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

Although the bell-shaped nectophores of the siphonophore Nanomia bijuga are clearly specialized for locomotion, their complex neuroanatomy described here testifies to multiple subsidiary functions. These include secretion, by the extensively innervated 'flask cells' located around the bell margin, and protection, by the numerous nematocytes that line the nectophore's exposed ridges. The main nerve complex consists of a nerve ring at the base of the bell, an adjacent column-shaped matrix plus two associated nerve projections. At the top of the nectophore the upper nerve tract appears to have a sensory role; on the lower surface a second nerve tract provides a motor input connecting the nectophore with the rest of the colony via a cluster of nerve cells at the stem. N. bijuga is capable of both forward and backward jet-propelled swimming. During backwards swimming the water jet is redirected by the contraction of the Claus' muscle system, part of the muscular velum that fringes the bell aperture. Contractions can be elicited by electrical stimulation of the nectophore surface, even when both upper and lower nerve tracts have been destroyed. Epithelial impulses elicited there, generate slow potentials and action potentials in the velum musculature. Slow potentials arise at different sites around the bell margin and give rise to action potentials in contracting Claus' muscle fibres. A synaptic rather than an electrotonic model more readily accounts for the time course of the slow potentials. During backward swimming, isometrically contracting muscle fibres in the endoderm provide the Claus' fibres with an immobile base.