in the cities.

The eggs are covered with a very heavy triple shell, the outer, layer being rough and mammillated. Within this the embryo may live safely for several years if no chance is offered for further development. The latter begins when the egg is taken into the stomach of the host and its introduction is due to the use of contaminated vegetables or fruits or of impure drinking water. It has been shown recently that the embryos which hatch out in the stomach penetrate to the lung of the host and after a period there return to the intestine where they become sexually mature and the life cycle is complete.

While the presence of such parasites may not be fatal, they exert a toxic influence on the host, and give rise to reflex nervous symptoms of a serious character so that prompt measures should be taken under the advice of a physician for their removal. Similar species are found in the pig, sheep, cat, dog and horse, and some of them rarely occur in man. The pinworm, Oxyurias vermicularis, is less frequently spoken of in medicine as a roundworm though it is a closely related member of the group of Nematoda. It inhabits the rectum and like Ascaris is most common in children. It is a much smaller worm, measuring only 3 to 12 millimeters in length. It has been known from remotest antiquity as a cosmopolitan parasite of man and is more abundant in the cities than in the country. An intense itching is induced by its presence and if large numbers are present this results in a serious inflammation which may lead to onanism or other similar disturbances.

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