lOO UNUERHILL: 



ter rediae, which again produce other rediae, until a large 

 number of these forms become lodged in the tissues of the 

 snail. Finally the redia gives rise to still another form, the 

 cercariae, which is somewhat heart-shaped and possesses a 

 long and flexible tail. The cercaria leaves the snail host and 

 proceeds to swim energetically about, eventually fixing itself 

 upon an aquatic plant, blade of grass, or other object within 

 or near the water. The tail now disappears, while from cer- 

 tain cells expelled from the body there is formed a cyst in 

 which the cercaria becomes enclosed, the cyst being attached 

 to the blade of grass or other object upon which the cercaria 

 has lodged. If now a herbivorous animal, in grazing over 

 the wet land or in drinking the water in which it may float, 

 swallows this cyst, the cyst wall is broken down in the 

 stomach by the action of the gastric juices, the young fluke 

 is set free, and, reaching the duodenum, finds its way to the 

 liver by way of the bile duct. Here it fixes itself and grows 

 to maturity and the egg-laying stage from which we started 

 upon this peculiar cycle. 



It will be noted that this life historj- is a very hazardous 

 one, and that its completion must depend upon the co-opera- 

 tion of numerous favorable conditions ; the eggs must reach 

 the exterior amid surroundings favorable to their hatching. 

 If hatched, the larva must escape its many aquatic enemies, 

 and within a few hours find its suitable snail host. Provid- 

 ing the snail is not eaten by a duck, or does not otherwise 

 perish during this phase of the cycle, it issues from its host 

 as the free-swimming cercaria, when it is again liable to fall 

 prey to various small aquatic animals. Kscaping this and 

 becoming encysted, the chance of any herbivorous animal 

 coming along and swallowing it is very small. The relation 

 of the enormous number of eggs and the number of individ- 

 uals which one egg may prtKluce, to the survival of the 

 species amid conditions fraught with such dangers, seems 

 quite evident. 



So varied are the conditions that surround the propagation 



