SUMMARY

1. The excitation pathways mediating the protective crumpling behaviour of Polyorchis penicillatus were studied with electrophysiological and ultra-structural techniques. 2. Stimulating the subumbrellar endoderm consistently resulted in a complex crumpling potential when recorded with suction electrodes from radial muscle (the prime effector). The potential represents the summation of a quick radial muscle potential (RMP) and a slow endodermal canal pulse (ECP). 3. The latencies of ECPs recorded from radial muscle during crumpling were directly proportional to the distance between the recording electrode and the subumbrellar stimulating electrode. Conversely, the latencies of RMPs, which were not tightly time-coupled to ECPs, were more directly related to the distance of the recording and stimulating electrodes from the marginal or apical termini of the radial muscle. 4. Stimulating the exumbrellar ectoderm resulted in a variable crumpling response, typically occurring after facilitation of numerous exumbrellar pulses (EPs). Since exumbrellar stimulation did not usually excite endoderm, the response recorded from radial muscle normally involved a simple RMP, un-associated with an ECP. 5. Typical synaptic junctions were observed between radial muscle pro-cesses and marginal neurites and between radial muscle and neurites of the radial nerve bundles along the length of the muscle. 6. The independence of the ECP and RMP conducting pathways demon-strates that endoderm does not provide the direct source of radial muscle excitation and the initiation of RMPs at points of known (marginal) and suspected (apical) nerve-muscle contact suggests the involvement of nerves in the control of crumpling behaviour. 7. These results are discussed in the light of other examples of active neuronal-epithelial interaction