296 General Biology 



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 Qgg by the secretions from the shell- 

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

 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 

 organs are absorbed. The proglottid is then said to be ripe. When 

 ripening 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 echinoeoccus. 



Each of these requires an intermediate host for the development of 

 the larval forms. The eating of the flesh of the intermediate host releases 

 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 in America. 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 

 four sucking discs on the worm's head. (There are no hooks in T. 

 saginata.) As it bores its way through into the blood-stream, it is 

 carried by the blood-stream throughout the entire system. Finally, the 

 worm comes to rest in various muscles and develops into a cystic larval 

 form. It is at this point that man becomes infected if raw beef is eaten 

 which contains these larvae. 



Taenia solium has less uterine pouches filled with eggs than 

 Taenia saginata. These eggs are ingested by pigs. This type of tape- 

 worm is rare in the human intestine in the United States, although it 

 does occur. The process of development is quite like that of Taenia 



