Mechanisms for Response – Protozoan
Protozoa are sensitive to many kinds of stimuli like touch, temperature changes, light, chemicals, etc. It is not clear how they do so but amoebae are constantly changing and their protoplasmic mass can receive stimuli as well as conduct them. Cilia and flagella are highly sensitive to touch. One special sensory organelle is the stigma or eyespot.
The stigma is a reddish body found in photosynthetic flagellates and usually located near the root of the flagellum. The stigma is often cupped shading the light sensitive area from one side. Again it is not clear how the stimulus is conducted from the point of reception to another point where activity may change.