Localization of a circadian pacemaker in the eye of a mollusc, bulla.

The eye of the marine mollusc Bulla contains a circadian pacemaker which, along with critical entrainment pathways, is located among a small group of neurons at the base of the retina. Long-term intracellular recording from cells of the organized photoreceptor layer, which constitutes most of the retinal volume, indicates that these cells are not involved in generating the rhythm since rhythmic changes in membrane potential were not observed. In addition, surgical removal of the entire photoreceptor layer does not alter the period of the circadian rhythm and does not prevent phase shifts by light pulses.