Complex synaptic configurations in planarian brain.

Complex synaptic configurations which appear to have especial evolutionary and functional significance are shown in the neuropil of the brain of the planarian Dugesia dorotocephala. Some of the endings in these synaptic attachments contain dense core vesicles, suggesting that nonadrenaline or serotonin or both are neurotransmitters at a more primitive phyletic level than reported hitherto. The spatial proximity and connectivity of the synapses suggest modes of action permitting greater functional complexity to the planarian brain than previously supposed. Closely adjacent cellular processes which contain polysomal ribosomes, unusual in the neuropil, suggest synaptic transmission-protein synthesis coupling and a possible role in memory.