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Journal of Experimental Biology 49,285-297 (1968)
Published by Company of Biologists 1968

Eyecup Withdrawal in the Crab, Carcinus, and its Interaction with the Optokinetic Response

M. BURROWS ¹ and G. A. HORRIDGE ²

¹ Gatty Marine Laboratory and Department of Natural History, University of St Andrews, Fife, Scotland; Department of Biology, University of Oregon, Eugene, Oregon. 97403 U.S.A.
² Gatty Marine Laboratory and Department of Natural History, University of St Andrews, Fife, Scotland

1. Protective withdrawal of the eyecup is caused by a burst of impulses in two axons of the optic tract, one to muscles 19a, 19b and 20a, the other to muscles 18, 20b, 21 and 22.

2. At a reflex eyecup withdrawal other concurrent activity is mechanically overridden ; the tonic activity in only one muscle is inhibited centrally. At a ‘spontaneous’ withdrawal, however, all motor activity to that eyecup is inhibited.

3. The largest muscle, 19a, inactive in other eyecup movements, is the prime mover in withdrawal, and some tonic fibres of this muscle hold the eyecup withdrawn.

4. Two muscles which move the eyecup toward the mid line on optokinetic responses are excited during a withdrawal. It is therefore possible for one muscle to contribute to movements in opposite directions.

5. Repeated reflex withdrawal of an eyecup moving towards the mid line inhibits the optokinetic response of the other eye.

6. Weak stimulation of an eyecup region by a variety of means, including withdrawal, improves the optokinetic response of that eyecup and sometimes of the other eyecup