PARASITIC METAZOA If we omit the insects which as occasional raiders for food act unwittingly as vectors of animal parasites (see ENTOMOLOGY, MEDICAL) and other arthropods, which are more permanent para sites of the skin, inducing irritation diseases such as pediculosis and scabies, the chief metazoan parasites are those commonly termed "parasitic worms." It should be noted, however, that the term "worm" is used very loosely for a large variety of animals which are, zoologically, unrelated: the "blind-worm" or "slow worm," "glow-worm," "round-worm," "wire-worm," "tape-worm" and "earth-worm" belong to different subdivisions of the animal kingdom. The parasitic worms found in animals belong either to the phylum Nemathelminthes or to the phylum Platyhelminthes (qq.v.).
Nematodes and Disease.—The class Nematoda is, however, by far the most important division of the Nemathelmia. It contains many thousand species of which a considerable proportion are free living in the soil and decaying organic matter. Specialized species are found in malt vinegar and paper-hangers' paste. All these forms are of microscopic size. Closely allied to them is a small group which is parasitic in many cultivated plants (Plate, fig. 4), e.g., potato, tomato, clover, wheat, oats, tulip, onion, etc., and cause considerable economic losses in Great Britain. On the European continent the sugar-beet is severely attacked, while in the Tropics coffee, bananas, oranges, rice and tobacco are fre quently infested by allied "nemas" or "eel-worms." Nematodes occur in practically all vertebrate animals. These round-worms are usually of larger size than those found free living or in the cultivated plants and may attain a length of two to three feet, e.g., Dracunculus medinensis, the Guinea worm (q.v.). The nematodes are unisexual, i.e., there are separate males and females. The eggs are simple and such yolk as may be present is incorporated as globules in the protoplasm of the ovum. Larval
development in the various species is essentially the same and proceeds by a series of metamorphoses marked by ecdyses. Some of these changes occur during the period of delay outside the body after the passage of the egg or embryo from the definitive host. The remainder occur after reinfection. In some cases the extra corporeal phase is undergone within the thick egg-shell as in the round-worms of dogs, cats, pigs and man. Infection results from the swallowing of these embryonated eggs as contaminations of food. In numerous other cases, however, as in the large group of so-called "bursate" nematodes (which include the hookworm of man and the red-worms of horses) the egg-shell is thin and the developing embryo emerges therefrom. After a short period outside the body, the hatched worm undergoes its further meta morphosis into the infective stage as a microscopical form in the soil, to re-enter its host later by the mouth. In a few instances re infection of the definitive host is accomplished by the penetration of the skin as in hookworm disease (q.v.).
In a third group of nematodes, viz., those which live in the tissues of the body and are commonly called filarial worms, the embryos are discharged into the lymph or blood and are sucked up by biting insects. In the bodies of these insects the same es sential metamorphosis takes place and the infection subsequently re-enters another definitive host through insect bites.
In a fourth group larval development takes place in an inter mediate host eaten by the definitive host, e.g., Trichinosis (q.v.). After gaining entry into the definitive host either by the mouth or through the skin, the larval nematodes in some cases, not in all, undergo strange migrations through the blood stream to reach their final habitat and during the course of these wanderings they may give rise to serious damage. Heavy losses are incurred in young pigs as result of pneumonia set up by the migration through the lungs of the young stages of the common round-worm Ascaris suilla. There is a similar migration on the part of young hook worms en route from the skin to the bowel, but there is relatively less damage as the number passing at a time is usually small and the associated symptoms are merely a transient cough. Similarly urticarial irritation is induced when large numbers of hookworm larvae penetrate the skin at the same time.