EFFECTS OF CARBON DIOXIDE 533 
they show considerable powers of recovery after a lengthy period 
of rest. In this respect they differ from the membranelles of 
Euplotes and Blepharisma which in these experiments never 
could be made to resume their beat after having having completely 
stopped. Doubtless this difference is due to the fact that in Vor- 
ticella they are accustomed to stopping every time the disc is 
retracted, while in the other two forms they normally remain in 
continuous motion and are stopped by nothing short of actual 
injury to the cell. 
The powers of recovery of Vorticella after all movements have 
ceased are quite considerable. In one experiment after three- 
quarters of an hour practically all of the individuals had become 
quiet, and many had been in this condition for half-an-hour or 
more. Five hours after exposure to the air in a moist chamber 
about half of them had recovered and many had begun to regen- 
erate the missing stalk, which was already one-tenth to one-half 
the length of the body. By the next day these stalks were one- 
half to two times the body length. In this experiment about 50 
per cent of the individuals never recovered and this is typical of a 
number of experiments that were tried. It may be said, there- 
fore, that certain of the movements of Vorticella are almost 1n- 
stantly affected and others only after a much longer time. Even 
after all visible movements have ceased the powers of recovery 
of the animals are considerable. 
In a few instances individuals were observed which, before the 
beginning of the experiment, had formed the circlet of cilia used 
in the free-swimming existence. Such cilia were about as resist- 
ant as the membranelles, though they were not observed to beat 
again after having stopped. In no case did individuals which 
broke from their stalks during the course of the experiment on 
account of the effect of the carbon dioxide, form such cilia, but 
their locomotion was entirely by means of their membranelles. 
The Vorticella cell apparently shows no tendency to burst and 
form droplets of protoplasm, but a considerable change in form 
may occur. In an atmosphere of carbon dioxide V. nebulifera 
inside a few minutes tends to assume the more rounded form 
characteristic of V. campanula. In the individuals which are 
