316 M. M. Mercatr 
0. dimidiata cysts cause infection of Rana esculenta tadpoles. 
Bufo vulgaris tadpoles. 
Rana temporaria tadpoles.* 
Bombinator pachypus tadpoles. * 
ss 4 adults* ,, m , the same tadpoles as the cysts. 
O. obtrigona adults* ,, Ps , Bufo vulgaris tadpoles. * 
Rana esculenta tadpoles. * 
Bombinator pachypus tadpoles. * 
In the preliminary notice of this work there were two errors: 
1) adult Rana esculenta were infected by cysts of O. caudata, not 
of O. intestinalis as there stated; 2) the Bufo vulgaris called “young 
toads” in that paper were not adult, but were metamorphosing tad- 
poles with fully formed legs, with the tails only beginning to 
diminish, and with mouths of the larval type. 
In the light of the results of these infection experiments, the 
restricted distribution of the parasites in the several hosts is very 
difficult to understand. It seems probable that any species of frog 
or toad can be infected by cysts or adults of any species of Opalina 
(except, of course, O. saturnalis). Why, then, is the distribution of 
the parasites so restricted? Why, for example, do not O. dimidiata. 
O. intestinalis and O. caudata all naturally occur in both Rana 
esculenta and Bombinator pachypus? The tadpoles of Rana esculenta 
and Bombinator pachypus live together in the same ponds and streams. 
Why does one species become infected only with O. dimidiata and 
the other species only with O. intestinalis and O. caudata, when all 
three kinds of cysts are present in the same ponds at the same 
time of year and must doubtless often be ingested by both species 
of tadpoles? The question deserves more attention than I had time 
to give it last spring. If, upon my return to America, I find con- 
ditions there favorable for experiment upon this point, I shall study 
it further. I hope also the matter will be further studied upon 
Europaean forms, the species mentioned above being especially favor- 
able for study. 
A Description of Opalina zelleri, NeREsHEIMER. 
In his fine paper upon the Opalinas, published in 1877, ZELLER 
describes finding, along with O. dimidiata, in the rectum of Rana 
esculenta, certain individuals much more stocky than the ordinary 
O. dimidiata. They were especially characterized by having the 
body folded posteriorly, with deep furrows between the folds, ordinary 
