PARAMOECIUM POSSESSING EXTRA VACUOLES 293 



instances, however, one extra vacuole was found in the anterior 

 end (fig. 3), but I have never found more than two vacuoles 

 located in the foremost half of the body. It is a curious fact 

 that, rare as this condition is in the normal medium of hay 

 infusion, in cultures to which a little sea salt has been added 

 paramoecia with two vacuoles in the anterior end have been found 

 to be much more numerous. As yet no plausible explanation 

 for the increase of this rare arrangement has been found — if, 

 indeed, it is not merely a coincidence. 



There can be no doubt that the contractile vacuoles are very 

 definitely located organs and fixed in position for a given indi- 

 vidual. When an animal is swimming the vacuoles can be seen 

 to turn with the cell and always maintain the same position in 

 relation to each other. As far as I have been able to determine, 

 both the radiating canals and what appears to be the excurrent 

 pore or tube aid in holding the vacuoles in position and particu- 

 larly the latter. That there is a connection with the outer wall 

 of the animal can be seen when a paramoecium bursts directly 

 opposite to a fully expanded vacuole, when the vacuole, drawn 

 by the outflowing protoplasm, can be seen straining at some 

 retaining fastening. 



2. The vacuoles. Under normal conditions the vacuoles in 

 this race measure on the average about 10 fx in diameter with a 

 capacity of 500 cubic micra. There may be some variation in 

 this size due to the age of the vacuole, i.e., the length of time 

 since it had appeared, but I think that it is safe to say that the 

 diameter of fully formed vacuoles is very nearly the same. There 

 can be no question that these vacuoles originate separately as 

 they generally lie at some distance from each other and no 

 cases have been seen when a vacuole appeared as though it had 

 been separated by di\dsion from another vacuole. The average 

 size of the vacuoles in the common race which I have observed is 

 11 /i with a cubic content of 664 cubic micra. In my fixed 

 preparation the vacuoles are surrounded by what appears to be 

 a definite morphological membrane (fig. 16). When animals 

 have been kept at a temperature a few degrees above freezing 

 for a few hours the vacuoles expand and do not contract and it is 



THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 23, NO. 2 



