by lack of oxygen (Thomas, 1942; Watson, 1944; Brown, 1939). For this reason the claims of Kolkwitz and Marsson (1909) and Nikitinsky and Mudrezowa-Wyss (1930) that V. microstoma survives the absence of oxygen must be qualified. V. microstoma survives only as a cyst or as a telotroch, it cannot feed or divide under such conditions. The presence of trophic vorticellids is an indication of the presence of oxygen.

Of the factors causing encystment of V. microstoma absence of food is generally accepted as essential. Enriques (1907) regarded chemical influences as also affecting the encystment of V. microstoma and putrefaction and desiccation have been suggested as possible factors (Brand, 1923). Brand himself inferred from his experiments that starvation and lack of oxygen were the principal causes of encystment and that desiccation also would cause encystment. Brand did not find that either the pH or the accumulation of Paramecium metabolites affected encystment. The present experiments have shown that lack of oxygen causes telotroch formation and not encystment and this is also true of desiccation. Brand (1946) himself recorded that V. nebulifera formed a telotroch when subjected to low oxygen tensions. Starvation therefore appears to be the principal cause of encystment. However, in a previous paper (Stout, 1954) encystment is recorded in dense bacterial suspensions and cysts were recovered from an anaerobic fluid which had ample bacterial food. In these cases the cysts were larger than those formed in well aerated cultures. It was suggested that encystment in these cases was due to the intense bacterial activity associated with a low redox potential. Hayes (1938) recorded encystment of Dileptus anser under similar conditions. It appears therefore that although starvation is normally the principal cause of encystment, under exceptional circumstances the vorticellids may encyst in the presence of bacteria, although in this case the cysts are larger than those formed under starvation conditions.

Brand (1923) found that excystment was affected by a number of factors including bacteria, oxygen and pH. Moreover he found that cysts differed in their response, some excysting under one set of conditions and others under different conditions. An alkaline pH he thought was inhibitory and neutralization, in removing this inhibitory factor, permitted excystment to take place. Aeration and neutralization were sufficient by themselves to cause excystment of over 30% of the cysts. The addition of bacteria caused the excystment of about 70%. The three factors accounted for the excystment of almost all the cysts and Brand therefore believed that they were the most significant. Cysts formed in the anaerobic fluid and in dense bacterial suspensions have been found to excyst following aeration (Stout, 1954) which implies that 30% of Brand's cysts may have been formed in response to such anaerobic or reducing conditions. In the present experiments cysts subjected to very low oxygen tensions were also found to excyst. This was also the case with cysts of Oxytricha pellionella and has been found to be the case with Colpoda steinii. No satisfactory explanation of this phenomenon is apparent. Normally, however, bacteria are required for excystment, neither yeast nor hay extract being effective (Stout, 1954). In this respect V. microstoma resembles Didinium (Beers, 1946) for which bacteria or bacterial metabolites are a necessary excystment factor. However in the case of Didinium, which is a carnivore, the bacteria cannot be thought of as a nutritional factor and although Vorticella is bacteriophagous it does not necessarily imply that the effect of the bacteria is nutritional.