Photoreceptors of the earthworm occur on their:

Explanation

Photoreceptors are restricted only to the dorsal surface. They are more numerous on the prostomium and peristomium of the earthworm and gradually reduce in number towards the posterior end of the body. They are totally absent in the clitellum. Each photoreceptor consists of a single ovoid cell, with a nucleus and clear cytoplasm containing a network of neurofibrillae and a small transparent L shaped lens or optic organelle made up of a hyaline substance. Photoreceptors enable worms to judge the intensity and duration of light.