Abstract
1. The muscular system of the sea urchin pluteus is stimulated by nicotinic agents. Excessive stimulation is followed by paralysis of the most powerful muscular strands. The effects are counteracted by antinicotinic agents. 2. Muscarinic agents keep the activity low, calm down initially unquiet larvae, and counteract the nicotinic effects. 3. The effects are quickly elicited and in general are also quickly reversible in sea water, which indicates that most of the receptors involved occur at the larval surface. 4. It is suggested that the nicotinic and muscarinic signals control certain ionic fluxes in opposite directions, and that the stimulatory effect on the most powerful muscular strands is amplified by a monoaminergic system.
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