Thé life history of Diplodiscns temporatiis Stafford. 609 



The formation o f t h e c y s t. 



Under normal conditions the cercariae swim quite directly to 

 the surface of the water and attach themselves to the sides of the 

 containing" vessel or to any floating- material, such as water Aveeds, 

 which may be present. When ready to begin the process of en- 

 cystment, the cercariae become firmly attached by means of the 

 ventral sucker. This attachment seems to be absolutely essential 

 for the formation of a perfect cyst. In a small dish where there 

 were many cercariae some of them would be unable to maintain 

 their attachment to the bottom, and Avhen these were separated 

 from the substratum the extrusion of the cystogenous material would 

 continue but a perfect cyst would never be formed. 



When rediae containing- nearly mature cercariae were taken 

 from the body of a snail and kept in water, the contents of the 

 cystogenous glands of the cercariae were extruded just as though 

 they were free in the water. The time which would elapse before 

 the extrusion of this material was practically the same whether the 

 cercariae were free in the water or still contained wdthin the bodj' 

 of the redia. A perfectly formed, or even completely closed cyst 

 was never formed by any of the cercaiiae which had attempted to 

 encyst while inside the redia. The cystogenous material would form 

 a straight hollow tube about the cercaria. There were no diflerences 

 in the diameter of this tube throughout its length. The tail was 

 seldom broken off from the body. In short, the attempt to encyst 

 resulted in the extrusion of the cyst forming material but the 

 movements necessary for the shaping of the cyst could not be 

 accomplished while the cercaria was unattached and the result of 

 the attempt was the open tube. 



The observations recorded in the previous paragraph suggested 

 that the stimulus causing the extrusion of the cystogenous material 

 was the change in the medium surrounding the cercariae. To test 

 this point fragments of infected snail livers were examined to 

 ascertain the stage of development of the contained cercariae. A 

 liver containing specimens of a favorable age was divided into two 

 parts, one of which was put in a moist chamber, while rediae teased 

 from the other part were put into w^ater. When the cercariae of 

 the second lot had extruded their cystogenous material, those of the 

 first lot were examined. None of these showed any appearence of 

 having extruded the cystogenous material; on the contrary, most 



