BIOLUMINESCENCE AND OTHER RESPONSES SPREAD BY EPITHELIAL CONDUCTION IN THE SIPHONOPHOREHIPPOPODIUS

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Abstract
1. Four responses are spread by through-conducting excitable epithelia in the nectophores: luminescence, blanching, muscular involution, and secretion. Swimming, which is independently controlled by the nervous system, is inhibited by epithelial impulses. 2. Luminescent flashes are correlated one for one with epithelial impulses. At least three impulses must be propagated before the first flash is recorded. Flashes sum and facilitate. Pacemaker-like after-discharges may continue after stimulation has ceased. Not all regions of the epithelium luminesce equally, and the active area can shift during a single luminescent episode, although the excitatory impulses pass across all regions equally. No steady luminescent glow has been observed. Comparisons are drawn with other luminescent systems. 3. Luminescence is generated intracellularly within the exumbrellar epithelium, but blanching (opacity) is associated with formation of granules in the adjacent mesogloea. The response builds up to saturation level within 10 sec, long before the exhaustion of luminescence. Fading is gradual, transparency returning within an hour. Some regions blanch more strongly and fade more slowly than others. 4. Involution of the margin involves contraction of radial muscle fibres in the velar ectoderm and subumbrellar endoderm. The two groups are functionally coupled, but coupling may break down with repeated stimulation. Excitation can pass from ecto- to endoderm at the margin and either way between the two layers on the axial side of the nectophore. Thus, the endodermal effectors, including the secretory epithelium, are excited concurrently with the ectodermal. 5. Impulse conduction in the excitable epithelia and myoepithelia is assumed to involve electrical coupling mediated by gap junctions, which have been found by electron microscopy in the regions concerned.

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