ABSTRACT
The eyes of the heteropod molluscs are unusually large and complex, compared with those of other gastropods (Hesse, 1900). They have a basic design not unlike fish eyes, with a large spherical lens of short focal length that forms an image on a retina of highly ciliated photoreceptors (Dilly, 1969). Perhaps the strangest feature of the eyes is the shape of the retina. It is not a hemispherical cup as in fish, cephalopods and alciopid worms, but a long narrow ribbon only a few receptors wide (6 in Pterotrachea coronata, only 3 in Oxygyrus). The ribbon is several hundred receptors long and in some species it is straight whilst in others it is curved into a horseshoe configuration. In either case this means that the field of view of the eye is a narrow strip of space a few degrees high and 80–180° long. Most of the surroundings are therefore not seen by the animal at any given time. In this paper I show that the problem of the narrow field of view is solved in one heteropod species at least by systematic scanning movements of the eyes, in which each retina ‘sweeps’ through a 90° arc.
