1. The role of the paired serotonergic cerebral giant cells (CGCs) in the feeding system of Lymnaea was examined by electrophysiological and pharmacological techniques. 2. The firing characteristics of the CGCs were recorded by fine wires attached to their cell bodies in freely moving intact snails (in vivo recording) and their "physiological" rates of firing determined during feeding and other behaviors. 3. The mean CGC firing rates recorded in vivo varied between 1 and 20 spikes/min but never reached the average rates seen in the isolated CNS (60-120 spikes/min). Maximum rates of firing were seen during bouts of radula biting/rasping movements characteristic of the consummatory phase of feeding (15 +/- 1.69 spikes/min, mean +/- SE, range 7-20 spikes/min), with lower rates seen during locomotion (6.7 +/- 0.75 spikes/min; range 5-9 spikes/min. The cells were rarely active when the animal was quiescent (1.45 +/- 0.91 spikes/min; range 0-2 spikes/min). 4. In vivo recorded CGC firing was phase locked to the feeding movements of the animal, with spikes occurring just before the opening of the mouth, during the protraction phase of the feeding cycle. 5. Evoking firing rates on the CGCs in the isolated preparation similar to those seen in vivo during rasping movements (7-20 spikes/min) did not elicit a fictive feeding pattern in an inactive preparation. Neither did bath application of 10(-9) M serotonin (5-HT; the transmitter of the CGCs). 6. To allow the modulatory role of the CGCs to be examined during patterned activity, the fictive feeding pattern was evoked in the isolated preparation by injecting depolarizing current into a modulatory neuron, the slow oscillator (SO). 7. The tonic firing activity of the CGCs was accurately maintained by current injection in the isolated preparation at rates equivalent to that occurring during feeding, locomotion, and quiescence in the intact snail. This was possible where the CGCs became silent after 1-2 h. Only when the CGCs activity was maintained at a rate (approximately 15 spikes/min) similar to that occurring during rasping, was the SO able to drive a full, high-frequency fictive feeding pattern (15-20 cycles/min). At lower rates of CGC firing, the SO-driven rhythm was either of lower frequency or no rhythm occurred at all (CGCs silent). 8. In many isolated preparations (80%) the CGCs remained active, and it was difficult to maintain specific levels of tonic activity by current injection.(ABSTRACT TRUNCATED AT 400 WORDS)

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M ost cephalopods have m ore chrom atophores on the dorsal body surface than on the ventral and these tend to be kept tonically expanded. As a result the dorsal surface is usually darker than the ventral, an effect shown by m any anim als and known as countershading. We report here th at when Sepia officinalis, Loligo vulgaris and Octopus vulgaris are rotated 180° around the longitudinal body axis the ventral chrom atophores expand, causing darkening, while the dorsal chrom atophores retract, causing paling. W hen anim als are ro tated through only 90° the chrom atophores on the upperm ost half of the ventral and dorsal surfaces expand, while those on the lower h alf retract. This response, which we term the countershading reflex, can be abolished by ablating the statocysts; and experim ents in which the direction of incident light is reversed show th at the reflex is not driven by sensory input from the eyes. T he function of the reflex is presum ably to m aintain countershading while the anim al is m om entarily disoriented; this idea is supported by the fact th a t it lasts only a few seconds.