188 



PARASITISM 



tached from the end of the tapeworm and are defecated with 

 the feces to the outside. Each proglottid has the power to 

 produce thousands of eggs which are fertilized when mature and 



FIG. 84. A single proglottid of Taenia solium enlarged to show the reproductive 

 organs. (From Leuckart.) 



stored up in the uterus of the proglottid ready for development. 



When detached a ripe proglottid then has thousands of embryos, 



each capable of giving rise to a new tape- worm. But these are 

 deposited with the feces and before they can 

 develop into a new Taenia must undergo 

 partial development in the pig. In one way 

 or other they find their way into the food 

 of a pig; the embryos are liberated by 

 action of the pig's digestive fluids, and 

 when liberated make their way through the 

 walls of the digestive tract into the muscles 

 of the pig. Here their development is 

 arrested and as cysticercids or bladder- 

 worms they give rise to what is called 

 measly pork. Such pork eaten in an un- 

 cooked state is the source of human 

 infection. The bladder worms are freed in 



the digestive tract, become attached as scolecids to the 



lining epithelium and begin to bud out proglottids. 



Here there is a very characteristic physiological adaptation 



in which the difficulties of maintaining the species are balanced 



by the enormous number of embryos formed. 



muscle tissue. (From 

 Hertwig after Boas.) 



