This is a cross-section of a so-called Dunn chamber with two silicone rings. The inner ring contained high concentrations of PAH, whereas the outer ring did not contain any PAH. During the experiments, the ciliates moved from the inner ring, where they absorbed the PAH, to the outer ring, where they released them. Dorothea Gilbert demonstrated this by measuring the content of PAH in the two silicone rings.
*Gap heigth 10 times enlarged