DR. ZENKER, ON INFUSORIA. 269 
water is constantly entering in active currents. Although 
this distribution of the vessels is apparent, only a few Infusoria 
with the equal distinctness (Paramecium aurelia, Nassula 
elegans), nevertheless indications of the existence of similar 
capillary vessels are manifest in other instances (Spirostomum 
ambiguum) ; and this leads to the supposition that the same 
disposition exists in other ciliated Infusoria. 
All that has been above adduced with respect to this pro- 
cess leads to the conclusion that it is of a respiratory nature, 
as suggested by Spallanzani and Dujardin (‘ Hist. des Inf.,’ 
p- 109). Whether the water be introduced through the 
mouth or integument, it is impossible that it should pervade 
the body of the animal through such a fine capillary network 
without it leaving something behind which, from analogy, 
can only be the oxygen contained with water. Thus we 
have a respiratory apparatus which may be compared with 
the branchie of fish or other animals. In every apparatus of 
this kind especial provision must be made for the expulsion 
of the water which has been used, and this object is answered 
by the contractile vesicle. But a difference exists between 
this kind of respiration and that which is effected by branchie, 
in the circumstance that, in the latter case, the current of 
water is introduced by mechanical means, and remains on the 
surface, the oxygen only penetrating into the interior ; whilst 
in the case of the Infusoria the whole of the water enters, and 
pervades the substance of the body throughout. It may also 
be said that there is no visible mechanical appliance, unless 
it be assumed that the current set in motion by the oval cilia 
is sufficiently powerful to carry the water through the tissues 
of the body, and that afterwards expelled by the pulsating 
vesicle. In such instances as Actinophry, more especially, 
this theory would leaye us completely in the lurch; conse- 
quently the propulsive force must be simply of a chemical 
nature, and in order to illustrate the notion I entertain of 
the process I would propose the following hypothesis :— 
** That the oxygenated water is more powerfully attracted by 
the tissues than when it is deprived of its oxygen.” ‘This 
being admitted, the reason why the de-oxygenated water is 
always impelled by the oxygenated ; and why one is always 
taken in, and the other expelled is at once apparent. 
I am reluctant to propose such an hypothesis without 
having subjected its correctness to the proof of experiment ; 
but this I have hitherto found it impossible to carry out. In 
order to establish it,it would be necessary to institute similar 
conditions experimentally. This might be done, for instance, 
if a cylinder of carbon were filled with pure water and placed 
