. A civic biology : presented in problems. Biology; Sanitation. THE ECONOMIC IMPORTANCE OF ANIMALS 227 of all mammals. Rats are believed to carry bubonic plague, the " Black Death " of the Middle Ages, a disease estimated to have killed 25,000,000 people during the fourteenth century. The rat, like man, is susceptible to plague ; fleas bite the rat and then biting man transmit the disease to him. A determined effort is now being made to exterminate the rat because of its connection with bubonic plague. Other Parasitic Animals cause Disease. - - Besides parasitic protozoans other form
. A civic biology : presented in problems. Biology; Sanitation. THE ECONOMIC IMPORTANCE OF ANIMALS 227 of all mammals. Rats are believed to carry bubonic plague, the " Black Death " of the Middle Ages, a disease estimated to have killed 25,000,000 people during the fourteenth century. The rat, like man, is susceptible to plague ; fleas bite the rat and then biting man transmit the disease to him. A determined effort is now being made to exterminate the rat because of its connection with bubonic plague. Other Parasitic Animals cause Disease. - - Besides parasitic protozoans other forms of animals have been found that cause disease. Chief among these are certain round and flat worms, which have come to live as parasites on man and other animals. A one-sided relationship has thus come into existence where the worm receives its living from the host, as the animal is called on which the parasite lives. Consequently the parasite frequently becomes fastened to its host during adult life and often is reduced to a mere bag through which the fluid food prepared by its host is absorbed. Sometimes a complicated life history has arisen from their parasitic habits. Such is seen in the life history of the liver fluke, a flatworm which kills sheep, and in the tapeworm. Cestodes or Tapeworms. - These para- sites infest man and many other vertebrate animals. The tapeworm (Tcenia 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 tape- worm. Thus during its complete life history it has two hosts. Another common tapeworm parasit
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