ABSTRACT
The response of Onchidoris fusca to monochromatic light in the range 400·650 m· was measured in behaviour experiments with two opposing lights.
The peak sensitivity was found to lie at 500−505 mμ. The thick, brown, dorsal skin of Onchidoris has the effect of reducing the sensitivity of the eye to ultraviolet and blue light.
After allowing for a 90% absorption of light by the dorsal skin, the absolute threshold of the naked eye of Onchidoris (at 501 mμ) was found to lie between 81 and 106 quanta/sec./eye.
If the lens and cornea absorb a further 10% of the available light, then each of five receptor cells in the eye of Onchidoris may be capable of responding to 12−18 quanta/sec. However, the effective threshold of the eye of Onchidoris is much higher, lying at 881−1148 quanta/sec.
