The Swimming of Nymphon Gracile (Pycnogonida)

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Abstract
1. During the swimming of intact Nymphon gracile the legs beat ventrally in a metachronal sequence starting from the rear, and with a phase delay of approximately one-third between successive appendages. The first and fourth ipsilateral legs thus beat together, usually in synchrony with either the second or the third contralateral leg, rather than halfway between them. 2. The pattern of leg movement in swimming involves a succession of such combinations, e.g. (R4 R1 L2) (R3 L4 L1) (R2 L3), and the preferred gait alternates between this sequence and its mirror-image combination. 3. Amputation of one or both of the third leg pair was followed by a lateral movement of the adjacent legs, but the rhythm of movement was little impaired, to the extent of reducing slightly the phase delay of leg 2 in leg 4 on the same side. 4. The effect of an imposed cycle of movement on the phase relationship of adjacent legs suggests that proprioceptive feedback plays only a little part in entraining the rhythm, but there is some evidence for reflex coupling between adjacent legs. This appears to be strongest within the segment. 5. The results are best explained in terms of negative coupling between oscillators controlling each leg.

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