WORMS. 



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rotary one, and around and around it spins, as though it were trying to 

 screw its way into the snail. This is really what it does do ; the papilla 

 on the head becomes shaped like an awl, and this gradually 

 forces aside the tissues of the snail, so that at last the whole 

 of the embryo squeezes in. 



Arrived in the body of the snail, a marked change takes 

 place. The embryo casts off the ciliated cells, secretes a 

 cuticular envelope, and becomes converted into a ' sporocyst,' 

 which rapidly increases in length to ten times its former 

 dimensions. During this growth the granular contents of 

 the cyst become converted into spores, and these in turn into 

 worm-like bodies with a mouth and stomach. These forms 

 are called 'redise,' and in each cyst will usually be found 

 one well-developed redia, and others less advanced. As each 

 redia becomes mature, it breaks through the walls of the 

 cyst and travels through the tissues of the snail. The redia 

 in turn has its own peculiar contents, tadpole-like bodies 

 called ' cercaria,' and one redia may produce twenty-three of 

 these. As the cercarioe reach the limit of their development, 

 they escape from the redia by an opening which exists for 

 the purpose ; and then, wriggling like a tadpole, these work 

 their way out from the host. 



Suppose that the infested snail during the processes 

 described has crawled out upon the land ; then the cercarice 

 escaping from the body will be distributed over the field. 

 They are fully prepared for this, for they soon 

 come to rest, the tail drops off, and the exter- 

 nal cells of the body secrete a mucilage which soon hardens 

 into an impervious envelope, fastening the cercaria to the 

 grass. Here it is able to exist for a considerable time ; but 

 when the grass to which it is attached is eaten by a sheep 

 or other animal, this external envelope is dissolved by the 

 digestive fluids, setting the worm free. It then bores 

 through the wall of the stomach, and makes its way to 

 the liver, where in the space of about six weeks it grows 

 into the adult fluke and begins to produce eggs, and the 

 cycle is complete. 



The history of this fluke has been worked out the most 

 thoroughly of any, but all have somewhat similar cycles so 

 far as known. In some there is no necessity for an intermediate host, 

 while in others there may be two. As will be seen, there is an alternation 



i 





Fig. 54. — R e d i a 

 stage of the liver- 

 fluke with sever- 

 al well-developed 

 cercariae in it. 



Fig. 55. — Cercaria 

 stage of the liver- 

 fluke. 



