228 



GENERAL ZOOLOGY 



just behind the head, and as they grow older and larger are 

 pushed backward by newer ones. The tapeworm is an inter- 

 esting example of a parasite that has become so completely 

 dependent on its host that one of the most important systems 

 of the body has totally disappeared, namely the digestive 

 system. The parasite maintains its hold on the inner wall of 

 the intestine by four sucking-disks on the head ; the body floats 

 free in the intestine, and food can be absorbed from all sides. 

 Every tapeworm has in each division both 

 eggs and spermatozoa. The spermatozoa ferti- 

 lize the eggs in the same division. When the 

 embryos reach a certain stage, a few terminal 

 divisions of the adult separate from the 

 rest, and pass out with the undigested 

 portion of the person's food. Sometimes 

 the embryos, inclosed in their thick mem- 

 branes, are swallowed by cattle while 

 drinking at pools. In the intestine the 

 membrane of the embryo is dis- 

 solved, and the freed larva bores 

 its way through the wall of the 

 intestine, and finally comes to 

 rest in the muscular tissue. It 

 remains there until the muscle 

 is consumed in a partially raw 

 state by man ; then in his intestine 

 the young animal develops into 

 the adult, mature tapeworm. An- 

 other species of tapeworm (Tcvnia 

 so'lium), found in human beings, 

 comes from uncooked pork. 



FIG. 112. Tapeworm. Left-hand 

 drawing, reduced ; right-hand 

 drawing, enlarged. (After 

 Leuckart) 



Tapeworms are not necessarily fatal. The annoyance is con- 

 siderable, however, until, by a period of fasting jgid medical 

 treatment, the life of the head of the parasite is destroyed. 



