The Swimming Response and Its Pacemaker System in the Anemone Stomphia Coccinea

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Abstract
1. The swimming reaction of the anemone Stomphia coccinea to Hippasteria phrygiana from Danish waters is identical with that of North American Pacific coast anemones to the starfish H. spinosa and Dermasterias imbricata. 2. The swimming reaction is also evoked by the nudibranch Aeolidia papillosa, which feeds on the anemone (K. W. Ockelmann, unpublished observations). The foot of the mollusc secretes a specific chemical which is not the same as that produced by the starfish, and probably acts at different receptor sites in the anemone. The swimming response is interpreted as an escape reaction to predators which is also evoked by certain starfish. 3. Cutting experiments show that parieto-basilar contractions during swimming arise from a pacemaker ring about halfway up the column, and not from sensory or nervous reflexes. Parts of the system are radially equivalent. Once excited by effective sensory or electrical stimuli, the pacemaker shows properties common to other sources of rhythmical nervous activity. Sites of firing are labile but influence others, and the activity of the intact system is correspondingly integrated.

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