Intracellular recording in the retina of the snail, Helix pomatia L., reveals the existence of two types of cell responsive to diffuse flashes of achromatic or monochromatic light: B-type cells, which respond with sustained depolarization that is sometimes accompanied by spikes, and D-type cells, which respond with sustained hyperpolarization. The peak of spectral sensitivity for both B- and D-cells falls in the 450-500 nm range and coincides with range of maximal sensitivity for the rhodopsin family of photopigments. Within a proposed two-channel model of snail achromatic vision, responses of the B- and D-cells are represented by a two-dimensional 'excitation vector'. The length of the 'excitation vector' is approximately constant, and its direction correlates with light intensity. The vector model of light encoding in the snail is discussed in relation to models of achromatic vision in vertebrates (fish, frog, monkey, and humans) based on psychophysical, behavioral and neurophysiological data. Intracellular data in the snail taken together with data from vertebrate animals support the hypothesis that a 2-dimensional model of brightness and darkness encoding utilizes a universal mechanism of 'vector encoding' for light intensity in neuronal vision networks.