FLATWORMS AND THREADWORMS [ \ 295 



outward so that it can fasten its hooks into its new host's intestine. It 

 is now ready to bud off proglottids again. 



At least one per cent of all cattle slaughtered in this country have 

 tapeworms. Certain species are also found in pork. All meat should 

 therefore be well cooked before eating. 



The structure of the tapeworm is quite similar to Planaria, the flat- 

 worm which served as our introduction to this phylum. 



It is well, however, to obtain a good description of the way tape- 

 worms reproduce, as it is due to their reproduction that infection takes 

 place. 



The mature proglottid is almost entirely filled with reproductive 

 organs. From the spherical testes (which are scattered throughout the 

 entire proglottid) the sperm cells are carried through the vas deferens, 

 after being gathered into fine tubules, and pass to the genital pore. 



Eggs arise in the two-lobed ovary, and pass into the oviduct. Yolk 

 from the yolk-gland then enters the oviduct and surrounds the eggs. 

 After this a shell is provided for the egg by the secretions from the shell- 

 gland, and the eggs pass into the uterus. The eggs have by this time 

 been fertilized and pass into the vagina. As the proglottid grows older, 

 the uterus becomes extended with eggs and even sends off uterine 

 branches likewise filled with eggs, while the rest of the reproductive or- 

 gans are absorbed. The proglotti-d is then said to be ripe. When ripen- 

 ing occurs, the proglottid is very likely to break off and be thrown out 

 with the faeces. 



Cestode Infections. 



There are four principal types of cestode worms (Fig. 184) which 

 infect the human being. These are : 



Taenia saginata or mediocanellata, 



Taenia solium, 



Bothriocephalus latus, 



Taenia echinococcus. 



Each of these requires an intermediate host for the development of 

 the larval forms. The eating of the flesh of the intermediate host re- 

 leases the larval forms and the mature worm forms in the human host. 



Taenia saginata (the common beef-tapeworm) is common in the 

 small intestine of man. As the segments (which are loaded with eggs) 

 ripen, they are discharged. The eggs are taken up with the food of the 

 ox. Then the embryo pierces the intestinal wall with the six hooks on 

 the worm's head. As it bores its way through into the blood-stream, 

 this blood-stream carries it throughout the entire system. Finally, they 

 come to rest in various muscles and develop into a cystic larval form. 

 It is at this point that mari becomes infected if raw beef is eaten which 

 contains these larvae. 



Taenia solium has less uterine pouches filled with eggs than 



