Simple photoreceptors in Limulus polyphemus.

The "olfactory nerve," the endoparietal eye, and the rudimentary lateral eyes of Limulus (polyphemus) contain simple photoreceptor cells that duplicate many of the electrical responses of the retinular cells of the lateral eye; the responses are a receptor potential consisting of aninitial transient phase and a subsequent steady phase,low-amplitude fluctuations, and a small locally regenerative response to pulses of both light and current. Photic stimulation does not induce conducted action potentials, but does increase the membrane conductance. The receptor potentialrequires the presence of sodium ions in the external medium. Measurements of action and absorption spectra indicate a photopigment whose maximum absorption is of light with wavelength of 535 nanometers. The functional significance of these cells has not been ascertained.