A NIMAL PA RA SITES 



23 



the external excretory vessel is the large longitudinal nerve. 

 Two smaller nerves accompany this but are hardly distinguishable 

 in the sections. 



Occupying the central portions of the section, between the 

 excretory vessels of the two sides, are sections of the reproductive 

 organs. In the sections furnished the major portions of this mass 

 consist of lobes of the testes. 



Life History. — Examine eggs from a ripe proglottis. These 

 are best studied in fresh material, even if another species is the 

 only one available; but prepared slides may be used. 



FlG; 



•Cysts of Taenia pisiformis in the liver of a rabbit 24 days after experi- 

 mental feeding. (Orig.) 



It will be found that in the eggs ready to be discharged from 

 the body, the six-hooked or hexacanth embryo is "already formed. 

 In the fresh material movements of the embryo can be readily 

 studied. It should be noted that the so-called shell of eggs of 

 tapeworms of the group under consideration is in reality an 

 embryonic membrane, the embryophore, formed from cells of 

 the egg itself. In Tamia pisiformis and related forms, this 

 embryophore is finely striated, appearing to be made up of many 

 fine rods placed side by side. The hexacanth embryo is also 

 commonly known as the oncosphere. 



Hatching of the Eggs. — When the eggs of the tapeworm are 

 taken up by an appropriate animal, the embryo is liberated by 

 the action of the digestive juices. In the case of Tamia pisiformis 



