304 M. M. Mercaur 
On the other hand the restriction of sexual phenomena in Opa- 
lina to one period of the year is quite different from what we find 
in most Ciliata. This is probably associated in some way with its 
parasitic habit. 
The gametes are true ciliates with characteristic basal granules 
upon their cilia. They are not formed during encystment, or in 
ereat numbers simultaneously from one individual, as is so usual 
among Plasmodroma. They result from ordinary, though very rapid, 
division. They differ in no very decided manner from the indivi- 
duals of the asexual generation, though the microgametes do show 
special adaptation in the naked sticky tail. The posterior end of 
O. saturnalis in the asexual generation is naked, so this character 
in the microgametes is not so much of a departure from the usual 
conditions. 
The formation of large numbers of minute individuals in the 
spring, and their encystment and extrusion from the rectum of the 
host into the water, is doubtless an adaptation to the fact that the 
most favorable time for infecting new hosts is a brief one in the 
spring while the adult frogs and toads are found almost exclusively 
in the water, and while the vegetarian tadpoles are browsing over 
the bottom of the ponds where the cysts lie. As my experiments 
have shown, adult frogs can be infected by Opalina cysts, but the 
feeding habits of the adult are such that infection of this sort must 
be very infrequent. Infection of the tadpoles, on the other hand, is 
very easy. 
The period of very rapid division in Opalina is followed by 
sexual union, as is usual among Ciliata, the restriction of copulation 
to the spring being probably due to the occurrence of such rapid 
divisions oniy in the spring. 
The formation of microgametes is by no means unknown among 
the Ciliata — witness the Vorticellas. In the Vorticellas we also 
find complete fusion of the gametes. 
The resemblance between the reproductive processes in Opalina 
and those in Plasmodroma seems superficial, the gametes in Opalina 
being true ciliates and the chief peculiarity being the restriction, 
for ecologic reasons, of the period of rapid division and copulation 
to a brief time in the spring, and the consequent false appearance of 
distinct asexual and sexual generations. There is no true alternation 
of generations in Opalina, any more than there is in Paramaecium. 
The question as to whether Opalina is a primitive or a highly 
modified genus of the Cilata also needs considering. Its parasitic 
