very rapidly It first becomes narcotized and later cytolysis takes place. This occurred in the following experiments where an oxygen-carbon dioxide mixture was employed.

A hanging drop containing six Oxytricha pellionella, twenty-six trophic vorticellids and many cysts was placed in the chamber and pure oxygen turned on for an hour. There was no change in either the encysted or trophic forms with the exception of a single vorticellid at the margin of the drop which formed a telotroch is the edge of the drop dried. The oxygen-carbon-dioxide mixture (about 1:1) vas then passed through for twenty minutes. There was no excystment or telotroch formation but the ciliates were narcotized and the gases were therefore turned off. Forty minutes later there were six telotrochs, the other ciliates were still narcotized and after an hour and twenty-five minutes there were no telotrochs all the ciliates being either settled and feeding, narcotized, or dead. Even when pure oxygen was passed through some of the ciliates failed to recover from the initial treatment of carbon dioxide. Pure carbon dioxide was then passed through the chamber for some minutes. After twenty minutes there were eleven telotrochs and after thirty minutes there were twenty telotrochs and no trophic ciliate. A trace of oxygen was let into the chamber but the ciliates were left in a predominately carbon dioxide atmosphere for seventeen hours when there were still thirty-five to forty telotrochs. The experiment was then stopped.

Further experiments subjecting trophic vorticellids to a mixed oxygen-carbon dioxid atmosphere for half an hour generally showed no telotroch formation but narcosis and death of the trophic ciliates. However in one experiment under such conditions about twelve trophic ciliates were placed in the chamber and the oxygen-carbon dioxide mixture turned on. After three hours these ciliates had formed telotrochs. It would appear that provided the carbon dioxide concentration does not narcotize the trophic ciliate it will react by forming a telotroch. This reaction takes place in the presence of oxygen so that the response appears to be due to the raised carbon dioxide tension and not to the removal of oxygen. Moreover the telotroch does not settle in a high carbon dioxide atmosphere even when oxygen is present.