314 M. M. Mercaur 
Infection Experiments. 
Under natural conditions the several species of Gpalina are found 
only in certain definite hosts, as noted in the table on page 207. 
In the hope of reaching a better understanding of this restricted 
distribution, many artificial infections were made with the cysts of 
O. intestinalis, O. caudata and O. dimidiata upon the larvae of Rana 
esculenta, Bufo vulgaris and Bombinator pachypus and upon the adults 
of several species of frogs and toads and Triton cristatus. Attempts 
were made also to infect the same larvae and some of the same 
adults with adult Opalinae of four species, O. intestinalis, O. caudata, 
O. dimidiata and O. obtrigona. 
Opalina intestinalis cysts cause infection of the tadpoles of Rana 
esculenta and Bufo vulgaris as readily as of the tadpoles of Bombi- 
nator pachypus. Under natural conditions Rana esculenta only very 
rarely contains this parasite. It has never been reported from 
Bufo vulgaris. In both of these hosts the Opalinae form normal 
gametes which copulate. After four weeks the infection appeared 
normal when studied from living material. Preserved material from 
older infections has not get been examined. 
Adult Hyla arborea and Rana temporaria, as well as tadpoles 
of the latter species, are also readily infected if forcibly fed with 
the cysts, the young Opalinae in the rectum being apparently entirely 
normal. The later history of these infections was not followed to 
see if copulation occurred. O. intestinalis has never been reported 
from Hyla arborea or Rana temporaria. 
Tadpoles of Bombinator pachypus, Bufo vulgaris, and Rana esculenta, 
when placed with foeces of either species of Bombinator containing 
adult O. intestinalis, ingest many of the parasites with the foeces. 
Many others of the parasites pass into the nostrils with the respira- 
tory current. Many of these ingested adult Opalinae are digested 
by the tadpoles of Bufo vulgaris, but some pass uninjured through 
the whole alimentary canals to the recta and there establish thriving 
colonies. Tadpoles of Bombinator pachypus digest a smaller proportion, 
and tadpoles of Rana esculenta digest almost none of the adult 
Opalinae, their recta becoming very richly infected with large 
Opalinae. There is no subsequent degeneration of the Opalinae in 
any of these infections, at least within four weeks. Adult Ayla 
viridis and adult Rana temporaria are also readily infected if forcibly 
fed with adult O. intestinalis. 
