The life cycle of a tapeworm. (1) The eggs are taken in with filthy food by the pig; (2) man eats undercooked pork by means of which the bladder worm (3) is transferred to his own intestine (4).
Cestodes or Tapeworms.—These parasites infest man and many other vertebrate animals. The tapeworm (Tænia solium) passes through two stages in its life history, the first within a pig, the second within the intestine of man. The developing eggs are passed off with wastes from the intestine of man. The pig, an animal with dirty habits, may take in the worm embryos with its food. The worm develops within the intestine of the pig, but soon makes its way into the muscle or other tissues. It is here known as a bladderworm. If man eats raw or undercooked pork containing these worms, he may become a host for the tapeworm. Thus during its complete life history it has two hosts. Another common tapeworm parasitic on man lives part of its life as an embryo within the muscles of cattle. The adult worm consists of a round headlike part provided with hooks, by means of which it fastens itself to the wall of the intestine. This head now buds off a series of segmentlike structures, which are practically bags full of sperms and eggs. These structures, called proglottids, break off from time to time, thus allowing the developing eggs to escape. The proglottids have no separate digestive systems, but the whole body surface, bathed in digested food, absorbs it and is thus enabled to grow rapidly.
Trichinella spiralis imbedded in human muscle. (After Leuckart.)
Roundworms.—Still other wormlike creatures called roundworms are of importance to man. Some, as the vinegar eel found in vinegar, or the pinworms parasitic in the lower intestine, particularly of children, do little or no harm. The pork worm or trichina, however, is a parasite which may cause serious injury. It passes through the first part of its existence as a parasite in a pig or other vertebrate (cat, rat, or rabbit), where it lies, covered within a tiny sac or cyst, in the muscles of its hosts. If raw pork containing these worms is eaten by man, the cyst is dissolved off by the action of the digestive fluids, and the living trichina becomes free in the intestine of man. Here it reproduces and the young bore their way through the intestine walls and enter the muscles, causing inflammation there. This causes a painful and often fatal disease known as trichinosis.
The Hookworm.—The discovery by Dr. C. W. Stiles of the Bureau of Animal Industry, that the laziness and shiftlessness of the "poor whites" of the South is partly due to a parasite called the hookworm, reads like a fairy tale.
The people, largely farmers, become infected with a larval stage of the hookworm, which develops in moist earth. It enters the body usually through the skin of the feet, for children and adults alike, in certain localities where the disease is common, go barefoot to a considerable extent.
A complicated journey from the skin to the intestine now follows, the larvæ passing through the veins to the heart, from there to the lungs; here they bore into the air passages and eventually work their way by way of the windpipe into the intestine. One result of the injury of the lungs is that many thus infected are subject to tuberculosis. The adult worms, once in the food tube, fasten themselves and feed upon the blood of their host by puncturing the intestine wall. The loss of blood from this cause is not sufficient to account for the bloodlessness of the person infected, but it has been discovered that the hookworm pours out a poison into the wound which prevents the blood from clotting rapidly (see page [315]); hence a considerable loss of blood occurs from the wound after the worm has finished its meal and gone to another part of the intestine.
