Colpoda can survive several days in the absence of oxygen and feeds and moves freely. On the exhaustion of food the ciliates will encyst. Rarely “unstable” cysts are formed. Division is totally inhibited. The division rate is greater in pure oxygen than in atmospheric air. Reduced oxygen tensions appear to stimulate excystment, but absence of oxygen inhibits excystment.

C. steinii and C. inflate are far more resistant than C. cucullus to carbon dioxide and show recovery after prolonged exposure to an atmosphere of pure carbon dioxide. C. steinii survived twenty-four hours in a mixed oxygen-carbon dioxide atmosphere moving slowly but not feeding or dividing.

C. steinii grew over a wide range of pH, although growth was inhibited by a tartrate buffer of pH 3.5 and a borate buffer of pH 9.2. Both encystment and excystment are independent of pH.

C. steinii can be acclimatized to salinities of 3% NaCl, but growth and division is progressively inhibited with increasing salinity and leads to the formation of “unstable” cysts. Adaptation to higher salinities is achieved by increased osmotic tension of the milieu interieur. Excystment will not take place unless the osmotic tension of the milieu exterieur is less than that of the milieu interieur. My experiments support the belief that the first critical reaction in excystment is osmotic diffusion.

C. steinii divides and excysts only within the temperature range 8 ° to 35 ° C. At 37 ° C. both division and excystment are inhibited and “unstable” cysts are formed. Similarly at 1 ° C. the ciliates move and feed but neither excyst nor divide, and “unstable” cysts are formed. “Unstable” cysts so formed divide and excyst when returned to room temperatures. In this case a second critical reaction is inhibited by temperature.

The dissociation of division and excystment from other physiological processes is discussed in relation to the “activity” and “resting” systems identified in other organisms. It is stressed that the mechanism of inhibition is distinct for each factor and possible interpretations are offered in the light of recent theories.