Fig. 5. Differences of maximal light emissions (L_max) of O. californica arm segments after stimulation by tryptamine (10^–7 mol l^–1 and 10^–8 mol l^–1) in normal ASW and in Ca²⁺-free ASW. Values (means ± S.E.M.) are expressed as a percentage of photogenesis triggered by 10^–7 mol l^–1 and 10^–8 mol l^–1 tryptamine, respectively, in normal artificial seawater. **P<0.01; N=8 stimulated aliquot parts for each concentration.