1. Tritonia has two eyes, each lying beneath the integument at the base of the rhinophore. Three cell types are present in the eye: large photoreceptors with nuclei up to 50 fizn. in diameter, pigment cells, and epithelial cells. A count of nuclei revealed a total of five receptors in each eye.
2. All receptors produced depolarizing generator potentials in the presence of light, regardless of intensity within the responsive range. Responses usually consisted of a fast early depolarization followed by a more slowly rising component. Spikes were uncommon in receptor somata. When present, spikes were superimposed on the slower phase of the response and measured no more than 12 mV. Depolarization was maintained at a steady level when illumination was sustained.
3. The receptors are coupled electrotonically, and synaptic inhibition was evident in a few cells. The functional consequences of these interactions are considered to be minimal.
4. Morphologically, the eye of Tritonia is very similar to that of the nudibranch, Hernissenda. But there are significant differences in the electrophysiology of the two eyes, particularly with respect to cell interactions.
5. Although the maximum recorded sensitivity was about five orders of magnitude less than that considered to be necessary for the detection of sunlight in Tritonia's natural habitat, a visual role in the regulation of periodic behavioural patterns cannot be definitely excluded. The eye may also function in the location or identification of luminescent pennatulids, the major source of food for Tritonia.