194 PARASITES IN THE INTESTINES 



excite wonder at the continued good health of the host. 

 More than 50 per cent, of the dogs examined harbor worm 

 parasites. 



It is essential for the taenise to have an intermediate host 

 to complete their life cycle. The following animals serve 

 as intermediate hosts for most of them: Sheep, ox, pig, 

 horse, goat, rabbit, and even man. 



The head (scolex) of a tsenia is generally expanded and 

 supplied with suckers ; some species in addition are provided 

 with hooks. From this head, by proliferation, is formed the 

 neck, a thin non-annulated constriction, which, continuing 

 and becoming wider produces the body or strobila. The 

 body is in the form of a long, narrow band divided into more 

 or less distinct segments, and as these become mature they 

 are cast off gradually from the distal end of the parasite as 

 new segments develop from the head. Each of the segments 

 contain numerous eggs (ova) which pass out with the feces. 

 Before becoming detached the segments can also liberate ova 

 through an opening on the lateral wall or ventral surface, 

 called the genital pore, forming two ways of disseminating 

 the ova. When deposited in a suitable medium, preferably 

 warm, moist soil, or filth, the ova undergo a series of compli- 

 cated changes which finally result in the first larval form. In 

 this form they are ingested by the intermediate host, most 

 commonly with the food or water. They burrow through the 

 intestinal walls and migrate into the adjacent tissues, or w r ith 

 the blood stream are carried to remote parts where they 

 develop into cysts. During the development of the cyst in 

 the various organs serious conditions, such as " gid" in sheep, 

 echinococcus disease of man, etc., may arise in the new host. 

 As the dog is the harborer of the parent tsenia, treatment 

 should be given not only to reduce the disorders they occasion 

 in other hosts but also to lessen the injury they produce in the 

 dog. The cyst form is the limit of development in the inter- 

 mediate host. The life cycle can be completed only in case 

 the cyst is ingested by and reaches the digestive tract of 

 another host in which it can develop. Dogs harboring the 

 adult tsenia cannot become reinfested by ingesting the larval 

 form, but the larval form may migrate from the intestines 



