| Abstract: | Parallel observations on synaptic and neurohaemal terminals in the polychaete annelids Nereis diversicolor and Harmothoe imbricata have revealed a remarkable identity of ultrastructure. Even features peculiar to the synaptic vesicles of polychaetes are mirrored by those of synaptoid inclusions. A wide range of terminal types show a clear duality of secretory inclusions, featuring both ‘storage granules’ and synaptic/synaptoid vesicles. The inclusions exhibit a marked zonation. Vesicles form tight clusters with interstitial dense material in many terminals and these make contact with release sites. Terminals containing larger, typically peptidergic granules often have mainly dense-cored synaptic/synaptoid vesicles, although some such inclusions are present in other endings also. A variety of synaptic associations are present, and ‘serial synaptoids’ are formed by neurohaemal terminals. The results are interpreted to suggest that synaptic and synaptoid vesicles have a common functional significance. |
