264: DR. ZENKER, ON INFUSORIA. 
opening directed outwards, and consequently of the evacuation 
of the contents of the vesicle into the surrounding water. 
His description is clear and distinct, and as convincing as the 
sight of the thing itself, which, with proper microscopical 
appliances, it must be confessed, is not difficult. 
Nevertheless, since the appearance of the important works 
of Stein, Lieberkuhn, and of Claparéde and Lackmann, the 
opposite view has obtained almost universal acceptance, 
Oscar Schmidt’s observation haying been regarded as based 
upon an optical delusion. 
The controversy would appear to have been definitively 
settled by an observation of Claparéde in a non-ciliated ani- 
mal, Actinophrys Hichhorni (¢ Mill. Arch., 1854). This 
observation showed that simultaneously with the sudden col- 
lapse of the projecting vesicle no movement was perceptible 
in the minute particles suspended in the surrounding water ; 
whence it was concluded that the contents must have been 
expelled, not in an outward but in an inward direction, and 
the pulsating vesicle was consequently proved to be a circu- 
latory organ. 
I am in a condition, however, to show that this generally 
accepted view is incorrect. It is said also that Lachmann, in 
the last days of his life, expressed himself in favour of my 
opinion. 
In the first place, then, it is not true that the evacuation 
of the pulsating vesicle in an outward direction must neces- 
sarily produce any visible movement in surrounding sus- 
pended particles. It is quite correct to say that a movement 
would be produced were the contents of the vesicle com- 
pressed air, or were the expulsive force very great. But as 
the vesicle contains water, which is virtually incompressible, 
and upon which no great degree of pressure is exerted, the 
impulse which would be given by the expansion of the con- 
tained fluid on the contiguous parts is entirely wanting. An 
opening is suddenly formed in the delicate outer membrane 
in consequence of which the vesicle collapses, so that the 
contained fluid simply occupies the same space as before. 
The sole movements undergone by the water contained in 
the vesicle is due to its being forced through the more or less 
narrow orifice, in order to diffuse itself on all sides, and fill 
up the vacuum arising from the moderately slow collapse of 
the membrane. The motion of the fluid is consequently 
limited te the space previously occupied by the pulsatiug 
vesicle itself; and it is merely a sort of vortex, and always 
very feeble. Consequently it is only in extremely minute 
