The photograph shows a roundworm, C. elegans. Magnification $$\(\times 200\)$$ (Source: © STEVE GSCHMEISSNER / SCIENCE PHOTO LIBRARY) These roundworms are simple animals with a nervous system consisting of 302 neurones. The roundworm can respond to stimuli with reflex actions. A reflex action is a rapid involuntary movement in response to a stimulus that involves receptors. Which receptors detect a change in light intensity and stimulate an action potential? (1)

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