174 METAZOAN PHYLA 



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 alimentary 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. 



The life history of the tapeworm seems best interpreted as including 

 an asexual generation of proglottids produced from the scolex and neck 

 by a process of budding. These proglottids in turn reproduce sexually. 

 Thus the life history illustrates the phenomenon of metagenesis. 



202. 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. 



203i Practical Aspects. — A knowledge of the hfe 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 when infection is known to exist, 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. The character of the 

 life history of the fluke explains the freedom from infection in the flocks 

 on the dry ranges of the western states. 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 

 it 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 

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



