Whilst the topic of parasitic worms will always remain near the low-end of high-brow conversation, who among us isn't fascinated by this netherworld of the biological sciences. Parasites are simply animals or plants, which live on or at the expense of other animals. The subject of Parasitism is vast, so our focus here will be primarily two "worm" phyla, the flatworms (Platyhelminthes) and the roundworms (Nemathelminthes).

Flatworms include non-parasitic worms such as planaria, which live in water, and parasitic flukes and tapeworms. The human liver fluke infects over 75% of the people in parts of Japan, China, and South Asia. The adult fluke, about 3/4 inch long, lives in the bile ducts of the liver; its eggs (1) pass from the body in the feces. The eggs, containing larvae, are eaten by water snails (2) and then develop into another form, which passes into the water. They then bore into the bodies of fishes (3). When raw fish is eaten (4) - as is common in the Orient - the young worms swim from the intestine into the fine branches of the bile duct and grow to maturity.

Blood flukes with similar life histories infest blood vessels of animals and humans in parts of Africa and the Orient. A related fluke larva burrows into the skin causing "swimmer's itch," making swimming almost impossible in some lakes of the North Central States.

Tapeworms are marvelously adapted to a parasitic existence inside intestines. They are highly specialized with no digestive tract (they live on food digested by the host) and only rudimentary nervous, muscular and excretory systems. However, their reproductive system is very elaborate. Each tapeworm produces both sperm and eggs.

The beef tapeworm, a parasite of man, has a head with sucking discs, which aid the worm in hanging to the intestine wall. Then follow many sections, all alike, which absorb food and produce myriads of sperms and eggs. Matured sections, gorged with fertilized eggs and embryos break off and pass out with the feces. Cattle may pick them up when grazing. If so, the tiny six-hooked larvae are liberated from the old section walls and bore through the cow's intestines to enter the blood. They move into the cow's muscles and there form cysts (bladder worms). If uncooked or undercooked beef containing cysts is eaten by a person, the bladder worm breaks out of the cyst, attaches to the intestine, and develops into a new worm.

Roundworms (Nemathelminthes) are found almost everywhere. Some are microscopic; one is 14 inches long. A spadeful of rich garden soil may contain millions. "Vinegar eels" and "threadworms" are other examples. But over 50 different species parasitize man and cause such diseases as hookworm, trichinosis and elephantiasis. Few plants or animals are spared. Many crop plants suffer from nematode infections of their roots.

Intestinal Roundworms, of which Ascaris is a common form, infect wild and domestic animals and man. Eggs drop to the ground in animal feces. Humans are infected by eating contaminated food. The diagram at right shows the life history of Ascaris. Eggs hatch into larvae in the intestinal tract and bore through intestine wall (1). Blood carries larvae to lungs (2), where they grow. They are then coughed up or crawl up windpipe to back of mouth (3), are swallowed (4), and mature in intestine (5). A female worm living in the intestine may have millions of eggs in its body. These pass out in feces (6) and, if eaten by other animals, start the life cycle over again.

Hookworm, once a serious disease in the southern U.S., is still common in the warm areas of the world. Infected people lack energy. The hook-worm life cycle is like that of intestinal roundworms except that infection occurs as larvae hatched in the warm soil bore through the skin, usually on the soles of the feet, to enter the blood.

Trichinosis is caused by the trichina worm, which infects pigs, rats and man. Pigs get the disease by eating infected rats or raw garbage. Man gets it from eating poorly cooked pork which contains larvae in microscopic cysts. Thousands of larvae may be in a single slice of infected pork. The cysts are digested away and the worms reproduce in the human intestines. Larvae then bore through intestine walls into the blood stream. They burrow into muscles all over the body and form cysts. Millions of larvae may be in the body at one time, causing sever pain, weakness and even death.

source: Zoology, A Golden Science Guide, 1958

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