EFFECTS OF CARBON DIOXIDE a2 
behavior just described. Loeb and Hardesty ('95) mention that 
Paramecium aurelia remains in the center of the drop fifteen min- 
utes. In all probability they were dealing with a larger drop 
than.the one used in these experiments, where in no case did either 
P. aurelia or P. caudatum require more than a few minutes to 
become adjusted to the new condition. 
After this primary response, the animals swim about in a more 
or less normal manner but more and more slowly, until they finally 
come to rest in a time that may vary from twenty or twenty-five 
minutes to several hours. Even after locomotion has ceased the 
cilia continue to beat for some time. As death approaches they 
beat slowly and irregularly and frequently show visible signs of 
injury. Often a group of them may keep on beating after the 
others have come to rest. There seems to be nothing constant 
about the part of the body where movements persist longest. 
After the cilia have completely stopped it seems to be impossible 
to start them again even by prolonged exposure to the air. The 
animals are to all intents and purposes dead. The same thing is 
true of the other ciliates studied, with the exception of Vorticella, 
whose membranelles are often stopped before the animal is seri- 
ously injured and consequently can be started again. In the 
majority of ciliates, however, thé vibratile structures are among 
the most resistant parts of the cell and when they have finally 
succumbed the life of the rest of the cell is practically extinct. 
In the meantime, certain other changes have been occurring. 
Among the mos: striking of these is the change in shape of the 
body, which becomes shorter and thicker. Some of the increase 
in the thickness is doubtless due to the shortening, but this does 
not account for all of it, and it is probable that an actual increase 
in volume occurs by the absorption of water. This phenomenon 
is more strikingly shown in some of the other forms studied than 
in P. caudatum. About the time that the swelling becomes notice- 
able, the nuclei begin to be very clearly visible, standing out 
sharply from the rest of the protoplasm by their greater opaque- 
ness. This is perhaps due to the acid nature of the medium sur- 
rounding the cell since other acids produce the same phenomenon. 
A further effect of the carbon dioxide often is apparent in the burst- 
