192 



PARASITISM 



tached from the end of the tapeworm and are defecated with the 

 faeces of the host to the outside. Each proglottid has the power 

 to produce thousands of eggs which are fertilized when mature, 



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



organs. (From Leuckart.) 



and stored up in the uterus of the proglottid, ready for develop- 

 ment. When detached, a ripe proglottid then has thousands of 

 embryos, each capable of giving rise to a new tapeworm. But 



these are deposited with the faeces, 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 ar- 

 rested, and, as cysticercids or bladder-worms, 

 they give rise to what is called measly pork. 



rr"-~^- Such pork eaten in an uncooked state is a 

 FIG. 85. Trichina 



spiralis, encysted in source of human infection. The bladder- 

 muscle tissue. (From r j . ,1 j. ,. i 



Hertwig after Boas.) worms are freed in the digestive tract, be- 

 come 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. 



