Abstract
1. Habituation of the withdrawal response in Nereis diversicolor has been investigated by photographing successive responses to sudden decreases and sudden increases in illumination.2. Responses to sudden decreases in illumination are usually anterior end withdrawals whereas sudden increases usually elicit posterior end withdrawals. Withdrawals to sudden decreases are faster and have shorter latencies than those to sudden increases.3. Most worms responded to the first stimulation in a series by withdrawing, but the number responding diminished with successive trials. Although there was an initial decline in the extent of withdrawals, subsequent contractions were approximately constant.4. Sudden decreases in illumination sometimes inhibit irrigating movements, particularly when the stimulus is novel. However, habituation is rapid and apparently independent of habituation of the withdrawal response; after the third trial irrigating activity was infrequently inhibited by stimulation although withdrawals continued to occur.
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