1. The effect exerted by the pH of their medium on Colpidium is determined quantitatively by counting the average number of food vacuoles formed in a given time when Colpidium is supplied with Indian ink.
2. Graphs obtained by plotting the rate of feeding against the pH of the medium show a characteristic depression on the alkaline side of neutrality (pH 7.5-8.5), on a curve which otherwise rises steadily from pH 4.5 to a maximum at pH 6.0, and falls from here gradually through the alkaline range.
3. Methods for measuring the rate of movement of ciliates are described, the most practicable being those in which their galvanotropic and geotropic reactions are used to control the direction of the movement. The effect of changes in the pH of the medium on the rate of movement of Colpidium was found to correspond very closely to the effect of similar pHs on the rate of food ingestion. It is, therefore, suggested that changes in the rate of ciliary movement are largely responsible for changes in the rates of food ingestion.
4. Mucus, produced for food collection, and probably also present in the fluid in which the cilia are working, is shown to have a maximum viscosity at pH 8.0. It is suggested that the depression in the region of pH 8.0, seen in curves representing changes in the rates of feeding and movement with pH, indicate that the viscosity of the fluid in which the cilia are beating is one of the limiting factors in the rate of food ingestion through the range of pH occupied by the depression.
Loss of endocytic capacity in aging Paramecium. The importance of cytoplasmic organelles.
