THE PHYLUM PLATYHELMINTHES 



217 



each proglottid becomes filled with eggs it breaks off and passes out of 



the host. 



Most tapeworms live in the intestine of vertebrates with the scolex 

 buried in the intestinal wall. They do not feed upon the host itself, but 

 soak up nutrients, competing with the host for food that the latter has 

 digested. 



The scolex contains a brain from which two lateral nerves extend 

 posteriorly through all of the proglottids. Excretory tubules also extend 

 the length of the body, opening posteriorly where the last proglottid 

 dropped off. Flame cells connected with these tubules occur throughout 

 the body. The body wall includes a cuticle and muscular tissue, with 

 which the tapeworm can make slow writhing movements. 



When a proglottid becomes sexually mature it usually mates with 

 itself by autocopulation, but mating between proglottids, either of the 

 same or of different worms, has been observed. As in the trematodes, 

 each egg is covered with a separate capsule. The eggs are retained in the 

 proglottid, which eventually becomes full, breaks off and bursts. 



Most cestodes have more than one kind of host. The larva hatches 

 from its capsule only after it is eaten by the appropriate first host, usu- 

 ally an arthropod, in whose digestive tract the capsular membrane is 

 digested away. The first stage is the oncosphere (Fig. 11.15), little more 

 than a ball of cells containing a few hooks. It bores through the in- 

 testinal wall and develops in various organs of the host. In some tape- 

 worms it is covered witli a ciliated epidermis while in others it is 

 covered with a cuticle. It also has a pair of flame cells. 



Scolex 



Sucker 



Scolex in 



wall of 



intestine 



Genltol 

 pore 



Figure 11.14. The pork tapeworm, Taenia solium. Insets show the head, an im- 

 mature and a mature section of the body. (Villee: Biology.) 



