Structure and function of the nervous system in nectophores of the siphonophore Nanomia bijuga.

i) Although Nanomia's bell-shaped nectophores are specialized for locomotion, their cellular elements and complex nerve structures suggest they have multiple subsidiary functions.ii) The main nerve complex is a nerve ring at the base of the bell, an adjacent columnar-shaped matrix plus two associated nerve projections. An upper nerve tract may provide a sensory input while a lower nerve tract connects with the rest of the colony apparently via a cluster of nerve cells at the stem.iii) The structure of the extensively innervated "flask cells" located around the bell margin suggests a secretory function.iv) The numerous nematocytes present on exposed ridges of the nectophore appear to have an entangling rather than a penetrating role.v) Movements of the velum, produced by contraction of the Claus' muscle system during backwards swimming, can be elicited by electrical stimulation of the surface epithelium even when the major nerve tracts serving the nerve ring have been destroyed (confirming Mackie, 1964).vi) Epithelial impulses generated by electrical stimulation elicit slow potentials and action potentials in the velum musculature. The slow potentials arise at different sites around the velum and give rise to action potentials in contracting Claus' muscle fibres.vii) A synaptic rather than an electrotonic model can more readily account for the time course of the slow potentials.viii) During backward swimming radial muscle fibres in the endoderm contract isometrically providing the Claus' fibres with a firm foundation.