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 s timulation of the skin evoked a biphasic general defense reaction. A w eak stimulation evoked only the first phase of the reaction, represent ed as a fast pulling of the shell towards the head. With stronger stim ulation, this phase was followed by the second phase that was comprise d of three components: detachment from the substrate, slow retraction of the body into the shell, and letting out of air from the lung throu gh the pneumostome. 2. About 70 motor neurons (MNs) of the columellar muscle have been revealed in different ganglia by means of their cobal t back-filling through the cut columellar nerve. A complicated pattern of electrical coupling was found for different groups of MNs. Excitat ion of individual MNs, evoked by current injection, resulted in contra ction of the columellar muscle (CNS-columellar muscle preparation). Th e strongest contraction was evoked by the cerebral MNs; fast small con traction by the parietal MNs; and slow, long-latency contraction, by t he pedal MNs. 3. In the same preparation, electrical stimulation of th e 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 evoke d only the first phase of the reaction, while a stronger stimulation e voked both phases. The amplitude of both the first and the second phas e was graded with the strength of stimulation. The two phases of muscl e response elicited by nerve stimulation were differently represented in the activity of MNs: the cerebral, parietal and lateral pedal MNs r eceived an excitatory input during both phases. The threshold for spik e response was much higher in the cerebral MNs. The medial pedal MNs r eceived 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 defe nse reaction.