CHAPTER XXXII 



WORMS AND OTHER CONTROLLABLE DISEASE AGENTS 



Questions. 1. Can people support parasites without knowing it ? 

 2. What effects produced by parasites are not commonly recognized as 

 disease? 3. Can food that has not been exposed to dust or dirt still 

 carry disease germs ? 4. Can disease be caused by organisms larger than 

 microbes (bacteria and protozoa) ? 5. Are any worms dangerous to 

 health ? If so, what kinds ? How ? 



248. Worms. Under older systems of classifying animals 

 the name worms or the Latin form vermes was applied to a 

 mixed group that had very little in common except the shape 

 of worms (see pages 8i, 82). The true ringed worms correspond 

 to the more common idea of a worm ; some of these are of great 

 practical importance (see sections 330, 332). The rotifers, or 

 wheel animalcules, are mostly microscopic and of no direct im- 

 portance to us. In the other two branches, flatworms and round- 

 worms, there are many species that live parasitically in the 

 bodies of higher animals, and so they come to be of importance 

 to us as injurious either to human beings or to domestic animals. 



The liver fluke destroys many sheep by its ravages upon the liver of 

 the host. The parasite reproduces sexually, discharging eggs and sperms. 

 Fertilization takes place within the liver of the host, and then the em- 

 bryo finds its way into the intestine by way of the bile duct. Discharged 

 from the body of the host, the young animal swims about in water, or 

 in dew upon grass, until it comes across a snail, into the body of which 

 it penetrates. Here it undergoes several changes, which include a rapid 

 multiplication by budding, and an escape from the snail into the grass, 

 to the blades of which it attaches itself. From the grass it is taken 

 up by grazing sheep and finds its way into the liver, again by way 

 of the bile duct. The losses occasioned by this parasite were formerly 

 enormous, for we knew neither cure nor prevention. But since we 

 know that there can be no liver fluke in our sheep unless there are 



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