DR. ZENKER, ON INFUSORIA. 267 
substance is compelled to retreat, the more is the membrane, 
formed by the viscous cement which is adherent to it, 
stretched until suddenly it gives way and is torn across from 
side to side, At the same time the parts resume their former 
position ; that is to say, the sides of the vesicle come together 
and remain invisible as long as it is open, that is, until the 
cemented material has again blocked up the orifice. The 
closure of the vesicle causes that of the adducent vessels, 
because the surrounding substance at the periphery, even of 
a still enlarged vesicle when forced to accommodate itself to 
that of a contracted one, must be compressed, and conse- 
quently must lose all vacuities. ‘The consequence of this is 
that the vessels in their turn become distended by the fluid 
which is poured uninterruptedly into them from their capillary 
ramifications. In any case they are obstructed, as is obvious 
from the violence and want of absolute simultaneousness 
observable in their outflow when it takes place. 
The change of colour above noticed indicates simply the 
presence or absence of the occluding mucus over the orifice. 
The pulsation may be altogether prevented by keeping the 
animal a little while in only a thin film of water beneath the 
covering glass, whose pressure at length puts a stop to all 
movement. Under’ these circumstances the vesicle remains 
about two thirds to three quarters full, the radiating vessels 
also remain constantly open, as well as does the external 
orifice. Consequently in such a case there is no possibility of 
the existence of an oscillatory circulation, but, on the con- 
trary, of a continuous uniform excretion. 
Thus, in both instances, we find that the closure of the 
vesicle is effected by a cementing substance which replaces, 
as it were, the sphincter muscles, by which a similar function 
is fulfilled in more highly organized animals. One would be 
tempted to regard this material as of an analogous nature in 
both cases, that is, an amorphous protoplasm, to adopt an ex- 
pression of Max Schultze’s, and which is certainly correctly 
applied in the case of Actinophrys. In the ciliated infusoria 
however, it appears to me more correct to regard the substance 
in question as a true product of secretion, since. particularly 
with the great Spirostomum ambiguum, it is easily seen how 
frequently mucoid excretions from the substance of the body 
are collected in the very large pulsating vesicle, and how these 
are again expelled from it. After seeing this animal it is 
incredible that the existence of an external orifice should 
have been so long a matter of doubt. 
The existence of an external orifice to the vesicle, and the 
circumstance that its contents are entirely evacuated out- 
