DEFENSE REACTION IN THE POND SNAIL PLANORBIS-CORNEUS .1. ACTIVITY OF THE SHELL-MOVING AND RESPIRATORY SYSTEMS

1. In the intact pond snail Planorbis corneus, tactile or electrical stimulation of the skin evoked a biphasic general defense reaction. A weak stimulation evoked only the first phase of the reaction, represented as a fast pulling of the shell towards the head. With stronger stimulation, this phase was followed by the second phase that was comprised of three components: detachment from the substrate, slow retraction of the body into the shell, and letting out of air from the lung through the pneumostome. 2. About 70 motor neurons (MNs) of the columellar muscle have been revealed in different ganglia by means of their cobalt back-filling through the cut columellar nerve. A complicated pattern of electrical coupling was found for different groups of MNs. Excitation of individual MNs, evoked by current injection, resulted in contraction of the columellar muscle (CNS-columellar muscle preparation). The strongest contraction was evoked by the cerebral MNs; fast small contraction by the parietal MNs; and slow, long-latency contraction, by the pedal MNs. 3. In the same preparation, electrical stimulation of the cutaneous (lip) nerve evoked biphasic contraction of the columellar muscle (a first phase lasting similar to 3 s, and a second phase of up to 1 min). The temporal pattern of this response was similar to that of the defense reaction in the intact animal. A weak stimulation evoked only the first phase of the reaction, while a stronger stimulation evoked both phases. The amplitude of both the first and the second phase was graded with the strength of stimulation. The two phases of muscle response elicited by nerve stimulation were differently represented in the activity of MNs: the cerebral, parietal and lateral pedal MNs received an excitatory input during both phases. The threshold for spike response was much higher in the cerebral MNs. The medial pedal MNs received a prolonged excitatory input during the second phase, while in the first phase a part of them received an inhibitory input. 4. In a CNS-lung preparation, stimulation of the lip nerve resulted in a long-lasting opening of the pneumostome during the second phase of the defense reaction.