IONIC REGULATION IN THE CILIATE SPIROSTOMUM AMBIGUUM

It is generally accepted that the cytoplasm of fresh-water Protozoa is hypertonic to the medium. This is based on circumstantial evidence and indirect measure-ments of the osmotic pressure of the body fluids. Inorganic ions are likely to account for a large part of the osmotic pressure, and in certain Protozoa radioactive isotopes may be used to determine the concentrations of ions. The isotope tech-nique has been used to demonstrate that cells in Metazoa, namely nerve, muscle and erythrocytes, accumulate ions and carry out ionic regulation. Evidence is presented in this paper concerning the tonicity of the body fluids of the hetero-trichous ciliate Spirostomum ambiguum and concerning its powers of ionic regulation. The concentration of sodium, potassium and bromide have been measured in animals gTown in medium containing these ions. The rates of entry and loss of these ions have been measured and the results are discussed in the light of current theories of the mechanisms of ionic regulation in other cells. MATERIALS AND METHODS Spirostomum ambiguum was chosen for the experiments as it is easily cultured, it is relatively large and it survives the experimental conditions. It was cultured in the following medium: „ 6 IUM/I