Class 11 NEET Biology Structural Organisation in Animals

(c) : Photoreceptors restricted only to dorsal surface, are more numerous on prostomium and peristomium of earthworm and gradually reduce in number towards posterior end of body. They are totally absent in 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 or phaosome, made up of a hyaline substance. Photoreceptors enable worms to judge the intensity and duration of light.