PARASITIC ADAPTATION. 99 



While in some cases the complete life cycle of a parasite 

 requires but one host, more often, for reasons above stated, 

 two successive and generally specifically different hosts are 

 required. A rather complicated example of the latter phe- 

 nomenon is the life history of the common liver fluke {Fasci- 

 ola hepatica), one of the flat worms infesting in its adult state 

 the livers of herbivora. This parasite is interesting in that it 

 must necessarily alternate between a mammal, usually the 

 sheep, and an aquatic snail, in completing its cycle. The 

 adult form occurring in the sheep's liver is leaf-like, about 

 three-quarters of an inch long, and has two prominent suck- 

 ers, one on the under side of the body and one surrounding 

 the mouth. The eggs, of which each individual fluke is cap- 

 able of producing in the neighborhood of 100,000, pass into 

 the bile duct of the sheep, from which they enter the aliment- 

 ary canal and reach the exterior with the excrement. Here, 

 if the proper conditions of moisture and temperature neces- 

 sary for further development are present, the embryo makes 

 its escape. At this stage it is active, ciliated, free-swimming, 

 and not unlike certain infusoria — the period of the miracidium 

 and a ver}- critical one in its history. It cannot survive in 

 this condition for more than twelve to eighteen hours, there- 

 fore if during this time it does not come in contact with a 

 snail it is doomed to perish. Finding its molluscan host 

 (^Limncea humidis) the larva, by means of its conical rostrum 

 and a rotary motion of its body, forces its way into the tissue, 

 most frequently that surrounding the lungs. Here it becomes 

 more oval in shape, the cilia disappear, and it enters the stage 

 of the sporocyst, which very earl}^ may by simple division 

 produce two individuals. Within the body of the sporocyst 

 there appear from five to eight cellular masses which, becom- 

 ing transformed into energetically moving bodies, finally 

 rupture the maternal sac and issue forth as so many radiae — 

 these, on obtaining their liberty, passing through the tissues 

 of the snail and becoming fixed in another organ, usually the 

 liver. Within the body of the redia there are formed daugh- 



