608 1^- R- ^Aß'^. 



tlie sporocysts or rediae contained young- cercariae which were in 

 practically the same stage of development, and which would come to 

 maturity at about the same time. The other embryos in the nurses 

 were always of a markedly younger stage in their development, so 

 that there would be no overlapping- of the broods and a consequent 

 continuous liberation of the cercariae. 



When the cercariae have been set free from the snails they 

 swim about actively for some time. Apparently the length of the 

 period of activity has a definite correlation with the depth of the 

 water in which the cercariae are liberated. In any event they show 

 a strong positive heliotropism, so that in a vessel into which light 

 is admitted only at the top, all of the cercariae become encysted at, 

 or near, the surface of the water, on the side nearest to the source 

 of the light. The strength of the heliotropic reaction was well 

 shown when the mature cercariae were placed in shallow watch 

 g-lasses for study. 



If the glass was left undisturbed on the table all of the cer- 

 cariae Avould become encysted on the sides of the glass nearest to 

 the source of light. If. on the other hand, the watch glass was 

 placed on the stage of a microscope and the light from the mirror 

 thrown up through the bottom of the glass, the cercariae would 

 become encysted on the bottom of the glass. By narrowing the 

 pencil of light admitted through the stage of the microscope it was 

 possible to segregate the most of the cercariae in a small area 

 directly over the centre of the diaphragm, where the illumination 

 was most intense. 



Looss and Lang have stated that the cercariae of Amphistoynum 

 suhcla€atnm may swim about for as many as fifteen to twenty eight 

 hours before encysting. From eight to ten hours is given by Looss 

 as being the usual extent of the free swimming period. At the end 

 of this time they sink to the bottom and form their cysts. 



The period of the active swimming of the cercaria of D. tem- 

 poratus was always much shorter than the least period mentioned 

 for Amphistomum. None of the cercariae were observed to remain 

 active for more than one hour, while in a small dish all of those 

 capable of forming a cyst would be attached within half an hour 

 from the time of their liberation from the snail. 



