Two-dimensional records of the eyecup movements of the crab Carcinus

Summary 1. The eyecup movements of the common shore crab, Carcinus maenas L., have been recorded in two dimensions. 2. Saccades and eyecup drift occur in all directions. Eyecup tremor is a complex irregular movement although horizontal tremor predominates in most crabs. 3. Although the response to the movement of the light in a circle is usually an approximate circle or ellipse, rectangular responses are occasionally obtained and the eyecup often moves in a stepwise fashion in response to a smooth diagonal movement of the light. 4. The angle of response to a diagonal movement depends on the ratio of the response to horizontal and vertical movements alone. The eye is virtually stabilized in all planes by a contrasting object but never fixates upon a moving stimulus. 5. No torsional or twisting movements of the eyecups were obtained in response to light stimuli