Opalina. 297 
(cf. Figs. 136, 138, 139, Pl. XXII). Many of the infection cysts are 
almost completely filled by the little animals within them (Fig. 130, 
Pl. XXIT). A faint appearance of striation, which is usually spiral 
but may be concentric, is often found in the infection cysts, being 
due to the rows of cilia and their basal granules, the direction of 
the course of the lines being determined by the position of the ani- 
mal within the cyst. These three features, which NerRESHEIMER gives 
as distinctive criteria between the copulation cysts and infection 
cysts, seem insufficient to establish the presence of copulation cysts. 
NERESHEIMER’S description may possibly apply in part to the pheno- 
mena of abnormal encystment. 
On the other hand, the description by EncrnmMann and by 
Zeuier, for multinucleated Opalinae, of uninucleated cysts with large 
nuclei, in the recta of the tadpoles, seems to indicate the presence 
of a second type of cysts, which might well be copulation cysts. If, 
however, there really are such copulation, cysts, I do not understand 
why I have not found them. The ease with which cross infections 
are secured in aquaria in which adults and tadpoles of different 
species of frogs are kept (see the chapter on infection experiments, 
p. 314) suggests that possibly some cysts of O. itestinalis or 
O. caudata might have been present in EnGreumann’s and 
ZELLER’S tadpoles, the infection cysts in these species having usu- 
ally a single nucleus. I have had collected tadpoles of Rana tem- 
poraria, R. esculenta and Bufo vulgaris become infected with both 
O. intestinalis and O. candata by leaving them an hour in a jar with 
adult Bombinator pachypus, while on the way from the field to the 
laboratory. Some of these tadpoles were already infected with Opa- 
linas of the natural species, so that, upon opening them, three spe- 
cies of Opalina were found living together. As the cysts of Opa- 
lina live in water for quite a time without injury, of course such 
cross infection could be secured by placing tadpoles in aquaria in 
which the Lombinator adults had been placed, perhaps only for a 
short time, many, days before. Encystment after copulation seems 
to me doubtful. The matter needs further study. 
Licer & Dusoscg (1904a) describe copulation cysts of an 
utterly different type in O. ranarum. These cysts are said to en- 
close two distinct gametes which lie side by side, each occupying 
one hemisphere of the cyst. I can suggest no satisfactory expla- 
nation of the phenomena, which were evidently not true copulation. 
