202 BOVINE PATHOLOGY. 



the Cystic, or larval taenise, are found in animals other 

 than the bearer of the adult tapeworm, and some con- 

 nection between the hosts of larval and adult forms must 

 occur or the race will die out. 



Under these circumstances it may seem astonishing 

 that this event has not happened long ago, but we must 

 recollect that each of the usually numerous segments of 

 a tapeworm contains an enormous number of eggs ; that 

 these eggs can remain exposed to weather and to other 

 such influences for a long time without injury ; that the 

 hydatids produced are sometimes many-headed, each head 

 being capable of becoming a distinct individual; that, in 

 other cases, the hydatids are capable of producing a 

 number of secondary cysts within themselves. 



To prevent excessive action of these favorable influences 

 many of the ova are lost ; the embryo does not always suc- 

 ceed in obtaining a bearer. Many of the bearers of hy- 

 datids are not sufficiently infested to die, and are not killed 

 and eaten sufficiently soon for the parasite, which, there- 

 fore, undergoes calcification after dying. The hydatids 

 are not always sufficiently fortunate to obtain a nidus for 

 development into the adult. Thus, under ordinary cir- 

 cumstances, the chances of invasion by TaBuise are about 

 even. When intermediary bearers are prevalent, when 

 conditions are favorable to development of the embryos, 

 and under other special circumstances, a severe outbreak 

 of parasitismus occurs. If we can break through the chain 

 of development at any one link we shall arrest parasitic 

 invasion, and the enemy, being thus cut off from rein- 

 forcements, will be defeated in its attempts to produce 

 disease. Thus, in the case of Taenias, communication of 

 dogs with cattle, and the former running over pastures 

 is to be avoided as much as possible. All parasites 

 passed, and all livers and lungs containing hydatids 

 should be destroyed, and not given to dogs or used as 

 manure. Thus, we may hope to succeed in eradicating 

 tapeworm. Flukes have a free stage of existence when, 

 as cercarians, they frequent the drinking-water, and thus 

 gain entry into the alimentary passages of the higher ani- 



