DR. ZENKER, ON INFUSORIA. 265 
and very closely contiguous particles that a trifling move- 
ment can possibly take place. 
And this in accordance with what I have observed. In 
making the observation, it 1s necessary to choose for its sub- 
ject, amongst those on the stage, an Actinophrys in which 
the pulsating vesicle is seen in profile, and at the same time 
turned slightly upwards. In this position the whole of the 
vesicle is sure to be visible, whilst when viewed exactly in 
profile, a considerable portion of it may, of course, frequently 
be overlapped. If the systole and diastole are now watched, 
it will be seen that immediately before the systole an opening 
is formed in the outer membrane, and always at the same 
spot, and that during the collapse of the wall, the free borders 
of the opening quiver in an outward direction. 
From this observation, the correctness of which can hardly 
be impugned upon the ground of its resting upon “ optical 
delusion,” it is directly proved that the contents of the vesicle 
are expelled by the systole into the exterior water. 
Assiduous observation will readily convince any one of the 
simple nature of the way in which the opening of the vesicle 
takes place. The orifice, that is to say, is nothing more than 
a slit, which is always reopened at the same spot, simply for 
the reason that the cicatrix, as it may be termed, of the pre- 
vious rupture always remains the weakest part. After the 
collapse of the vesicle, a short period elapses before any in- 
dication whatever of it is again visible. As it must be as- 
sumed that the secretion of fluid into the vesicle is pretty 
nearly continuous, we must suppose that its outward flow is 
for a certain time unimpeded. ‘The vesicle does not fill again 
until the fissure is entirely and firmly closed. If the site of 
the fissure be now brought accurately into focus, it will be 
clearly seen that the wall of the vesicle is at that spot very 
thin, but at some distance from it much thicker; and hs 
difference of thickness becomes more and more apparent as 
the vesicle continues to expand. But I have never been able 
to perceive any manifestly elastic extension, as in caoutchouc 
The observer at once feels that the vesicle will rupture at the 
thinnest part when the expansion has reached a further stage, 
as actually takes place, as above described. 
In the true ciliated Infusoria a higher degree of organiza- 
tion is observable, although the process is essentially the 
same. Among this class, the species selected by Oscar 
Schmidt (Bursaria leucas and Paramecium Aurelia) afford 
particularly favorable subjects for observation, owing to the 
circumstance that they may be held captive for a considerable 
