EXCYSTMENT IN THE CILIATE BURSARIA TRUNCATELLA

¹ Department of Zoology, University of North Carolina, Chapel Hill, and the Mt. Desert Island Biological Laboratory, Salsbury Cove, Maine

A supply of cysts of Bursaria truncatella was obtained in autumn by depriving active specimens of food in spring water. The cysts were densely granular and opaque. In the following months many attempts were made to induce the excystment of samples of them, but with practically no success. Examination of the remaining cysts when they were 11 months old showed that fully 80 per cent of them had become relatively sparsely granular and semi-transparent. They could be activated dependably in plant infusions which had been ineffective earlier. Two months later the remaining 20 per cent of the cysts became excystable, not only in plant infusions, but also in peptone solution, yeast-extract solution, and distilled water. The results indicate that in autumn after the passage of many generations Bursaria produces cysts which require a long period of dormancy before they become excystable. These may be called stable cysts, but not all cysts are of this type.

Specimens which emerged from stable cysts, if not fed, re-encysted. These cysts, as well as others which were produced by specimens collected in spring, did not require a period of dormancy, but could be excysted within a week or less. These may be called unstable cysts.

The excystment process is the same in either type of cyst. It involves the rupture of the membrane which covers a special emergence pore, and the escape of the organism by way of this pore. Excystment is a remarkable feat, in that the spherical, encysted bursaria, measuring about 155 µ in diameter, makes its way outward through a circular opening measuring only 25 µ in diameter. Upon emergence, the organism is so different in shape and structure from a typical specimen that it could never be recognized as a member of the genus Bursaria. Its development is completed within an hour.