Summary.1. The trichocysts are the means wherebyParameciumadheres to surfaces. Adherence takes place whenParameciumare attracted to a particular spot. The cause of this attraction is largely thepH of the water, but it may be masked in certain circumstances.2. The cause of the extrusion of the trichocysts is shown to be slight pressure such as may be set up by theParameciumcolliding with an object in the water. Verworn's view that the trichocyst consists of semi‐liquid material which hardens on being extruded into the water, is here adopted. It can be shown from the experimental evidence that the tip of the trichocyst thread is sticky, but the rest of the thread is not.3. Ciliary motion does not cease whenParameciumis attached by the trichocysts. The speed of movement, which is dependent on ciliary activity, is reduced when the/>H of the water in which the animals are living reaches 80. This reduction in ciliary activity results in the slender trichocysts being able to hold theParameciumfast, when the pH is within 0.1 of 7.9. A further increase in thepH of the water above 8.0 results in the speed of theParameciumbeing so much reduced as to render the force of a collision insufficient to expel the trichocyst. For theParameciumstudied the criticalpH for adherence was 7.95.4. It is shown that the observations of previous workers on the structure of the trichocyst accord very well with their functioning as organs of adherenc