THE OPALINID CILIATE INFUSOKIANS. 443 



spring of the year. Not all the Opalinids encyst and go out of the 

 tadpoles ; some remain in the tadpole and continue its infection [as is 

 the case with the adult Anura in the spring] . Cysts were found in 

 tadpoles of Bufo hufo after 16 days ; and of Discoglossus pictus after 

 one month; of Bana esculenta after 6 weeks; of Hyla arhorea^ natur- 

 ally infected, which were about a month old. Tadpoles of Rana tem- 

 poraiia and Bufo hufo, about two months old and ready to metamor- 

 phose, almost always showed some cysts. All of these cysts are iden- 

 tical morphologically (biologically ? ) with those found in the adult 

 Batrachians [but are different from the " copulation cysts with large, 

 spindle-shaped nuclei" described by Neresheimer] . Brumpt has not 

 seen cysts containing more than one Opalinid, though Leger and Du- 

 boscq and also Lowenthal have described them [I have seen one cyst 

 containing an individual which was in process of division, probably 

 having been in fission when encystment occurred] . 



Cysts of Opalinids from tadpoles, ingested by young tadpoles of 

 several species develop in every way typically. They form gametes 

 after 24 to 48 hours and their further development is normal. After 

 about three weeks, in one instance, new infection cysts were formed 

 in the tadpoles. The following infections were made: Young tad- 

 poles of Bufo hufo with cysts from tadpoles of Rana temporaria; 

 young tadpoles of Discoglossus pictus with cysts from tadpoles of 

 Hyla arborea; young tadpoles of Discoglossus pictus with cysts of 

 Protoopalin-a intestinalis from tadpoles of Discoglossus pictus. In 

 the last case, after three weeks, the tadpoles ejected the cysts. 



If infected tadpoles, in which the Opalinids have gone through the 

 sexual process, are reinfected with more cysts, the later cysts hatch, 

 but their Opalinids after emergence do not go through the sexual pro- 

 cess, but apparently develop directly to adults. But previously un- 

 infected tadpoles, newly infected from the same lot of cysts used in 

 the preceding experiment, do show gametes after 48 hours. 



Young tadpoles of numerous species infect themselves with cysts 

 of diverse species of Opalinids. When a tadpole is already infected 

 with its accustomed species of Opalinid, it is resistant to further in- 

 fection. How is it that one species opposes itself to the adoption of a 

 second species which is able to develop when the first species is absent? 

 We may believe that there is some vital interaction between parasite 

 and host which varies with the species of host. Two species {Proto- 

 opalina intestinalis^ Opalina ranarum) , which do not seem able to 

 live together in tadpoles of Ram-a esculenta and R. temporaria, live 

 very well in tadpoles of Discoglossus pictus whose adults in a state 

 of nature show both of these species of Opalinids. 



Cross infections (with unaccustomed species of Opalinids) are of 

 comparatively short duration, as are also infections of tadpoles by 



