2O M. H. JACOBS. 



practical use in converting a mixed culture of Paramcecium and 

 Colpidium into a pure culture of Colpidium. In case it is desired 

 to make such a separation, it is only necessary to wash the animals 

 in distilled water, concentrate them by gentle centrifugalization, 

 and then introduce them suddenly into distilled water one half 

 saturated with carbon dioxide. At the end of a few minutes 

 Paramoecium will be found to be dead and Colpidium living; the 

 CO 2 may then be removed from the water, a little dry hay added, 

 and an excellent pure culture of the latter organism will be 

 obtained. 



Another minor difference between Colpidium and Paramoecium 

 is that the former in its natural state appears to offer greater re- 

 sistance to the separation of the food vacuoles by centrifugal force. 

 While an exposure of two minutes, in the culture used, brought 

 about a very distinct separation in Paramcecium, one of four min- 

 utes was only slightly effective in the case of Colpidium. One 

 result of this greater difficulty in bringing about separation in the 

 normal animals is that while it is very easy in Colpidium to demon- 

 strate the preliminary liquefaction followed by an increasing solidi- 

 fication which finally leads to a viscosity at least as great as that 

 at the start of the experiment, it is not possible to show conclu- 

 sively, as in the case of Paramcecium, that this viscosity produced 

 by CO 2 is actually greater than that in the untreated animals. 



A third point of difference between the two forms is that in 

 Colpidium at least in the individuals studied there are present, 

 in addition to the food vacuoles, many fine dark particles scattered 

 irregularly through the protoplasm. The effect of centrifugal 

 force after liquefaction has occurred is to throw these particles to 

 the anterior end of the body, while the food vacuoles containing 

 ink go to the posterior end, giving a very distinct banded appear- 

 ance to the animals, the anterior band being gray, the middle one 

 transparent and the posterior one black. The appearance is very 

 striking and can be seen even with a hand lens. With the centrif- 

 ugal force used it was never obtained except in animals that had 

 been exposed a moderate length of time to carbon dioxide. In 

 individuals not exposed at all, or exposed a longer time, the band- 

 ing was indistinct or entirely lacking. 



