PARAMOECIUM POSSESSING EXTRA VACUOLES 303 



days all of the animals isolated in the fresh medium had divided 

 twice producing four animals, while one of the animals in the 

 old culture had divided once, one had not divided at all, and 

 two had died. Even in the new medium the division rate was 

 apparently very slow, but the animals were so starved that it 

 took them several days to return to the normal condition. 

 After starved paramoecia have been abundantly fed I have 

 always found their division rate to increase to normal. 



This experiment was repeated using animals that were not 

 starved. The division rate for a period of five days was 2.7 in 

 the old infusion against 5.6 divisions of sister paramoecia in 

 freshly made hay medium. A more complete account of the 

 effect of various media will be given under Physiology. 



V. PHYSIOLOGY 



a. General 



It has been over two years since the original animals of the' 

 new race were discovered and they not only show no indication 

 of dying off but have given much evidence of being a line su- 

 perior in vitality to any of the two-vacuoled paramoecia with 

 which they have been compared. Cultures of this new race 

 have been kept in various laboratories of the Zoological Depart- 

 ment of the University of Pennsylvania and in the course of 

 time, since no particular care was taken to prevent the mixture 

 of various cultures, the multi-vacuoled animals found their way 

 into the general cultures and it was not long before they had 

 displaced the common form. For some time this year it was 

 impossible to find a normal two-vacuoled paramoecium in any 

 of the cultures maintained in this laboratory. This extra 

 vacuoled race is made up of rather large animals and under many 

 adverse conditions these paramoecia surpass the common races 

 in viability and vigor. 



One of the characteristics of the race in question that was 

 first observed was the extreme resistance it seemed to possess 

 to culture media which were so old that they would probably 

 not have supported the common race of caudatum. Several 

 cultures made up at the beginning of the summer recess came 



