362 University of California Publications in Zoology [Vol. 19 



that the cercariae need not reenter the same snail from which they 

 came. This leaving and reentering seems to be an unnecessary con- 

 dition, equivalent to going up hill to go down again. It is even worse 

 than that, since there is a decided waste of cercariae, and thus the 

 possibility of the continuation of the life cycle is lessened. It is cer- 

 tainly safe to say that many more cercariae fail to find their way to 

 another suitable host or back to the same snail than those that succeed. 

 Such a waste of cercariae would be entirely avoided if the cercariae 

 encysted in the same snail. Faust (1917, p. 80) found that Cercaria 

 biflexa encysted within the host immediately upon breaking through 

 the birth pore of the redia. Such a step seems an obvious and probable 

 shortening of the process. I believe that it does take place quite often 

 with the cercaria of Echinostome revolutum. The evidence is from 

 two sources: first, from the fact that in some cases the number of cysts 

 exceed four hundred, while three hundred and two hundred cysts in 

 the same snail is not uncommon. That four hundred or three hundred 

 cercariae could find their way into a single snail from the open water 

 seems impossible. Even for two hundred cercariae so to enter seems 

 extremely improbable. I should say that fifty cercariae so finding 

 entrance to one snail would be unlikely and in fact in the majority 

 of eases less than fifty cysts are present. The second evidence is more 

 direct, since a still further shortening of the process of encystment 

 has been seen many times. When this shortening occurs the cercariae 

 encyst within the rediae without ever emerging through the birth pore. 

 Almost by accident fifty-eight rediae were found with cysts inside, 

 since in the great majority of snails examined no special attempt was 

 made to see how many could be so obtained. Of these fifty-eight 

 cysts inside rediae. twenty-six occurred singly (fig. 32). six rediae 

 had two cysts each (fig. 33). four contained three cysts each (fig. 

 34), and two had four cysts each inside (fig. 35). That cercariae 

 coming from the open water could penetrate the rediae and there 

 encyst, two. three or four in number is entirely impossible. In 

 fact, that one cercaria should penetrate a redia and encyst is quite 

 improbable. These rediae containing cysts were found in thirteen 

 different snails. The rediae containing these cysts were of different 

 shapes and sizes, being small, large, of normal shape or constricted at 

 one or more places. They also differed much in color and activity. 

 The majority of these rediae, however, were almost lifeless and of a 

 dark brown color. In the rediae with the cysts were sometimes found 

 active cercariae nearly ready to escape and germ balls of different sizes. 



