THE PLANARIANS, FLUKES, AND TAPEWORMS 265 



the scale through man that are free from attack by the 

 various species of these worms. 



The Beef Tapeworm of Man. The best-known tapeworm is 

 the one that is sometimes found to inhabit the intestine of 

 man, Tse'nia sagina'ta (Fig. 138). 

 This parasite frequently grows 

 to the length of many feet. The 

 figure seems to show that the 

 body is divided into somites, but 

 these divisions are not consid- 

 ered true somites. The head of 

 the animal is, except for the nar- 

 row neck behind it, the smallest 

 portion of the body ; the poste- 

 rior portion is the oldest part of 

 the parasite, except the head it- 

 self. New somite-like divisions 

 of the body form just behind the 

 head, and as they grow older and 

 larger are pushed backward by 

 newer ones. The tapeworm is an 

 interesting 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. When the embryos reach a certain stage, a 

 few terminal divisions of the adult separate from the rest, 



Fig. 138. Tapeworm. Left- 

 hand drawing, reduced; right- 

 hand drawing, enlarged 



After Leuckart 



