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(a) Hypothetical ancestor of all metazoans (Urmetazoa) with a diversity of secretory cells capable of volume transmission and early radiation of basal metazoan lineages preserving both non-synaptic transmission and the parallel development of systems for intercellular communications including synapses. (b) Non-specific injury response and repair signalling as an evolutionary predecessor of neurotransmitters [19]. Shadows show gradients of neurotransmitters from their release sites.
