For many lower animals the optokinetic response of the eye or head to a movement of the whole visual field is a useful indication that the. movement has been seen. The lower velocity limit of the optokinetic response is extraordinarily low in some animals. In the dog, ter Braak (1936) reports responses to drum movements at 6·4°/hr., which is comparable to the lower limit in the crab. The question arises as to whether a memory of successive positions is involved.

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