OBSERVATIONS ON HYDRA AND PELMATOIIYDRA UNDER DETERMINED HYDROGEN ION CONCENTRATION. W. L. THRELKELD AND S. R. HALL, UNIVERSITY OF VIRGINIA.' Much has been written recently concerning reduction, de-differentiation and resorption in Hydra. It is generally conceded that reduction in hydra is accompanied by a loss of tentacles. The literature enumerates the following causes by which hydras lose their tentacles. N. Annandale ('07) observed, in studying Hydra orientalis, that during the hot season of the year this species has but four tentacles while during the cold season it has six tentacles. G. Entz ('12) observed that an infection with Amoeba hydroxena may lead to a degeneration of tentacles. Reynolds and Looper ('28) have come to the conclusion that this parasite is responsible for the degeneration of the tentacles. Certain ciliates recorded by E. Reukauf ('12) and P. Shultze ('13) also caused the loss of tentacles. E. Shultz ('06) observed that hunger set up a process of dedifferentiation within the tentacles. Huxley and DeBeer ('23) observed that adverse environmental conditions accelerate dedifferentiation and resorption of the tentacles of Obelia and Campanularia . They also found that this process of dedifferentiation and resorption might involve not only the tentacles but also part of the zooid. Berninger ('10) found that, in response to inanition, hydra lost its tentacles. Finally Kepner and Jester ('27) also observed that the loss of tentacles was brought about in response to inanition. This loss, according to them, was accomplished by ingestion of the tips of the tentacles through the mouth. This may occur, but undoubt-edly is not the usual method, as Hyman ('28) indicated. It is a well known fact that the concentration of the hydrogen ion medium that bathes the protoplasm or protoplasmic tissue 1 These investigations were carried on under the direction of Professor W. A. Kepner. Acknowledgments are due Mr. Carl H. McConnell of this laboratory, for the preparations of the photomicrographs. 419
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