PARASITISM 177 



2. The proglottids usually break or disintegrate, and the minute eggs 

 become scattered over the ground. If the proglottids or the eggs are 

 accidentally eaten by a cow, the eggs will hatch, and the six-hooked larvae 

 will be liberated in the intestine. The larvae bore their way through the 

 wall of the alimentary canal and, migrating through the tissues, reach the 

 voluntary muscles, especially the muscles of mastication, where they 

 become encysted. 



3. The encysted larva in from three to six weeks develops a bladder- 

 like sac filled with a clear watery fluid and so becomes a bladder worm, 

 or cysticercus. One side of the wall of this sac gradually becomes thick- 

 ened, is inverted, and forms a hollow papilla projecting into the sac. 

 From the outer surface of the wall and in the cavity of this papilla 

 develops a scolex, with hooks and suckers. 



4. If the flesh of the cow has been insufficiently cooked and is eaten 

 by man, the cysticercus is freed in the ahmentary canal, the papilla 

 becomes everted, the scolex and neck project from the side of the bladder, 

 and the latter is destroyed. 



5. This scolex attaches itself to the wall of the intestine, begins to 

 develop proglottids, and thus a new tapeworm is produced. 



204. Behavior of Parasites. — The life history of these parasites shows 

 a changing behavior in the passage from one larval form to another and 

 from one host to another. Only changing chemical and contact reactions 

 can explain the entrance of a miracidium into the body of the snail and 

 the leaving of it by the cercaria. Changing physiological states undoubt- 

 edly accompany this changing behavior. 



205. Practical Aspects. — A knowledge of the life histories of such 

 parasites as those that have been considered is evidently of great value, 

 since it dictates the character of the control measures which must be 

 taken. It is evident, for example, that if the fluke infection is discovered 

 in a flock of sheep, all infected animals should be removed from the flock. 

 It is also evident that to avoid infection, a flock should be removed to a 

 pasture which contains no standing water and, if possible, to one which 

 has never had sheep upon it before. Destruction of snails by copper 

 sulphate is an aid in control of the parasite. In man the liver fluke may 

 produce no serious symptoms, but in some cases it has produced fatal 

 results. It has been removed from superficial abscesses. In the sheep 

 heavy infection causes serious functional disturbances and often death, 

 though the parasite may pass out of the host and spontaneous recovery 

 ensue. 



Man is subject to infection by tapeworms acquired from pork, fish, 

 and, less frequently, other animals, but the beef tapeworm is the most 

 common human tapeworm in this country. Most human tapeworm 

 infection can be avoided by measures that will insure careful meat inspec- 

 tion and the consumption of no meat that is not well cooked. 



