1. On either side of the nervous system in Aplysiathere is a giant cell (RGC and LGC) whose axon branches within the nervous system. The distribution of these branches has been traced in experiments involving stimulation and recording and the use of intracellular electrodes (in the soma) and extracellular electrodes (on nerves containing the axons).
2. On the right side the RGC sends an axon along the pleuro-visceral connective to the pleural-pedal ganglia where it divides and gives branches to the cerebro-pleural connective and each of the main nerves supplying the foot and parapodium.
3. Stimulation of any nerve containing a branch of this axon produces a large spike in the right connective and an antidromic potential in the soma. Transmission between the different branches is not always easy following antidromic stimulation but is always present in the orthodromic direction whether produced synaptically, by direct stimulation of the soma, or by mechanical pressure applied to the somatic membrane.
4. The LGC soma is in the left pleural ganglion near the origin of the left pleurovisceral connective. Similar techniques have shown that this cell sends branches to the corresponding nerves on the left side.
5. An hypothesis is suggested to account for the presence of the cell bodies of the RGC and LGC in two different ganglia, despite the similarities in branching of their axons. Possibly differences during torsion and detorsion in the fate of the ganglia in which these cells originate may account for their different locations in the adult.
6. The function of the RGC was investigated in whole-animal preparations. Although it tends to discharge when the animal makes spontaneous protective movements or is touched anywhere on its surface, stimulation of the cell directly through an intracellular electrode gives no overt movements of the animal.